gsm-frequency-planning

GSM Radio Network Planning and Optimization Chapter 6 GSM Frequency Planning

For internal use only

Table of Contents

Table of Contents ................................................................................................................... 1

List of Figures ................................................................................................................... 2

List of Tables ..................................................................................................................... 3

Chapter 6 GSM Frequency Planning ................................................................................... 4

6.1 Overview .................................................................................................................... 4

6.2 Frequency Division and C/I Requirement .................................................................. 5

6.2.1 Frequency Division .......................................................................................... 5

6.2.2 C/I ................................................................................................................... 5

6.3 Frequency Planning Principle .................................................................................... 9

6.4 Normal Frequency Reuse Technology ..................................................................... 10

6.4.1 C/I under 4 x 3 Frequency Reuse Pattern ..................................................... 10

6.4.2 10MHz Bandwidth 4 x 3 Frequency Reuse ................................................... 12

6.4.3 19MHz Bandwidth 4 x 3 Frequency Reuse ................................................... 13

6.4.4 6MHz Bandwidth 4 x 3 Frequency Reuse ..................................................... 13

6.4.5 4 x 3 Frequency Reuse Conclusion .............................................................. 14

6.5 Aggressive Frequency Reuse Technology ............................................................... 15

6.5.1 3 x 3 Frequency Reuse Pattern ..................................................................... 15

6.5.2 2 x 6 Reuse Pattern ...................................................................................... 16




6.5.6 A + B Frequency Reuse Pattern .................................................................... 24

6.6 Concentric Cell Technology ..................................................................................... 26

6.6.1 Concept ......................................................................................................... 26

6.6.2 General Underlay Overlay ............................................................................. 27

6.6.3 Intelligent Underlay Overlay .......................................................................... 28

6.6.4 Characteristics of Concentric Cell Technology .............................................. 29

6.7 Multiple Reuse Pattern Technology ......................................................................... 30

6.7.1 Basic Principle ............................................................................................... 30

6.7.2 MRP Sequence Grouping ............................................................................. 33

6.7.3 MRP Space Grouping ................................................................................... 34

6.7.4 Characteristics of MRP Technology ............................................................... 35

6.7.5 Comparison between MRP and 1 X 3 Frequency Reuse Pattern ................. 36

6.8 Network Capacity Comparison ................................................................................ 36

11/12/2010 All rights reserved Page1 of 37



List of Figures

Figure 1.1Intra-frequency reuse of the omni-directional base station ..............................6

Figure 1.2Intra-frequency interference for the omni-directional base station...................7

Figure 1.3Normal 4 x 3 frequency reuse pattern................................................................11

Figure 1.43 x 3 frequency reuse pattern.............................................................................15




Figure 1.8A + B frequency reuse pattern............................................................................25

Figure 1.9Schematic diagram of concentric cell................................................................26

Figure 1.10Structure of general underlay overlay..............................................................28

Figure 1.11Structure of intelligent underlay overlay..........................................................28

Figure 1.12Layering aggressive frequency reuse..............................................................30

Figure 1.13Frequency planning under MRP (7.2MHz bandwidth).....................................32

Figure 1.14Frequency planning under MRP space grouping............................................34




List of Tables

Table 3.1Frequency planning under 4 x 3 frequency reuse pattern (a)............................12

Table 3.2Frequency planning under 4 x 3 frequency reuse pattern (b)............................13

Table 3.3Frequency planning under 4 x 3 frequency reuse pattern (c)............................14

Table 4.1Frequency planning under 3 x 3 frequency reuse pattern.................................15



Table 7.11 X 3 frequency reuse space grouping (a)..........................................................21

Table 7.21 x 3 frequency reuse sequence grouping (a).....................................................22

Table 7.31 x 3 frequency reuse space grouping (b)..........................................................22

Table 7.41 x 3 frequency sequence grouping (b)...............................................................23


Table 8.1Frequency planning under A + B frequency reuse pattern................................25

Table 9.1Channel number grouping for 6MHz bandwidth concentric cell (a)..................27

Table 9.2Channel number grouping for 6MHz bandwidth concentric cell (b)..................27

Table 11.1A comparison between GUO and IUO................................................................29

Table 12.1Channel number allocation for each layer.........................................................30

Table 13.1MRP sequence grouping.....................................................................................33

Table 14.1Comparison of the network capacity under various frequency reuse pattern

................................................................................................................................................36




Chapter 6 GSM Frequency Planning

6.1 Overview

Frequency resource is scarce for the mobile communication, so how to maximize

the spectrum utilization ratio is a great concern for many carriers, equipment

providers, and scholars. And their research into this problem has accelerated the

development of the communication technologies. By now, the mobile

communication has experienced three phases: analog TACS/AMPS,

GSM/CDMA IS95, and WCDMA/CDMA2000.

The purpose to enhance the spectrum utilization ratio is to expand the network

capacity based on the limited spectrum resource while ensuring the network

quality. If not considering adding frequencies to the network, you can expand the

capacity of a GSM network using the two methods. One is to increase the

number of base stations in the network; the other is to use the frequency reuse

technologies. This chapter mainly describes the GSM frequency reuse

technologies, namely, frequency planning technologies.

To expand the network capacity, you must reuse the limited frequency resources.

Though frequency reuse is beneficial for network expansion, it brings into

another problem. That is, it deteriorates the conversation quality. The more

aggressive the frequencies are reused, the greater the interference will arise in

the network. Therefore, how to seek a balance between network capacity and

conversation quality is a demanding task in frequency planning.

Currently, the 4 x 3, 3 x 3, 2 x 6, 1 x 3, 1 x 1, MRP, and concentric circles are the

GSM frequency technologies in common use. For the 4 x 3 frequency reuse

pattern, the frequency utilization ratio is relatively low, but the higher carrier-to-

interference ratio (C/I) can be obtained, so you can enjoy better conversation

quality. Compared with the 4 x 3 frequency reuse pattern, the 1 x 3 frequency

reuse pattern ensures a relatively high frequency utilization ratio, but the reuse

distance is shorter, so interference is greater and the conversation quality is

poorer. In this case, you should take some measures, such as the frequency

hopping and DTX, against the interference.

The frequency planning is a key technology for GSM network, so the quality of

the frequency planning will determine the network quality.

This chapter introduces the rules of frequency reuse based on the frequency

reuse patterns and the network requirement. Meanwhile, it also provides




examples to detail the frequency division, C/I, frequency reuse degree under

each reuse pattern.

6.2 Frequency Division and C/I Requirement

6.2.1 Frequency Division

The GSM cellular system can be divided into GSM 900MHz system and DCS

1800MHz system in terms of the band to be used. The carrier spacing is 200

KHz.

I. GSM 900MHz

It has 124 channel numbers. The absolute radio frequency channel number

(ARFCN) is 1–124, and a protection band with 200 KHz in width is reserved at

the two ends. According to the documents prescribed by the relative government

department of China, China Mobile uses the 890–909/936–954MHz band, and

the corresponding ARFCN is 1–95 (generally, the channel number 95 is for

reservation only). For China Unicom, it uses the 909–915/954–960MHz band,

and the corresponding ARFCN is 96–124. For the bands defined for the carriers

from other countries, they can be calculated by the following formulas:

Base station reception: f1 (n) = [890.2 + (n – 1) x 0.2] MHz

Base station transmit: f2 (n) = [f1 (n) + 45] MHz

II. DSC 1800MHz

It has 374 channel numbers. The ARFCN is 512–885. The relationship between

the frequency and the channel number (n) are listed in the following:

Base station reception: f1 (n) = [1710.2 + (n – 512) x 0.2] MHz

Base station transmit: f2 (n) = [f1(n) + 95] MHz

China Mobile uses the 1710–1720 MHz band, and the corresponding ARFCN is

512–561. China Unicom uses the 1745–1755 MHz, and the corresponding

ARFCN is 687–736.

6.2.2 C/I

C/I stands for carrier-to-interference ratio. In the GSM system, frequency reuse

will cause intra-frequency interference. The intra-frequency is related to both the

reuse distance and the cell radius. Hereunder is an example.

6.2.2 shows the intra-frequency reuse of the omni-directional base station.




Figure 1.1 Intra-frequency reuse of the omni-directional base station

Suppose that the coverage radius of all base stations is the same, the

relationship of the intra-frequency reuse distance (D), the cell radius (R), and

number of each frequency reuse cluster (N) can be expressed by the following

equation:

NRDq 3/ ==

Here,

22 jijiN ++= (“i” and “j” are positive integers)

“q” is the intra-frequency interference attenuation factor.

For the directional cell, the physical meaning of the N stands for the number

of base stations in the frequency reuse clusters.

If the intra-frequency cell and the service cell work at the same time, the MS

locating in the center of the service cell will receive both the useful signals from

this service cell and the interfering signals from the intra-frequency cells. In this

case, the C/I can be expressed by the following equation:

∑=

=k

ikI

C

I

C

1




Here, kI is the Kth interfering signal. This equation can also be expressed as:

∑=

−= k

i

rkq

I

C

1

)(

1

Here,

kq is the intra-frequency interference attenuation factor of the Kth intra-

frequency interference cell.

r is the path loss slop according to actual geographical environment. In

moving environment, it ranges from 3 to 5. Generally, it is 4.

As shown in 6.2.2, for the omni-directional base station with regular frequency

reuse, there are 6 intra-frequency interference sources at the first layer, namely,

the 6 intra-frequency reuse cells in orange. There are 12 intra-frequency

interference sources at the second layer, namely, the 12 intra-frequency reuse

cells in yellow. However, the 12 intra-frequency interference sources has only a

little effect on the 6 interference sources at the first layer, so it can be neglected.

Figure 1.2 Intra-frequency interference for the omni-directional base station




If the radio propagation environment between the 6 intra-frequency reuse cells

and the service cell is the keeps stable, the following three equations are

present:

rqI

C−=

6

1

r

I

Cq

1

)6( ×=

6

rq

I

C =

Based on the three equations, the relationship between the C/I and the number

of the base station in the frequency reuse clusters can be expressed by the

following equation:

6

)3( rN

I

C =

When the MS locates at the edge of the service cell, it will receive the poorest

signals form the service cell but the strongest interfering signals. In this case, the

needed C/I can be expressed by the following equation:

6

)1( rq

I

C −=

If the cellular layout is improperly designed, the interfering sources will increase

and the C/I will decrease. According to the previous equations, the more the cells

in each cluster, the greater the C/I and the better the network quality are, but the

frequency utilization ratio will be lower. In addition, the GSM interference is

related to the traffic load. The intra-frequency interference reaches the greatest

when the traffic load reaches the peak.

Generally, the 4 x 3 frequency reuse pattern is used in GSM frequency planning.

For the areas where the traffic is great, you can use other frequency reuse

patterns, such as 3 x 3 and 1 x 3. No matter which frequency reuse pattern you

take, you must meet the requirement on interference-to-protection ratio.

Apart from the intra-frequency interference caused by normal frequency reuse,

there are other abnormal interferences. They are listed in the following:

Multipath signal interference (It occurs when useful signals fall outside the

delay equalizer of the system.)

Outside signal interference (It refers to the signals from the radar, illegal

wireless equipments, and environment noises.)

In the GSM system, the requirements on the C/I are listed in the following:




For intra-frequency C/I, it must be equal to greater than 9 dB. In actual

projecting, a margin of 3 dB is needed, namely, it is equal to or greater than

12 dB.

For adjacent-frequency C/I, it must be equal to or greater than -9 dB. In

actual projecting, a margin of 3 dB is needed, namely, it is equal to or

greater than -6 dB.

When the carrier offset reaches 400 KHz, the C/I must be equal to or

greater than -41 dB.

6.3 Frequency Planning Principle

Generally, when planning the frequency for the network, you will divide the

geographic area into smaller slices, but you must reserve a certain amount of

channel number at the intersection area between slices if the frequency resource

is adequate.

The intersection area must be far away from the areas where the traffic is great

and the areas where the networking is complex. Generally, you should begin the

planning with the area where base stations are intensively distributed. If there

are rivers or big lakes in the planning area, you must consider the refection effect

of the surface.

Generally, base stations irregularly distributed, so you cannot perform the

frequency planning completely according to 4 x 3 frequency reuse pattern or 3 x

3 frequency reuse pattern. Instead, you must make flexible adjustment according

to actual conditions.

No matter which reuse pattern you take, you must obey the following principles:

Generally, the intra-frequencies and adjacent channel numbers are allowed

to appear within a base station.

The frequency spacing between the BCCH and TCH must be greater than

400 KHz within a cell.

The frequency spacing between the TCHs must be greater than 400 KHz

within a cell. (When frequency hopping is used, you can meet this by

properly setting the mobile allocation index offset.)

The adjacent base stations cannot use the same frequency.

Considering the complexity of the antenna height and radio propagation

environment, the base stations near each other cannot use the same

frequency.

Generally, if using the 1 x 3 frequency reuse pattern, you must ensure that

the number of frequency hopping channel numbers is at least twice that of

the frequency hoping carriers.

Pay special attention to the intra-frequency reuse. The adjacent areas are

not allowed to share the BCCH and the BSIC.




6.4 Normal Frequency Reuse Technology

6.4.1 C/I under 4 x 3 Frequency Reuse Pattern

The spectrum utilization ratio can be expressed by frequency reuse degree,

which reveals the aggressiveness of the frequency reuse. The frequency reuse

degree can be expressed by the following equation:

TRX

ARFCNreuse N

Nf =

Here NARFCN is the total number of the available channel numbers, and NTRX is the

number of TRXs configured for the cell.

For the n x m frequency reuse pattern, “n” indicates the number of the base

stations in the reuse clusters, and “m” indicates the number of the cells under

each base station. In this case, the frequency reuse degree can be expressed by

the following equation:

reusef = n x m

In actual planning, however, the allocated number of channel numbers will be

greater than n x m, so the actual reusef is usually greater than n x m.

Therefore, the smaller the reusef , the more aggressive the frequency is reused

and the higher the frequency utilization ratio is. As the aggressiveness of the

frequency reuse grows, however, it will bring greater interference to the network.

In this case, you must enable the technologies, including DTX and power control,

to solve this problem. The more aggressive the frequency is reused, the lower

the spectrum utilization ratio is, but the conversation quality is better at this time.

The purpose the frequency planning is to reach a balance between the

frequency utilization ratio and the network capacity. Based on the assurance of

the network quality, you must take measures to maximize the network capacity.

In the GSM system, the 4 x 3 frequency reuse pattern is in basic use. Here “4”

indicates 4 base stations (each base station consists of 3 cells), and “3” indicates




the 3 cells under the control of each base station. Therefore, there are 12 sectors

are available. And the 12 sectors makes up of a frequency reuse cluster, but the

frequency in the same cluster cannot be reused.

For the 4 x 3 frequency reuse pattern, the intra-frequency spacing is great, so it

can meet GSM system’s requirement on the intra-frequency interference

protection ratio and adjacent frequency interference protection ratio. As a result,

this frequency reuse pattern is good for the network quality and security. Under

the 4 x 3 frequency reuse pattern, the frequency reuse aggressiveness is 12.

For the aggressive reuse introduced hereunder, because the BCCH plays an

important role in the network and you cannot use the apply the anti-interference

measures, such as downlink power control and DTX, to the BCCH, you must

apply the 4 x 3 frequency reuse pattern or looser reuse patterns to the BCCH

carriers.

6.4.1 shows the normal 4 x 3 frequency reuse pattern.

Figure 1.3 Normal 4 x 3 frequency reuse pattern

Under this frequency reuse pattern, N is 4, so the following equation is present:

46.3433 ＝×== Nq

Under this frequency reuse pattern, each cell is a 120º-directional cell. At this

time, the number of the interference source is reduced by 2, sot the C/I in the

poorest condition can be expressed by the following equation:

dBqq

IC 20)7.0(

1/

44=

++= −−

In actual conditions, because the base station are irregularly distributed, the

antenna height is different, and the effect from the radio environment, the C/I

cannot reach a so high value.




6.4.2 10MHz Bandwidth 4 x 3 Frequency Reuse

Hereunder are several assumptions:

The available bandwidth is 10MHz.

The channel number is 45–94.

If the channel numbers ranging from 81–94 (14 channel numbers in total)

are allocated to the BCCH, and the other channel numbers are allocated to

TCH.

If the previous assumptions are present, the frequency planning under 4 x 3

frequency reuse pattern is provided in 6.4.2.

Table 3.1 Frequency planning under 4 x 3 frequency reuse pattern (a)

Frequency group

number

A1 B1 C1 D1 A2 B2 C2 D2 A3 B3 C3 D3

Channel number of

each frequency

group

94 93 92 91 90 89 88 87 86 85 84 83

80 79 78 77 76 75 74 73 72 71 70 69

68 67 66 65 64 63 62 61 60 59 58 57

56 55 54 53 52 51 50 49 48 47 46 45

According to this table, the channel numbers in the first line are BCCH numbers,

in which the channel numbers 81 and 82 are standby channel numbers. The

frequency groups correspond to the cell numbers in 6.4.1. The channel number

of BCCH of the cell A1 is 94. It is 80, 68 and 56 for other carriers, and so on.

In a cluster which contains 12 cells, the frequency group for base station A is

{A1, A2, and A3}; the frequency group for base station B is {B1, B2, and B3}; the

frequency group for base station C is {C1, C2, and C3}; and the frequency group

for base station D is {D1, D2, and D3}.

Therefore, as listed in this table, no channel number is reused within a cluster. In

addition, the intra-frequency and adjacent frequency are not available for the

adjacent cells and the same cell.

However, the drawbacks of this frequency reuse pattern are that the frequency

reuse ratio is low and the capacity expansion needs a great amount of the

frequency resources. Therefore, this reuse pattern is not used in the areas where

the network capacity needs to be constantly expanded.

If the bandwidth is 10MHz, the maximum base station configuration is S4/4/4

under the normal 4 x 3 frequency reuse pattern, and the frequency reuse degree

is 12.5 (50/4 = 12.5).




Note:

The maximum base station type mentioned in the chapter refers to the

configuration type that most continuous base stations can reach. It does not

include standalone base station.


For the 19MHz frequency (1 to 94) used by China Mobile, the 4 x 3 frequency

reuse pattern are used for the frequency planning. The channel numbers ranging

from 79 to 94 (16 channel numbers in total) are allocated to the BCCH, and other

channel numbers are allocated to TCH. No channel number is reserved for micro

cells. In this case, the frequency planning solution is provided in 6.4.3.

Table 3.2 Frequency planning under 4 x 3 frequency reuse pattern (b)

Frequency group

number

A1 B1 C1 D1 A2 B2 C2 D2 A3 B3 C3 D3

Channel number of

each frequency

group

94 93 92 91 90 89 88 87 86 85 84 83

78 77 76 75 74 73 72 71 70 69 68 67

66 65 64 63 62 61 60 59 58 57 56 55

54 53 52 51 50 49 48 47 46 45 44 43

42 41 40 39 38 37 36 35 34 33 32 31

30 29 28 27 26 25 24 23 22 21 20 19

18 17 16 15 14 13 12 11 10 9 8 7

6 5 4 3 2 1

As listed in this table, the channel numbers ranging from 79 to 82 are standby

channel numbers. For the 19MHz bandwidth, the maximum base station type

can be S8/7/7 under 4 x 3 frequency reuse pattern. The frequency reuse

degrees are 11.75, 13.43, and 13.43, so the average value is 12.87.


For the 6MHz frequency (96 to 124) used by China Unicom, the 4 x 3 frequency

reuse pattern is used for the frequency planning. The channel numbers ranging

from 111 to 124 (14 channel numbers in total) are allocated to the BCCH, and

other channel numbers are allocated to TCH. No channel number is reserved for

micro cells. In this case, the frequency planning solution is provided in




Table 3.3 Frequency planning under 4 x 3 frequency reuse pattern (c)

Frequency group

number

A1 B1 C1 D1 A2 B2 C2 D2 A3 B3 C3 D3

Channel number of

each frequency

group

124 123 122 121 120 119 118 117 116 115 114 113

110 109 108 107 106 105 104 103 102 101 100 99

98 97 96

As listed in this table, the channel numbers ranging from 111 to 112 are standby

channel numbers. For the 6MHz bandwidth, the maximum base station type can

be S3/2/2 under 4 x 3 frequency reuse pattern. The frequency reuse degrees are

9.67, 13.5, and 13.5, so the average value is 12.22.

6.4.5 4 x 3 Frequency Reuse Conclusion

The 4 x 3 frequency reuse pattern is a basic technology applied in frequency

planning. It is applicable to other frequency aggressive reuse technologies that

are used for the BCCH.

Theoretical analysis shows that when the base stations are regularly distributed

and azimuths of the cells are consistent with each other, the interference can be

reduced to the minimum. Therefore, if you intend to expand the network capacity,

you can keep the base stations to be distributed as regular as possible and plan

the azimuths of the cells along the same direction. In addition, you can also

maintain the antennas at a similar height. However, sometimes you need to

adjust the azimuth of the antenna to improve the coverage, which seems

contradicts to the capacity expansion. Therefore, sometimes you must make find

a balance between the coverage and capacity.

If the network capacity needs to be further expanded, you can take the following

measures:

Split a cell into smaller cells. At present, however, the average coverage

radius of the macro cell base stations in urban areas is already shorter than

500m, so further cell splitting will meet difficulty in cost and technology.

Utilize new frequency resources. For example, you can employ the

1800MHz band to establish a DSC 1800MHz network.

Under the current 900MHz network, use more aggressive frequency reuse

technology to expand the network capacity.

At present, the aggressive frequency reuse technology works as the most

economical and convenient way to expand the network capacity, so it is also the

most popular with carriers.




The typical frequency reuse technology includes 3 x 3, 2 x 6, 2 x 3, 1 x 3, and 1 x

1.

6.5 Aggressive Frequency Reuse Technology

6.5.1 3 x 3 Frequency Reuse Pattern

The 3 x 3 frequency reuse pattern can be used in the areas with high traffic. That

is, three base stations form a group, and each base station has three cells, so

there are 9 cells, which form a frequency reuse cluster. However, the 9 cells use

different frequencies. Compared with the 4 x 3 frequency reuse pattern, the intra-

frequency reuse distance under the 3 x 3 frequency reuse pattern is small, so

on-line interference is greater.

6.5.1 shows the 3 x 3 frequency reuse pattern.

Figure 1.4 3 x 3 frequency reuse pattern

If the available bandwidth is 10MHz and the channel numbers are from 45 to 94,

you can use normal 4 x 3 frequency reuse pattern on BCCH. In this case, the

frequency ranges from 81 to 94, so 14 channel numbers are available. For TCH,

you can use 3 x 3 frequency reuse pattern. In this case, the frequency ranges

from 45 to 80, so 36 channel numbers are available.

For the frequency planning under 3 x 3 frequency reuse pattern, see 6.5.1.

Table 4.1 Frequency planning under 3 x 3 frequency reuse pattern

Frequency group

number

A1 B1 C1 A2 B2 C2 A3 B3 C3

Channel number of

each

80 79 78 77 76 75 74 73 72

71 70 69 68 67 66 65 64 63




frequency group

62 61 60 59 58 57 56 55 54

53 52 51 50 49 48 47 46 45

If 3 x 3 reusing the 10MHz band, you can configure the maximum base station

type as S5/5/5, and the frequency reuse degree is 10.

According to previous equations, because the number of base stations is 3 (N =

3), the intra-frequency interference attenuation factor is 3 (q = 3). In this case,

the number of the intra-frequency interference sources is 2 at the first layer. If the

radius of the cell is 4, the theoretical carrier-to-interference ratio (C/I) can be

expressed by the following equation:

dBq

IC 07.162

1/

4=

⋅= −

In actual conditions, because base stations are irregularly distributed, the

antenna height varies, and the effect from the radio environment, the value of C/I

can not be as high as 16.07 dB.

When the bandwidth is 10MHz, the base station type can be configured as

S5/5/5 under 3 x 3 frequency reuse pattern. For 4 x 3 frequency reuse pattern,

the maximum base station configuration type can only be configured as S4/4/4/.

Therefore, network capacity under 3 x 3 frequency reuse pattern is greater than

that under 4 x 3 frequency reuse pattern when the bandwidth is the same.

When the number of subscribers in a network is not great, you can use the 3 x 3

frequency reuse pattern to ease the pressure of network capacity. In actual

conditions, however, because base stations are irregularly distributed, the

antenna height is different, and the coverage area of each base station varies,

the interference in the network will increase. In this case, if you intend to obtain

better voice quality, you must take some anti-interference measures, such as

using frequency hopping and DTX.

The characteristic of the 3 x 3 frequency reuse pattern are as follows:

The adjustment for network structure is unnecessary.

The frequencies can be easily grouped and the system capacity is great.

Compared with 4 x 3 frequency reuse pattern, 3 x 3 frequency reuse pattern

brings greater interference, but the overall interference can be controlled to

a lower level.

If frequency hopping is used, adequate bandwidth is needed.

6.5.2 2 x 6 Reuse Pattern

The 2 x 6 frequency reuse pattern is developed from the 4 x 3 frequency reuse

pattern. Under the 4 x 3 frequency reuse pattern, you can add anther 2 cells to




each base station, so 2 base stations (each base station has 6 60°-sectorized

cells) has 12 cells, which form a frequency reuse cluster. In this case, a

frequency reuse cluster contains 12 60°-sectorized cells, and this is defined as 2

x 6 frequency reuse pattern.



Under the 2 x 6 frequency reuse pattern, 45.2233 ＝×== Nq .

Because each cell is 60°-directional cell under 2 x 6 frequency reuse pattern, the

interference source of each cell is reduced to 1 at the first layer. In this case, the

theoretical C/I can be expressed by the following equation:

dBq

IC 6.151

/4

== −

In actual conditions, because base stations are irregularly distributed, the

antenna height is different, and the effect from radio environment, the value of

C/I cannot be as high as 15.6 dB.

If the available bandwidth is 10MHz, the channel numbers range from 45 to 94,

you can also use 2 x 6 frequency reused pattern. Considering the characteristics

of the 2 x 6 cellular structures, you can also use the 2 x 6 frequency reuse for

BCCH. The frequencies are from 81 to 94, 14 channel numbers in total, and the

others are TCH numbers.



Frequency group

number

A1 B1 A2 B2 A3 B3 A4 B4 A5 B5 A6 B6




Channel number of

each frequency

group

94 93 92 91 90 89 88 87 86 85 84 83

80 79 78 77 76 75 74 73 72 71 70 69

68 67 66 65 64 63 62 61 60 59 58 57

56 55 54 53 52 51 50 49 48 47 46 45

As listed in this table, when allocating frequency to the base station, you can

select the frequency according to the regularity of {A1, A2, A3, A4, A4, A6} and

{B1, B2, B3, B4, B5, B6}. Note that intra-frequency and neighbor frequency

cannot be present within the same cell and adjacent cells.

Under the 2 x 6 frequency reuse pattern, you can enhance the system capacity

by adding new cells to the base station. Compared with 4 x 3 frequency reuse

pattern, the maximum base station type can be configured as S4/4/4/4/4/4 under

2 x 6 frequency reuse pattern, so the capacity of a single base station is twice

that of the base station under the 4 x 3 frequency reuse pattern.

Under this frequency reuse pattern, however, the intra-frequency reuse distance

is further shortened, which increases network interference greatly. In addition, as

the number of cells increases, the requirements on the half-power angle and

other antenna indexes are higher. Moreover, you must add antenna feeders to

the system if using the 2 x 6 frequency reuse pattern, which brings great difficulty

to project implementation. Therefore, the 2 x 6 frequency reuse pattern is seldom

used.

For the 2 x 6 frequency reuse pattern, the frequency reuse degree is 12.5. And

its characteristics are listed in the following:

Through add more cells to each base station, you can enhance the capacity

of the base station greatly.

The antennas with smaller half-power angle and good performance are

needed and the requirement on antenna and base station address is strict.

The signals radiated by antennas are more concentrated, which is good for

indoor coverage.

The BSS system must support 6 sectors.

More antennas are needed under the 2 x 6 frequency reuse pattern than

that under 4 x 3 frequency reuse pattern, so you must adjust and optimize

the planning for antenna system and frequencies.

The times of handovers under the 2 x 6 frequency reuse pattern are more

than that under the 4 x 3 frequency reuse pattern.

The intra-frequency reuse distance is small, so the interference within the

network is great. Therefore, you must take anti-frequency measures, such

as using DTX and frequency hopping.





Under 2 x 3 frequency reuse pattern, there are 2 base stations. Each one has 3

cells, so 6 cells form a frequency reuse cluster. The cells in the same cluster use

the different frequencies, and the cells in different clusters use the same

frequency group. This is defined as the 2 x 3 frequency reuse pattern.



Under 2 x 3 frequency reuse pattern, each intra-frequency cell is interfered by 3

cells. Because the number of base stations in each frequency cluster is 2 (N =

2), the intra-frequency interference attenuation factor (q) can be expressed by

the following equation:

45.22*33 === Nq

For regularly-arranged cells, the theoretical carrier-to-interference ratio (C/I) can

be expressed by the following equation:

dBq

IC 8.103

1/

4=

⋅= −

Even if the cells are regularly arranged, however, the value of C/I cannot meet

the requirement of the network. Therefore, you must take anti-frequency

measures, such as frequency hopping, power control, and DTX.

For 10MHz bandwidth, the available channel numbers are from 45 to 94. If the

14 channel numbers (81-94) are BCCH numbers, and the others are TCH

numbers, the frequencies are planned according to 6.5.3 under 2 x 3 frequency

reuse pattern.





Frequency group number

A1 B1 A2 B2 A3 B3

Channel number of each frequency

group

80 79 78 77 76 75

74 73 72 71 70 69

68 67 66 65 64 63

62 61 60 59 58 57

56 55 54 53 52 51

50 49 48 47 46 45

You can use looser 4 x 3 frequency reuse pattern and allocate 14 channel

numbers for BCCH. If the bandwidth is 10MHz, you can configure the maximum

base station type as S7/7/7 under the 2 x 3 frequency reuse pattern. In this case,

the frequency reuse degree is 7.14.

The network capacity is great under the 2 x 3 frequency reuse pattern, but small

intra-frequency reuse distance will cause great interference. In addition, the cell

traffic cannot 100% reach the designated value. In actual conditions, therefore,

you can use the looser 4 x 3 frequency reuse pattern for BCCH and the 2 x 3

frequency reuse pattern for TCH.

The characteristics of the 2 x 3 frequency reuse pattern are listed below:

The network capacity is relatively great.

The adjustment for the network structure is unnecessary.

The network capacity can be expanded without wide frequency band.

Small intra-frequency reuse distance will cause great interference, so you

must take anti-interference measures to ensure network quality.

Radio frequency (RF) hopping technology must be used to support the

equipments.

The antennas must be directed to the same direction as much as possible.


1 x 3 frequency reuse pattern is also called fractional reuse. For 1 x 3 or 1 x 1

frequency reuse pattern, the reuse distance is quite small, so the interference in

the network is quite great. Therefore, to avoid frequency collision, you must use

RF hopping technology and set the parameters, including MA (mobile allocation),

HSN (hopping sequence number), and MAIO (mobile allocation index offset).

The ratio of number of the TRXs to that of the frequency hopping is FR LOAD

(generally, it is smaller than 50%).

Under the 1 x 3 frequency reuse pattern, the interference in the network can also




indicates the probability of the collision of intra-frequencies and neighbor

frequencies. Emulation shows that probability of the collision is related to FR

only.

According to 1 x 3 frequency reuse pattern, the 3 cells of a base station form a

frequency reuse cluster. The same-directional cells of each base station use the

same frequency group.



For the 1 x 3 frequency reuse pattern, the number of base station is 1 (N = 1), so

73.13 == Nq , and dBq

IC 8.43

1/

4=

⋅= − .

Because the value of C/I here is far lower than the protection value required by

the system, you must take anti-interference measures, such as frequency

hopping, power control, and DTX, to enhance the value of C/I.

If the available bandwidth is 10MHz, the available channel numbers are from 45

to 94. Because RF hopping must be used under 1 x 3 frequency reuse pattern,

considering the importance of BCCH, you can use 4 x 3 frequency reuse pattern

for BCCH and 1 x 3 frequency reuse pattern for TCH.

For BCCH, 14 channel numbers (81-94) are available; for TCH, 36 channel

numbers (45-80) are available.

The channel numbers used for TCH are divided according to two ways. They are

space grouping and sequence grouping. For the 1 x 3 frequency reuse spacing

grouping, see 6.5.4.

Table 7.1 1 X 3 frequency reuse space grouping (a)


Channel number MAIO

A 80, 77, 74, 71, 68, 65, 62, 59, 56, 53, 50, 47 0, 2, 4,6, 8, 10




B 79, 76, 73, 70, 67, 64, 61, 58, 55, 52,49, 46 1, 3, 5, 7, 9, 11

C 78, 75, 72, 69, 66, 63, 60, 57, 54, 51, 48, 45 0, 2, 4, 6, 8, 10

For the 1 x 3 frequency reuse sequence grouping, see 6.5.4.

Table 7.2 1 x 3 frequency reuse sequence grouping (a)


Channel number MAIO

A 80, 79, 78, 77, 76, 75, 74, 73,72, 71, 70, 69 0, 2, 4, 6, 8, 10

B 68, 67, 66, 65, 64, 63, 62, 61, 60, 59, 58, 57 0, 2, 4, 6, 8, 10

C 56, 55, 54, 53, 52, 51, 50, 49, 48, 47, 46, 45 0, 2, 4, 6, 8, 10

Because the ratio of the number of carriers to that of frequency hopping is

required to be 1 to 2, if the bandwidth is 10MHz, you can configure the maximum

base station type as S7/7/7. In this case, the frequency reuse degree is 7.14.

The 3 cells of the same base station use the same HSN, and the cells of different

base stations use different HSNs. To avoid the interference from neighbor

frequencies, you can configure a proper MAIO for the cells of the same base

station.

If the available bandwidth is 6MHz, the available channel numbers are from 96 to

124. In this case, you can use 4 x 3 frequency reuse pattern for BCCH (the

available channel numbers are from 111 to 124, namely, 14 in total). For TCH,

you can use 1 x 3 frequency reuse pattern (the available channel numbers are

from 96 to 110, namely, 15 in total.

For the 1 x 3 frequency reuse space grouping when the bandwidth is 6MHz, see

6.5.4.

Table 7.3 1 x 3 frequency reuse space grouping (b)


Channel number MAIO

A 96, 99, 102, 105, 108 0, 2, 4

B 97, 100, 103, 106, 109 1, 3

C 98, 101, 104, 107, 110 0, 2

When the bandwidth is 6MHz, you can configure the maximum base station type

as S4/3/3 under 1 x 3 frequency reuse space grouping. In this case, the

frequency reuse degree is 7.25/9.67/9.67, with 8.86 in average.




For the 1 x 3 frequency reuse sequence grouping, see

Table 7.4 1 x 3 frequency sequence grouping (b)


Channel number MAIO

A 96, 97, 98, 99, 100 0, 2

B 101, 102, 103, 104, 105 0, 2

C 106, 107, 108, 109, 110 0, 2

Because the ratio of the number of carriers to that of frequency hopping is

required to be 1 to 2, if the bandwidth is 6MHz, you can configure the maximum

base station type as S3/3/3. In this case, the frequency reuse degree is 9.67.

For TCH, both the space grouping and sequence grouping have drawbacks.

Generally, for the urban areas where base stations are regularly and densely

distributed, you should better use sequence grouping. For the areas where base

stations are fragmentary and irregularly distributed, you should better use space

grouping.

The characteristics of 1 x 3 frequency reuse pattern are listed below:

The frequencies are more aggressively reused, so the network capacity is

great.

The network capacity under space grouping is a little greater than that under

sequence grouping.

When planning a network, you need to plan channel numbers for BCCH

only.

Re-planning for frequencies is unnecessary during network optimization.

The efficiency for network planning is high.

Wideband combiner must be used, but the cavity combiner with frequency

selectivity is inapplicable.

This frequency reuse pattern requires wideband repeater.

The interference among intra-frequencies and neighbor frequencies

increases as the frequency reuse distance decreases.

RF hopping must be used, and the channel numbers participating frequency

hopping is twice that of the number of carriers at least.

In actual conditions, you cannot take anti-interference measures, such as

RF hopping, DTX, and power control, for BCCH. Therefore, to ensure

network quality, you can use the looser 4 x 3 frequency reuse pattern for

BCCH only.





One cell of one base station forms a frequency reuse cluster, and this is defined

1 x 2 frequency reuse pattern. Other cells and this cell use the same frequency

group.

If the available bandwidth is 6MHz, the available channel numbers are from 96 to

124. Because RF hopping must be used under 1 x 1 frequency reuse pattern,

considering the importance of BCCH, you can use 4 x 3 frequency reuse pattern

for BCCH and 1 x 1 frequency reuse pattern for TCH.

If 4 x 3 frequency reuse pattern is used for BCCH, the available channel

numbers are from 111 to 124, 14 in total. The channel numbers from 96 to 110

are used for TCH, 15 in total.


Table 7.5 Frequency planning under 1 x 1 frequency reuse pattern.

Frequency group

number

Channel number MAIO

A 96,97,98,99,100,101,102,103,104,105,106,107,108,109,110 0,2,4

B 96,97,98,99,100,101,102,103,104,105,106,107,108,109,110 6,8

C 96,97,98,99,100,101,102,103,104,105,106,107,108,109,110 10,12

If the bandwidth is 6MHz, you can configure the maximum base station type as

S4/3/3/ under 1 x 1 frequency reuse pattern. In this case, the frequency reuse

degree is 7.25/9.67/9.67, so the average value is 8.86.

Therefore, the maximum base station configuration under 1 x 1 frequency reuse

pattern is the same as that under 1 x 3 frequency reuse space grouping pattern,

so is the network capacity.

6.5.6 A + B Frequency Reuse Pattern

The A + B frequency reuse pattern is developed from 1 x 3 frequency reuse

pattern. When the bandwidth is narrow but the capacity is great, you can use this

frequency reuse pattern. In this case, you must use RF hopping. Under the A + B

frequency reuse pattern, the frequencies can be divided into three groups. They

are {f1}, {f2}, and {f3}. For frequency planning, see 6.5.6.




Figure 1.8 A + B frequency reuse pattern

According to A + B frequency reuse pattern, you can increase frequency diversity

gain by increasing the number of channel numbers participating frequency

hopping within the cell, because the increase of the frequency diversity gain can

improve the carrier-to-interference ratio. To avoid interference among intra-

frequencies and neighbor frequencies, you can configure a proper MAIO for the

cells within the same base station. The probability of the collision of the intra-

frequencies and neighbor frequencies will decrease as the number of channel

numbers participating frequency hopping increases among cells of different base

stations.

If the available bandwidth is 6MHz, the available channel numbers are 96 to 124.

For A + B frequency reuse pattern, you must use RF hopping, but the BCCH

does not participate in RF hopping. Therefore, in actual planning, to ensure good

network quality, you can use looser 4 x 3 frequency reuse pattern for BCCH and

A + B frequency reuse pattern for TCH.

If you use 4 x 3 frequency reuse for BCCH, the available channel numbers are

111 to 124, 14 in total, in which two channel numbers are standby ones. For

TCH, the available channel numbers are 96 to 110, 15 in total.

For the frequency planning under A + B frequency reuse pattern, see 6.5.6.

Table 8.1 Frequency planning under A + B frequency reuse pattern


Channel number MAIO

A 96, 97, 98, 99, 100, 101, 102, 103, 104, 105 0, 2, 4

B 101, 102, 103, 104, 105, 106, 107, 108, 109, 110 1, 3

C 96, 97, 98, 99, 100, 106, 107, 108, 109, 110 5, 7




When the bandwidth is 10MHz, you can configure the maximum base station

type as S4/3/3 under A + B frequency reuse pattern. In this case, the frequency

reuse degree is 7.25/9.67/9.67, so the average value is 8.86.

In actual conditions, the irregular distribution of base stations and antenna height

may deteriorate the performance of parts of the network. Therefore, the A + B

frequency reuse pattern are not recommended in large networks.

6.6 Concentric Cell Technology

6.6.1 Concept

In the GSM network, concentric cell technology is used to divide the service area

into two parts: overlay and underlay. In essence, the concentric cell technology

concerns channel allocation and handover. When combining this technology with

various frequency planning technologies, you can both expand network capacity

and improve network quality.

The underlay covers the traditional cells, and the overlay covers the areas near

the base station. Generally, 4 x 3 frequency reuse pattern is used for the

underlay. For overlay, the frequency reuse patterns, such as 3 x 3, 2 x 3, or 1 x 3,

are used. Therefore, all carriers can be divided into two groups, one for underlay,

and the other one for overlay. The overlay and underlay share the same base

station address, one set of antenna feeder system, and one BCCH, so you must

set the BCCH on the underlay.

6.6.1 shows the schematic diagram of the concentric cell.

Figure 1.9 Schematic diagram of concentric cell

If the capacity of the overlay is great, you can group the channel numbers

according to 6.6.1. In this case, the overlay has more channel numbers, which is

beneficial for the base station to absorb nearby traffic volume.


Overlay



Table 9.1 Channel number grouping for 6MHz bandwidth concentric cell (a)

If traffic volume is evenly distributed, you can enhance the underlay capacity

through grouping the channel numbers according to 6.6.1. In this case, the

underlay can absorb more traffic volume.

Table 9.2 Channel number grouping for 6MHz bandwidth concentric cell (b)

6.6.2 General Underlay Overlay

General underlay overlay (GUO) aims to restrict the intra-frequency interference.

To realize this purpose, you can reduce the overlay coverage area. That is, if the

transmit power of the overlay carriers is lower than that of the underlay carriers,

the coverage area of the overlay is smaller than that of the underlay.

The handover between the overlay and underlay is related to the receiving level

of the MS and the TA (timing advance) from the MS to the base station. You

should allocate the channel numbers (such as BCCH number) with looser

frequency reuse aggressiveness to the MSs in the underlay. For the MSs in the

overlay, you should allocate the channel numbers with aggressive frequency

reuse to them. In this case, you can expand the network capacity by using

aggressive frequency reuse pattern in overlay.

6.6.2 shows the structure of the general underlay overlay.


Logical channel

Channel number

Underlay

(12)

66 67 68 69 70 71 72 73 74 75 76 77

Overlay (18)

78 79 80 81 82 83 84 85 86 87 88 98 90 91 92 93 94 95

Logical channel

Channel number

Underlay (24)

66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89

Overlay (6 )

90 91 92 93 94 95



Figure 1.10 Structure of general underlay overlay

For general underlay overlay, the coverage area of the underlay is inconsistent

with that of the overlay, so problems concerning traffic and handover control are

often caused. The general underlay overlay is applicable to the areas near the

base station where the traffic is concentrated. The more concentrated the traffic

near the base station, the more apparent the effect of capacity expansion is.

However, the transmit power of the carriers in the overlay is low, so it is hard for

the base station to absorb indoor traffic volume. In this case, when the traffic

volume is evenly distributed, the general underlay overlay has little effect on

capacity expansion.

6.6.3 Intelligent Underlay Overlay

Intelligent underlay overlay (IUO) technology can ensure that the coverage areas

of call carriers are the same. For an IUO, the transmit power of the carriers in the

underlay and overlay is the same. For the structure of the IUO, see 6.6.3.

Figure 1.11 Structure of intelligent underlay overlay

In an IUO, the frequencies of a base station are divided into two layers: one is

regular layer, and the other one is supper layer. At the regular layer, the

frequency reuse distance is large, so you can use looser frequency reuse

pattern, such as 4 x 3 frequency reuse pattern. At the supper layer, the frequency

reuse distance is relatively small, so you can use aggressive frequency reuse

patterns, such as 2 x 3 and 1 x 3 frequency reuse pattern.




In an IUO, the interference at the supper layer is great, so designated

equipments and handover algorithms on C/I must be provided.

In an IUO, the conversation is first established at the supper layer, and then the

BSC monitors the C/I of the channels at the supper layer without any stop. If the

C/I is greater than the Good C/I Threshold, the conversation seizes a channel at

the supper layer. If the C/I is smaller than the Bad C/I Threshold, the

conversation seizes a channel at the regular layer. In addition, you can control

the traffic volume at the supper layer and the regular layer by adjusting the

handover threshold.

For an IUO, the transmit power of the carriers at the regular layer is the same as

that at the supper layer, so the network can absorb the traffic flexibly, which is

beneficial for the expansion for actual network capacity.

If the IUO technology is used, you must add the functions, including the

estimation of intra-frequency protection C/I for downlink channels and the

handover algorithms related to IUO, to the system.

6.6.4 Characteristics of Concentric Cell Technology

The characteristics of concentric cell technology are listed below:

Any change of the network structure is unnecessary.

Special software and designated algorithms on channel allocation and

handover are needed.

The system has no special requirement on hardware.

GUO is applicable to the areas near the base station where the traffic is

concentrated.

The overlay coverage of the GUO is small, so the intra-frequency reuse

attenuation factor (q) is great, which increases interference in the network.

The transmit power of the overlay carriers in the GUO is low, so it is hard for

the carriers to absorb indoor traffic.

The transmit power of the underlay carriers in the GUO is the same, so the

carriers can absorb indoor traffic, which contributes to network capacity

expansion and good conversation quality.

For the comparison between the GUO and IUO, see 6.6.4.

Table 11.1 A comparison between GUO and IUO.

Coverage area

Frequency reuse pattern

Transmit power

Logical channel allocation

Handover algorithm

GUO Underlay 4 x 3 High BCCH/TCH Power& Distance

Overlay 3 x 3/2 x 3/1 x 3 Low TCH




IUO Underlay 4 x 3 Same BCCH/TCH C/I

Overlay 3 x 3/2 x 3/1 x 3 Same TCH

6.7 Multiple Reuse Pattern Technology

6.7.1 Basic Principle

According to multiple reuse pattern (MRP), the carriers are divided into several

groups. The carries in each group work as an independent layer, and each layer

uses a different frequency reuse pattern. During frequency planning, you can

configure the carriers layer by layer, with reuse aggressiveness increases layer

by layer, as shown in 6.7.1.

Figure 1.12 Layering aggressive frequency reuse

In this figure, the ellipse in the same color indicates a frequency group, in which

the frequencies are reused, and the size of an ellipse indicates its coverage

area. L1, L2…L3 indicates frequency layers in the cell. As shown in this figure,

the degree of frequency reuse aggressiveness is greater at upper layers. If the

bandwidth is certain, the number of carriers is greater under the layering

aggressive frequency reuse pattern than that under the frequency reuse pattern

where the reuse degree is the same between layers.

MRP has no special requirement on hardware. It is developed from the concept

of carrier layering. That is, the available channel numbers are divided into

multiple groups, and each group works as a carrier layer. According to the rules

of the aggressive frequency reuse pattern, the channel numbers allocated for

each layer are listed in 6.7.1.

Table 12.1 Channel number allocation for each layer

Layer Channel number

BCCH n1




TCH 1 n2

TCH 2 n3

… …

TCHm-1 nm

Note:

n1 ≥ n2 ≥ n3 ≥ n4 ≥…≥nm.

For MRP, first you must divide an available band into several sub-bands.

Generally, the sub-bands work as the bands for BCCH. The reasons are listed

below:

BSIC decoding will not be affected by traffic. TCH numbers cannot affect

separated BCCH numbers, which is helpful for the MS to decode the BSIC.

The planning for adjacent cell list can be simplified. The separated BCCH

numbers contributes the simplification of adjacent cell list, so the MS can

capture the useful BCCH quickly.

Maximum gain can be obtained from power control and DTX. Downlink

power control and DTX can be applied to TCH carriers only, so the

separated BCCH numbers can maximize the function of downlink power

control and DTX.

The re-planning for TCH numbers will not affect BCCH. When a TRX is

added to the system, if not considering the isolation of combiner and

adjacent frequency interference, you do not have to change the BCCH

numbers.

After that, you must divide the remaining channel numbers into multiple TCH

bands. For MRP, different frequency reuse patterns must be used for different

TCH bands.

According to the carrier allocation in the network, you can decide the average

frequency reuse degree. According to the maximum number of carriers

configured in each cell and the number of cells configured in the network, you

can adjust the average frequency reuse degree to a proper value. In this way,

you can effectively control network quality.

The increase of the carries has little effect on the frequency allocation plan. The

increased channel numbers affect other cells that have more carriers than the

service cell has. For example, if a cell has four carriers, the cells that have been

configured with more than four cells will be affected.

MRP technology enables carriers to be configured flexibly. According to MRP, the

frequencies of a cell can never be completely identical with that of the adjacent

cells. Therefore, the MRP improves both the intra-frequency interference

protection ratio and frequency hopping effect.




According to the requirements defined in GSM protocols, all the downlink

timeslots of the BCCH carriers must transit with full power and the interference

features of the BCCH are different from that of the TCH. Therefore, to ensure

network quality and security, you are recommended to use 4 x 3 frequency reuse

pattern for BCCH. In this case, the channel numbers used for BCCH are equal to

or more than 12. In actual conditions, they are from 12 to 15.

If the available bandwidth is 7.2MHz, the available channel numbers are from 60

to 95, 36 in total, and they can be divided into 4 groups, as shown in 6.7.1.

Figure 1.13 Frequency planning under MRP (7.2MHz bandwidth)

As shown in this figure, 12 channel numbers can be reused for BCCH. Traffic

channels are divided into TCH1, TCH2, and TCH3. For TCH1, 9 channel

numbers can be reused; for TCH2, 8 channel numbers can be reused; and for

TCH3, 7 channel numbers can be reused.

To ensure network security, you must finish BCCH number allocation first. To be

specific, plan the 12 channel numbers according to 4 x 3 frequency reuse pattern




and allocate 1 BCCH number to each of the 12 cells. After that, you should

allocate 1 carrier at the TCH3 layer to each cell, and then you should allocate the

TCH2 and TCH1 numbers to the cells. In this case, you can configure four

channel numbers for each cell of a base station (S4/4/4). The remaining 3

channel numbers can be configured for micro cells or mini-micro cells.

6.7.2 MRP Sequence Grouping

Because BCCH numbers and TCH numbers are selected in different ways, the

MRP can be divided into two types. They are MRP sequence grouping and MRP

space grouping, the first of which is introduced hereunder.

If the available bandwidth is 10MHz, the channel numbers are from 46 to 94. In

this case, you can plan the frequencies at the BCCH and TCH carrier layers

according to the sequence of the channel numbers. If using the sequence

planning, you should add 1 to 2 extra channel numbers to the BCCH numbers.

For the MRP sequence grouping, see

Table 13.1 MRP sequence grouping

Carrier type ARFCN of the available channel number

Available channel numbers

BCCH 83–94 12

TCH1 74–82 9

TCH2 66–73 8

TCH3 58–65 8

TCH4 52–57 6

TCH5 46–51 6

Note:

ARFCN stands for absolute radio frequency channel number.

According to this table, the channel numbers can be divided into 6 groups. For

BCCH, 12 channel numbers can be reused at the carrier layer. Traffic channels

can be divided into 5 groups, from TCH1 to TCH5. For TCH1, 9 channel

numbers can be reused; for TCH2 and TCH3, 8 channel numbers can be

reused; and for TCH4 and TCH5, 6 channel numbers can be reused.

Therefore, when the bandwidth is 10MHz, the base station type can be

configured as S6/6/6. If the traditional 4/12 frequency reuse pattern is used, the

maximum base station type can be configured as S4/4/4 only.

For MRP sequence grouping, intra-frequency and neighbor frequency

interference may exist within the frequency layer, and the interference between




frequency layers exist at the critical points of the frequencies.

6.7.3 MRP Space Grouping

6.7.3 shows the frequency planning under MRP space grouping. According to

this figure, 37 channel numbers are available for the BCCH, 12 of which are

allocated to the BCCH, and the remaining of which are allocated to TCH1, TCH2,

TCH3, and MICRO.

For MRP space grouping, neighbor frequency interference does not exist within

the frequency layer, but exist between frequency layers. When the traffic is not

busy, this frequency reuse pattern can reduce network interference.

Figure 1.14 Frequency planning under MRP space grouping

If the available bandwidth is 10MHz, the available channel numbers are from 46

to 94. In this case, the frequencies can be allocated according to

Carrier type

ARFCN of the available channel number Available channel numbers

BCCH 46, 48, 50, 52, 54, 56, 58, 60, 62, 64, 66, 68 12

TCH1 70, 72, 74, 76, 78, 80, 82, 84, 86 9

TCH2 88, 90, 92, 94, 47, 49, 51, 53 8

TCH3 55, 57, 59, 61, 63, 65, 67, 69 8

TCH4 71, 73, 75, 77, 79, 81 6

TCH5 83, 85, 87, 89, 91, 93 6




Note:

ARFCN stands for absolute radio frequency channel number.

At the very beginning, not each cell needs the TRX of the last layer, so the TRX

of the last layer can reuse the frequencies more aggressively. In addition, though

interference increases after the MRP is enabled, the TRXs in the cells also

increase. In this case, more the channel numbers will participate in frequency,

which enhances frequency hopping gain.

If both the channel numbers with a little interference and the channel numbers

with great interference exist simultaneously within a cell, the frequency hopping

technology will average the interference through mixing these channel numbers.

In this case, the system can still decode the signals normally.

When allocating the frequencies according to MRP, you must notice that the

minimum frequency reuse degree at the TCH layer must be equal to or greater

than 6. In actual conditions, however, the minimum average frequency reuse

degree at the TCH layer ranges from 7 to 8. Therefore, when the frequency

resource is adequate, you can reserve some channel numbers to for future use

during frequency planning.

Fixed MRP means that the channel numbers allocated to each TCH are fixed.

They are independent of each other, as shown in 6.7.3. For MRP, you should

plan the channel numbers layer by layer so that the TCH numbers can be easily

adjusted. In this case, if interference is present at a TCH layer, you need to

adjust the channel numbers allocated to that layer only.

6.7.4 Characteristics of MRP Technology

MRP technology can enables you to plan the frequencies flexibly according to

traffic distribution. Compared with 3 x 3 frequency reuse pattern, MRP

contributes to greater network capacity. Compared with 2 x 3 and 1 x 3

frequency reuse pattern, MRP has little effect against network quality. In addition,

MRP technology is compatible with the technologies, such as frequency hopping,

power control, DTX. Moreover, it has no special requirement on hardware and

software.

Generally, the advantages of the MRP are listed below:

The network capacity is great and frequency utilization rate is high.

The channel configuration is flexible. The frequency reuse pattern is

selected according to network capacity and traffic distribution. In the areas

where the traffic is high, you can add carriers to these areas.

No two cells have the same channel numbers, so no intra-frequency cell

exists in the system if the MRP is used.




Baseband hopping and RF hopping can be used.

The base station type can be configures flexibly, which is good for network

quality.

The channels to be allocated are weighted, which enhances the network

quality.

6.7.5 Comparison between MRP and 1 X 3 Frequency Reuse Pattern

In fact, 1 x 3 frequency reuse pattern is a special kind of MRP. The configuration

for the equivalent MRP is 12/3/3/3/3/3. The following is a comparison between

MRP and 1 x 3 frequency reuse pattern.

The network capacity under 1 x 3 frequency reuse pattern is greater than

that under MRP.

For 1 x 3 frequency reuse pattern, you need to plan a group of frequencies

for TCH only. If you have to add new carriers to the system without adding

new base stations, you do not have to re-plan the frequencies. Therefore,

the frequency planning is simpler under 1 x 3 frequency reuse pattern than

that under MRP.

If the network is irregular in landforms and traffic distribution, you should

better not use 1 x 3 frequency reuse pattern. In most cases, a base station

is interfered by many base stations nearby. If the 1 x 3 frequency reuse

pattern is used, you will find it hard to position the interference source.

Therefore, when adding new base stations to the network, you cannot

eliminate the interference by adjusting some channel numbers only. If using

MRP, however, you can easily solve this problem.

6.8 Network Capacity Comparison

For the comparison of the network capacity under various frequency reuse

patterns, see

Table 14.1 Comparison of the network capacity under various frequency reuse pattern

Bandwidth Frequency reuse

pattern

Frequency

reuse

degree

Base station

configuration

type

Loadable

traffic

volume

Admissible

subscribers

Capacity

ratio

6MHz 4×3 12 3/2/2 27.9 1188 1

3×3 9 3/3/3 34.5 1380 1.16

4×3 + 1×3 7.5 4/4/3 53.5 2140 1.8

MRP(12, 9, 6) 9 3/3/3 34.5 1380 1.16

2 × 6 12 2/2/2/2/2/2 49.2 1968 1.66




IUO: 4 × 3 + 2 × 3 9 4/4/3 53.5 2140 1.8

7.2MHz 4 × 3 12 3/3/3 34.5 1380 1

3 × 3 9 4/4/4 62 2480 1.8

4 × 3 + 1 × 3 7.5 5/5/5 81.9 3276 2.37

MRP(12, 9, 8, 7) 9 4/4/4 62 2480 1.8

2 × 6 12 3/3/3/2/2/2 60.1 2404 1.74

IUO: 4 × 3 + 2 × 3 9 5/5/5 81.9 3276 2.37

9.6MHz 4 × 3 12 4/4/4 62 2480 1

3 ×3 9 5/5/5 81.9 3276 1.32

4 × 3 + 1 × 3 7.5 7/7/7 123.6 4944 1.99

MRP(12,9,8,7,6,6) 8 6/6/6 104.1 4164 1.70

2 × 6 12 4/4/4/4/4/4 126 5040 2.03

IUO: 4 × 3 + 2 × 3 9 7/7/7 123.6 4944 1.99

Note:

GoS = 0.02; a = 0.025 Erl.
