frequency hopping planning guide

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Frequency Hopping Network Planning Guide


Version Date Page

1.0.0 Oct 23, 1998 2/80

HISTORY

Version Date Author Comments

0.0.1 21 Sep, 1998 MaSa The first draft

0.0.2 24 Sep, 1998 JRy Modifications: The whole document restructured,

Chapter 2.3: PC and DTX gains, Chapter 7.2: RXQual

distribution, Table 9: Ho Threshold Interference.

Added: Figure 5-13, Figure 5-14, Figure 7-3, Figure 7-4,

Figure 7-8, Table 10, Table 11, Table 12.

Added: History , Chapter 2.1.6, Chapter 3.6, Chapter

3.9.1, Chapter 3.9.6, Chapter 3.9.7, Chapter 3.9.8,

Chapter 5.1, Chapter 5.2, Chapter 5.6.2, Chapter 6.3,

Chapter 7.1, Chapter 7.2, Chapter 7.9.

1.0.0 23 Oct, 1998 JRy The first accepted version


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CONTENTS

1. INTRODUCTION ........................................................................................................................... 5

1.1 GENERAL DESCRIPTION OF FREQUENCY HOPPING ........................................................................... 5

1.2 FREQUENCY HOPPING MODES ........................................................................................................ 6

1.3 CELL ALLOCATION ........................................................................................................................ 8

1.4 MOBILE ALLOCATION .................................................................................................................... 9

1.5 HOPPING SEQUENCE NUMBER ........................................................................................................ 9

1.6 MOBILE ALLOCATION INDEX OFFSET ............................................................................................. 9

1.7 MAIO STEP ................................................................................................................................. 10

2. THEORETICAL PERFORMANCE OF FREQUENCY HOPPING ......................................... 11

2.1 FREQUENCY DIVERSITY ............................................................................................................... 11

2.1.1 Coherence Bandwidth.......................................................................................................... 11

2.1.2 Effect of Interleaving ........................................................................................................... 13

2.1.3 Cyclic vs. Random Hopping Sequences ................................................................................ 14

2.1.4 Simulated Frequency Diversity Gains .................................................................................. 14

2.1.5 Effect in Cell Coverage Area ............................................................................................... 16

2.1.6 Effect of Mobile Speed ......................................................................................................... 16

2.2 INTERFERENCE DIVERSITY ........................................................................................................... 16

2.3 EFFECT OF POWER CONTROL AND DTX ........................................................................................ 18

3. NOKIA’S SUPPORT FOR FREQUENCY HOPPING IN GSM ................................................ 20

3.1 BSS LEVEL IMPLEMENTATION...................................................................................................... 20

3.2 THE 2ND GENERATION BASE STATION .......................................................................................... 20

3.3 TALK FAMILY BASE STATION ....................................................................................................... 21

3.4 PRIMESITE ................................................................................................................................... 22

3.5 BASE STATION CONTROLLER........................................................................................................ 23

3.6 NPS/X ......................................................................................................................................... 23

3.7 MAXIMUM CONFIGURATIONS ....................................................................................................... 23

3.8 RADIO NETWORK FAULT MANAGEMENT ...................................................................................... 24

3.8.1 The 2nd Generation Base Station......................................................................................... 25

3.8.2 Talk Family Base Stations and PrimeSite ............................................................................ 25

3.9 RESTRICTIONS ON THE USAGE OF FH ............................................................................................ 25

3.9.1 DL Power Control with BB FH ............................................................................................ 25

3.9.2 Downlink DTX ..................................................................................................................... 26

3.9.3 Extended Range Cell (DE34/DF34/DG35) .......................................................................... 26

3.9.4 MS Speed Detection ............................................................................................................. 26

3.9.5 Half Rate ............................................................................................................................. 26

3.9.6 Frequency Sharing .............................................................................................................. 26

3.9.7 RTC Combiner .................................................................................................................... 26

3.9.8 NPS/X .................................................................................................................................. 26

4. SELECTING THE RIGHT HOPPING STRATEGY ................................................................. 27

5. FREQUENCY PLANNING OF FREQUENCY HOPPING NETWORKS ............................... 29

5.1 NETWORK PLANNING PROCEDURE ................................................................................................ 29


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5.2 FREQUENCY PLANNING PROCEDURE WITH NPS/X ........................................................................ 30

5.3 FREQUENCY REUSE ON FREQUENCY HOPPING NETWORK .............................................................. 33

5.3.1 Effective Reuse .................................................................................................................... 34

5.3.2 Frequency Allocation Reuse (RF FH only) .......................................................................... 34

5.4 LOAD ON NETWORKS UTILISING FRACTIONAL LOADING (RF FH ONLY) ........................................ 35

5.4.1 Frequency Load................................................................................................................... 35

5.4.2 Hard Blocking Load ............................................................................................................ 36

5.4.3 Fractional Load................................................................................................................... 37

5.5 TRUNKING EFFECT AND EFFECTIVE REUSE ................................................................................... 38

5.6 FREQUENCY ALLOCATION STRATEGIES ........................................................................................ 40

5.6.1 BCCH Allocation ................................................................................................................. 40

5.6.2 Selecting the Effective Reuse (BB FH) ................................................................................. 43

5.6.3 Selecting the Frequency Allocation Reuse and the Frequency Load (RF FH) ....................... 44

5.6.4 Frequency Sharing by Using MAIO Management (RF FH only) .......................................... 46

5.6.5 Frequency Sharing in the Single MA-list Scheme (RF FH only) ........................................... 50

6. RADIO NETWORK PARAMETERS ......................................................................................... 52

6.1 PARAMETERS FOR MA-LIST DEFINITIONS IN BSC ......................................................................... 52

6.2 BTS LEVEL FH RELATED PARAMETERS ....................................................................................... 54

6.3 POWER CONTROL ......................................................................................................................... 56

6.4 HANDOVER .................................................................................................................................. 58

6.5 DTX ............................................................................................................................................ 59

6.5.1 Uplink DTX ......................................................................................................................... 59

6.5.2 Downlink DTX ..................................................................................................................... 59

7. OPTIMISATION .......................................................................................................................... 60

7.1 TOOLS FOR NETWORK MONITORING ............................................................................................. 60

7.2 KPIS FOR HOPPING NETWORK ...................................................................................................... 60

7.3 RXQUAL IN FH NETWORKS ........................................................................................................ 61

7.4 IDLE CHANNEL INTERFERENCE MEASUREMENT ............................................................................ 65

7.5 CYCLIC AND RANDOM HOPPING SEQUENCES ................................................................................ 66

7.6 INTRACELL HANDOVER ................................................................................................................ 69

7.7 POWER CONTROL ......................................................................................................................... 69

7.7.1 Downlink Power Control with BB Hopping ......................................................................... 70

7.8 HANDOVER CONTROL .................................................................................................................. 70

7.9 HSN PLANNING WITH RANDOM HOPPING ..................................................................................... 70

8. PLANNING CASES ..................................................................................................................... 71

8.1 PLANNING CASE 1: SINGLE MA-LIST ............................................................................................ 71

8.1.1 Frequency Planning ............................................................................................................ 71

8.1.2 MAIO Planning ................................................................................................................... 72

8.2 PLANNING CASE 2: RF FH WITH FRACTIONAL LOADING (FAR 3 – 5) ............................................ 75

8.2.1 Defining the Frequency Band and the Number of Frequencies Needed in Each Cell ............ 75

8.2.2 Frequency Allocation and Analysis ...................................................................................... 77

8.3 PLANNING CASE 3: RF FH WITH FREQUENCY SHARING ................................................................. 78

8.3.1 Frequency Planning ............................................................................................................ 78

8.3.2 MAIO Planning ................................................................................................................... 79

8.3.3 Analysis ............................................................................................................................... 80


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1. INTRODUCTION

The purpose of this document is to explain the theory behind the frequency hopping (FH), how the

frequency hopping is implemented in Nokia’s network elements, how to choose the right frequency

hopping strategy, parameters related to FH, frequency allocation procedure, how to analyse the

quality of the network and the optimisation process. Also some practical planning examples are

presented.

Frequency hopping is one of the standardised capacity enhancement features in GSM system. It

offers a significant capacity gain without any costly infrastructure requirements. It is also compatible

with all the existing GSM mobile phones, since the frequency hopping support has been required by

the GSM specifications from the beginning. Frequency hopping can co-exist with most of the other

capacity enhancement features and in many cases it significantly boosts the effect of those features.

All these factors make frequency hopping a very tempting capacity enhancement solution.

Figure 1-1. Solutions to enhance network capacity.

1.1 General Description of Frequency Hopping

Frequency hopping can be briefly defined as a sequential change of carrier frequency on the radio

link between the mobile and the base station.

In GSM, one carrier frequency is divided into eight time slots. Each time slot provides one physical

channel, which can be assigned to one link between a mobile and a base station. The communication

between the mobile and the base station occurs in bursts inside the assigned time slot. Each burst

lasts about 577 µs. When frequency hopping is used, the carrier frequency may be changed between

each consecutive TDMA frame. This means that for each connection the change of the frequency

may happen between every burst. This is called Slow Frequency Hopping (SFH), because more than

one bit is transmitted using the same frequency. In Fast Frequency Hopping (FFH), the carrier

frequency is allowed to change more than once during a bit duration, but this is not implemented in

GSM.

CAPACITY GAIN

Effective Network Planning

Channel-Bandwidth Spectrum Cell Size Reuse-Factor (C/I)

Dual-Band-/Dual-Band-/

Dual-Mode-Dual-Mode-

NetworksNetworks

PC DTX FHPC DTX FH

Smart AntennasSmart Antennas

IUOIUOIFHIFH

Half-RateHalf-Rate

NetworksNetworksAntennas Down Antennas Down

Ant.Ant. Downtilting Downtilting

Micro-CellMicro-Cell

Pico-Cell / IndoorPico-Cell / Indoor


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At first, the frequency hopping was used in military applications in order to improve the secrecy and

to make the system more robust against jamming. In cellular network, the frequency hopping also

provides some additional benefits such as frequency diversity and interference diversity. The basic

principle of frequency hopping is presented in Figure 1-2.

Frequency

Time

F1

F2

F3

Call is transmitted through severalfrequencies in order to • average the interference (interference diversity)• minimise the impact of fading (frequency diversity)

Figure 1-2. Basic functionality of frequency hopping.

1.2 Frequency Hopping Modes

The requirement that the BCCH TRX must transmit continuously in all the time slots sets strict

limitations on how the frequency hopping can be realised in a cell. The current solutions are

Baseband Frequency Hopping (BB FH) and Synthesised Frequency Hopping (RF FH).

In the baseband frequency hopping the TRXs operate at fixed frequencies. Frequency hopping is

generated by switching consecutive bursts in each time slot through different TRXs according to the

assigned hopping sequence. The number of frequencies to hop over is determined by the number of

TRXs. Because the first time slot of the BCCH TRX is not allowed to hop, it must be excluded from

the hopping sequence. This leads to three different hopping groups. The first group doesn’t hop and it

includes only the BCCH time slot. The second group consists of the first time slots of the non-BCCH

TRXs. The third group includes time slots one through seven from every TRX. This is illustrated in

Figure 1-3.


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B

RTSL 0 1 2 3 4 5 6 7

TRX-1

TRX-2

TRX-3

TRX-4

f1 B = BCCH timeslot. It does not hop.

f2

f3

f4

Time slot 0 of TRX-2,-3,-4 hop over f2,f3,f4.

Time slots 1...7 of all TRXs

hop over (f1,f2,f3,f4).

Figure 1-3. Baseband hopping (BB FH).

In the synthesised frequency hopping all the TRXs except the BCCH TRX change their frequency

for every TDMA frame according to the hopping sequence. Thus the BCCH TRX doesn’t hop. The

number of frequencies to hop over is limited to 63, which is the maximum number of frequencies in

the Mobile Allocation (MA) list covered in Section 1.4. Synthesised hopping is illustrated in Figure

1-4.

BTRX-1

Non-BCCH TRXs are hopping over

the MA-list (f1,f2,f3,...,fn) attached to the cell.

TRX-2

B = BCCH timeslot. TRX does not hop.

f1,

f2,

f3,

fn

f1,

f2,

f3,

fn

. . . .

Figure 1-4. Synthesised hopping (RF FH).

The biggest limitation in baseband hopping is that the number of the hopping frequencies is the same

as the number of TRXs. In synthesised hopping the number of the hopping frequencies can be

anything between the number of hopping TRXs and 63. However in synthesised hopping the BCCH

TRX is left completely out of the hopping sequence. The differences between BB and RF hopping

are further illustrated in Figure 1-5.

HSN2

HSN1

HSN1


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MSC

BB-FHF1(+ BCCH)

F2

F3

Dig. RF

TRX-3

TRX-1

RF-FH

F1, F2, F3

Dig. RF

TRX-1

TRX-2

BSCTCSM

BCCH

Frequency

Time

F1F2F3

MS does not seeany difference

BB-FH is feasible with large configurations RF-FH is viable with smaller configurations

Figure 1-5. The difference between BB and RF FH.

1.3 Cell Allocation

The Cell Allocation (CA) is a list of all the frequencies allocated to a cell. The CA is transmitted

regularly on the BCCH. Usually it is also included in the signaling messages that command the

mobile to start using a frequency hopping logical channel. The cell allocation may be different for

each cell.

In GSM 900 the CA list may include all the 124 available frequencies [GSM 04.08]. However, the

practical limit is 64, since the MA-list can only point to 64 frequencies that are included in the CA

list as presented in the next section. The only signaling method allowed in the GSM 900 systems to

transmit the CA list is the “bit map 0” method presented in Table 1.

Table 1. The signalling method for transmitting the CA list in GSM 900 system.

CA signaling

method

Lowest

ARFCN

Max. ARFCN range Max. number of

frequencies in the CA list

bit map 0 0 124 124* * Practical limit is 64, because the MA-list can only point to 64 frequencies.

In GSM 1800 and GSM 1900 systems the frequency band is so large that the CA list cannot include

all the frequencies available in a system. In these systems the “bit map 0” method is not available, but

five other methods can be used [DCS 04.08] [J-STD 7]. Each of these methods has different

limitations that limit the maximum frequency range and the maximum number of frequencies. These

signaling methods together with their limitations are presented in Table 2. In Nokia implementation

the variable bit map and the 512 range signaling methods are available. The CA list is always

automatically generated and it includes the BCCH frequency and the frequencies that are defined for

the MA-list.


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Table 2. Different signalling methods for transmitting the CA list in GSM 1800/1900.

CA signaling

method

Lowest

ARFCN

Max. ARFCN

range

Max. number of

frequencies in the CA list

1024 range 0 1024 16 (17 if ARFCN 0 is included in the CA list)

512 range selectable 512 18



variable bit map selectable 112 112* * Practical limit is 64, because the MA-list can only point to 64 frequencies.

1.4 Mobile Allocation

The MA is a list of hopping frequencies transmitted to a mobile every time it is assigned to a hopping

physical channel. The MA-list is a subset of the CA list. The MA-list is automatically generated if the

baseband hopping is used. If the network utilises the RF hopping, the MA-lists have to be generated

for each cell by the network planner. The MA-list is able to point to 64 of the frequencies defined in

the CA list. However, the BCCH frequency is also included in the CA list, so the practical maximum

number of frequencies in the MA-list is 63. The frequencies in the MA-list are required to be in

increasing order because of the type of signaling used to transfer the MA-list.

1.5 Hopping Sequence Number

The Hopping Sequence Number (HSN) indicates which hopping sequence of the 64 available is

selected. The hopping sequence determines the order in which the frequencies in the MA-list are to be

used. The HSNs 1 - 63 are pseudo random sequences used in the random hopping while the HSN 0 is

reserved for a sequential sequence used in the cyclic hopping. The hopping sequence algorithm takes

HSN and FN as an input and the output of the hopping sequence generation is a Mobile Allocation

Index (MAI) which is a number ranging from 0 to the number of frequencies in the MA-list subtracted

by one. The HSN is a cell specific parameter. For the baseband hopping two HSNs exists. The zero

time slots in a BB hopping cell use the HSN1 and the rest of the time slots follow the HSN2 as

presented in Figure 1-3. All the time slots in RF hopping cell follow the HSN1 as presented in Figure

1-4.

1.6 Mobile Allocation Index Offset

When there is more than one TRX in the BTS using the same MA-list the Mobile Allocation Index

Offset (MAIO) is used to ensure that each TRX uses always an unique frequency. Each hopping TRX

is allocated a different MAIO. MAIO is added to MAI when the frequency to be used is determined

from the MA-list. Example of the hopping sequence generation is presented in Figure 1-6. MAIO and

HSN are transmitted to a mobile together with the MA-list. In Nokia solution the MAIOoffset is a cell

specific parameter defining the MAIOTRX for the first hopping TRX in a cell. The MAIOs for the

other hopping TRXs are automatically allocated according to the MAIOstep -parameter introduced in

the following section.


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GSM Hopping algorithm

MAI(0...N-1) =

f1 f2 f3 f4 fNfN-1MA

0 1 2 3 N-1N-2MA INDEX(MAI)

TRX-1 TRX-2 TRX-3

FN & HSN

MAIOTRXTRX-1 0

TRX-2 1

TRX-3 2

For this TDMA frame the output from the algorithm is 1

1

1

+ MAIOTRX

Figure 1-6. Example of the hopping sequence generation.

1.7 MAIO Step

The MAIOstep is a Nokia specific parameter used in the MAIO allocation to the TRXs. The MAIO for

the first hopping TRXs in each cell is defined by the cell specific MAIOoffset parameter. MAIOs for

the other hopping TRXs are assigned by adding the MAIOstep to the MAIO of the previous hopping

TRX as presented in Equation (1.1).

)1()( −⋅+= nMAIOMAIOMAIO stepoffsetnTRX (1.1)

An example of the MAIO assignment is presented in Figure 1-7. More examples can be found in

Section 5.6.4.

Sector TRX # HSN MAIO stepMAIOoffsetl MAIO

1 1 Non-hopping BCCH TRX

2 7 2 0 0

3 2

4 4


2 7 2 6 6

3 84 10


2 7 2 12 12

3 14

4 16

MAIO step indicates the

difference between the MAIOs of

successive TRXs in a cell.

+MAIO step

Figure 1-7. Example of the use of the MAIO related parameters.

MAIOOFFSET ,

User definable

These parameters

are set

automatically


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2. THEORETICAL PERFORMANCE OF FREQUENCY HOPPING

Frequency hopping is a powerful countermeasure in order to overcome the harmful effects introduced

by the propagation channel and interference. The quality gain achieved by employing frequency

hopping can be traded for capacity gain by tightening the frequency reuse in the network.

2.1 Frequency Diversity

The fast fading is a significant problem especially in the downlink direction since the mobiles do not

employ antenna diversity, which is commonly used in base stations. Fluctuations of the received

signal strength are especially harmful for the slow moving mobiles because they tend to stay in a

fading dip much longer than the faster moving mobiles. Frequency hopping causes the consecutive

bursts to be transmitted on different frequencies. If the separation between these frequencies is

sufficient, the fading characteristics of these frequencies are different.

For the fast moving mobiles, the consecutive bursts have different fading characteristics even

without frequency hopping, because the spatial movement between the consecutive bursts is

significant and the locations of the fading dips are relatively constant in most environments. Thus the

frequency diversity gain for the fast moving mobiles is not significant.

2.1.1 Coherence Bandwidth

Coherence bandwidth represents a bandwidth that is required between two frequencies in order to

ensure that their fading characteristics are different enough to provide properly uncorrelated

amplitudes and phases. The coherence bandwidth depends strongly on the mean delay spread of the

environment.

Because of the multipath scattering, the transmitted impulse signal spreads in time domain before it is

received. A typical signal delay envelope of a transmitted impulse is presented in Figure 2-1. The

parameters as defined in [Lee82] are

d tE t dtm =∞

∫ ( )0

(2.1)

∆2 2

0

2= −∞

∫ t E t dt dm( ) , (2.2)

where:

• dm = mean excess delay time

• t = excess delay time

• E( ) = signal power density

• ∆ = delay spread


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0

0 dB

dMean delay time

tDelay time

Powerdensity

E(t)

Delay spread

∆

Figure 2-1. Typical delay envelope.

The delay spread is thus defined as the standard deviation of the mean delay time. The measurements

indicate that the delay spread is highly dependent on the environment. Typical values are presented in

Table 3 [Lee89].

Table 3. Mean delay spreads

Type of environment Delay spread ∆, µs Open area < 0.2

Suburban area 0.5

Urban area 3

The coherence bandwidth is often defined as the frequency separation that yields an autocorrelation

coefficient value of 0.5 or less [Pen95]. If the propagation environment is also time dependent, the

time separation of signals has to be taken into account. The autocorrelation coefficient based on the

frequency and time separation can be written as follows [Lee82]

ρ ω τβ τωr

J v( , )

( )

( )∆

∆ ∆=

+0

2

2 21, ( 2.3 )

where

• J0 ( ) = Bessel function of 0th order

• β = 2π/λ , λ = signal wavelength

• v = velocity of the mobile

• τ = time separation

• ∆ = delay spread of the environment

• ∆ω = 2π*∆f, ∆f = frequency spacing

Adequate coherence bandwidth, where signal autocorrelation coefficient between bursts equals to

0.5, can be derived from Equation (2.3) assuming τ = 0 as

BWC ( . )ρπ

= =⋅

051

2 ∆. ( 2.4 )


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Equation (2.4) can be fully applied only in an ideal case, and it is therefore only a theoretical model.

However, it gives an idea about how the coherence bandwidth differs in different types of

environments. In Figure 2-2 the autocorrelation coefficient has been plotted for several different

values of delay spread (∆) assuming τ = 0. It can be seen that in the urban environment even the

adjacent channel having separation of 200 kHz appears to be adequately uncorrelated and in the

suburban environment the channel separation of 400 kHz is adequate. In open environments the

channel separation should be at least 800 kHz corresponding to four GSM carriers.

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

0

100

200

300

400

500

600

700

800

900

1000

1100

1200

1300

1400

1500

frequency spacing (kHz)

autocorrelation coefficient

0.2

0.5

1

2

3

delay

spread (µs)

Figure 2-2. The autocorrelation coefficient as a function of carrier spacing.

2.1.2 Effect of Interleaving

In GSM the speech frame is transmitted over eight consecutive bursts. The fast fading causes bursty

bit errors that degrade the efficiency of the convolutional coding. The interleaving is designed to

spread these errors over longer time. However, the decoding performance is not significantly

improved if consecutive bursts are exposed to the similar radio channel. If the mobile moves fast

enough, the fading of successive bursts is uncorrelated due to spatial movement. Frequency hopping

causes consecutive bursts to be transmitted on different frequencies. If these frequencies have

sufficient separation the fading of successive bursts is uncorrelated as presented in Section 2.1.1.

Since the interleaving depth is eight, the frequency diversity gain of cyclic hopping doesn’t

significantly improve if more than eight frequencies are used in a hopping sequence.

In data calls, the interleaving length is 19. Therefore, the gain for data calls compared to speech

calls might be bigger when more than 8 frequencies are used in a hopping sequence.

The signalling channels have an interleaving depth of four. The frequency diversity gain for the

signalling channels is thus smaller.


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2.1.3 Cyclic vs. Random Hopping Sequences

Both cyclic and random hopping modes are available in GSM.

• In the cyclic mode the frequencies are changed sequentially from the lowest frequency to the

highest as defined in the MA-list.

• In random mode the frequency to be used for each burst is selected from the MA-list by a

predefined pseudo random sequence. This means that the same frequency may be used for a

couple of consecutive bursts and the frequencies are not used evenly in a short time scale.

Thus, the optimum frequency diversity gain is possible to achieve only if the cyclic hopping is

used. As the number of frequencies becomes larger the difference between the cyclic and the random

mode becomes small.

2.1.4 Simulated Frequency Diversity Gains

0

1

2

3

4

5

6

7

8

9

10

No hop 2 3 4 5 6 8 Infinite

Number of carriers

∆∆ ∆∆Eb/N0 (dB)

FLAT 3

FER = 3%

TU3

FER = 3%

FLAT3

RBER

Cl 1b = 0,3%

TU3RBER

Cl 1b = 0,3%

Figure 2-3. Frequency diversity gain of frequency hopping link against thermal noise

compared to a non-hopping link.


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0

1

2

3

4

5

6

7

8

9

No hop 2 3 4 5 6 8 Infinite

Number of carriers

∆∆ ∆∆C/Ic (dB)

FLAT 3

FER = 3%

FLAT3

RBERCl 1b = 0,2%

TU3

FER = 3%

TU3

RBERCl 1b = 0,2%

Figure 2-4. Frequency diversity gain of frequency hopping link against co-channel

interference compared to a non-hopping link.

The simulations show a very significant gain for FLAT3 channel compared to the TU3 channel. This

happens because the TU3 channel includes several propagation paths having statistically independent

fading conditions and it is thus providing path diversity that helps to achieve the performance targets

even in the non-hopping case. The results of this simulation represent a best possible case, because

the fading on the used frequency channels is assumed uncorrelated and the cyclic hopping mode is

used. In real life, the frequencies are not necessarily uncorrelated as explained in Section 2.1.1 and

the random hopping is used to maximise the interference diversity gain. Also, the presented

gains are not achievable in uplink direction if a proper diversity reception (about 4 dB gain) method

is already in use at base stations.

According to the simulations, the performance of the SACCH / SDCCH and TCH for the cases of

non hopping and ideal FH as a function of C/I (according to 05.05 test conditions and TU3) are

presented in the following:

Table 4. The frequency diversity gain of the SACCH / SDCCH against TCH for the

cases of non hopping and ideal FH as a function of C/I, with 2%FER.

TCH/FS SACCH

No FH 15dB 11.5dB

FH 8dB 8dB

In the non hopping mode, the SACCH is more robust than the TCH/FS, whereas in the FH mode they

perform equal.


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2.1.5 Effect in Cell Coverage Area

In coverage limited cells the frequency hopping may increase the cell coverage area because of

the frequency diversity gain, but since the BCCH time slot doesn’t hop, the increased coverage area

is relevant only for the ongoing calls that have been successfully established and are allocated a

hopping TCH. According to the simulations, see Table 4, the non-hopping signalling channel (BCCH

/ SDCCH) has a better performance than a non-hopping TCH but a worse performance than a

hopping TCH channel. Therefore, the cell coverage area could be increased, but not according to

the full FH gain, but by considering the performance of the BCCH time slot.

In RF FH case, the whole BCCH carrier is non-hopping. Thus, the frequency diversity gain should

be considered as a quality gain in the cell border area rather than the gain increasing the cell service

area.

2.1.6 Effect of Mobile Speed

As mentioned earlier, the frequency diversity gain for the fast moving mobiles is not significant. The

movement as itself causes the same gain which is lost from the frequency diversity gain. Therefore,

the fast moving mobiles get the same gain than the slow moving ones, the gain just comes more or

less from the moving as itself.

In GSM, the speed of Power Control (PC) is slow. When moving fast, the PC cannot follow anymore

the slow fading dips so efficiently. Therefore, the fast moving mobiles might loose in PC gain. Also

the Handover (HO) performance may be degraded with high speed.

2.2 Interference Diversity

In a conventional non-hopping network, each call is transmitted on a single fixed frequency. This

means that the interference situation in a network is also quite stable. Some calls may experience

very little interference and the other calls may be interfered severely. Severe interference can be

avoided by a handover, but the probability of finding an interference free channel decreases as the

network load increases. In a non-hopping network, the interference tends to be continuous, so that the

same interference source affects several consecutive bursts. If this interference is strong enough it

may lead to a corruption of several consecutive bursts. The error correction measures used in GSM

can not usually tolerate several corrupted bursts in a speech frame and thus these frames are likely to

be erased causing significant deterioration in speech quality.

In random hopping network, the interference sources vary from burst to burst. Thus, the interference

tends to get averaged over all the calls in the network. As a consequence, the interference affecting

each call in the network has a lower standard deviation around its mean value. This effect is

illustrated in Figure 2-5. Another advantage of random frequency hopping is that the severely

interfered bursts occur randomly. Because of this, the probability of several consecutive corrupted

bursts and erased frames decreases.


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0

5

10

15

20

25

30

Call 1 Call 2 Call 3

Average C/I (dB)

0

5

10

15

20

25

30

Call 1 Call 2 Call 3

Average C/I (dB)

Figure 2-5. Interference averaging between users in a random frequency hopping network.

In order to use the available frequency spectrum efficiently, the frequencies are reused in a network.

The sufficient distance between the cells using the same frequency depends on the minimum C/I ratio

tolerated by the system, the surrounding environment and the network topology. In practice the

minimum reuse for a non-hopping macro cells is about 12. This means that the same frequency may

be used in every 12th cell. Because the interference levels for each user vary considerably, a large

interference margin has to be included to guarantee sufficient quality for each user in the network.

When the random frequency hopping is employed the deviation of interference level is decreased as

illustrated in Figure 2-5. This means that the interference margin used in the frequency planning

can be reduced allowing the usage of tighter frequency reuse as illustrated in Figure 2-6.

Field strenght

Serving carrier

averageweakestinterference

averagestrongestinterference

interferencemargin

worstinterference

FH with tighterfrequencyreuse

FH withimprovedquality

no FH

Figure 2-6. The gain of frequency hopping.

How big is the interference diversity gain is a subject for a further study.

f3 f1

f2

f3

f2

f1

f3

f2

f1

f2

f3

f1

Ave

FH No FH


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2.3 Effect of Power Control and DTX

Both the power control and the DTX are standard GSM features, which are designed to minimise

the interfering transmission when possible. They are both mandatory features in the mobile

terminals, but it is up to the network operator to decide whether to use them or not. DTX prevents

unnecessary transmissions when there is no need to transfer information. Power control is used to

optimise the transmitted signal strength so that the signal strength at the receiver is still adequate. The

both features can be individually activated for uplink and downlink. Operators have been widely

using both features in UL direction mainly in order to maximise the battery life in mobiles.

In a non-hopping network these features provide some quality gain for some users, but this gain

cannot be transferred effectively to increased capacity, since the maximum interference experienced

by each user is likely to remain the same. Also the power control mechanism doesn’t function

optimally because the interference sources are stable causing chain effects where the increase of

transmission power of one transmitter causes worse quality in the interfered receiver, which in turn

causes the power increase in another transmitter and so on. This means that, for example, one mobile

located in a coverage limited area may severely limit the possibility of several other transmitters to

reduce their power.

In a random hopping network the quality gain provided by both features can be efficiently

exploited to capacity gain because the gain is more equally distributed among the users. Since the

typical speech activity factor (also called DTX factor) is less than 0.5, DTX effectively cuts the

network load in half when it is used. In a soft blocking limited network this means that the DTX can

theoretically provide up to 100% capacity increase. Also, the power control works more efficiently

because each user has many interference sources. Thus, if one interferer increases its power, the

effect on the quality of the connection is not seriously affected. In fact, it is probable that some other

interferers are decreasing their powers at the same time. Thus, the system is more stable and chaining

effects mentioned earlier do not occur frequently.

The simulated gain for power control and DTX with different mobile speeds can be seen in the

following Figure 2-7.

GAIN:PC on 1.4 dBDTX on 2.3 dBPC on, DTX on 3.7 dB

GAIN:PC on 1.0 dBDTX on 2.3 dBPC on, DTX on 3.5 dB

Reuse 3/9, TU 3km/h Reuse 3/9, TU 50km/h

C/I improvement

Figure 2-7. The simulated gain of PC and DTX with FH.


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DTX has some effect on the RXQual distribution. Normally the BER is averaged over the duration

of one SACCH frame lasting 0.48 seconds and consisting of 104 TDMA frames. However, four of

these TDMA frames are used for measurements, so that only 100 bursts are actually transmitted and

received. When DTX is in use and there is no speech activity, only the bursts transmitting the silence

descriptor frame (SID-frame) and the SACCH are transmitted. When there are periods of no speech

activity, the BER is estimated over just the bursts carrying the silence descriptor frame and the

SACCH. This includes only 12 bursts over which the BER is averaged (sub quality). This means that

the BER gets averaged much more effectively when DTX is not used yielding to a quality

distribution where the proportion of moderate quality values is enhanced. The sub quality

distribution is wider than the full quality distribution, meaning that more good and bad quality

samples are experienced.

The differences between full and sub quality distributions are largest in frequency hopping networks

utilising low frequency allocation reuse, since in that kind of networks the interference situation

may be very different from burst to burst. A couple of severely interfered bursts may cause very bad

quality for the sub quality sample when they happen to occur in the set of 12 bursts over which the

sub quality is determined. The full quality sample of the same time period has probably only

moderate quality deterioration because of the better averaging of BER over 100 bursts. The

differences between full and sub quality distributions can be seen in Figure 2-8.

In a real network utilising DTX the quality distribution is a mixture of full and sub quality samples.

The proportions of full and sub samples depend on the speech activity factor also known as the DTX

factor. The differences in the BER averaging processes cause significant differences in the RXQUAL

distributions. These differences should be taken into account when the RXQUAL distributions of

networks utilising and not utilising DTX are compared.

1/1 reuse 15 freqs

0.00 %

5.00 %

10.00 %

15.00 %

20.00 %

25.00 %

30.00 %

35.00 %

40.00 %

Q0 Q1 Q2 Q3 Q4 Q5 Q6 Q7

RxQ full

RxQ sub

Figure 2-8. The distribution of normal RXQual and subRXQual values in a frequency

hopping network.

The limitations in the usage of DL PC and DTX can be seen in Chapters 3.9.1 and 3.9.2.


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3. NOKIA’S SUPPORT FOR FREQUENCY HOPPING IN GSM

The support for frequency hopping is a standard feature of Nokia Base Station Sub-System (BSS). In

this chapter the frequency hopping support of different base station generations and the BSC are

described. Also the current and upcoming frequency hopping support of Nokia’s radio network

planning tool NPS/X is presented.

3.1 BSS Level Implementation

In GSM only the BSS is responsible of the implementation of frequency hopping. The Network Sub-

System (NSS) including the Mobile Switching Centre (MSC) is not involved in it. The Operation and

Maintenance Centre (OMC) is involved in managing the FH related parameters, but their

management in the OMC doesn’t differ from any other cell level parameter. The fault management in

the OMC of a frequency hopping network is identical to that of a non-hopping network. The primary

network elements in GSM are presented in Figure 3-1.

MSC

MS

OMC

BSC

BTS

BTS

NSSBSS

A

interface

Abisinterface

Figure 3-1. The primary network elements in GSM.

3.2 The 2nd Generation Base Station

The second generation base station supports only baseband hopping. The main functional blocks in

the second generation BTS considering frequency hopping are the Frame Units (FU), the Frequency

Hopping Unit (FQHU) and the Carrier Units (CU) [Nok96]. The frame unit performs all the control

and the baseband functions for frames of up to 8 full rate or 16 half rate logical channels. Each carrier

unit contains a transmitter and two receivers. The main function of the transmitter is to convert the

digital data from the frame unit into a modulated carrier signal. The receiver is responsible for the

down conversion from the RF frequency band to baseband followed by A/D conversion and

serialising I and Q signals and sending them to the demodulation part in the corresponding frame unit

[Nok95]. The number of frame units and carrier units corresponds to the number of installed TRXs in

the BTS.


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The frequency hopping connects the frame units and the carrier units as illustrated in Figure 3-2. The

hopping function is realised by multiplexing baseband digital bit streams between the frame units and

the carrier units. The multiplexing is done according to the hopping sequence, which is calculated in

FQHU. The hopping unit is common for the BTS; all the sectors of a BTS use the same FQHU. The

FQHU can be duplicated for reliability or because of diversity reception. If the diversity is not

used, the other FQHU acts as a hot redundancy, which means that it is automatically taken into

operation if the other FQHU fails. When diversity reception is used, the other FQHU is used for

carrying the signal from the diversity receiver.

FU1

FU2

FU3

FU12

CU1

CU2

CU3

CU12

F

Q

H

U

Figure 3-2. Functional units for frequency hopping in 2nd generation BTS.

The FQHU is capable of supporting a maximum of 12 hopping groups at a time. This is sufficient as

in three sector configuration the number of hopping groups used is nine (including the non-hopping

zero time slots on the BCCH carriers). Both random and cyclic hopping modes are supported but not

simultaneously, meaning that all the sectors under the same BTS must use either cyclic or random

hopping sequences. With random hopping the hopping sequence numbers (1-63) can be selected

freely for each hopping group.

The timing of sectors is derived from a common clock unit, so the different sectors are frame- and

bit-synchronised enabling the use of synchronous handovers. Consequently, the hopping sequences

are synchronised as well. The combiners used in the 2nd generation BTSs limit the minimum

channel spacing to 600 kHz!

3.3 Talk Family Base Station

The Talk family base stations are capable of both baseband hopping and RF hopping. Baseband

hopping implementation is slightly different compared to the implementation on the 2nd generation

base station. Functionality inside one TRX is divided between burst level operations (EQDSP) and

block level operations (CHDSP). The burst level operations cover all the operations done for a single

burst, such as ciphering/deciphering, equalisation, bit detection, diversity combining etc. The block

level operations deal with blocks of information, such as a speech block or a signaling block. These

operations include interleaving/deinterleaving, block coding/decoding etc. The baseband hopping

interface resides between this logical division.


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FHDSP is a digital signal processor dedicated to controlling the frequency hopping operation. In

baseband hopping the FHDSP controls the information transfer between the EQDSP and the

CHDSP realising the frequency hopping as illustrated in Figure 3-3. The FBUS is a two-way parallel

bus dedicated for this purpose and dimensioned to support a maximum of 12 TRXs.

TRX1,

CHDSP

TRX1,

EQDSP

F

B

U

S

FHDSP

TRX2,

EQDSP

TRX3,

EQDSP

TRX12,

EQDSP

TRX2,

CHDSP

TRX3,

CHDSP

TRX12,

CHDSP

Figure 3-3. Baseband hopping implementation in the Talk family base stations.

With RF hopping the FBUS is also used, but the connections are always made one-to-one. For

example, the EQDSP of TRX1 is always connected to the CHDSP of TRX1. The FBUS is then used

for sending the RF channel number from the FHDSP to be used on the next time slot. Two

synthesiser banks are used, while one is in use the other is being tuned to the frequency used in the

next time slot. Delivery of channel numbers from FBUS to synthesisers is done by hardware.

RF hopping and BB hopping cannot be used simultaneously. This means that all the sectors under

the same Base Station Control Function (BCF) must use the same hopping method, if any.

However, some sectors may be hopping while others remain non-hopping. The used combiner

type may also restrict the possibility of utilising RF hopping. If Remote Tuned Combiners (RTC) are

used, the RF hopping cannot be used. This is because the RTC is based on tuneable cavities, which

cannot be retuned dynamically according to the used hopping sequence. The minimum channel

spacing when RTC is used is 600 kHz. The other combiner option for the Talk family base stations

is the wide band Antenna Filter Equipment (AFE). AFE supports both BB and RF hopping and there

are no minimum channel spacing requirements.

3.4 PrimeSite

PrimeSite is a small highly integrated base station based on the Talk family technology. It contains

only one TRX and the hardware is reduced, so that the FBUS have been removed and the functions

of FHDSP have been integrated to the CHDSP.


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The RF hopping can be implemented by connecting two or more PrimeSites together as a multi-

TRX configuration. In this case the first PrimeSite provides the BCCH carrier and is thus in a non-

hopping mode, whereas the other connected PrimeSites are hopping according to the hopping

sequence.

The BB hopping is also possible to arrange with the PrimeSites by using properties of RF hopping.

This pseudo-BB hopping appears outwards similar to the pure BB-hopping. Pseudo-BB hopping is

possible when two or more PrimeSites have been connected for a multi-TRX configuration. The

PrimeSite is able to transmit the first time slot (RTSL 0) by using a different frequency than the other

time slots. The pseudo-BB hopping is realised by transmitting the RTSL 0 on the BCCH TRX on one

fixed frequency and the other time slots by using a frequency determined according to the hopping

sequence. The other TRXs use the HSN1 for the RTSL 0s and HSN2 for the RTSLs 1-7 as described

in Section 1.2. The number of frequencies in the pseudo-BB hopping equals the number of connected

PrimeSites for RTSLs 1-7 and one less for the RTSL 0. A dummy signal is sent on the BCCH

frequency in the non-active TCH time slots.

3.5 Base Station Controller

The BSC functionality related to frequency hopping is implemented by software. There are no

hardware dependencies. The frequency hopping management in the BSC is quite simple. The main

principle is that the BSC is handling logical channels on the cells under its control. The logical

channels may then be assigned on the frequency hopping physical channels, but they are provided by

the base stations. The basic requirement for the BSC is to handle the additional parameters (MA,

MAIO and HSN) needed to define a hopping logical channel. The parameters are stored in the BSS

Radio Network Configuration Database (BSDATA) in the BSC, maintained by the Operation and

Maintenance (O&M) software.

The radio resource management doesn’t know about frequency hopping. It allocates the logical

channels as usual. The hopping related parameters are attached later by the Abis interface program

block, which reads the needed hopping related parameters from the database. The parameters

defining a frequency hopping channel are then attached to Abis and Air interface signaling messages.

In Abis and Air interface radio resource management signaling the frequency hopping is affecting the

CHANNEL_ACTIVATION (Abis), IMMEDIATE_ASSIGNMENT (Air),

ASSIGNMENT_COMMAND (Air) and HANDOVER_COMMAND (Air) messages.

3.6 NPS/X

NPS/X is an integrated software package for the cellular network planning developed by Nokia. See

more details of the FH support and the planning and frequency allocation process in Chapters 5.1 and

5.2.

3.7 Maximum Configurations

Maximum BTS configurations are presented in Table 5.

Table 5. Maximum BTS configurations in different BSS software releases.


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BSS 6

BTS type Combiner type BCFs

Antennas/cell

(polarisation

diversity used) Combiner type BCFs

Antennas/cell

(polarisation

diversity used)

2nd generation RTC (BB FH)

omni 10 TRXs 1 1

sectorised 4+4+4 TRXs 1 1

Talk family RTC (BB FH) AFE (BB & RF FH)

omni 6 TRXs 1 1 4 TRXs 1 1

sectorised 6+6+6 TRXs 2 1 4+4+4 TRXs 1 1

Prime Site Standard

(BB & RF FH)

sectorised n*y TRXs 1) 1 1

BSS 7 2nd generation RTC (BB FH)

omni 10 TRXs 1 1





Prime Site Standard

(BB & RF FH)


BSS 8 2nd generation RTC (BB FH)

omni 10 TRXs 1 1





Prime Site Standard

(BB & RF FH)


1) The amount of sectors is not limited; even each TRX can be a sector of its own. Max. 16 TRXs per BCF are allowed.

They can be freely divided into sectors of different sizes. Only rule is that n*y must be less than or equal to 16.

3.8 Radio Network Fault Management

The radio network configuration management in the BSC determines the recovery actions in

abnormal situations in the BSS radio network, such as faults, fault cancels and initialisations. The

recovery actions are executed if errors occur in the functional blocks of the BTS, such as the carrier

unit, the frame unit, the tranceiver, functional blocks common to the whole cell or the functional

blocks common to the whole BTS site. In addition to this, the recovery options are executed if the D-

channel of the Abis interface fails or if there are failures detected by the call control of the BSC in the

connection with the radio channel allocation procedure. The recovery actions are determined based

on the type of the faulty functional block and they are based on the radio facilities configured to the

faulty block.


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3.8.1 The 2nd Generation Base Station

A frame unit fault may be either internal or external. The internal fault could be caused, for

example, by a FU hardware malfunction and the external fault could happen, for example, because of

a lost LAPD-link to the TRX. In both cases the recovery procedure is similar. The procedure is as

follows:

1. The BTS alarms the BSC or the BSC detects a non-functional LAPD-link.

2. The BSC clears all the calls that are allocated to those Abis circuits corresponding to the faulty

TRX. Calls on the other TRXs proceed normally.

3. The BSC blocks the faulty frame unit in order not to allow new traffic for the Abis circuits

corresponding to it.

The calls on the other TRXs can proceed normally and the hopping parameters can be left untouched,

because all the carrier units are still functioning. The mobiles on the cell can still hop over all the

frequencies originally allocated to that cell.

In case of a carrier unit fault one tranceiver doesn’t work properly. Thus, one of the frequencies in

the hopping sequence cannot be transmitted and/or received properly. In this case the procedure is as

follows:

1. The BTS alarms the BSC.

2. The BSC blocks all the TRXs of the cell for a while. This causes clearing of all the ongoing calls

on that cell.

3. The BSC calculates new hopping parameters including a new MA-list in which the frequency of the

faulty CU is removed.

4. The BSC unblocks the TRXs that have functioning CUs and the new hopping parameters are

transferred to the BTS.

5. The BSC allows new traffic for the functioning TRXs.

3.8.2 Talk Family Base Stations and PrimeSite

In a case of BB hopping the procedure is similar to the carrier unit fault in the 2nd generation BTS as

described in the previous section.

If the BTS is RF hopping, the recovery procedure is similar to the frame unit fault in the 2nd

generation BTS as described in the previous section.

3.9 Restrictions on the Usage of FH

3.9.1 DL Power Control with BB FH

In BB FH the BCCH carrier is involved in the hopping sequence. The BCCH carrier is always sent in

the downlink direction with the maximum power defined for the cell. When the PC is used in the

other than BCCH carrier, there is a big difference in the sent / received power between the carriers.

The gain control of some mobiles cannot follow so big and sudden changes in the received power.

Therefore, it is recommended to restrict the PC range in DL direction to 10-15 dB with BB FH.


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3.9.2 Downlink DTX

Baseband hopping combined with downlink DTX causes problems in the mobile stations, because in

the silent phase the dummy frames are sent on the BCCH frequency causing malfunction in the

mobile stations. ETSI has approved a solution to solve the problem and it is implemented in Nokia

BSS. The solution is to use a special training sequence code in the dummy burst but it does not

guarantee that all mobile station models of different manufacturers are working error free.

3.9.3 Extended Range Cell (DE34/DF34/DG35)

Only RF hopping is supported, and only for the TRXs serving the normal coverage area. The TRXs

serving the extended coverage area cannot hop.

3.9.4 MS Speed Detection

The speed detection algorithm in the BTS works only for non-hopping channels. In a case of

frequency hopping the speed information in the Measurement Result message from BTS to BSC is

set to value 'non-valid' indicating that speed information is not available from that particular cell.

3.9.5 Half Rate

The interleaving depth of the TCH/HS is four instead of eight as it is in TCH/FS. Because the

interleaving has a significant effect on the successful error correction of the speech frame, especially

on the frequency hopping networks utilising low frequency allocation reuse and fractional loading,

the performance of frequency hopping may be reduced.

The use of cyclic hopping with even number of hopping frequencies should be avoided in networks

utilising half rate. Since the half rate channel is transmitted on every other TDMA frame, the usage of

cyclic hopping with even number of frequencies means that one half rate connection uses only half of

the frequencies. This problem doesn’t occur if random hopping sequences are used.

3.9.6 Frequency Sharing

The basic requirement in frequency sharing (1/1 reuse, 3/3 reuse) is that the cells at one site have to

be controlled by the same BCF, so that they are frame synchronised. With the current Nokia

equipment this requirement limits the maximum TRX configuration to 12 TRXs per site.

3.9.7 RTC Combiner

In the 2nd

generation and Talk Family base stations, the RTC combiners have the limitation of the

minimum channel spacing of 600 kHz.

3.9.8 NPS/X

NPS/X 3.2 and the older versions don’t support frequency allocation for a fractional loaded network

(= more frequencies than TRXs). NPS/X 3.2 can estimate the quality of the fractional loaded

frequency plan.

NPS/X 3.3 can make the channel allocation for a fractional loaded network.


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4. SELECTING THE RIGHT HOPPING STRATEGY

The goal in the selection of the hopping strategy is to maximise the effectiveness of frequency

hopping in order to achieve a maximum capacity and/or quality gain. The basic requirement for

the maximum FH gain is to make sure that each cell has a sufficient number of frequencies in the

hopping sequence. Equally important is that a good frequency plan minimising the interference can

be produced.

The BTS hardware may severely restrict the possibilities. Second generation base stations are only

capable of BB hopping. The Talk family (3rd

gen) base stations support also RF hopping, but only if

wide band combiners (AFE) are used.

The maximum TRX configurations which can be used with different hopping modes (combiners)

and hopping schemes (maximum TRX amounts under the same BCF) can easily become also

restricting factors.

The amount of antennas and antenna feeder cables can be a limiting factor. With AFE combiner,

about three times more antennas are required than with RTC combiner.

The utilisation of RF hopping is preferable if downlink power control is used. In BB hopping the

DL PC causes dramatic changes in DL field strength as some of the bursts are transmitted by the full

power BCCH TRX and the rest of the bursts by low power TRXs. The mobile receivers cannot

tolerate quick changes of field strength resulting to poor DL quality. To avoid this problem the

maximum power reduction for DL PC in conjunction with BB hopping should be limited to 10 – 15

dB. This limitation is likely to reduce the achievable gain from DL PC.

As in conventional network, the successful implementation of RF hopping with fractional loading

requires a good frequency plan that minimises the interference in the network. Usually the best

results can be achieved with a help of a frequency allocation tool. However, the frequency allocation

is not possible for fractionally loaded networks if the frequency allocation tool doesn’t support

fractional loading. For NPS/X this support is available in version 3.3.

There is, however, one special case of RF hopping with fractional loading that doesn’t require any

frequency planning at all. In this single MA-list scheme all the frequencies are allocated to every cell

so that the frequency allocation reuse is 1. In many cases this scheme may not provide the best

possible gains, but the gain compared to a non-hopping network is still significant as verified in a

trial that was conducted in a real network. If the frequency band is extremely limited, the application

of a single MA-list may be the only sensible way to implement FH, because it always provides the

maximum number of frequencies to hop over in every cell.

Another possibility is to utilise frequency sharing arrangement. In this scheme all the cells of one

site share the same MA-list in a controlled manner so that interference between the cells of the same

site can be avoided. Frequency sharing makes it possible to have enough hopping frequencies in

every cell without a need to utilise fractional loading. Thus, the frequency planning is possible with

tools that don’t support fractional loading.

The main factors affecting the decision of the frequency hopping strategy are presented in Figure 4-1.


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Figure 4-1. Flow chart for hopping strategy decision.

Min TRX configuration

3 TRX/cell

or more

2 TRX/cell

BTS generation2nd gen.

3rd gen. only

Planning tool supports

FH and fractional loading

YesNo

Easy planning preferred

over maximum capacity

Yes

No

BB FH used on the cells

having more than 2 TRXs

max 6 TRX / cell with RTC

or 12 TRX with AFE

RF FH with frequency

allocation reuse 1

(=single MA list scheme)

max 12 TRX / site!

(under the same BCF)RF FH with frequency

allocation reuse 3 ~ 5

max 12 TRX / cell

Combiner type /

Amount of antennas

RTC

AFE

Yes

No

Maximum gain from

DL PC required

RF FH with frequency

sharing (no fractional

loading)

max 12 TRX / site!

(under the same BCF)

<=12 TRXs/site

configurations

YesYes

No


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5. FREQUENCY PLANNING OF FREQUENCY HOPPING NETWORKS

Frequency hopping requires some new considerations in the frequency planning process. This is

especially important if the RF hopping with fractional loading is used. The frequency planning of

fractionally loaded networks requires special attention to the load control. On the other hand, the RF

hopping allows some new planning concepts like frequency sharing and the control over frequency

allocation reuse while the effective reuse in the network remains the same.

Large TRX configurations make baseband hopping feasible. In order to achieve a proper

frequency hopping gain, a minimum of three TRXs in a cell should be used with the baseband

hopping [Tun97]. The benefit of the baseband hopping is that the TCHs located on the BCCH TRX

are included in the frequency hopping sequence. The BCCH frequencies have a high frequency reuse

in order to guarantee a successful signaling and a fast decoding of the base station identification code.

It is beneficial to have this interference free BCCH frequency included in the hopping sequence,

because it is likely to improve the quality of reception on the hopping logical channels.

In frequency planning point of view, the planning of a baseband hopping network differs less than

the planning of a RF hopping network from the planning of a conventional non-hopping network.

The main difference is that the fractional loading is not possible when the baseband hopping is used.

Because of this, it is possible to use the conventional frequency planning tools when planning the

baseband hopping network. However, because of the interference and frequency diversity gains,

lower C/I ratios and therefore smaller frequency reuse distances can be allowed in the baseband

hopping network compared to a non-hopping network.

5.1 Network Planning Procedure

The network planning and monitoring process for a baseband frequency hopping network is

basically the same than for a non-hopping network. The planning of an RF hopping network can

be a little more complex, if the maximum capacity is wanted to get out from the network. The

suitable frequency allocation scheme have to be selected and the frequency load must be equalized to

guarantee an equal quality distribution.

If a tight frequency allocation scheme has been chosen then the estimation of the subjective speech

quality can become a more challenging task compared to a non-hopping network. When FH is used

the RXQual distribution is not anymore comparable to the non-hopping network.

NPS/X is an integrated software package for the cellular network planning developed by Nokia. It

provides the basic tools for coverage prediction, frequency allocation and interference analysis. The

propagation modeling is based on digital maps presenting both the terrain type information and the

height data of the target area. Available propagation models include Okumura-Hata, Juul-Nyholm,

Walfish-Ikegami and a ray-tracing model. The ray-tracing model is specifically for microcell

planning and it is available in NPS/X version 3.2.

NPS/X versions before version 3.2 don’t include any frequency hopping specific support. New

versions called NPS/X 3.2 and 3.3 have some new functionalities to make the frequency planning and

the quality analysis an easier task, see Chapter 5.2.


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Figure 5-1. Network planning and monitoring process.

Capacity PlanningCapacity Planning

Frequency PlanningFrequency Planning Parameter PlanningParameter Planning

MonitoringMonitoring

NPSXNetdimNDW

NPSXNetdimNDW

NPS/X 3.3NPS/X 3.3

NDWNDW

PlanEditCDW

PlanEditCDW

NMS/2000NMS/2000

5.2 Frequency Planning Procedure with NPS/X

NPS/X versions before version 3.2 don’t include any frequency hopping specific support. However,

since the frequency hopping doesn’t affect the propagation, the coverage planning phase is not

different when planning frequency hopping networks compared to non-hopping networks. In

coverage limited cells the frequency hopping increases the cell coverage area because of the

frequency diversity gain. Since the BCCH time slot doesn’t hop, the increased coverage area must

be dimensioned according to the performance of BCCH time slot instead of hopping TCHs, see

Chapter 2.1.5. For this reason, the frequency diversity gain should be considered as a quality gain in

the cell border area rather than a gain increasing the cell service area.

For the planning of baseband hopping networks the traditional frequency allocation and

interference analysis tools are also sufficient. Due to the frequency diversity and interference

diversity gains the hopping allows somewhat worse C/I ratios compared to a non-hopping network.

This can be taken into account when setting parameters for the frequency allocation tool leading to a

tighter frequency plan. When analysing the resulting plan, higher interference levels can be tolerated.

Frequency hopping specific planning tool support is needed when RF hopping with fractional

loading is used. Fractional loading means that a cell is allocated with more frequencies than there are

TRXs.

The quality prediction tool in NPS/X 3.2 estimates the downlink RXQUAL for every pixel in the

work area. These values can be displayed in the digital map using different colours for particular

RXQUAL levels. From the map overlay the areas potentially suffering from interference can be

easily identified. To make the comparison between different plans easier, a statistics window is also

implemented. This window presents the distribution of predicted RXQUAL values in the work area.


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The prediction is based on the C/I ratio that is calculated by using the field strengths of the serving

carrier and the interfering carriers. The corresponding Bit Error Ratio (BER) is determined from the

calculated C/I ratio. The calculations take the DTX factor and the load factor into account where

appropriate. When the BER for the pixel is calculated it is converted to RXQUAL value according to

the mapping specified in GSM specifications [GSM 05.08]. The input parameters needed for the

calculation are the frequencies allocated for the cells, the DTX factor and the blocking probability for

each cell. Both base band and RF hopping modes are supported. Note, that the frequency

allocation for a fractional loaded network is not supported in NPS/X 3.2.

Figure 5-2. Example output from the RXQUAL prediction tool.


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Figure 5-3. RXQUAL statistics window.

NPS/X 3.3 will include a new frequency allocation tool, which is capable of allocating frequencies

utilising low frequency reuse and fractional loading. Also the MAIO Management can be taken into a

use. The MA list lengths can be defined manually in cell basis, or NPS/X can define them

automatically by a certain criteria. After the MA list length has been chosen the allocation algorithm

tries to produce an optimal allocation. In high interfered areas longer MA list lengths can be tried to

average the interference.

Also the network simulator of NPS/X 3.3 includes a support for FH.


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Figure 5-4. Frequency allocation procedure.

5.3 Frequency Reuse on Frequency Hopping Network

Since the frequency band is always limited, the frequencies have to be reused in the network. As the

reuse distance becomes smaller, there are more frequencies available for each cell. Because each

TRX in a cell requires a unique frequency, the capacity potential of a cell is increased, as there are

more frequencies available for each cell. However, when the reuse distance becomes small enough,

all the frequencies available for the cell cannot be utilised because of too severe interference in the

cell border areas. For a conventional non-hopping network this is the practical frequency reuse limit.

The BB hopping network has this same limit, but because of frequency hopping gain, somewhat

lower reuse distances are allowed before the quality reaches the minimum acceptable limit.

The advantage of RF hopping is that the frequency reuse distance can be set as low as wanted. This

can be done, because a RF hopping cell can use more frequencies than there are TRXs installed. This

means that the used frequencies are only fractionally loaded as presented in Section 5.4.3. For a

fractionally loaded RF hopping network, two reuse figures have to be defined. These are effective

reuse and frequency allocation reuse. They are presented in the following sections.

Capacityestimation,cell basis

Capacityestimation,cell basis

Planningconceptdecision

Planningconceptdecision

Estimation ofneeded numberof frequencies

Estimation ofneeded numberof frequencies

Coverage dataCoverage data

Neighbour cellmeasurements with

GPA tool

Neighbour cellmeasurements with

GPA tool

InterferenceCalibration Tool

InterferenceCalibration Tool

Interference matrixgeneration

Interference matrixgeneration

Automatic interferergeneration for IUO

Automatic interferergeneration for IUO

Frequencyrequirements

Frequencyrequirements

FrequencyAllocation

FrequencyAllocation

Spectrumand HWconstraints

Spectrumand HWconstraints

NPS/X 3.3

Planning of otherparameters

Planning of otherparameters

OMC / CDW/ NDW

Quality Analysis

AutomaticParameter tuning

Quality Analysis

AutomaticParameter tuning

NetDim /NPS/X


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5.3.1 Effective Reuse

The effective reuse is essentially the same as the conventional frequency reuse distance. It is

calculated as

RN

Neff

freqsTOT

TRXave

= , ( 5.1 )

where:

• Reff = effective reuse

• NfreqsTOT = total number of used frequencies

• NTRXave = average number of TRXs in a cell

Since the effective reuse takes the actual number of frequencies together with the number of TRXs

into account, it can be also used as a capacity index, provided that the TRXs can be loaded at least to

the hard blocking limit as presented in Section 5.4.2. The smaller the effective reuse, the higher the

capacity in terms of the number of TCHs provided by one frequency in the network.

5.3.2 Frequency Allocation Reuse (RF FH only)

Frequency allocation reuse indicates how closely the frequencies are actually reused in a network.

Thus, it indicates the severity of a worst case C/I in the cell border. It is calculated as

FARN

N

freqsTOT

freqs MA

=/

, ( 5.2 )

where:

• FAR = frequency allocation reuse

• NfreqsTOT = total number of used frequencies

• Nfreqs/MA = average number of frequencies in MA-lists

If the network doesn’t utilise fractional loading, the frequency allocation reuse is the same as the

effective reuse. Example of the reuse calculations for the fractionally loaded RF hopping network is

presented in Figure 5-5.


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Frequency Allocation Reuse = Total # offrequencies / # of frequencies in MAL

Effective Reuse = Total # of frequencies/Number of TRXs per cell

Frequency Allocation Reuse ≠≠≠≠ Effective Reuse

Total # of freqs = 30

4 TRX¨s / cell

10 frequencies / cell

32

1

1 12 2

33

1/3

FAR = 30/10 = 3

Eff.reuse = 30/4 =7.5

Example:Example:

Figure 5-5. Example of reuse calculations.

5.4 Load on Networks Utilising Fractional Loading (RF FH only)

One of the most essential parameters of the fractionally loaded RF hopping network is the load. The

load on the frequencies is the most important one since it determines the probability of collisions.

Collision means that the serving cell and an interfering cell are transmitting at the same frequency at

the same time so that the potential interference becomes reality.

5.4.1 Frequency Load

When designing a network with low frequency allocation reuse, the interference sources are very

close. Even a neighboring cell may be an interferer by sharing at least some of the frequencies. In

that kind of situations the C/I is very low when the collisions occur. In order to guarantee an adequate

quality, the collision probability has to be made low. The closer the interferers, the more infrequent

the collisions must be in order to maintain a proper quality. The collision probability depends on the

load of the hopping frequencies called a frequency load. The frequency load describes the probability

that a frequency channel is used for transmission at one cell at one time.

The frequency load is a product of two other loads: the average busy hour TCH occupancy, which

should in most cases be equal to the hard blocking load that is presented in Section 5.4.2, and the

fractional load that is presented in Section 5.4.3. The frequency load can be written as

L L Lfreq HW frac= ⋅ , ( 5.3 )

where:

• Lfreq = frequency load

• LHW = the busy hour average hard blocking load

• Lfrac = fractional load


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Each frequency allocation reuse corresponds to a different C/I at the cell border, thus requiring a

different maximum allowed frequency load in order to keep the collision probability low enough.

5.4.2 Hard Blocking Load

Hard blocking means that all the available traffic channels in the cell are in use and all the new call

attempts fail because of the lack of available traffic channels. If it is assumed that the call attempts

occur randomly, then the number of call attempts in a time interval is Poisson distributed. If the call

attempts are Poisson distributed and the length of the calls is exponentially distributed, then the hard

blocking probability (that is also known as the grade of service) can be calculated by using the Erlang

B formula

B

T

N

T

n

N

TCH

n

n

N

TCH

TCH

=

=∑

!

!0

, ( 5.4 )

where:

• B = hard blocking probability

• T = offered traffic (Erl)

• NTCH = number of TCHs in the cell

In order not to exceed the predefined hard blocking probability, the average busy hour TCH

occupancy may not exceed the threshold defined by the offered traffic at the desired blocking

probability and the number of TCHs. When determining the hard blocking load, only the non-BCCH

TRXs should be considered as illustrated in Figure 5-6. That’s because the BCCH TRX is non-

hopping in RF hopping cell and the calculation of the loads is only relevant in soft blocking limited

network. Currently soft blocking limited BB hopping networks should not be designed because of the

lack of the gatekeeper algorithm, which prohibits the initialisation of new calls if the load in the

network is about to exceed the load threshold at the soft blocking limit. The hard blocking load is

calculated as

LT

NHW

hopTCH

hopTCH

= , ( 5.5 )

where:

• LHW = hard blocking load

• ThopTCH = average number of used TCHs in the busy hour

• NhopTCH = total number of TCHs in the hopping TRXs


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BCCH SDCCH SDCCH TCH TCHTCHTCH

TCH TCH TCH TCH TCHTCHTCH



TRX-1

TRX-2

TRX-3

TRX-4

f1

f2,f3,f4

f3,f4,f2

f4,f2,f3

TCH

TCH

TCH

TCH

Active slots Empty slots

75 % 25 %

Load on the BCCH TRXnot considered, sincethe BCCH frequenciesare planned separately

Figure 5-6. Hard blocking load of 75% on RF hopping TRXs.

The average busy hour TCH load, as defined in Equation (5.5), can be used as the maximum TCH

occupancy. In reality, there are times when the TCH occupancy is over the busy hour average LHW.

However this happens randomly and since the LHW limit is an average there is about an equal time in

which the load is less than the LHW. If the offered traffic is Poisson distributed, the frequency

allocation can be quite safely dimensioned by using the LHW as the maximum TCH occupancy. In an

environment where the offered traffic is known not to be randomly generated, a higher figure should

be used.

5.4.3 Fractional Load

Fractional loading means that the cell has been allocated more frequencies than there are TRXs as

illustrated in Figure 5-7. This is only possible for RF hopping TRXs. The fractional loading is very

useful when the number of TRXs is low. By utilising fractional loading, it is possible to provide

enough frequencies to hop over (to get FH gain) to even a cell with just one hopping TRX. Fractional

load can be calculated as

LN

Nfrac

TRX

freqs cell

=/

, ( 5.6 )

where:

• Lfrac = fractional load

• NTRX = number of TRXs in a cell

• Nfreqs/cell = number of frequencies allocated to a cell (MA-list length)


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BCCHTRX-1

TRX-2

TRX-3

TRX-4

f1

f2, f3, f4, f5, f6

f2, f3, f4, f5, f6

f2, f3, f4, f5, f6

Active slots Empty slots Frac. load = 3/5 = 0.6

Figure 5-7. Fractional load of 0.6.

In a soft blocking limited network the fractional load is used to tune the frequency load down to a

desired level, which is determined by the used frequency allocation reuse.

5.5 Trunking Effect and Effective Reuse

For Poisson distributed call attempts, it is characteristic that the hard blocking load providing the

same blocking probability increases as the number of traffic channels increases as presented in Figure

5-8. This is called trunking effect. For a hard blocking limited network this is a real gain since the

network is able to serve more traffic with the same grade of service and the same effective reuse.

However, for a soft blocking limited network utilising fractional loading the trunking effect doesn’t

provide any gain. As the hard blocking load increases, the fractional load must be decreased in order

to keep the frequency load and thus the collision probability acceptable. Decreasing the fractional

load is done by adding more frequencies than TRXs to the cells. This has a direct effect on the

effective reuse. The effective reuse can be rewritten as

RN

N

N

N

N

N

FAR

Leff

freqsTOT

TRX

freqsTOT

freqs MA

freqs MA

TRX frac

= = ⋅ =/

/. ( 5.7 )

Equation (5.7) shows the fixed relation between the effective and frequency allocation reuses and the

fractional load. The required increase in the effective reuse in a soft blocking limited network as the

trunking efficiency increases is presented in Figure 5-9. It should be noted that although the effective

reuse increases, the number of frequencies required to handle a certain amount of traffic stays

constant. The effective reuse doesn’t take the trunking efficiency into account.


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0.0 %

10.0 %

20.0 %

30.0 %

40.0 %

50.0 %

60.0 %

70.0 %

80.0 %

90.0 %

100.0 %

1 4 7

10

13

16

19

22

25

28

31

34

37

40

43

46

49

52

55

58

61

64

67

70

73

76

79

82

85

88

91

94

97

100

Number of TCH's

TCH occupancy at the hard blocking limit

Hard blocking prob. 5%



Figure 5-8. Average busy hour TCH occupancy at the hard blocking limit.

5

6

7

8

9

10

11

12

2 3 4 5 6 7 8 9 10 11 12

TRX's/cell

effective reuse

FAR 1 (2% Blocking, Freq.load 7,5% (trialed))FAR 1 (1% Blocking, Freq.load 7,5% (trialed))FAR 3.65 (2% Blocking, Freq.load 30% (trialed))FAR 3.65 (1% Blocking, Freq.load 30% (trialed))FAR 3 (2% Blocking, Freq.load 30% (simulated)) FAR 3 (1% Blocking, Freq.load 30% (simulated))

Figure 5-9. Increase of required effective reuse on a soft blocking limited network due to the

better trunking efficiency on bigger cell configurations.


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5.6 Frequency Allocation Strategies

When preparing for a frequency allocation, some decisions have to be made concerning the wanted

frequency allocation reuse and the corresponding frequency load. Also, it must be decided

whether to use a separate frequency band for the BCCH carriers or use a common band for both the

BCCH and the normal TCH TRXs.

5.6.1 BCCH Allocation

The BCCH carriers are special in a sense that the transmission to the downlink direction is constant

and always active on them. There are two basic approaches in the BCCH allocation. The BCCH

frequencies may be allocated from a separate dedicated frequency band or the frequencies for the

BCCH TRXs and the TCH TRXs (TRXs not carrying the BCCH) may be allocated from one

common band.

Both approaches have been simulated for frequency hopping network in [Kro97]. In this simulation,

the used frequency band was 27 frequencies corresponding to 5.4 MHz. For the dedicated band

strategy 12 frequencies were dedicated to the BCCH TRXs and the remaining 15 frequencies were

used as TCH frequencies, which were allocated by using a slow Adaptive Channel Allocation (ACA)

algorithm presented in [Alm96]. In the common band case, the BCCH frequencies were first

allocated by using a reuse of 27. The ACA algorithm was then used to select the TCH frequencies for

each cell. The BCCH frequencies were not changed during this procedure. In both cases three

different TRX configurations were simulated. The cells had 3, 4 or 5 TCH TRXs depending on the

case. In every case, the average reuse is the same in both strategies, so the results are easily

comparable.

In the simulation, the signal powers were averaged over a period of 0.48 seconds. During this period,

all the frequencies in the hopping sequence have been used several times. Thus, the fast fading can be

assumed to have been removed by averaging. The slow fading was assumed to be constant over the

averaging period. Both, the co-channel and the adjacent channel interference were considered. The

simulated hopping mode was random BB hopping. Frequency diversity effect was not considered.

The used interference limited network consisted of 108 cells in three sectorised configuration having

a radius of 1 km. The mobiles were randomly generated and static. Power control and DTX were

used in the both directions.

The system performance was measured by determining the 10 percent Cumulative Distribution

Function (CDF) value of the C/I ratio. The load measure was defined as the number of served users

per cell using the time slot one.

The uplink performance as a function of served traffic is presented in Figure 5-10. It can be seen that

the common band strategy performs better. The improvement is 1-2 dB. The more uniform reuse

provided by the common band strategy is more effective, because the continuous transmission on the

BCCH TRXs is only employed in downlink direction. The BCCH reuse of 12 forces the reuse on the

TCH TRXs to be very tight. This is unnecessary in uplink direction since the load is about the same

on the BCCH and TCH TRXs. The common band strategy is better when the uplink is

considered. However, the uplink is not usually the limiting link in interference limited networks,

since antenna diversity is normally utilised at base stations.


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The downlink performance on the TCH TRXs as a function of served traffic is presented in Figure

5-11. For a downlink direction the dedicated bands strategy is superior. The improvement is on

the order of 1-5 dB depending on the traffic load. The degradation of the C/I ratio is quite slow as the

traffic load increases in common bands case. This indicates that the BCCH transmitters are the main

interference source. It was also shown in additional simulations that the performance gain from the

power control and the DTX in the common band systems were smaller than in corresponding

dedicated band systems. This happens, because the BCCH frequencies, which are the dominating

interference source, cannot utilise the PC or the DTX.

The downlink performance on the BCCH TRXs as a function of served traffic is presented in Figure

5-12. The downlink performance on the BCCH TRXs is important, because the call initialisation

always starts on the BCCH frequency and the BCCH frequencies have to be clean enough to

guarantee successful decoding of the cell identification for handover purposes. The common band

strategy performs clearly better when the load is small. As the load increases on the interfering TCH

TRXs, the performance degrades rapidly. The dedicated bands strategy provides a very stable

behavior as the traffic load doesn’t have any effect on the performance. In the dedicated band case

the C/I of the BCCH frequencies in the downlink direction is exclusively determined by the used

frequency reuse on the BCCH TRXs. Because of the stable and easily predictable behavior on the

BCCH frequencies in the downlink direction, the dedicated bands strategy is preferable.

Figure 5-10. UL C/I at the 10 % level.


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Figure 5-11. DL C/I at the 10 % level.

Figure 5-12. DL C/I at the 10 % level on the BCCH frequency.

Still one, not common used method is to use separate but not continuous band for the BCCH

frequencies. For example, every 4th frequency is allocated for BCCH. Thus, adjacent channel

interference is avoided between BCCH frequencies. On the other hand, TCH band causes adjacent

channel interference for the BCCH frequencies and vice versa, but the interference might not be too

significant.


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Figure 5-13. Different BCCH allocation strategies.

5.6.2 Selecting the Effective Reuse (BB FH)

With BB hopping, the fractional loading cannot be utilised and the number of hopping frequencies

is always the same as the number of TRXs in a cell, except for TCHs on the zero time slots, which

always have one hopping frequency less than the other TCHs. Thus, in a BB hopping network the

frequency allocation reuse always equals the effective reuse in the network.

Since frequency and interference diversity gains significantly depend on the number of hopping

frequencies, it is recommended to have at least three hopping frequencies as a minimum

configuration. If the cell TRX configurations are smaller than that, BB FH is not recommended to be

used. In that case, RF FH or IUO might offer a better solution to increase the capacity.

Before making the actual frequency plan by using the frequency allocation tool like NPS/X, an

estimation of the minimum effective reuse might be needed, for example in tendering phase. The

following Figure 5-14 gives an estimation of an applicable reuse compared to the situation before

implementing BB FH. For example, if we have in the non-hopping network reuse 15, after

implementing BB FH with 4 TRX average configuration per cell, we end up to reuse 9. The bigger is

the TRX configuration, the smaller reuse we can use, since the reuse is dependent on the number of

hopping frequencies (=TRXs with BB FH).

BCCH TCH

BCCH + TCH

BCCH TCH

Dedicated band

Common band

Dedicated mixed band


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MIN Effective Reuses with different TRX configurations in BB

FH case

0

2

4

6

8

10

12

14

16

18

20

3 6 9 12 15 18

Original reuse

New reuse

No FH

3 TRX

4 TRX

5 TRX

6 TRX

Figure 5-14. Effective reuse after implementing BB FH.

5.6.3 Selecting the Frequency Allocation Reuse and the Frequency Load (RF FH)

If the RF hopping is used, the frequency allocation reuse has a great impact on the required fractional

load and thus, on the number of frequencies allocated to each cell. With BB hopping, the fractional

loading cannot be utilised and the number of hopping frequencies is always the same as the number

of TRXs in a cell, except for TCHs on the zero time slots, which always have one hopping frequency

less than the other TCHs. Thus, in a BB hopping network the frequency allocation reuse always

equals the effective reuse in the network.

Since frequency and interference diversity gains significantly depend on the number of hopping

frequencies, it is important to ensure that each cell has enough hopping frequencies. If the cell TRX

configurations are small, RF hopping with fractional loading makes it possible to still provide

sufficient number of hopping frequencies to the cells even with small TRX configurations.

Fractional loading reduces the average channel utilisation in the network, thus reducing the

probability that interference will occur, making it possible to significantly decrease the frequency

reuse distance. The average channel utilisation is also known as frequency load as explained in

Section 5.4.1. The relationship between the frequency allocation reuse distance and the

corresponding maximum frequency load is illustrated in Figure 5-15.


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32

1

1 12 2

33

3

11

1

1 11 1

11

1

32

1

14

4

3

132

4

1

32

4

4 2

2

2

1

3

1

31 1

1

11

1

1

1

1

1

3

5

1327

13

4

27

6 5

12

3

47

7FARFAR

Worsening C/I at the cell border

Increasing collision probability

Max.Max.frequency loadfrequency load 8% 30% 40?% 70?%

Figure 5-15. Relationship between frequency allocation reuse and maximum allowed

frequency load in the network.

A good approach is first to determine the number of frequencies to hop over in each cell. To

maximise the frequency and interference diversity gains, it is recommended to use at least four

frequencies in MA-lists. This is likely to require fractional loading, especially if the TRX

configurations in the cells are small. Fractional loading means that the frequencies are not

continuously used, which allows the reuse of the same frequency closer. Thus, as the fractional load

decreases, the frequency allocation reuse must be tightened to maintain the same effective reuse. The

relation between the effective reuse, fractional load and the frequency allocation reuse is presented in

Equation (5.3). However, it is beneficial to avoid big differences in the frequency loads caused by

each cell. If the frequency load across the network is kept relatively constant then the

interference will be distributed more evenly in the network.

In practice, the network layout and the surrounding environment have a significant effect on the

highest possible frequency load. Highly irregular network layout makes it very difficult to find a

good frequency allocation that minimises interference in all parts of the network. In that case, it

might be necessary to restrict the maximum frequency load in order to keep interference acceptable.

Generally, in dense propagation environments such as microcells, the path loss slope is steeper. This

naturally reduces interference as the distant interferers are attenuated more. Thus, in these cases

somewhat higher frequency load may be possible. This doesn’t necessarily apply to frequency

allocation reuse of 1, since in that case the worst interferers are the closest neighbors. On the cell

border the interference coming from the neighboring cell attenuates just as much as the signal from

the serving cell regardless of the path loss slope. Because of this, it is not possible to obtain

significant gain from increased path loss slope and it might not be possible to increase the frequency

load. The recommended approach is to start with a low frequency load and then increase it

gradually until the quality threshold is reached.


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Important is also to ensure that the effective reuse is not too low to ensure a good quality. The

following Table contains an example of choosing the right F.A. reuse scheme to give the best

capacity gain. As can be seen, the best capacity is got with the F.A. reuses 2-5. The minimum

effective reuse and maximum frequency load values are still under further consideration. They

might be too optimistic for some environments!

Table 6. Limits for the effective reuse and the frequency load values with different

frequency allocation reuses.

5.6.4 Frequency Sharing by Using MAIO Management (RF FH only)

The MAIO management makes it possible to share the same MA-list between the cells of the same

RF hopping site without co- or adjacent channel collisions. This can be done by utilising the user

definable MAIOoffset and MAIOstep parameters presented in Sections 1.6 and 1.7. MAIOOFFSET helps

to avoid the interference between the cells inside the site, whereas, MAIOSTEP avoids the

interference inside the cell. The cell level MAIOoffset parameter defines the MAIOs for the first

TRXs in each cell. The remaining TRXs are given MAIOs according to the Equation (3.1). In Nokia

implementation the default MAIOstep is 1, but it will be adjustable after the BSC software release S7.

The frequency sharing makes it possible for a cell to hop over all the frequencies allocated to that site

as presented in Figure 5-16. All the cells on a site share the same MA-list. Thus, in a case of a three

sectorised site, the site can be allocated three times less frequencies and still the number of

frequencies to hop over in a cell remains the same. Since less frequencies are needed per site, the

frequency allocation reuse distance can be bigger. The bigger reuse distance leads to less

interference, so the fractional loading is not necessarily needed.

MA-list: 3 6 9

1

3

2

3

6

9

TDMA frame n-1

1

3

2

TDMA frame n+1

1

3

2

3

6

9

TDMA frame n

6

3

9

Example: 21 frequencies

F.A. reuse MA list length Min. Eff. reuse Max. Freq. load Traffic (Erl) TCHs

1 21.0 8.5 8% 13.4 21

2 10.5 7.5 20% 16.8 25

3 7.0 7 30% 16.8 25

4 5.3 6.5 40% 16.8 25

5 4.2 7.5 50% 16.8 25

6 3.5 8.5 55% 15.4 23

7 3.0 10.5 60% 14.4 22

8 2.6 12 65% 13.7 21

9 2.3 13 70% 13.1 20


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Figure 5-16. The principle of frequency sharing.

However, there are some requirements that have to be fulfilled. First of all, the basic requirement is

that the cells at one site have to be controlled by the same BCF, so that they are frame

synchronised. With the current Nokia equipment this requirement limits the maximum TRX

configuration to 12 TRXs per site.

The number of frequencies (MA-list length) have to be at least equal (equal if fractional loading is

not to be used) to the total number of TRXs in the site. If the MAIOstep parameter is more than one,

even more frequencies are needed. The requirement can be formulated as follows

stepsiteTRXsitefreqs MAIONN ⋅= //min , (5.8)

where:

• min Nfreqs/site = minimum number of frequencies needed for a site

• NTRX/site = total number of TRXs on a site

• MAIOstep = the value of the MAIOstep parameter

In Equation (5.8) it is assumed that the MAIO separation between the cells is equal to the used

MAIOstep. In that case, the MAIOoffset parameters are allocated as follows

∑−

=

⋅=1

1

/

n

i

cellTRXstepcell inNMAIOMAIO , (5.9)

where:

• MAIOoffset n = MAIOoffset for the n th cell in a site


• NTRX/cell i = number of TRXs in i th cell

If the number of frequencies is less than min. Nfreqs, then co- or adjacent channel interference might

occur. Example of this is presented in Figure 5-18. In a normal frequency sharing arrangement, the

goal is to minimise the number of frequencies needed per site, so that the frequency allocation

reuse distance can be kept high. For this reason, the MAIOstep should be normally 1. This should be

taken into account in the frequency planning process, because an intracell adjacent channel

interference should not be allowed. Since the frequencies have to be in the increasing order in the

MA-list, the list may not contain adjacent channels if the MAIOstep is 1.

The cells at one site have to use the same HSN. Otherwise, co-channel interference between

the cells will occur. However, the HSNs should be different in interfering sites in order to

ensure the interference diversity. An example of a correct parameter assignment for

frequency sharing is illustrated in Figure 5-17.


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INDEX NO: 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

MA_LIST1: 1 4 8 10 15 20

TDMA 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

MAI 1 5 1 1 2 3 1 2 0 2 3 4 5 1 5 2 4

TDMA-FRAMES ->

SECTOR MA-LIST HSN MAIO TRX 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

1 1 3 1 bcch frequency 1...

0 2 4 20 4 4 8 10 4 8 1 8 10 15 20 4 20 8 15

1 3 8 1 8 8 10 15 8 10 4 10 15 20 1 8 1 10 20

2 1 3 1 bcch frequency 2 ...

2 2 10 4 10 10 15 20 10 15 8 15 20 1 4 10 4 15 1


3 2 15 8 15 15 20 1 15 20 10 20 1 4 8 15 8 20 4

4 3 20 10 20 20 1 4 20 1 15 1 4 8 10 20 10 1 8

5 4 1 15 1 1 4 8 1 4 20 4 8 10 15 1 15 4 10

Figure 5-17. Example of frequency sharing when MAIOstep is 1.

INDEX NO: 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

MA_LIST1: 1 4 8 10 15

TDMA 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

MAI 1 1 1 1 2 3 3 4 0 2 3 0 1 1 1 4 4

TDMA-FRAMES ->



0 2 4 4 4 4 8 10 10 15 1 8 10 1 4 4 4 15 15

1 3 8 8 8 8 10 15 15 1 4 10 15 4 8 8 8 1 1


2 2 10 10 10 10 15 1 1 4 8 15 1 8 10 10 10 4 4


3 2 15 15 15 15 1 4 4 8 10 1 4 10 15 15 15 8 8

4 3 1 1 1 1 4 8 8 10 15 4 8 15 1 1 1 10 10

5 4 4 4 4 4 8 10 10 15 1 8 10 1 4 4 4 15 15

Figure 5-18. Example of frequency sharing when the site is allocated with too few

frequencies and co-channel interference between sectors exists.

Since the cells on the same site share the same frequencies, all the hopping frequencies are

transmitted in every cell on the same site. This has to be taken into account when the frequency

planning is done. This can be modeled in NPS/X 3.2 or older by utilising power dividers so that the

site has only one cell having as many TRXs as there are non-BCCH TRXs in all the sectors of the

actual site. The cell is distributed to multiple antennas forming multiple sectors by using power

dividers. Special care has to be taken to compensate the losses of power divider. In frequency

allocation phase one common interference probability is determined for the entire site and the site is

then allocated one common set of frequencies that form the MA-list. To avoid interference, the

minimum channel separation has to be at least 1. Since each cell has its own BCCH, the BCCH

allocation has to be done separately without the power divider arrangement.

Simulation results of the performance of a network utilising frequency sharing have been presented

in [Nie98]. In this simulation, the network utilising frequency sharing at a nominal reuse of 3/9 was

compared to the RF hopping network using 1/3 frequency allocation reuse at 33 % frequency load.

The reuse on the BCCH carriers was 4/12 in both cases. The served traffic was also the same in both

cases. The simulated network consisted of 48 3-sectorised sites. Power control was utilised in DL

direction, but the DTX was not activated. Downlink FER statistics reported by each mobile every

0.48 seconds from the non-BCCH carriers were collected for analysis. Mobile speeds of 3 km/h and

50 km/h were simulated.


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The resulting cumulative density functions of DL FER have been presented in Figure 5-19 and Figure

5-20. In both mobile speeds, the performance of the two simulated arrangements is very similar until

the FER gets close to 10 %. For the mobile speed of 3 km/h the percentage of FER samples

indicating FER above 15 % is 2 % for the frequency sharing case and 3 % for the 1/3 reuse case. For

the mobile speed of 50 km/h, the corresponding values are about 1.1 % and 1.5%. The difference in

favor of frequency sharing is clear, although not dramatic. However, as higher FER percentages are

studied, the difference gets bigger.

The effect of the mobile speed on the FER distribution can be clearly seen. As the speed increases to

50 km/h, the share of both the low FER percentages and the high FER percentages increases. The

higher mobile speed provides better performance against fast fading. This increases the

proportion of low FER. The higher speed also means that the changes caused by slow fading are

faster and the ability of power control to compensate the fluctuations of signal strength is

reduced. This along with the relatively slow handover algorithm causes the proportion of high FER

to increase at the higher mobile speeds. However, the mobile speed doesn’t have significant effect on

the relative performance of the network utilising frequency sharing.

It may be concluded according to this simulation that the frequency sharing provides better quality

compared to the 1/3 reuse case.

0.001

0.01

0.1

1

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

FER

CDF

BCCH reuse 4/12, TCH reuse 1/3

BCCH reuse = 4/12, TCH reuse = 3/9 by using MAIO-management

Figure 5-19. CDF of DL FER for a mobile speed of 3 km/h.

0.001

0.01

0.1

1

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

FER

CDF

BCCH reuse 4/12, TCH reuse 1/3

BCCH reuse = 4/12, TCH reuse = 3/9 by using MAIO-management

Figure 5-20. CDF of DL FER for a mobile speed of 50 km/h.


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5.6.5 Frequency Sharing in the Single MA-list Scheme (RF FH only)

Frequency sharing can also be used to realise the usage of only one MA-list in the networks utilising

sectorised base station configurations. In the single MA-list scheme all the cells use the same set of

frequencies. If the cells in one site use the same MA-lists without the frequency sharing functionality,

occasional co-channel collisions will happen between the cells of one site. When frequency sharing is

used, it can be ensured that no unnecessary co- or adjacent channel collisions will occur provided that

the cells on the same site use the same HSN.

When the single MA-list scheme is employed, a continuous frequency band is usually allocated to

the cells. In order to avoid intracell adjacent channel interference, the MAIOstep should be set to at

least 2. Preferably, even bigger step should be used, especially if uplink power control is not in use.

Because interference between the cells of the same site is much less likely to occur than intracell

interference, a smaller channel separation can be used between the cells of the same site.

Consequently, the number of needed frequencies is reduced. When this possibility is taken into

account, the Equation (5.8) can be rewritten in more general form as follows

( ) SNMAIONNN sitecellstepsitecellsiteTRXsitefreqs ⋅+⋅−= ////min , (5.10)

where:

• min Nfreqs/site = minimum number of frequencies needed for a site

• NTRX/site = total number of TRXs on a site


• Ncell/site = total number of cells in the site

• S = MAIO separation between cells

A good approach is to set the MAIOstep as high as possible. However, it should be checked that the

requirement presented in Equation (5.10) is still fulfilled. An example of a good MAIO plan is

presented in Figure 5-21. In this example, the MAIO separation between cells is 2 and the MAIOstep

is set to its maximum value, which is 3 in this case. If a MAIOstep of 4 would have been used instead,

constant adjacent channel interference would have occurred between the second TRX of sector one

and the fourth TRX of sector three as shown in Figure 5-22.

INDEX NO: 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

MA_LIST1: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

TDMA 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

MAI 0 2 6 2 2 11 4 0 8 9 3 12 8 8 10 6 8

TDMA-FRAMES ->



0 2 1 3 7 3 3 12 5 1 9 10 4 13 9 9 11 7 9

3 3 4 6 10 6 6 15 8 4 12 13 7 1 12 12 14 10 12


5 2 6 8 12 8 8 2 10 6 14 15 9 3 14 14 1 12 14


7 2 8 10 14 10 10 4 12 8 1 2 11 5 1 1 3 14 1

10 3 11 13 2 13 13 7 15 11 4 5 14 8 4 4 6 2 4

13 4 14 1 5 1 1 10 3 14 7 8 2 11 7 7 9 5 7

Figure 5-21. Example of frequency sharing when MA-list consists of consecutive

frequencies and MAIOstep is set to 3.


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INDEX NO: 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

MA_LIST1: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

TDMA 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

MAI 0 2 6 2 2 11 4 0 8 9 3 12 8 8 10 6 8

TDMA-FRAMES ->



0 2 1 3 7 3 3 12 5 1 9 10 4 13 9 9 11 7 9

4 3 5 7 11 7 7 1 9 5 13 14 8 2 13 13 15 11 13


6 2 7 9 13 9 9 3 11 7 15 1 10 4 15 15 2 13 15


8 2 9 11 15 11 11 5 13 9 2 3 12 6 2 2 4 15 2

12 3 13 15 4 15 15 9 2 13 6 7 1 10 6 6 8 4 6

16 4 2 4 8 4 4 13 6 2 10 11 5 14 10 10 12 8 10

Figure 5-22. Example of too few frequencies compared to the size of the MAIOstep.

Often, it is possible to achieve higher intracell frequency separations, by using bigger MAIOstep and

by not defining the MAIOoffset parameters in increasing order. If this approach is used, the Equations

(7.8) - (7.10) are not valid anymore. Instead, each configuration should be evaluated case by case. An

example of this approach is presented in Figure 5-23. In this example, the used MAIOstep is 6 and the

required MAIO separation between cells is 2. Compared to the example in Figure 5-21, a bigger

MAIOstep can now be used while the number of required frequencies is still the same.

INDEX NO: 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

MA_LIST1: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

TDMA 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

MAI 0 2 6 2 2 11 4 0 8 9 3 12 8 8 10 6 8

TDMA-FRAMES ->



2 2 3 5 9 5 5 14 7 3 11 12 6 15 11 11 13 9 11

8 3 9 11 15 11 11 5 13 9 2 3 12 6 2 2 4 15 2


4 2 5 7 11 7 7 1 9 5 13 14 8 2 13 13 15 11 13


0 2 1 3 7 3 3 12 5 1 9 10 4 13 9 9 11 7 9

6 3 7 9 13 9 9 3 11 7 15 1 10 4 15 15 2 13 15

12 4 13 15 4 15 15 9 2 13 6 7 1 10 6 6 8 4 6

Figure 5-23. Example of customised MAIO allocation.


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6. RADIO NETWORK PARAMETERS

The BSS radio network parameters related to frequency hopping are presented in Table 7.

Table 7. FH related BSS radio network parameters.

Object Parameter Description MA Frequency MA-list. Used with RF FH BTS, max. 63 frequencies per list.

BCCH frequency must not be included in the list.

MA Identification of MA-list MA-list identification number in a BSC (1 - 128).

MA Type of MA-list Frequency band of the MA-list (GSM900, GSM1800, GSM1900).

BTS BTS is hopping (HOP) The hopping mode of the BTS (BB, RF or N).

BTS Hopping sequence

number 1 (HSN1)

Hopping sequence number of the hopping group 1. In BB FH for

the 0 time slots except the BCCH time slot and in RF FH all the

time slots of hopping TRXs (0 - 63).

BTS Hopping sequence

number 2 (HSN2)

Hopping sequence number of the hopping group 2. For the time

slots 1-7. BB FH only (0 - 63).

BTS MAIO offset Defines the MAIO for the first TRX in the cell (0 - 62). Allows the

sharing of the same MA-list between multiple sectors of one BTS

without intrasite collisions. Sectors must be under the same BCF.

Relevant in RF FH only.

BTS MAIO step Defines the step size that is used when the MAIO is calculated for

the TRXs in the cell. Relevant in RF FH only. (Available in BSS7)

BTS Identification of MA-list MA-list id number identifying the MA-list that is allocated to that

BTS. Relevant in RF FH only.

TRX Frequency (FREQ) Assign a frequency to a TRX (GSM900 1 - 124, 975 - 1023;

GSM1800 512 - 885; GSM1900 313 - 810)

To define a BB-hopping cell the following parameters have to be set:

• BTS hopping mode (HOP) = BB

• Hopping sequence number 1 (HSN1) = 0..63 (0 for cyclic hopping and 1..63 for random

sequences)


sequences) (in most cases HSN1 may equal HSN2)

• Fixed frequencies for each TRX (FREQ)

To define a RF-hopping cell the following parameters have to be set:

• MA-list, MA-list ID and MA-list type must be defined in BSC (max. 63 frequencies)

• BTS hopping mode (HOP) = RF


sequences)

• MAIO offset = 0..62

• MAIO step = 0..62 (available in BSS7)

• MA-list ID used by the BTS = 0..128

Examples of MAIO offset and MAIO step definitions are presented in Chapter 8.

6.1 Parameters for MA-list Definitions in BSC

MA-list Description: Mobile Allocation Frequency List, used with RF hopping BTS,


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max. 63 frequencies. Object class: Mobile Allocation Frequency List (MA) GSM reference: GSM 04.08 10.5.2.12,GSM 05.02 6.2.2 Option: - Release: before S4 Modification: When BTS is locked, if used in a RF hopping BTS Restriction: BCCH frequency must not be included in the list. MML name: frequency MML range: 1..124 and 975..1023 (GSM) 512..885 (DCS) 512..810 (DCS19) MML default: - MML command: EBE,EBT,EBI NMS GUI name: Frequencies NMS GUI range: 1..124 and 975..1023 (GSM) 512..885 (DCS) 512..810 (DCS19) NMS GUI dialog name: MAL Parameter Window NMS DB name: frequency NMS DB range: 0..1023 NMS DB mapping: 1:1

MA-list ID Description: Identification of a Mobile Allocation Frequency List in a BSC. Object class: Mobile Allocation Frequency List (MA) GSM reference: GSM 04.08 10.5.2.12,GSM 05.02 6.2.2 Option: - Release: before S4 Modification: Read only Restriction: - MML name: Identification of mobile allocation frequency list MML range: 1..128 MML default: - MML command: EBE,EBR,EBT,EBI,EQA NMS GUI name: MAL ID NMS GUI range: 1..128 NMS GUI dialog name: MAL Parameter Window NMS DB name: object_instance NMS DB range: String up to 10 characters NMS DB mapping: 1:1

Type of MA-list Description: Frequency band of the list. The band is either GSM, DCS or DCS19 band. Object class: Mobile Allocation Frequency List (MA) GSM reference: GSM 04.08 10.5.2.12,GSM 05.02 6.2.2 Option: - Release: before S4 Modification: Read only Restriction: - MML name: type of the mobile allocation frequency list MML range: GSM, DCS, DCS19 MML default: - MML command: EBE,EBI NMS GUI name: Frequency Band in Use NMS GUI range: GSM, DCS 1800, PCS 1900 NMS GUI dialog name: MAL Parameter Window NMS DB name: freq_band_in_use NMS DB range: 0..3


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NMS DB mapping: GSM (0), DCS 1800 (1), PCS 1900 (2)

6.2 BTS Level FH Related Parameters

BTS hopping mode Description: The hopping mode of the BTS. RF and BB hopping cannot be active simultaneously at the same site (BCF). Object class: BTS GSM reference: GSM 04.08 10.5.2.5 Option: - Release: before S4 Modification: BTS site types DE21/DF12 and DE45/DF45: when BTS is locked BTS site type DE34/DF34: when BCF and BTS are locked Restriction: BTS site type DE21/DF12 does not support RF hopping. MML name: BTS hopping mode (HOP) MML range: BB baseband hopping is used RF radio frequency hopping is used N hopping is not used MML default: - MML command: EQC,EQE,EQO NMS GUI name: Hopping Mode NMS GUI range: Non-hopping, Baseband, RF NMS GUI dialog name: BTS Parameter Window NMS DB name: hopping_mode NMS DB range: 0..2 NMS DB mapping: Non-hopping (0), Baseband (1), RF (2) Hopping sequence number 1 Description: HSN1 is used in the frequency hopping sequence generation algorithm and it is located in the Frequency Hopping System 1 (time slots 0 except BCCH time slot). Object class: BTS GSM reference: GSM 04.08 10.5.2.5,GSM 05.02 6.2.2 Option: - Release: before S4 Modification: When BTS is locked Restriction: Check that either cyclic or random hopping is used in the whole site (2

nd gen BTS). Parameter is only used with BB and RF hopping.

See parameter BTS hopping mode. MML name: hopping sequence number 1 (HSN1) MML range: 0 cyclic hopping 1..63 random hopping MML default: 0 MML command: EQC,EQE,EQO NMS GUI name: HSN-1 NMS GUI range: 0..63 NMS GUI dialog name: BTS Parameter Window NMS DB name: hsn NMS DB range: 0..63 NMS DB mapping: 1:1 Hopping sequence number 2 Description: HSN2 is used in the frequency hopping sequence generation algorithm and it is located in the Frequency Hopping System 2 (time slots 1-7). Object class: BTS


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GSM reference: GSM 04.08 10.5.2.5,GSM 05.02 6.2.2 Option: - Release: before S4 Modification: When BTS is locked Restriction: Check that either cyclic or random hopping is used in the whole site (2

nd gen BTS). Parameter is used only with BB hopping. See

parameter BTS hopping mode. MML name: hopping sequence number 2 (HSN2) MML range: 0 cyclic hopping 1..63 random hopping MML default: 0 MML command: EQC,EQE,EQO NMS GUI name: HSN-2 NMS GUI range: 0..63 NMS GUI dialog name: BTS Parameter Window NMS DB name: hsn NMS DB range: 0..63 NMS DB mapping: 1:1 MAIO offset Description: The parameter sets the MAIO offset which is the lowest MAIO in the cell. With MAIO offset it is possible to use the same MA frequency list for two or more sectors of the site without collisions. Object class: BTS GSM reference: No ref. Option: - Release: S6 Modification: The parameter can be modified only when the BTS is locked or not RF hopping. Restriction: - MML name: MAIO offset (MO) MML range: 0..62 MML default: 0 MML command: EQA,EQO,EFO NMS GUI name: MAIO Offset NMS GUI range: 0..62 NMS GUI dialog name: BTS Parameter Window NMS DB name: maio_offset NMS DB range: 0..62 NMS DB mapping: 1:1 MAIO step Description: The parameter sets the MAIO step. Object class: BTS GSM Reference: No ref. Option: - Release: S7 Modification: On-Line Restriction: - MML name: MAIO step (MS) MML range: 1..62 MML default: 1 MML command: EQA,EQO,EFO NMS GUI name: MAIO Step NMS GUI range: 1..62 NMS GUI dialog name: BTS Parameter Window NMS DB name: maio_step NMS DB range: 0..62


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NMS DB mapping: 1:1 Note: OPTIONAL (Flexible MAIO management) MA-list used by BTS Description: The parameter defines the mobile allocation frequency list to which the BTS will be attached. Relevant when RF hopping is used. See chapter Mobile Allocation Frequency List. Object class: BTS GSM reference: No ref. Option: - Release: before S4 Modification: If BTS is RF hopping, then BTS must be locked Restriction: - MML name: mobile allocation frequency list (MAL) MML range: 0..128 (the value 0 detaches the BTS from any mobile allocation frequency list) MML default: No MA-list attached MML command: EQA,EQO NMS GUI name: Used Mobile Allocation NMS GUI range: Not Assigned; Assigned ID(1..128) NMS GUI dialog name: BTS Parameter Window NMS DB name: used_mobile_alloc_list_id NMS DB range: 0..128 NMS DB mapping: Assigned ID (1..128), Not Assigned (0)

6.3 Power Control

Table 8. Example PC parameters for RF FH network.


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GROUP EXPLANATION Q3 NAME RANGE UNIT Setting

General Enable BTS power control powerCtrlEnabled Yes / No Yes

Min time interval between PC's powerControlInterval 0 ... 31 sec 1

Power increase step size powerIncrStepsize 2,4 or 6 dB 2

Power decrease step size powerRedStepsize 2 or 4 dB 2

BS tx max pwr preattenuation rfMaxPowerReduction 0 ... 12 dB 0

optional ave UL signal quality (BER)< 0.2 % pwrDecrLimitBand0 0 ... 38 dB 38

ave UL signal quality (BER) 0.2 % - 0.4 % pwrDecrLimitBand1 0 ... 38 dB 20

ave UL signal quality (BER) > 0.4 % pwrDecrLimitBand2 0 ... 38 dB 8

pwrDecrQualFactor 0 / 1 1

optional MS Power optimisation after HO msPwrOptLev -110 ... -47/ N dBm -79

BTS power range Max attenuation bsTxPwrMin 0 ... 30 dB 30

Min attenuation bsTxPwrMax 0 ... 30 dB 0

Averaging windows pcAveragingLevDL 1 ... 32 SACCH 1

weighting 1 ... 3 1

pcAveragingLevUL 1 ... 32 SACCH 1

weighting 1 ... 3 1

pcAveragingQualDL 1 ... 32 SACCH 1

weighting 1 ... 3 1

pcAveragingQualUL 1 ... 32 SACCH 1

weighting 1 ... 3 1

Thresholds pcLowerThresholdsLevDL -110 ... -47 dBm -101

px 1 ... 32 1

nx 1 ... 32 1

pcLowerThresholdsLevUL -110 ... -47 dBm -101

px 1 ... 32 1

nx 1 ... 32 1

pcLowerThresholdsQualDL 0 ... 7 4

px 1 ... 32 1

nx 1 ... 32 1

pcLowerThresholdsQualUL 0 ... 7 4

px 1 ... 32 1

nx 1 ... 32 1

pcUpperThresholdsLevDL -110 ... -47 dBm -47

px 1 ... 32 1

nx 1 ... 32 1

pcUpperThresholdsLevUL -110 ... -47 dBm -47

px 1 ... 32 1

nx 1 ... 32 1

pcUpperThresholdsQualDL 0 ... 7 1

px 1 ... 32 1

nx 1 ... 32 1

pcUpperThresholdsQualUL 0 ... 7 1

px 1 ... 32 1

nx 1 ... 32 1


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6.4 Handover

Table 9. Example HO parameters for FH network utilising aggressive power control. GROUP EXPLANATION Q3 NAME RANGE UNIT Setting

Averaging adjacent Averaging window size for adj cells averagingWindowSizeAdjCell 1 ... 32 SACCH 8

cells Number of zero results allowed numberOfZeroResults 0 ... 7 7

Adj cells averaging: 6 best or 32 allAdjacentCellsAveraged Yes / No No

Averaging Method enaFastAveCallSetup Yes / No No

enaFastAveHo Yes / No No

enaFastAvePC Yes / No Yes

Minimum Intervals minIntBetweenUnsuccHoAttempt 0 ... 30 sec 3

minIntBetweenHoReq 0 ... 30 sec 5

Periodic Handovers hoPeriodPBGT 0 ... 63 SACCH 6

HoPeriodUmbrella 0 ... 63 SACCH 6

HO types allowed enableIntraHoInterfUL Yes / No Yes

enableIntraHoInterfDL Yes / No Yes

enablePwrBudgetHandover Yes / No Yes

enableUmbrellaHandover Yes / No No

enableMSDistanceProcess Yes / No No

enableSDCCHHandover Yes / No Yes

Margins Enable HO margin for Lev and Qual enableHoMarginLevQual Yes / No Yes

hoMarginPBGT -24 ... 63 dB 4

hoMarginLev -24 ... 24 dB 3

hoMarginQual -24 ... 24 dB 0

Averaging windows hoAveragingLevDL 1 ... 32 SACCH 6

and weighting values weighting 1 ... 3 1

hoAveragingLevUL 1 ... 32 SACCH 6

weighting 1 ... 3 1

hoAveragingQualDL 1 ... 32 SACCH 1

weighting 1 ... 3 1

hoAveragingQualUL 1 ... 32 SACCH 1

weighting 1 ... 3 1

msDistanceAveragingParam 1 ... 32 SACCH 10

msSpeedAveraging 1 ... 32 SACCH 4

Thresholds hoThresholdsLevDL -110 ... -47 dBm -95

px 1 ... 32 1

nx 1 ... 32 1

hoThresholdsLevUL -110 ... -47 dBm -95

px 1 ... 32 1

nx 1 ... 32 1

hoThresholdsQualDL 0 ... 7 5

px 1 ... 32 3

nx 1 ... 32 4

hoThresholdsQualUL 0 ... 7 5

px 1 ... 32 3

nx 1 ... 32 4

hoThresholdsInterferenceDL -110 ... -47 dBm -85

px 1 ... 32 1

nx 1 ... 32 1

hoThresholdsInterferenceUL -110 ... -47 dBm -85


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px 1 ... 32 1

nx 1 ... 32 1

msDistanceHoThresholdParam 0 ... 63 TA 63

px 1 ... 32 1

nx 1 ... 32 1

6.5 DTX

6.5.1 Uplink DTX

The status of the uplink DTX can be defined in miscellaneous BTS parameters in BSC. The mode

of the MS for using the discontinuous transmission (DTX) can be selected in DTX parameter as

following:

0 - MS may use DTX

1 - MS shall use DTX

2 - MS shall not use DTX.

The default for the parameter is 1, meaning that the mobile have to use DTX. Only a few operators

in the world use the value 0, where the default setting of the mobile chooses the uplink DTX mode.

6.5.2 Downlink DTX

The status of the downlink DTX can be defined in BTS parameters of MSC. This DTX parameter

can receive one of the following values:

ON – Downlink DTX enabled by MSC

OFF – Downlink DTX disabled by MSC.

The current default value for the parameter is OFF. If the activation of the downlink DTX doesn’t

cause any special harm for the functioning of the network, the usage of the DTX function is

recommendable.

Here is one example of BTS parameters in MSC including DTX function:

DX 220 DX2x0-LAB 1990-11-1 10:28:56

BASE TRANSCEIVER STATION BTS3 NUMBER 00456 IS CREATED

BSC NAME : - NUMBER : -

LA NAME :LAREA3 LAC :00004

CELL IDENTITY (CI) :00003

BTS ADMINISTRATIVE STATE :LOCKED

ROUTING ZONE (RZ) : -

TARIFF AREA (TA) :000

DOWNLINK DTX DISABLED BY MSC (DTX) :OFF , etc…


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7. OPTIMISATION

7.1 Tools for Network Monitoring

The following tools for example can be used for monitoring the quality and the traffic of the network:

• Cell Doctor version 1.18.41 or later in NMS/2000. The tool extracts data in text format from the

database.

• NDW can be used for Quality / traffic monitoring. It uses the database of NMS/2000.

• TIM / TOM monitoring SW can be used for indoor / outdoor drive tests

• A special DL FER monitoring tool can be used internally, consisting of a Nokia 8110i with SW,

a laptop with FMON and postprocessing SW

• Ericsson TEMS monitoring tool can be used for the normal drive tests and DL FER monitoring

7.2 KPIs for Hopping Network

The KPIs to analyse the performance and the quality of the network are basically the same than in the

non-hopping network. Only the RXQUAL and Drop Call Rate measures differ from the non-

hopping case. Worse RXQUAL can be tolerated when FH is used. Drop call rate doesn’t neither

correlate directly to the quality, since with FH the drop call rate tends to stay low eventhough the

subjective speech quality were not anymore acceptable.

New quality measures are under development and in testing phase to measure the subjective speech

quality more accurately. In the following Table, the normal BSS and NSS level KPIs are presented.

These KPIs are more informational than Nokia’s official values!

With FH, the criteria for the cumulative uplink and downlink quality distribution could be the

following:

Table 10. KPIs for the uplink and the downlink RXQUAL distribution.

BSS related indicators Short term criteria Long term criteria

Uplink quality distribution 0…5, 95% 0…5, 98%

Downlink quality distribution 0…5, 95% 0…5, 98%

Table 11. BSS and NSS related KPIs.


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BSS RelatedIndicators

Shorttermcriteria

Longtermcriteria

TCH Availability [%], 24h 95 99*

SDCCH Success Ratio [%],24h

95 97

TCH Success Ratio [%],24h

95 97

Call Setup Success Rate [%],24h

90 95

SDCCH Blocking [%], cellBH

0.5 0.2

TCH Call Blocking [%], cellBH

5 2

TCH HO Blocking [%], cellBH

TBD TBD

Access Grant Blocking [%], cellBH

0 0

TCH Drop Ratio [%],24h

5 3

Cumulative UL Quality distribution,24h

0… 4, 95%

0… 4, 98%Cumulative DL Quality distribution,

24h0… 4, 95%

0… 4, 98%Average Interference Band,

24hTBD TBD

BSC Controlled Outgoing HO Success [%],24h

93 97

MSC Controlled Outgoing HO Success [%] ,24h

90 95

Intra Cell HO Success [%],24h

96 99

Ratio of BTSs Exceeding 5% Blocking in BH [%],24hSMS Success Rate,24h

95 98*Note, objects which are Locked by User are counted as non available and will reduce the availability value.

NSS RelatedIndicators

Shorttermcriteria

Longtermcriteria

Intra MSC HO Success Ratio[%]

91 96

Inter MSC HO Success Ratio[%]

85 94

Paging Success Ratio[%]

TBD TBD

Technically Successful Calls[%]

TBD TBD

MSC CGR Availability [%] 100 100

PSTN CGR Availability[%]

100 100

A-if CGR Availability [%] 100 100

VMS CGR Availability [%] 100 100

MSC CGR Blocking [%],BH

1 0.1

PSTN CGR Blocking [%],BH

1 0.1

A-if CGR Blocking [%], BH 1 0.1

VMS CGR Blocking [%],BH

1 0.1Intra VLR LU Success Ratio for Home Subscriber[%]

97 99

Intra VLR LU Success Ratio for Roaming Subscriber[%]

97 99

Inter VLR LU Success Ratio for Home Subscriber[%]

96 98

Inter VLR LU Success Ratio for Roaming Subscriber[%]

90 95

Periodic LU Success Ratio[%]

97 99

Home Subscriber LU Success ratio when visitingdifferentPLMN

TBD TBD

Home Subscriber LU Success ratio when comingHomefrom different PLMN

TBD TBD

GeneralStatistics

CriteriaNumber of alarms per Network Element, exc.transmission

Eki

Number of transmission alarms pernode

Eki

Customer complaints of NWproblems

< 1/1000subscriber/dayCustomer complaints of

billing< 1/1000subscriber/year

7.3 RXQUAL in FH Networks

Frequency hopping causes some changes in the RXQUAL distribution. Also, there are some

differences in a way the RXQUAL distribution should be interpreted.

The Frame Erasure Ratio (FER) is a ratio of discarded speech frames compared to all the received

speech frames. A speech frame is generally discarded if after the decoding and error correction

process any of the category 1a bits is found to be changed based on the three parity bits following

them in a speech frame.


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he FER is a measure of how successfully the speech frame was received after the error correction

process and it is thus a better indication of the subjective speech quality compared to the RXQUAL

which gives an estimate of the link quality in terms of BER. The RXQUAL doesn’t indicate how the

bit errors were distributed in a speech frame. The bit error distribution affects the ability of the

channel decoding to correct the errors.

The following table gives an idea of the correlation between RXQUAL and FER and between

subjective speech quality and different FER classes.

Table 12. RXQUAL vs. FER comparison according to the laboratory tests.

Subjective quality, laboratory tests

Steady quality/FER value (fast mobile or frequency hopping)

RXqual FER

0 - 4 good 0 - 4% good

5 slightly degraded 4 - 15% slightly degraded

6 degraded 15 - 35% degraded

7 useless >35% useless

The relation of downlink FER and RXQUAL was measured during a FH trial. The relation is

clearly different in the hopping case compared to the non-hopping case. The distributions of FER in

each RXQUAL class are presented in Figure 7-1 and Figure 7-2. One clear observation can be made;

in the non-hopping case there are significant amount of samples indicating deteriorated quality

(FER>10%) in RXQUAL class 5 while in the hopping case the significant quality deterioration

(FER>10%) happens in RXQUAL class 6. Thus, it may be concluded that in the frequency hopping

networks significant quality deterioration starts at RXQUAL class 6 while in non-hopping network

this happens at RXQUAL class 5.

This difference is a consequence of interference and frequency diversities that affect the frequency

hopping network. Because of these effects, the interference or low signal strength tend to occur

randomly, while in a non-hopping network it is probable that interference or low field strength will

affect several consecutive bursts making it harder for the error correction to actually correct errors.

The successful error correction leads to less erased frames and thus improves the FER.


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"0-1" "1-5""5-10"

"10-

15""15-

100"

Q0

Q1

Q2

Q3

Q4

Q5

Q6

Q7

0.00 %

10.00 %

20.00 %

30.00 %

40.00 %

50.00 %

60.00 %

70.00 %

80.00 %

90.00 %

100.00 %

FER %

RXQUAL

DL FER / RXQUAL (No hopping)

Figure 7-1. Measured relation of FER and RXQUAL in a non-hopping case.

"0-1" "1-5""5-10"

"10-

15""15-

100"

Q0

Q1

Q2

Q3

Q4

Q5

Q6

Q7

0.00 %

10.00 %

20.00 %

30.00 %

40.00 %

50.00 %

60.00 %

70.00 %

80.00 %

90.00 %

100.00 %

FER %

RXQUAL

DL FER / RXQUAL (ave 3.6 hopping carriers / cell)

Figure 7-2. Measured relation of FER and RXQUAL in frequency hopping case.


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This improvement of FER means that the higher RXQUAL values may be allowed in a frequency

hopping network. RXQUAL thresholds are used in the handover and power control decisions.

Because of the improvement in the relative reception performance on the RXQUAL classes 4-6, the

RXQUAL thresholds affecting handover and power control decisions should be set higher in a

network using frequency hopping network. In a frequency hopping network RXQUAL classes 0-5

are indicating good quality.

Typically, the share of the RXQUAL classes 6 and 7 may increase after FH is switched on, even if

no other changes have been made. This may seem to be surprising since it is expected that frequency

hopping improves the network quality. However, in most cases the quality is actually improved, but

the improvement is more visible in the call success ratio. The improved tolerance against interference

and low field strength in FH network means that it is less likely that the decoding of SACCH frames

fails causing increment in the radio link timeout counter. Thus, it is less likely that a call is dropped

because of the radio link timeout. Instead, the calls generating high RXQUAL samples tend to stay

on. This may lead to increase in the share of RXQUAL 6-7. However, at the same time the call

success rate is significantly improved.

In the Figure 7-3, there are presented some trial results of a DL RXQUAL distribution with different

frequency allocation reuse patterns. As can be seen from the figures, the tighter the reuse becomes,

the less samples fall in quality class 0 and more samples fall in quality classes 1-6. There’s bigger

difference in downlink than in uplink direction.

Figure 7-3. DL RXQUAL distribution of a trial with different frequency allocation reuse

patterns (no FH, 1/1, 1/3 fixed, 1/3 heuristic allocation)

DL RXQUAL Distribution

0.00

10.00

20.00

30.00

40.00

50.00

60.00

70.00

80.00

90.00

100.00

q 0 q 1 q 2 q 3 q 4 q 5 q 6 q 7

Quality Classes

Percentage (%)

No FH

1/3 pure

1/3 heuristic

1/1


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Figure 7-4. UL RXQUAL distribution of a trial with different frequency allocation reuse

patterns (no FH, 1/1, 1/3 fixed, 1/3 heuristic allocation)

Frequency hopping forces each call to use all the frequencies in the hopping sequence. If some of

those frequencies are more interfered than others, it may happen that after FH is switched on the

quality of the calls suffers. When FH is not used, the calls tend to be allocated to the TRXs using

interference free carriers (the TRX and the time slot are selected based on the UL idle channel

interference measurement). Especially outside the busy hours, it is probable that time slots are always

available on the TRXs having interference free carriers. Frequency hopping forces all the calls to

use all the frequencies in the hopping sequence. This means that the interfered frequencies are

always used as much as the interference free frequencies. This is likely to lead to worse quality

outside the busy hours. During the busy hours in a non-hopping case, some of the calls have to be

allocated to a TRX using interfered frequency, because interference free TRXs may be full. These

calls are likely to experience significantly worse quality. The frequency hopping tends to average the

quality, so in the frequency hopping case all the calls experience average quality instead of some very

high quality calls and some very low quality calls. It is thus important to compare only busy hour

statistics and to keep in mind that the interference problems may not show up outside the busy hours

in the non-hopping case, while in the FH case the effect of interference is always present.

Note! In BB FH and RF FH case the frequency specific RXQUAL cannot be measured anymore. The

quality is averaged over the hopping sequence.

7.4 Idle Channel Interference Measurement

When a new call is established or a handover is performed, the BSC selects the TRX and the time

slot for the traffic channel based on the idle channel interference measurements. The frequency

hopping has a significant effect on the idle channel interference measurement results.

UL RXQUAL Distribution

0.00

10.00

20.00

30.00

40.00

50.00

60.00

70.00

80.00

90.00

100.00

q 0 q 1 q 2 q 3 q 4 q 5 q 6 q 7

Quality Classes

Percentage (%)

No FH

1/3 pure

1/3 heuristic

1/1


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When the frequency hopping is used, the frequency of a hopping logical channel is changed about

217 times in a second. The frequency of the idle time slots changes according to the same sequence.

In a case of the random hopping, this means that the measured idle channel interference is likely

to be the same for all the TRXs that use the same MA-list. If the interference is averaged over

more than one SACCH frame, the averaging effect is even stronger. However, normally the

interferers are mobiles located in interfering cells. In this case, there are probably differences in the

measured idle channel interferences between different time slots in the cell. This happens,

because the interfering mobiles are only transmitting during the time slot that has been allocated to

them. This is illustrated in Figure 7-5.

If the cyclic hopping sequence is used, there might occur differences on the measured idle channel

interference levels between the TRXs on the same time slot as explained in the following section.

0 1 2 3 4 5 6 7RTSL

Path loss to the

interfered BTS

TRX 1

TRX 2

TRX 3

f1, f2, f3

f1, f2, f3

Interfering

mobiles using

the same

frequencies:

Timeslot #

0 1 2 3 4 5 6 7

Path loss to the

interfered BTS

TRX 1

TRX 2

TRX 3

f1, f2, f3

f1, f2, f3

Interfering

mobiles using

the same

frequencies:

Timeslot #

Low

High

Idle channelinterference level

Figure 7-5. Idle channel interference in a case of the random RF hopping.

7.5 Cyclic and Random Hopping Sequences

If the cyclic hopping mode is used, the interference caused by a mobile is not necessarily spread

evenly on all the hopping TRXs as can be seen in Figure 7-6. If the random hopping sequences are

used, interference is always evenly spread on all the TRXs using the same MA-list as presented in

Figure 7-7. The distribution of interference presented in this section is the same for both uplink and

downlink directions.


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1

interference

TRX 1

TRX 2

TRX 3

TRX 4

TRX 5

TRX 6

TDMA frames

Base station

Mobile

4

7

11

15

18

6 frequencies

3 frequencies

4

7

11

15

18

1

7

11

15

18

1

4

11

15

18

1

4

7

15

18

1

4

7

11

18

1

4

7

11

15

1

4

7

11

15

18

4

7

11

15

18

1

7

11

15

18

1

4

11

15

18

1

4

7

15

18

1

4

7

11

18

1

4

7

11

15

1

4

7

11

15

18

4

7

11

15

18

1

7

11

15

18

1

4

11

15

18

1

4

7

15

18

1

4

7

11

18

1

4

7

11

15

1 8 15 1 8 15 1 8 15 1 8 15 1 8 15 1 8 15

Figure 7-6. Example of interference distribution in one cyclic hopping case.

5 2 1 4 1 6 1 4 3 4 2 5 5 4 3 4 1 2 1 6 3 2 2 6

3 5 1 1 2 1 4 2 6 3 3 4 1 2 3 3 1 6 2 2 5 3 5 5

TRX 1

TRX 2

TRX 3

TRX 4

TRX 5

TRX 6

TDMA frames

Base station

Mobile

16 3 2 5 2 1 2 5 4 5 3 6 6 5 4 5 2 3 2 1 4 3 3

4 21 4 3 6 3 2 3 6 5 6 4 1 1 6 5 6 3 4 3 2 5 4

5 5 32 5 4 1 4 3 4 1 6 1 5 2 2 1 6 1 4 5 4 3 6

1 6 6 43 6 5 2 5 4 5 2 1 2 6 3 3 2 1 2 5 6 5 4

5 2 1 1 54 1 6 3 6 5 6 3 2 3 1 4 4 3 2 3 6 1 6

6 frequencies

6 frequencies

interference

Figure 7-7. Interference distribution when random hopping sequences are used.

The drawback of the cyclic hopping is that the interference coming from one interferer may affect

only some of the TRXs as seen in Figure 7-6. This limits the number of interferers and compromises

the interference diversity. For this reason, it is recommended that cyclic hopping sequences are

not used in the areas where the network is interference limited.


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Figure 7-8. Guide to choose between cyclic and random hopping sequences.

Frequency hopping makes it possible to change the interference sources for each TDMA frame. The

result of this is a beneficial effect called interference diversity, which was presented in Section 2.1.6.

The more different interferers the link has, the better interference is averaged and the better the

interference diversity gain. Time division used in the GSM systems limits the interference diversity.

Because of the TDMA principle, the interference diversity is only possible among the mobiles that

share the same time instant for transmission. However, the base stations that are located on different

sites, are not usually synchronised. This means that the time slots may be partially overlapping each

other as presented in Figure 7-9. Thus, the interference from one interfering cell may consist two

interference sources (mobiles) in uplink direction or two different power levels in downlink direction

if downlink power control is used. This enhances interference diversity. The degree of overlapping in

non-synchronised network is random but constant between any non-synchronised cell pair and it may

be anything between 50 % and 100 % as presented in Figure 7-10.

RTSL

5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4

Serving cell

0 1 2 3 4 5 6 7

RTSL

0 1 2 3 4 5 6 7

Interfering cell

0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7

Figure 7-9. Interference from non-synchronised cell.

Cyclic:

• In the areas where the interference is NOT a problem (low traffic areas)

Random:

• In the areas where the interference is a problem (high traffic areas)


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50% 50% 100%

Serving cell

Interfering cell

Figure 7-10. The two extreme cases: 50 % and 100 % overlapping of bursts.

7.6 Intracell Handover

The lack of synchronisation has a positive effect on the interference diversity. However, interference

is still averaged only between the mobiles sharing the same time instant for transmission. Because of

this, the intracell handover to another time slot changes the interference sources and is feasible if the

overall interference situation in the target time slot is better. During busy hours when the traffic in

the network is at the maximum, it is likely that there are no significantly better time slots available.

Thus, significant gain can not be accomplished by intracell handover to another time slot.

Normally the intracell handover is triggered by poor RXQUAL. In order to avoid unnecessary

intracell handovers, the RXQUAL threshold for intracell handover should be set so high that the

handover is not attempted before the quality of the call is seriously threatened. Example HO

parameters are presented in Table 9.

7.7 Power Control

Power control has been found to improve the quality in FH networks and thus, it is recommended to

be used in both UL and DL directions. Power control is the most effective when the used TX

power level is kept as low as possible while still maintaining an acceptable link quality. To achieve

this, a fast and mainly RXQUAL driven power control is recommended.

In order to make the PC as fast as possible, the measurement averaging in BTS should be disabled

and aggressive power control parameters should be used. Example PC parameters are presented in

Table 8.


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7.7.1 Downlink Power Control with BB Hopping

In the baseband hopping the BCCH TRX is included in the hopping sequence. This means that

occasionally some bursts are transmitted by the BCCH TRX using the time slots from 1 to 7. The

GSM specifications require that the BCCH TRX must transmit continuously and always at the full

power. This is required, because the BCCH frequencies are used in the downlink level measurements

of the neighboring cells by the mobiles. Consequently, if the downlink power control is used, the

downlink signal level may fluctuate dramatically since the BCCH TRX is not using the power

control. This may cause serious problems in the mobile receiver if the mobile is located close to the

cell site. To avoid such problems, the maximum base station power decrease (bsTxPowerMin)

should be limited to 10 - 15 dB when downlink power control is used together with baseband

hopping.

7.8 Handover Control

Since FH and an aggressive power control cause significant changes in the RXQUAL distribution,

the RXQUAL thresholds triggering handovers have to be adjusted accordingly. Normally, the

RXQUAL thresholds have to be increased by 1 or 2 classes (RXQUAL 4 -> RXQUAL 5). Also

the HO speed should be fast enough but still slower than the PC speed, to ensure that the PC will

become triggered before HO. An example of HO parameters for FH networks utilising aggressive

power control are presented in Table 9.

7.9 HSN Planning with Random Hopping

The HSN defines the used FH sequence. The HSN value 0 means cyclic hopping and the values from

1 to 63 mean different random hopping sequences.

In random hopping case, the same HSNs should be used in different cells inside the site. Thus, if a

common MA list is shared inside the site, the frequency collisions can be avoided.

Neighboring sites should use different HSNs, especially those sites, which use common

frequencies. It ensures the collisions to happen randomly between the sites.


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8. PLANNING CASES

8.1 Planning Case 1: Single MA-list

In this example a single MA-list implementation is planned for a small network consisting of 7 sites

and 16 sectorised cells. The benefit in single MA-list implementation is that no frequency planning is

required, because each cell has the same MA-list containing all the allocated frequencies. Since

fractional loading is required, only RF hopping can be used. In order to avoid interference between

the cells of the same site, a MAIO plan is made for each site. The number of TRXs can be

maximum 12 TRXs per site.

8.1.1 Frequency Planning

Although actual frequency plan is not needed, it must be checked that the used frequency band is

sufficient to provide acceptable quality. Also in some cases the differences in cell level traffic

distributions may require that some frequencies are reserved to be used only in the highly loaded

cells.

Site Ce ll TRX count

A 1 2

2 3

B 1 4

C 1 4

2 4

3 3

D 1 3

2 4

3 2

E 1 3

2 4

F 1 4

2 3

3 4

G 1 4

2 3

Hopping

TRXs

1

2

3

3

3

2

2

3

1

2

3

3

2

3

3

2

Average hopping TRXs/ce ll : 2.4

Figure 8-1. Network layout and TRX configurations.

The BCCH frequency plan is made separately and it is not considered here. On the average there are

2.4 hopping TRXs per cell in the example network. 21 frequencies are to be allocated to the hopping

TRXs. The effective reuse on the frequency hopping TRXs can be calculated by using Equation (5.1)

as follows:


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8.84.2

21==effR

Effective reuse of 8.8 is reasonable for frequency hopping network and it can be expected that the

network will have good quality, see Table 6.

In order to keep the collision probability low enough, it is recommended that the average frequency

load caused by each cell in the network doesn’t exceed 8 %. The load distribution in the network is

calculated by using Equations (5.3), (5.4) and (5.5). Here, also the BCCH TRX is included in the

traffic estimations.

Tra ffic a t

2% blocking

Number of

time slots HW load

Fractiona l

load

Frequency

load

9.8 16 61.4 % 4.8 % 2.9 %

16.6 24 69.3 % 9.5 % 6.6 %

23.7 32 74.1 % 14.3 % 10.6 %

23.7 32 74.1 % 14.3 % 10.6 %

23.7 32 74.1 % 14.3 % 10.6 %

16.6 24 69.3 % 9.5 % 6.6 %

16.6 24 69.3 % 9.5 % 6.6 %

23.7 32 74.1 % 14.3 % 10.6 %

9.8 16 61.4 % 4.8 % 2.9 %

16.6 24 69.3 % 9.5 % 6.6 %

23.7 32 74.1 % 14.3 % 10.6 %

23.7 32 74.1 % 14.3 % 10.6 %

16.6 24 69.3 % 9.5 % 6.6 %

23.7 32 74.1 % 14.3 % 10.6 %

23.7 32 74.1 % 14.3 % 10.6 %

16.6 24 69.3 % 9.5 % 6.6 %

Average frequency load: 8.1 %

Site Ce ll

A 1

2

B 1

C 1

2

3

D 1

2

3

E 1

2

F 1

2

3

G 1

2

Figure 8-2. Load calculations.

The average frequency load in the network is 8.1 %. This is acceptable, because it is only very

slightly above the 8 % recommendation. The maximum frequency load is 10.6 %. This doesn’t

exceed the average frequency load significantly. If the maximum frequency load exceeded 13 % -14

%, it might make sense to reserve some extra frequencies that would be used only on the highly

loaded cells. By doing this, the frequency load distribution in the network can be kept more even.

Cells causing high frequency loads tend to deteriorate the quality in the neighboring cells.

8.1.2 MAIO Planning

Since all the cells of a sectorised site are usually controlled by the same BCF, they are frame

synchronised. This means that the TDMA frame number is always the same in the sectors of one site.

Since the hopping sequence is derived from the HSN and the TDMA frame number, the

synchronisation makes it possible avoid interference between the sectors of one site.


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To prevent intra site and intra cell interference the following requirements have to be fulfilled:

• All the sectors of one site have to controlled by the same BCF

• All the sectors of one site have to use the same HSN

• MAIO planning have to be properly made

In order to guarantee interference diversity, a different HSN should be used in the different sites

located in the same area.

MAIO planning should be done for each site. The HSN parameter has to be defined for each site and

MAIOoffset and MAIOstep for each cell. These parameters and their functionality are presented in

Section 1. MAIOstep defines the channel separation between the TRXs of the same cell. It is thus

used to guarantee that intra cell interference doesn’t occur. MAIOoffset is used to control the

channel separations between the sectors of the same site. However, MAIOoffseet doesn’t directly

define the channel separation between the cells. Instead it defines the MAIO of the first hopping TRX

of the cell.

At first it should be checked that proper channel separations are possible with allocated frequency

band. The minimum requirement for channel separation between sectors is 1. However, in order to

avoid constant adjacent channel interference between the sectors of the same site, a separation of 2 is

highly recommended. In order to avoid intra cell interference, the channel separation between the

TRXs of the same cell should be at least 2. Preferably the separation should be 3 or more,

especially if UL power control is not used. In this case the goal is to have a minimum channel

separation of 2 between the sectors and 3 between the TRXs of the same cell. To check if that is

possible with the current frequency band of 21 carriers, Equation (5.10) is used. The site to be

investigated is the site with biggest TRX configurations that is in this case site C having 3 sectors and

8 hopping TRXs.

( )min / / / /N N N MAIOstep N Sfreqs site TRX site cell site cell site= − ⋅ + ⋅

21233)38( =⋅+⋅−→

As a result, it can be seen that the frequency band of 21 carriers is just enough to allow the

implementation of wanted channel separations even in the site with the biggest TRX configurations.

The MAIO plan is now made for the Site C by using MAIOstep 3 and by selecting the MAIOoffset

parameters for the sectors so that the channel (=MAIO) separation of 2 is realised between the

sectors. Example MAIO plans are presented in the following pictures.


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Site C

The sectors share

the same HSN

MAIO Offset determines the

MAIO of the first hopping

TRX in each sector

MAIOs for the rest of the hopping TRXs

are determined by adding MAIO Step to

the MAIO of the previous hopping TRX

MAI value for each TDMA frame is calculated by BTS

and MS by using HSN and TDMA frame number

No co- or adjacent channel

interference between sectors

Transmitted frequencies for each TRX

during each TDMA frame

Figure 8-3. Example of MAIO planning.

Site D

Site F

Site G

Figure 8-4. Example MAIO plans.


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Table 13. Parameters for each cell in the example network.

Site Cell TRX count

HSN MAIOoffset MAIOstep

A 1 2 2 0 3 2 3 2 8 3

B 1 4 5 0 3

C 1 4 6 0 3 2 4 6 8 3 3 3 6 16 3

D 1 3 7 0 3

2 4 7 8 3 3 2 7 16 3

E 1 3 4 0 3 2 4 4 8 3

F 1 4 3 0 3 2 3 3 8 3 3 4 3 13 3

G 1 4 1 0 3 2 3 1 8 3

If more TRXs are later added, it should me made sure that the MAIO plan for that site is still valid.

Failure to do so may lead to continuous co- or adjacent channel interference between the TRXs of the

site.

8.2 Planning Case 2: RF FH with Fractional Loading (FAR 3 – 5)

The network in this case is the same as in the first planning case. The goal is to achieve the highest

capacity by employing very tight frequency reuse. Low effective frequency reuse is possible because

each cell has enough frequencies in the hopping sequence to provide good frequency hopping gain

and the usage of frequencies can be planned so that the worst potential interferers do not use the same

frequency. Also power control is to be used both in uplink and in downlink.

The target frequency allocation reuse is between 3 and 5, meaning that the frequencies are repeated

in every 3 to 5 cells. This makes it possible to avoid interference between the strongest interferers.

The frequency allocation can be done by utilising a frequency allocation tool that supports RF

hopping with fractional loading, such as NPS/X 3.3.

8.2.1 Defining the Frequency Band and the Number of Frequencies Needed in Each Cell

The BCCH frequency plan is made separately and it is not considered here. On the average there are

2.4 hopping TRXs per cell in the example network. In order to end up with an effective reuse of 8, 19

frequencies are to be allocated to the hopping TRXs. The effective reuse on the frequency hopping

TRXs can be calculated by using Equation (5.1) as follows:


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9.74.2

19==effR

Effective reuse of 7.9 is quite low, but it can be expected that with implementation of frequency

allocation reuse of 3 to 5 the network will have acceptable quality provided that the network model in

the frequency allocation tool is accurate. However, the minimum achievable reuse also depends very

much on the environment and the network layout. For example, if the antennas in the urban

environment are located too high so that the isolation between the interfering cells provided by the

surrounding environment is not exploited, a higher effective reuse may have to be used in order to

maintain good quality.

As a rule of thumb, the frequency load caused by each cell should range from 30 % to 50 % as

the frequency allocation reuse ranges from 3 to 5, see Table 6. This is used as a basis when the

number of frequencies to be allocated in each cell is defined. In the following figure, MA-list length

definitions are made for the example network. Also the BCCH TRX is included in the traffic

calculation.

Site Ce ll TRX count

A 1 2

2 3

B 1 4

C 1 4

2 4

3 3

D 1 3

2 4

3 2

E 1 3

2 4

F 1 4

2 3

3 4

G 1 4

2 3

Hopping

TRXs

1

2

3

3

3

2

2

3

1

2

3

3

2

3

3

2

Tra ffic a t

2% blocking

Number of

time slots HW load

MA list

length

Fractiona l

load

Frequency

load

9.8 16 61.4 % 3 33.3 % 20.5 %

16.6 24 69.3 % 4 50.0 % 34.6 %

23.7 32 74.1 % 6 50.0 % 37.1 %

23.7 32 74.1 % 6 50.0 % 37.1 %

23.7 32 74.1 % 6 50.0 % 37.1 %

16.6 24 69.3 % 4 50.0 % 34.6 %

16.6 24 69.3 % 4 50.0 % 34.6 %

23.7 32 74.1 % 6 50.0 % 37.1 %

9.8 16 61.4 % 3 33.3 % 20.5 %

16.6 24 69.3 % 4 50.0 % 34.6 %

23.7 32 74.1 % 6 50.0 % 37.1 %

23.7 32 74.1 % 6 50.0 % 37.1 %

16.6 24 69.3 % 4 50.0 % 34.6 %

23.7 32 74.1 % 6 50.0 % 37.1 %

23.7 32 74.1 % 6 50.0 % 37.1 %

16.6 24 69.3 % 4 50.0 % 34.6 %

Average MA list length: 4.9

Average frequency load: 34.1 %

Average frequency load 34.1 %

(max. 37.1 %) OK

Effective reuse = 19 frequencies / 2.4 hopping TRXs per cell = 7.9 OK

Frequency allocation reuse = 19 frequencies / 4.9 FH freqs per cell = 3.9 OK

Figure 8-5. Load and reuse calculations.

In this example, the MA-list lengths were selected so that the frequency load caused by each cell

falls between 30 % and 40 %. However, the minimum length was 3 in order to guarantee sufficient

FH gains. The resulting average MA-list length is 4.9 carriers per cell. The frequency allocation

reuse can now be calculated by using Equation (5.2) as follows:


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9.39.4

19==FAR

The average frequency load is 34.1 %. In most cases this should provide low enough collision

probability for a network having a frequency allocation reuse 3.9. Actually, it might even be possible

to reduce the frequency band a little bit. With 17 carriers the frequency allocation reuse would reduce

to 3.47. This would match the rule of thumb perfectly, as we would now have a frequency load of

34.1 % and frequency allocation reuse 3.47. The effective reuse with 17 carriers would be 7.1.

Whether the quality will still be acceptable depends on the quality of the frequency plan as well as

the network layout and surrounding environment.

8.2.2 Frequency Allocation and Analysis

Now, once the number of frequencies to be allocated for each cell is defined, the allocation should be

performed with help of an frequency allocation tool that supports fractional loading and is able to

minimise the interference in the network such as NPS/X 3.3.

The allocation parameters can be similar as in the normal non-hopping case. The minimum channel

separation between the frequencies in the MA-list should be at least one carrier in order to avoid

intracell adjacent channel interference. Preferably the separation of two should be used unless that

requirement significantly degrades the allocation result (=increases the resulting value of the cost

function). However, if the fractional load on a cell is 50 % or less, then it is advantageous to allow

consecutive frequencies in the MA-list and set the MAIOstep parameter to 2 in that cell. The

MAIOstep of 2 ensures that adjacent carriers are not used at the same time. Thus, adjacent channel

interference is prevented. The removal of the intra cell channel separation requirement makes it

possible for the allocation tool to find a better allocation that minimises the interference more

effectively. The difference can be so significant, that it might make sense to deliberately force the

fractional load to 50 % or less so that the intracell separation requirement can be removed. An

example of how intra cell adjacent channel interference is avoided is presented in the following

figure.

Consecutive carriers

allowed in the MA lists

MAIOstep is set to 2

Fractional load in every sector is 50% or less

(fractional load = MAL_length / Nb_TRX)

No intra cell adjacent

channel interference!



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Adjacent channels may be allowed between the sectors of the same site especially if all of them are

not adjacent to each other. The interference diversity and fractional loading ensures that even if

adjacent channel interference occurs, it won’t be continuous and thus its effect on the quality is

reduced.

Since frequency sharing is not used in this case, all the cells in one area using the same frequency

band should have a different HSN in order to maximise interference diversity.

Since fractional loading is used, it is very difficult to analyze the frequency plan with conventional

C/I analysis. Instead, more advanced analysis tool such as the RXQUAL analysis tool available in

NPS/X 3.2 and 3.3 should be used. The RXQUAL tool estimates the typical RXQUAL for every

pixel on the digital map. It supports frequency hopping and fractional loading. The RXQUAL

analysis tool is suitable for comparing different frequency allocations and for finding the locations

of possible interference spots where the quality is likely to be the worst. However, the RXQUAL

analysis tool is sensitive to the fractional loading! The lower the fractional load, the better quality

seem to be predicted even when it is likely that the quality in reality should be worse. Because of this,

the tool is only suitable to analysing different frequency plans while the fractional loading (=MA-list

lengths) remain the same. It should also be noted that the indicated RXQUAL doesn’t necessarily

correspond to the actual measured RXQUAL but is still gives an indication of the overall quality of

the frequency plan and the locations of the probable interference areas.

8.3 Planning Case 3: RF FH with Frequency Sharing

The network in this case is the same as in the first planning case. The frequency sharing arrangement

makes it possible to use FH with sufficient number frequencies in the hopping sequence even with

small TRX configurations without need to utilise fractional loading that requires special planning tool

support. Since this is RF hopping, downlink power control can also be fully utilised.

8.3.1 Frequency Planning

In this scheme all the sectors of the same site use a common MA-list. Fractional loading is not

utilised, since it is usually possible to get a sufficient number of frequencies in the hopping sequence

even without it. The benefit is that the frequency planning can be accomplished by using

conventional frequency planning tools that don’t support fractional loading. Since fractional

loading is not used, each MA-list will have as many frequencies as there are hopping TRXs in all

the sectors of each site. The BCCH frequencies are planned normally.

From the interference point of view the frequency sharing effectively combines all the sectors into

one virtual cell that covers the combined coverage area of all the sectors in that site. This can be

modeled in the planning tool (NPS/X 3.2 or older) by creating one virtual cell for each site and by

transmitting this cell through multiple directional antennas for example by using power divider

feature in the planning tool. It should be made sure that no power dividing losses are included since

in the reality there are no power splitters. Each of these virtual cells should have as many TRXs as

there are hopping TRXs in all the sectors of that site. As a result, it is now possible to create an

interference matrix that describes how much the sites interfere each other. In order to avoid intracell

interference, the channel separation should be set to at least 2. The frequency allocation can be now

performed normally, resulting in one common MA-list for each site.


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A B

C

DE

F G

1

2 11

1 1

1

1

2

2

2

2

2

3

3

3

Site Cell TRX count

A 1 2

2 3

B 1 4

C 1 4

2 4

3 3

D 1 3

2 4

3 2

E 1 3

2 4

F 1 4

2 3

3 4

G 1 4

2 3

Hopping

TRXs

1

2

3

3

3

2

2

3

1

2

3

3

2

3

3

2

19 frequencies reserved for non-BCCH TRXs19 frequencies reserved for non-BCCH TRXs

Network layout:Network layout:

Effective reuse = 19 frequencies / 2.4 hopping TRXs per cell = 7.9 OKEffective reuse = 19 frequencies / 2.4 hopping TRXs per cell = 7.9 OK

Ave rage hopping TRXs/ce ll : 2.4

The same MA list is shared among

all the sectors of one site

The same MA list is shared among

all the sectors of one site

MAIO planning neededMAIO planning needed

Each cell has a sufficient number of hopping

frequencies even without fractional loading

Each cell has a sufficient number of hopping

frequencies even without fractional loading

Figure 8-7. Network layout and the calculation of the needed MA-list lengths.

8.3.2 MAIO Planning

MAIO planning is needed in order to avoid mutual interference between the sectors of the same

site. Since the sectors of the same site use the same MA-list, there will be co-channel interference

between those sectors unless MAIO planning is properly done.

To prevent intra site and intra cell interference the following requirements have to be fulfilled:

• All the sectors of one site have to controlled by the same BCF

• All the sectors of one site have to use the same HSN

• MAIO planning have to be properly made

In order to guarantee interference diversity, a different HSN should be used in the different sites

located in the same area.

MAIO planning is simple in this case. MAIOStep should be set to 1 in every sector and MAIOOffset

must be selected for each sector so that the MAIOs of the hopping TRXs in one site will be in

consecutive order.


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MAIOStep is 1

The sectors share

the same HSN

MAIOoffset for each sector is set so that the

MAIOs for TRXs are in consecutive order

Frequencies for the MA list are planned with help of

frequency planning tool. Minimum separation is 2.


8.3.3 Analysis

Since fractional loading is not used, conventional C/I analysis is possible. However, the same virtual

cell with power dividers –setup that was used in the frequency allocation phase must be used in the

analysis.

Alternative option is to use the RXQUAL analysis tool of NPS/X 3.2. Since it is not possible to take

the benefits of MAIO management into account (=no interference between the cells of the same site),

similar setup as in the C/I analysis has to be used.

frequency hopping planning guide

Documents

polarisation bcfs diversity

error correction process

nms gui dialog

low field strength

silence descriptor frame

1 3 heuristic allocation

primary network elements

nms db mapping