gsm frequency planning
DESCRIPTION
GSM Frequency Planning, Neighbor Cell Planning and BSIC PlanningTRANSCRIPT
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GSM_P&O__01_200904 GSM Frequency Planning
Objective
Frequency Planning
Neighbor Cell Planning
BSIC Planning
Reference
GSM Cellular Network Design and Optimization
GSM Frequency Planning
GSM Frequency Hopping Principles
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i
Contents
Chapter 1 Frequency Planning .................................................................................................................... 1
1.1 Cellular Structure Formation Rule ....................................................................................................... 1
1.2 Interference Model............................................................................................................................... 3
1.3 Frequency Multiplexing Technology and Interference Analysis ......................................................... 8
1.4 Packet Frequency Multiplexing Technology ....................................................................................... 8
1.4.1 4 x 3 Multiplexing Technology .................................................................................................. 8
1.4.2 3 x 3 Multiplexing Technology ................................................................................................ 14
1.4.3 1 x 3 Multiplexing Technology ................................................................................................ 16
1.4.4 2 x 6 Multiplexing Technology ................................................................................................ 17
1.4.5 MRP Multiple Frequency Multiplexing MRP ......................................................................... 18
1.4.6 Concentric Cell Technology .................................................................................................... 25
1.5 Cell Splitting ...................................................................................................................................... 30
1.6 Several Common Immunity Technology to Interference ................................................................... 31
1.6.1 Discontinuous Transmission (DTX) ........................................................................................ 32
1.6.2 Frequency Hopping (FH) ......................................................................................................... 32
1.6.3 Dynamic Power Control (DPC) ............................................................................................... 37
1.6.4 1 x 3 Multiplexing + Radio Frequency FH + DTX + DPC ..................................................... 37
1.7 Conclusion on Principles of Frequency Assignment in Engineering ................................................. 38
Chapter 2 Neighboring Cell Planning ....................................................................................................... 41
Chapter 3 BSIC Planning ........................................................................................................................... 49
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1
Chapter 1 Frequency Planning
Key points
Frequency multiplexing and interference model analysis based on ideal cellular
structure
Several common immunity technologies to interference
1.1 Cellular Structure Formation Rule
In ideal situation, a base unit (base station area) of cellular structure is a
regular hexagon (handoff border). A certain number of regular hexagons
constitute a radio cluster. A full mobile network coverage is composed of two
adjacent radio clusters.
The radio cluster, a base unit of Frequency Multiplexing (FR), allocates all of
the available channels in a radio cluster to every base station area or sectoral
cell. Two same radio clusters are able to be adjacent to each other and ensure
mapping relationship between each base station areas or sectoral cells. The
channel group assigned to every base station area or sectoral cell is fixed.
Therefore mapping base station areas or sectoral cells in any adjacent radio
clusters are all co-frequency areas. This forms a comprehensive co-frequency
multiplexing pattern.
The radio cluster must meet the following conditions:
1) The radio clusters are able to be adjacent to each other.
2) The distance between any two co-frequency multiplexing area centers in
the adjacent radio clusters should be equal.
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GSM_P&O_ _01_200904 GSM Frequency Planning
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A
B
C
D
E
F
G
A
B
C
D
E
F
G R
60o
i
j
D
Figure 1.1-1 Constitution of a radio cluster
As shown in the above figure, i and j are two parameters. Given the two
parameters different values (cannot be 0 at one time), any area can be
reached from a certain area. Based on triangular relationship shown in the
above figure, the distance D between two co-frequency multiplexing areas
is:
22 jijiD
The number of base stations N included in the radio cluster based on the
above distribution is:
22 jijiN
Given the distance between the centers of two adjacent base station areas is
1, and semi diameter of base station area is R, then:
3/1R
Define RDq / as co-frequency multiplexing distance protection
coefficient or called as co-channel interference attenuation factor:
NR
Dq 3
(
1
-
1
) (
1
-
2
) (
1
-
3
) (
1
-
4
)
-
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3
1.2 Interference Model
1. Co-frequency interference protection ratio B
Under the condition wherein wanted signals from the Tx end of a receiver
meet the defined quality, the parameter indicates the minimum ratio of
wanted RF signals to unwanted RF signals. Usually, the value of this
parameter is represented as dB.
2. Estimation on carrier-to-interference ratio in an N-multiplexing radio
cluster
A
B
C
D
E
F
G
A
B
C
D
E
F
G
A
B
C
D
E
F
G
A
B
C
D
E
F
G
A
B
C
D
E
F
G
A
B
C
D
E
F
G
A
B
C
D
E
F
G
A
B
C
D
E
F
G
A
B
C
D F
G
A
B
C
D
E
F
G
A
B
C
D
E
F
G
A
C
D
E
F
G
A
B
C
D
E
F
G
A
B
C
D
E
F
G
A
B
C G
A
B
E
F
G
A
B
C
D
E
F
G
A
D
E
F
A
B
C
D
E
Figure 1.2-1 Interfering resource
Regarding wave propagation characteristic, it could be described with the
preceding general model:
DiffkkkHdkHkdkkPL effeff 765loglog4log3log21
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GSM_P&O_ _01_200904 GSM Frequency Planning
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Since it is the ideal cellular system that has been observed and studied, both
emission power of each cell and antenna antenna apparent height are the
same, and there is no diffraction loss. Therefore the carrier-to-interference
ratio can be calculated as follow:
M
k
dkk
dkk
M
k
PL
PL
M
k
PL
PL
M
k
PLP
PLP
M
kk
kHeff
Heff
k
kkt
t
I
C
I
C
1
10/log)log42(
10/log)log42(
1
10/
10/
1
10/
10/
1
10/)(
10/)(
1
10
10
10
10
10
10
10
10
Indicate effHkkk log42'2
, d as cell semi diameter R, kd as
propagation distance D from each interfering resource to this cell.
As shown in figure 1-2, there are 6 the most intense interfering resources
around each cell, and 6 (or 12) the secondary most intense interfering
resources.
10/'210/'2
10'2
12
1
10/2log'26
1
10/log'2
10/log'2
)2(126
1010
10
kk
lk
k
Dk
k
Dk
Rk
DD
R
I
C
Indicate '2k/10 ( This is so-called propagation path loss slope
determined by the actual terrain environment.)
2
126)2(126
q
DD
R
I
C
Logarithm, it is:
)2
126log(10log'2)(
qkdB
I
C
(
1
-
6
)
(
1
-
5
)
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5
No loss generality, indicate 40'2 k , 4 .
So
dB5.06log10)2
126log(10
4
.
We can see that contribution to interference made by the secondary most
intense interfering resource in the second circle is much less than that of the
most intense interfering resource in the first circle, which can be negligible.
Now we have established an interference model under ideal cellular
environment. We will use this model to study its interference when various
common multiplexing methods are introduced later.
3. Co-frequency interference possibility )/( BICP
Actually, because of non-ideal site location and rise-and-fall characteristic topography,
when mobile station is on the move, received signals are influenced by Rayleigh fast
fading and Gauss slow fading. No matter it is signal or interference, before it reaches
mobile station, its field strength instantaneous value and median value are all random
variables. Even though mobile station stays still, as a result of various existed
interference including movement of surrounding moving objects, its field strength
instantaneous value and median value are still random variables.
We can see that the value of receiver Rx end IC / is not static but a random variable.
Only if BIC / , there is no interference. Co-frequency interference appears with
certain possibility.
According to CCIR740-2 report, in 1979 France comes up with the idea that
when multipath fading complies with Rayleigh distribution and shadow
fading complies with Gauss distribution, co-frequency interference
possibility is :
du
uBICP
uBIC 10/)2(
2
101
}exp{1)/(
In the formula, u is integration variable, is standard deviation of
signal and interference, IC
.
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GSM_P&O_ _01_200904 GSM Frequency Planning
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BICZ p
100
10-1
10-2
10-3
10-4
20 40 60
dB
= 12
= 0 = 6 = 8
Figure 1.2-2 Co-frequency interference possibility
Co-frequency interference possibility in the typical circumstance is shown
as above.
Without losing its generality, indicate = 6, interference possibility
)/( BICP =0.1,
dBZ p 12 concluded from the chart, GSM
network requires co-frequency interference protection ratio B be less than 9
dB, generally B = 12dB in engineering. Therefore, in ideal interference
model carrier-to-interference ratio must be more than: 9(12) + 12 = 21 dB
(24dB).
William C.Y. Lee believes that indicating 6 dB margin is enough, so it is
concluded that in ideal interference model carrier-to-interference ratio must
be more than 9(12)+6=15dB (18dB).
4. Near End- Far End interference
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7
A
B
C
D
d2
d1
2
1 d2
d1
Figure 1.2-3 Near end- Far end interference
1 Cell 1
2 Cell 2
According to interference model, indicate mobile station B relative to
mobile station A
dBd
dkdB
I
C9log'2)(
2
1
, then
69.11
2 d
d
. If
frequency used by mobile station B is adjacent to that of mobile station A,
when
69.11
2 d
d
, adjacent frequency interference protection ratio does not
match the condition, which causes call drop. The same circumstance also
appears in adjacent cells.
Lets check another extreme circumstance: given that Tx power of two
antennas in a cell is 34 dBm, the level received on spot D is -85 dBm, base
Comment [m1]:
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GSM_P&O_ _01_200904 GSM Frequency Planning
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station sensitivity is -110 dBm If uplink and downlink power are balanced,
then transmission power of mobile station D is -110+(34-(-85))=9dBm.
Now, when the very near mobile station C powers on, if it is working with
the maximum transmission power 30 dBm (1 W), given that the path loss
when the signal reaches cell 2 is same to that reaching mobile D, then
interference signal received by cell 2BTS is : 30-(34-(-85)= -89 > -110 + 9.
Therefore, call drop occurs.
1.3 Frequency Multiplexing Technology and Interference Analysis
Frequency multiplexing is a kind of technology commonly used in a GSM
network. It applies the same frequency to cover different areas. In addition,
it keeps certain distance between these areas using the same frequency, and
the distance is called co-frequency multiplexing distance.
If directional antenna is used, it is recommended to adopt 4 x 3 multiplexing
method. In certain areas with heavier traffic, other multiplexing methods can
be adopted according to machine capability, such as 3 x 3 and 2 x 6. No
matter which method it is adopted, its basic principle is that it should meet
the requirements of interference protection ratio after considering different
propagation conditions, different multiplexing methods, and multiple
interference factors. They are shown as follows:
Co-frequency protection ratio C/I 9 dB
Adjacent frequency interference protection ratio C/I -9 dB
400 kHz adjacent frequency interference protection ratio C/I -41 dB
1.4 Packet Frequency Multiplexing Technology
1.4.1 4 x 3 Multiplexing Technology
There are a variety of frequency multiplexing structures used by GSM, such
as 4 x 3, 3 x 3, and 2 x 6. Usually, all multiplexing methods are to classify
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9
limited frequencies into a certain number of groups. In sequence these
groups form a cluster of frequency assigned to adjacent cell (shown in the
following figure). According to advices of GSM system criteria, 4 x 3 is
commonly used in various GSM systems. 4 x 3 multiplexing method is to
divide frequencies into 12 groups assigned to 4 stations in turn. That means
3 frequency groups can be used in each station. As result of long
multiplexing distance in this frequency multiplexing method, it can reliably
meet the specifications of co-frequency protection ratio and adjacent
frequency interference protection ratio required by GSM system. Therefore,
it makes GSM network operate in fine quality and good security.
A3
D2B1
D1
D3
C1B3
C2
B2
C3
A1
A2
A3
D2B1
D1
D3
C1B3
C2
B2
C3
A1
A2
A3
B1
B3B2
A1
A2
A3
B1
A1
A2A3
D2B1
D1
D3
A1
A2
A1
A3
D2B1
D1
D3
C1B3
C2
B2
C3
A1
A2
Figure 1.4-1 43 multiplexing
Indicating the value of cellular hexagon side length as 1, from the above
figure and the preceding interference models, it can be concluded as:
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GSM_P&O_ _01_200904 GSM Frequency Planning
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dBdBI
C18
)2.7(28
2log10)(
52.352.3
52.3
Subtracting the margin of 6dB suggested by William C.Y. Lee, the value is
exactly 12 dB.
Discussion on 4 x 3 frequency packet and multiplexing model applied in
engineering:
As the name implies, 4 x 3 multiplexing divides usable frequencies into 4 x
3 = 12 groups, and respectively marks them as A1, B1, C1, D1, A2, B2, C2,
D2, A3, B3, C3, and D3. Take the following table as an example:
A1 B1 C1 D1 A2 B2 C2 D2 A3 B3 C3 D3
1 2 3 4 5 6 7 8 9 10 11 12
13 14 15 16 17 18 19 20 21 22 23 24
25 26 27 28 29 30 31 32 33 34 35 36
Indicate A1, A2, and A3 as a large group, and assign it to 3 sectors in a base
station. Indicate B1, B2, B3, C1, C2, C3, D1, D2, and D3 as a large group,
and assign it to 3 sectors in an adjacent base station. Obviously, there are 6
frequency multiplexing methods as follows.
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11
A1
A2
A3
D1
D2
D3
B1
B2
B3
C1
C2
C3
A1
A2
A3 B1
B2
B3
C1
C2
C3
A1
A2
A3
A1
A2
A3
1
A1
A2
A3
C1
C2
C3
B1
B2
B3
D1
D2
D3
A1
A2
A3 B1
B2
B3
D1
D2
D3
A1
A2
A3
A1
A2
A3
2
A1
A2
A3
D1
D2
D3
C1
C2
C3
B1
B2
B3
A1
A2
A3 C1
C2
C3
B1
B2
B3
A1
A2
A3
A1
A2
A3
3
A1
A2
A3
B1
B2
B3
C1
C2
C3
D1
D2
D3
A1
A2
A3 C1
C2
C3
D1
D2
D3
A1
A2
A3
A1
A2
A3
4
A1
A2
A3
C1
C2
C3
D1
D2
D3
B1
B2
B3
A1
A2
A3 D1
D2
D3
B1
B2
B3
A1
A2
A3
A1
A2
A3
5
A1
A2
A3
B1
B2
B3
D1
D2
D3
C1
C2
C3
A1
A2
A3 D1
D2
D3
C1
C2
C3
A1
A2
A3
A1
A2
A3
6
(1~6) Method (1-6)
If following the above frequency sequence packet method, there will be no
problems about co-frequency occurred in adjacent base stations. However,
adjacent frequency in end-on cells still exists: (see the positions indicated by
red arrowheads in the above figure)
Method 1: D1---A2; Method 2: D2---A3; Method 3: D1---A2;
Method 4: D2---A3; Method 5: D3---A1; Method 6: D3---A1.
Therefore, lets switch to another frequency packet method. See it in the
following table:
Comment [m2]:
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GSM_P&O_ _01_200904 GSM Frequency Planning
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A1 B1 C1 D1 A2 B2 C2 D2 A3 B3 C3 D3
1 2 4 3 5 8 7 6 9 11 10 12
13 14 16 15 17 20 19 18 21 23 22 24
25 26 28 27 29 32 31 30 33 35 34 36
The same 6 multiplexing methods:
No end-on adjacent frequency in Methods 1 and 4; Method 2: C1---A2;
Method 3: B2---A3;
Method 5: C1---A2, B2---A3, D3---A1; Method 6: D3---A1.
(1~6) Method (1-6)
A1
A2
A3
D1
D2
D3
B1
B2
B3
C1
C2
C3
A1
A2
A3
B2
B3
C1
C2
C3
A1
A2
A3
A1
A2
A3
1
A1
A2
A3
C1
C2
C3
B1
B2
B3
D1
D2
D3
A1
A2
A3 B1
B2
B3
D1
D2
D3
A1
A2
A3
A1
A2
A3
2
A1
A2
A3
D1
D2
D3
C1
C2
C3
B1
B2
B3
A1
A2
A3 C1
C2
C3
B1
B2
B3
A1
A2
A3
A1
A2
A3
3
A1
A2
A3
B1
B2
B3
C1
C2
C3
D1
D2
D3
A1
A2
A3 C1
C2
C3
D1
D2
D3
A1
A2
A3
A1
A2
A3
4
A1
A2
A3
C1
C2
C3
D1
D2
D3
B1
B2
B3
A1
A2
A3 D1
D2
D3
B1
B2
B3
A1
A2
A3
A1
A2
A3
5
A1
A2
A3
B1
B2
B3
D1
D2
D3
C1
C2
C3
A1
A2
A3 D1
D2
D3
C1
C2
C3
A1
A2
A3
A1
A2
A3
6
Comment [m3]:
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13
Therefore, we recommend the above frequency packet multiplexing
methods 1 and 4. Base station of each system may not be right on the grid,
hence it can be all right if we adopt the preceding packet method classified
by frequency sequence. However, adjacent frequency problem occurred in
adjacent cells should be avoided.
We can see from the above example table that the largest station type is of
7.2M bandwidth. It can be concluded that this multiplexing method cannot
satisfy the requirement of network capacity expansion in the areas with
heavy traffic, as a result of its low frequency utilization rate. In some large
and medium cities with high population density, after many times
expansion, station distance is less than 1 km, coverage semi diameter is no
more than several hundred meters and some sites even cover 300 m.
Therefore, it is not realistic to increase network capacity by adopting
large-scaled cell splitting technology. There are two methods to solve the
problem of ever-increasing network capacity demand. One is to develop
GSM900/1800 two-frequency network, and the other is to adopt the closer
frequency multiplexing technology.
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1.4.2 3 x 3 Multiplexing Technology
A3
C2B1
C1
C3
B3B2
A1
A2
A3
C2B1
C1
C3
B3B2
A1
A2A3
C2B1
C1
C3
B3B2
A1
A2
A3 C1
A1
A2
A3
C2B1
C1
C3
B3B2
A1
A2
A3 C1
A1
A2
A3
B1
B3B2
A1
A2
Figure 1.4-2 3 3 multiplexing
Indicating the value of cellular hexagon side length as 1, from the above
figure and the preceding interference models, it can be concluded as:
dBdBI
C3.13
)57.5(2)7(2
2log10)(
44
4
Discussion on 3 x 3 frequency packet and multiplexing model applied in
engineering:
3 x 3 multiplexing generally adopts baseband frequency-hopping, or it
adopts without frequency-hopping. However, it does not perform well. 3 x 3
multiplexing divides usable frequencies into 9 groups, and respectively
marks them as A1, B1, C1, A2, B2, C2, A3, B3, and C3, as follows:
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15
A1 B1 C1 A2 B2 C2 A3 B3 C3
1 2 3 4 5 6 7 8 9
10 11 12 13 14 15 16 17 18
19 20 21 22 23 24 25 26 27
28 29 30 31 32 33 34 35 36
There are the following two multiplexing methods:
Method 1: Adjacent frequency in no-end-on cell; Method 2: C1---A2,
C2---A3, C3---A1.
Obviously, multiplexing method 1 is better.
A1
A2
A3
B1
B2
B3
C1
C2
C3
C1
C2
C3
B1
B2
B3 A1
A2
A3
A1
A2
A3
C1
C2
C3
B1
B2
B3
A1
A2
A3
C1
C2
C3
B1
B2
B3
B1
B2
B3
C1
C2
C3 A1
A2
A3
A1
A2
A3
B1
B2
B3
C1
C2
C3
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1.4.3 1 x 3 Multiplexing Technology
A3
A1
A2
A3
A1
A2
A3
A1
A2
A3
A1
A2A3
A1
A2
A3
A1
A2
A3
A1
A2
Figure 1.4-3 1 x 3 multiplexing
Indicating the value of cellular hexagon side length as 1, from Figure 1-7
and the preceding interference models, it can be concluded as:
dBdBI
C43.9
)36.4(25
2log10)(
44
4
Discussion on 1 x 3 frequency packet and multiplexing model applied in
engineering:
1 x 3 is one of the closest methods in frequency multiplexing. It is generally
adopted in synthesizer hopping system. Meanwhile DTX, power control,
antenna diversity and other immunity technologies to interference are used
to make up for interference degradation caused by shortened multiplexing
distance. All non_bcch frequencies are divided into three groups: A1, A2,
and A3. Each of them is MA of three sectors in each base station, as shown
below:
-
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17
A1 1 4 7 10 13 16 19 22 25 28 31 34
A2 2 5 8 11 14 17 20 23 26 29 32 35
A3 3 6 9 12 15 18 21 24 27 30 33 36
When frequency-hopping load (number of cell frequency/MA length) is less
than 50%, MAIO of 3 cells in the same base station should not be adjacent
frequency. In addition, MAIO of cells in the same direction in each station
and HSN of 3 cells in the same base station should be the same, and HSN in
adjacent base stations should be different. Base station distance with the
same HSN should be as far as possible, all of which should be guaranteed.
1.4.4 2 x 6 Multiplexing Technology
A1
A2
A3
A4
A5
A6
B1
B2
B3
B4
B5
B6
A1
A2
A3
A4
A5
A6
B1
B2
B3
B4
B5
B6
A1
A2
A3
A4
A5
A6
B1
B2
B3
B4
B5
B6
A1
A2
A3
A4
A5
A6
B1
B2
B3
B4
B5
B6
A1
A2
A3
A4
A1
A2
A3
A4
A1
A2
A3
A4
A5
A6
B1
B2
B3
B4
B5
B6
A1
A2
A1
A2A6
A1
A2
A3
A4
A5
A6
B1
B2
B3
B4
B5
B6
Figure 1.4-4 2 x 6 multiplexing
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GSM_P&O_ _01_200904 GSM Frequency Planning
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Obviously, 2 x 6 multiplexing model is not symmetrical model. Therefore,
the multiplexing distance of cells A1 and A4 are different from that of other
cells.
Indicating the value of cellular hexagon side length as 1, from Figure 1-8
and the preceding interference models, carrier-to-interference ratio of cells
A1 and A4 can be concluded as:
dBdBI
C86.16
)64.2(
1log10)(
4
4
Carrier-to-interference ratio of other cells can be concluded as:
dBdBI
C04.12
)2(
1log10)(
4
4
1.4.5 MRP Multiple Frequency Multiplexing MRP
The Multiple Multiplexing Pattern (MRP) technology divides the full band
of frequency into BCCH frequency band and a certain number of TCH
frequency bands, and these frequency bands are mutually orthogonal. In
addition, each band of load frequency is an independent layer. Frequencies
in different layers adopt different multiplexing method and frequency
multiplexing is closer and closer by layers.
This method divides the full band of frequency into two mutually
orthogonal bands, that is, BCCH frequency band and TCH frequency band,
planning with different multiplexing methods respectively. One of methods
to improve system capacity is to use closer multiplexing method. BCCH
channel plays a decisive role in the process of mobile station access and
switching. Therefore, in order to ensure the quality of BCCH channel, the
following benefits can be enjoyed, if using the frequency orthogonal to TCH
frequency band:
BCCH can use 4 x 3 or higher multiplexing coefficient to ensure the
quality of BCCH channel, while TCH uses relatively close
multiplexing method.
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19
Separation among each layer of BCCH and TCH frequency band
reduces planning workload. Therefore, planning by layer is available.
In addition to that, a section of frequency may be kept for micro cell.
BSIC decoding has nothing to do with voice channel load. BCCH
frequency band and TCH frequency band are mutually orthogonal.
Therefore, the increase in TCH channel load has little influence on
BCCH channel. In addition, it does not have an impact on BSIC
decoding, and thereby improving switching performance.
Simplify the configuration of adjacent cell list. Some documents
indicate that long adjacent cell list will reduce switching performance,
while this method can simplify adjacent cell list, and thereby
improving switching performance.
BCCH independently uses a segment of frequency (12 frequency
points in 4 x 3 method), and thus length of adjacent cell list
(composed of BCCH frequency points) can be greatly reduced.
Give full play to immunity technologies to interference, such as
power control and DTX. BCCH cannot use dynamic power control
and DTX and it has been transmitting signal in the highest
transmission power. Therefore, BCCH and TCH will influence the
effect of these anti-interference technologies by using the same
frequency band.
Each layer in BCCH and TCH is comparatively independent, which
helps maintenance and expansion by layer. Increasing or deleting sites or
TRX in cells will not have an impact on existed BCCH frequency planning
and thus facilitating network maintenance.
MRP segmenting with 6 MHz frequency band
1 2 3 4 5 6 7 8 9 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30
1 2 3 4 5 6 7 8 9 11 12
13 14 15 16 17 18 19 20
21 22 23 24 25 26
27 28 29 30TCH3(4)
BCCH(12)
TCH1(8)
TCH2(6)
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Carrier No.
MRP is one of the hotspots in frequency planning technology development
in recent years. Some documents indicate that by using MRP simultaneously
integrated with frequency-hopping, DTX, power control and other immunity
technologies to interference can reduce average frequency multiplexing
coefficient to around 7.5, without influencing network quality.
Example:
TRX 2 3 4
20% 30% 50%
MRP 12/8 12/8/6 12/8/6/4
12+8/2=10 (12+8+6/3=8.7 (12+8+6+4)/4=7.5
TRX TRX quantity of the cell
Proportion of the cell
MRP MRP segments
Average frequency multiplexing coefficient
Frequency-hopping diversity gain
Large, medium, small
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21
In the above table, the number of cells with 2TRX is 20%, that with 3TRX
is 30%, and that with 4TRX is 50%. Given that these cells are distributed
equally, thus average frequency multiplexing coefficient must be less than
actual multiplexing coefficient. Take the cells with 3TRX as an example.
Since the number of cells with 3TRX or above is actually 80%, and they are
distributed equally, thus the actual multiplexing coefficient on the third layer
is 6/0.8=7.5.
Extended MRP is the development of MRP concept. After being segmented,
each layer can include frequencies of each layer thereafter: Layer TCH0
includes frequency points in each layer from TCH1 to TCHn, layer TCH1
includes frequency points in each layer from TCH2 to TCHn, and so forth.
First, assign frequency points in Layer TCHn, then frequency points in
Layer TCHn-1, and so forth. However, this will have an impact on the
structure of MRP planning.
Extended MRP segmenting with 6 MHz frequency band
1 2 3 4 5 6 7 8 9 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30
1 2 3 4 5 6 7 8 9 11 12
13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30
21 22 23 24 25 26 27 28 29 30
27 28 29 30TCH3(4)
BCCH(12)
TCH1(8)
TCH2(6)
Carrier No.
Example:
Take frequency bandwidth of 7.2 MHz as an example. Classify 36 pairs of
carrier frequencies into four groups according to 12/9/8/7 by using MRP, as
follows:
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TCH1 TCH2 TCH3
60 61 62 63 64 65
66 67 68 69 70 71
72 73 74 75 76 77
78 79 80
81 82 83 84 85
86 87 88
89 90 91 92
93 94 95
Channel Type
Channel No.
Logic Channel
TCH1 TCH1 Traffic Channel
TCH2 TCH2 Traffic Channel
TCH3 TCH3 Traffic Channel
Channel BCCH adopts 4 x 3 multiplexing (Figure 1.4-5A), traffic channel
TCH1 adopts 4 x 3 multiplexing (Figure 1.4-5B), traffic channels of TCH2
and TCH3 adopt 2 x 3 multiplexing (Figure 1.4-5A and Figure 1.4-5B),
classify them into four groups.
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23
60
64
68
62
66
7063
67
7161
65
69
72
75
78
73
76
7972
75
787477
80
12-carrier frequencies of BCCH adopt 4 x
3 multiplexing method
(A)
9-carrier frequencies of TCH1 adopt 3 x 3
multiplexing method
(B)
Figure 1.4-5
89
91
93
9092
94 9092
9489
91
93
8183
85
8284
8682
84
8183
85
86
8-carrier frequencies of TCH2 adopt 2 x 3
multiplexing method
(A)
7-carrier frequencies of TCH3 adopt 2 x 3
multiplexing method
(B)
Figure 1.4-6
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60728189
64 75 8391
85 93 6878
62738290
66 76 8492
70808594
63 7282 90
67 75 92 84
71 8678 94
65 77 8391
61748189
85936980
Figure 1.4-7 Configuration diagram of MRP carrier frequency with a frequency bandwidth of 7.2
MHz
Comparison about system capacities between packet multiplexing and MRP
technology
According to the preceding various analysis and introduction on
multiplexing technologies, now lets make a comparison on capacity
increasing among these four multiplexing methods (4 x 3 multiplexing
method).
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43 3/2/2 3/3/2 1440 1
33 3/3/3 1788 124
13 4/4/4 2640 183
MRP1296
**
3/3/3 1788 124
6MHZ
26 2/2/2/2/2/2 2160 15
43 4/4/4 2628 1
33 5/5/5 3384 129
13 6/6/6 4272 163
MRP1296
**
6/6/6 4272 163
9.6MHZ
26 3/3/3/4/4/4 4416 168
Note: GOS = 0.02, 0.025 Erl/User
** () herein indicates multiplexing method of each carrier frequency
Multiplexing Method
Based Station Configuration
Average Capacity per Site
Capacity Ratio
1.4.6 Concentric Cell Technology
(1) Basic principle
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So-called concentric cell is to divide a common cell into two areas: external
layer and internal layer, or called as overlay and underlay. The scale of
overlay covers traditional cellular, while that of underlay covers mainly
around base stations. The differences between overlay and underlay are not
only on coverage scale, but also on frequency multiplexing coefficient.
Overlay generally adopts traditional 4 x 3 multiplexing method, while
underlay adopts closer multiplexing method, such as 3 x 3, 2 x 3 or 1 x 3.
Therefore, all carrier channels are classified into two groups. One is for
overlay and the other is for underlay. The reason why this structure is called
concentric cell is that overlay and underlay share co-location, a set of
antenna system and the same BCCH channel. However, public control
channel must belong to external channel group, which means call
establishment must operate on external channel. Diagram of concentric cell
structure is shown as follows:
f5
f2
f3
f6
f9
f12
f10
f11
f1
f7
f4
f8
/
/
Figure 1.4-8 Diagram of concentric cell structure
/ Overlay
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/ Underlay
According to different methods to realize concentric cell, they are common
concentric cell and intelligent underlay overlay (IUO). The difference
between these two concentric cells is mainly about underlay transmission
power and handoff algorithm between underlay and overlay.
Generally overlay transmission power of common concentric cell is lower
than overlay power, and thereby reducing coverage scale, increasing
distance ratio and satisfying co-frequency interference requirement. Handoff
between underlay and overlay in common concentric cell is based on power
and distance.
Transmission power in underlay of IUO (frequency adopts closer
multiplexing method, therefore, this layer is usually called as super layer) is
the same as that in overlay (usually called as regular layer), as a result of
handoff algorithm. Handoff algorithm of IUO is switched based on C/I. The
simple description of its realization process is as follows:
A call is established at the regular layer. Then, the BSC continuously
monitors the downlink super group channel C/I ratio of the call. When a
certain super channel C/I ratio reaches the available threshold (good C/I
threshold defined in IUO), the channel for the call is switched to the super
channel. At the same time, the BSC continues to monitor the channel C/I
ratio. When the C/I ratio reaches a bad threshold (bad C/I threshold defined
in IUO), the channel is switched back to the regular channel. Therefore, to
use IUO, the system must have the following functions:
A. Estimation on downlink co-frequency C/I ratio
B. Handoff algorithm concerning IUO
Handoff in cell from regular layer to super layer (measured C/I greater
than good C/I threshold)
Handoff in cell from super layer to regular layer (measured C/I less than
bad C/I threshold)
(2) Capacity
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Since underlay adopts closer multiplexing method, each cell can be assigned
to more TRX, and thereby improving frequency utilization rate and
increasing network capacity. However, it should pay attention to the fact that
coverage semi diameter of underlay in concentric cell is less than common
cell and its traffic absorption is confined by call operation distribution and
coverage scale. The following table shows distribution on different call
operations. Under different coverage scales, make a comparison on capacity
between concentric cell and traditional 4 x 3 method. Indicate Si as underlay
coverage, Sout as outlay coverage area, the measure of capacity as Erlang:
Si / Sout
3TRX 2TRXout+2TRXin 4TRX 3TRXout+2TRXin
0.3 14.04 10.57 21.04 20.05
0.7 14.04 20.55 21.04 28.25
0.9 14.04 21.04 21.04 28.25
0.3 14.04 15.09 21.04 21.92
0.7 14.04 21.04 21.04 28.25
Coverage Ratio
Uniform distribution of traffic
Linear distribution of traffic
What needs to explain is that coverage ratio is concerned with frequency
multiplexing type. The closer frequency multiplexing type is, the more
intense co-frequency interference is and the less underlay coverage ratio is.
In addition to that it is concerned with configuration of handoff parameter
and surrounding environment. Therefore, coverage semi diameter is not
configured at will. It needs giving comprehensive consideration upon the
quality of network, which is hardly more than 50%.
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From the preceding analysis, the concentric cell technology improves the
capacity a little or even reduces the capacity when traffic is well distributed.
The denser the traffic around a cell, the more obvious the effect is. Above all,
capability increasing is limited. For a common concentric cell, the Tx power
of its internal layer is low, which is hard to absorb the traffic indoor.
Therefore, the frequency efficiency is low and the actual capacity is
increased by about 10% to 30%. For IUO, the Tx power of its internal layer
remains unchanged, which can absorb the traffic indoor and the handoff
based quality for capacity absorbing is flexible. Therefore, the actual
capacity is increased by 20% to 40% (relatively high).
(3) Features and application
A. Common concentric cell
The features of common concentric cell are as follows:
No need to change network structure.
Need to increase some special handoff algorithm, but generally the
realization is simple.
No specific requirements on mobile phone.
A limitation on capacity increase, generally it is 10-30%. It is concerned
with call operation distribution. Because of small power, underlay is hard to
absorb indoor traffic.
It is applicable in the situation that call traffic is highly concentrated around
base station and distributed outdoor.
Notice in application
Make a good network planning. On one hand, it should be applied in areas
of high call operation concentration, on the other hand, making a good
planning about coverage area of underlay. The area cannot impact quality
because of interference caused by close multiplexing, and it should absorb
enough call operation. If it is a bad planning, it will not only hardly increase
capacity, but also reduce network quality.
It would be better to integrated with technologies about reducing
interference, such as power control and DTX.
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B. IUO
IUO mainly has the following features:
As a kind of concentric cell, IUO can utilize existing site, have little
modification on network and no specific requirements on mobile phone.
System function needs to increase measure and estimation on C/I and
special handoff algorithm.
Capacity has an augment of 20% - 40%, and it has nothing to do with call
operation distribution and call traffic absorbed by super layer. In addition to
that it can ensure quality on the basis o f increasing capacity.
Super layer can adopt closer multiplexing method. When the frequency is
enough wide, it can keep a segment of frequency for micro cellular.
It is applicable for the areas where high density of call operation is and
concentrated around base stations.
Notice in IUO application:
Make a good planning. Cell should be planned based on call operation
distribution and notice to reduce interference.
When cell channel is being assigned, reasonable configuration between
super layer frequency and regular layer frequency should be noticed.
In order to reduce interference, power control and DTX technology should
be integrated in application.
It would be better to adopt handoff based on C/I in regular layer.
1.5 Cell Splitting
In the beginning period of GSM network establishment, since there are not
too many users, channels are surplus. With ever-increasing users, blockage
occurs in the channels that were assigned to each base station. At this time
new channels can be added and assigned to the original base stations. If
users are constantly increasing, while the usable channels are exhausted,
then only cell splitting, increase in base stations and co-channel
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multiplexing can meet demand of users. Usually, the semi diameter of
newly-splitted cell is only half of that of original cell.
Semi diameter of new cell=semi diameter of old cell/2 (1-5-1)
based on formula (1-4-1), the following formula works:
Coverage area of new cell=coverage area of old cell/4 (1-5-2)
Given that the highest traffic load of each new cell and old cell are the same,
then principally speaking, it is concluded as:
New traffic/unit area= 4 old traffic/unit area (1-5-3)
Therefore, the relation between cell splitting and increase on user capacity
can be represented as follows:
Tn = 4n T (1-5-4)
In the formula: Tn- network capacity after n times cell splitting
T0- network capacity prior to cell splitting
Formula (1-5-4) is applicable for cellular grid that is splitted to 4 less cells
at a ratio of 1:4. Simply speaking, after one time splitting, the number of
users can be increased to 4 times of the original one and the actual capacity
is less than that of its four times.
1.6 Several Common Immunity Technology to Interference
GSM system itself has many immunity technologies to interference, such as
frequency hopping, power control, discontinuous transmission based on
voice activity detection and so on. If effectively applicable, it will improve
C/I, thereby it can form a closer frequency multiplexing method, in addition
to increase on frequency multiplexing coefficient and frequency utilization
rate. Herein, we will introduce these technologies one by one and analyze
their gain through absolute mathematics model and artificial model.
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1.6.1 Discontinuous Transmission (DTX)
TRAU BTS
BTS MS
480 ms
Comfort noise frame
Voice frame
During the period of voice activation, discontinuous transmission encodes
voice at 13 kbit/s. During silence period, it encodes comfort noise at 500
bit/s.
During silence period, discontinuous transmission has little contribution to
interference. It can be regarded that its power is zero (none activation). If
DTX activity factor is p
, then its gain is
pI
C
pI
CdBIC log10log10log10)(/
1.6.2 Frequency Hopping (FH)
Frequency hopping is one kind of spread spectrum communication. It is
applied in cell mobile communication system to improve system
anti-multipath fading capability. In addition to that it can curb co-frequency
interference on communication quality. Therefore, it is highly applicable.
Especially, nowadays when spectrum resource is more and more insufficient,
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frequency hopping becomes one of the most effective methods to improve
spectrum utilization rate.
In GSM, data of each logic frame is sent in a way of decentralized and
interleaved in 8 TDMA frames, while all of these data has been encoded in
convolution. If a part of these 8 blocks of burst is interfered or damaged, it
can recover the data that has been sent through convolution encoder.
However, if too many blocks are damaged, it is hard to recover the original
data. By frequency hopping, it is unlikely to make burst in one channel in
heavy fading area too long (it easily occurs in a still or moving-at-low-speed
mobile station that works on a fixed carrier), or to be interfered by a certain
strong co-frequency signal. Thus it is possible to get good transmission
effect by using channel coding and encoding. It is the simple principle to
improve communication quality by adopting frequency hopping technology.
Frequency hopping sequence used by GSM system is a kind of Poisson false
random variable sequence. It can provide 64 frequency hopping sequence at
most. Length is same to hyperframe (lasting 3 hours 28 min 53 sec 760ms)
to ensure, as soon as possible, that each sequence is orthogonal to each other,
so that ensure the effect of frequency hopping. Frequency hopping sequence
in GSM is mainly described by two parameters: HSN (Hopping-frequency
Sequence Number) and MAIO (Mobile Assignment Index Offset). Usually,
different HSN is assigned to different cell and different value of MAIO is
assigned to different channel of cells.
It is noticed that every channel in a same cell adopts a same HSN and only
value of MAIO offset is different, which ensures that every channel in a
same cell will not occupy same frequency points at a same time. In different
cells, as a result of different HSN, it adopts different categories of frequency
hopping sequence. Then it makes frequency hopping sequence in every cell
is not relevant as far as possible, so that interfering resource signals are
assigned to many channels to ensure coding and encoding effect. When
HSN=0, MAI is dup loop from low to high, it is called as Cyclic Hopping.
Since frequency hopping gain in this method is very low, usually it is not
adopted in GSM
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GSM supports baseband frequency hopping and RF frequency hopping (or
called as Synthesized Frequency Hopping). Baseband frequency hopping
means that many transmitters work on their own fixed frequency points,
while on baseband signals from different channels switched to different
transmitters are sent according to frequency sequence. Baseband frequency
hopping can be easily realized, however, frequency hopping points are few
as a result of the limited TRX number. Frequency hopping artificial system
established by ZTE is mainly to support RF frequency hopping. Baseband
frequency hopping is only regarded as an exception of RF frequency
hopping (that is, the number of frequency points equals to the number of
TRXs). The advantage brought by frequency hopping is mainly about the
effect of Frequency Diversity and Interference Diversity. Frequency
diversity actually improves network coverage scale, and Interference
diversity increases network capacity
The number of available frequency hopping in baseband frequency hopping
is equal to the number of TRX. Therefore, it can only bring frequency
diversity gain, not interference diversity gain. However, now GSM
operators are more concerned about capacity problem. Coverage is not a
problem in most of cities. RF frequency hopping is a very effective method
to solve capacity problem.
RF frequency hopping is a trend of application in network planning.
Frequency diversity gain
Frequency diversity means its immunity ability to Rayleigh fading. Since
Rayleigh fading on different carriers is certain irrelevance (the more
frequency differential is, the less irrelevance is), then burst distributed on
different carriers will not influenced by the same Rayleigh fading. It means
a lot to the still and moving-at-low-speed mobile station. It is said that it can
provide a gain value of 6.5dB. However, MS moving at high speed and two
successive burst in a same channel are different on timing position, which
means they are irrelevant to Rayleigh change. They are seldom influenced
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by the fading at a time when frequency diversity provided by low-speed
frequency hopping is very little.
Under the condition that MS moves at high speed, frequency points
assigned in cells have little impact on frequency hopping performance.
While under the condition of no frequency hopping, there are about
frequency diversity gain of 1 dB to 2dB. When MS moves at low speed
(TU3), because of frequency diversity effect, the number of assignment
frequency points has significant influence on system performance.
Frequency points increased by a time will obtain about gain value of
0.2~1dB, load rate can be increased by 10% or so.
(1) Interference diversity gain
Interference diversity means that it curbs capability of interfering signals in
other co-frequency multiplexing cell, that is, to provide frequency hopping
and interfering differential on the transmission path in order to improve
interference under the harshest conditions. It makes all users evenly enjoy
good communication quality, which is very important for the mobile
communication system with lots of users, especially critical to increase
communication traffic through augmenting frequency multiplexing rate.
Usually interference diversity effect should be provided, and the number of
frequency hopping points should not be less than 3.
MA},...,,,{ 321 nffff ,
TRX m (mn)
Regarding the above figure, given that mobile station is in air with fk at time
t. At that time, the possibility of interfering cell fk being activated is
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GSM_P&O_ _01_200904 GSM Frequency Planning
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nmCCp mnmn //
11
m
n
I
C
pI
CdBIC log10log10log10)(/
Gain
(3) Frequency hopping planning and capacity analysis
If co-frequency point is 10 MHz, frequency hopping planning and capacity
analysis without adopting frequency hopping are as follows:
The multiplexing method of BCCH is 4X3, and the multiplexing method of
traffic channel is 3X3. 10 MHz has 50 frequency points. It leaves 37
frequency points after losing 1 protection frequency point and 12 BCCH
frequency points. Thus, each cell can be assigned 4 traffic frequency points
((37-1)/9), and only one frequency point is left. Then its most assignment
should be 5+5+5. Each cell can provide 37 channels
(1BCCH+2SDCCH+37TCH).
When RF frequency hopping technology is adopted, traffic channel can
adopt 1X3 multiplexing. When load is 50%, each cell can provide 6 service
logic frequency points. The reason why it is called logic frequency point is
that they all adopt the same 12 frequency hopping collection ((37-1)/3).
Only HSN is different from MAIO, then one frequency point is left, and the
most assignment becomes 7+7+7. It can provide 53 service traffic channels
(1BCCH+2SDCCH+53TCH) with increase on capacity by 43%. At this
time more than 90% of areas can have C/I with a value of 9dB. When DTX
and ZTE distinctive fast power control algorithm are adopted at the same
time, system capacity can be improved much better. If intelligence traffic
control technology is adopted, GSM can acquire soft capacity, and gain
more system capacity by sacrificing certain voice quality in hot traffic areas.
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1.6.3 Dynamic Power Control (DPC)
A3
A1
A2
A3
A1
A2
A3
A1
A2
A3
A1
A2A3
A1
A2
A3
A1
A2
A3
A1
A2
Seen from the above figure, in dynamic power control interfering only when
mobile station is at the border of a cell, BTS can work with the most
transmission power.
Obviously, the position of interfering mobile station is a possibility. The
circumstance is even more obvious in frequency hopping.
Indicate DPC factor as p:
pI
C
pI
CdBIC log10log10log10)(/
Gain
1.6.4 1 x 3 Multiplexing + Radio Frequency FH + DTX + DPC
Lets observe and study 13multiplexing interference in specific and
check the contribution made by immunity technologies to interference to
reduce interference and increase system capacity.
Differentials on interfering degradation between 1 x 3 and 4 x 3
multiplexing methods:
CIR 43- CIR 13 =18 - 9.43 8.57 dB
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Interference diversity gain made by 1 x 3 frequency hopping with 50% of
frequency carrier:
10log10(2/1) = 3dB
Indicate the length of frequency hopping is 12 frequency points, then
frequency diversity gain is about 2 dB.
Indicate DTX activation factor is 0.5, then its gain is:
-10log10(0.5) = 3dB
Indicate DPC factor is 0.9, then its gain is:
-10log10(0.9) =0.5dB
The total gain is:
3+2+3+0.5=8.5Db
From the above analysis we can see that utilizing immunity technologies to
interference basically can recover interference degradation made by
intensified multiplexing methods.
1.7 Conclusion on Principles of Frequency Assignment in Engineering
Adjacent frequencies cannot be identical in the same base station.
Directly adjacent base station should avoid co-frequency (even
though the direction of their antenna central lobes are different, side
lobe and back lobe can also bring much interference.):
End-on cells cannot be co-frequency and should avoid adjacent
frequency, especially for BCCH and SDCCH carrier frequency
(usually they are 1st and 2nd carrier frequencies of the cell). When
frequency hopping is adopted, the starting hopping points of adjacent
base stations can be the same, while the algorithm of frequency
hopping cannot be the same.
When in common frequency hopping (that is synthesizer frequency
hopping), frequency hopping algorithm (HSN) of each cell in the
same station are all identical. However, starting frequency hopping
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39
points (MAIO) cannot be adjacent frequency. Note: whatever adopts
CCB combiner cannot support frequency hopping.
Design of BSIC should also be noticed. BSIC = 8 x NCC + BCC,
BCC is available from 0-7, thus the near co-frequency and adjacent
frequency cell should not be the same as far as possible.
Co-frequency (especially BCCH carrier frequency) and the same
BSIC in short distance should be avoided as far as possible.
There are high mountains between base stations, which is not
regarded as adjacent station. If there are large scale of water between
base stations, which should be regarded as adjacent station.
Prior to frequency hopping, and no limitation on the using scale of
BCCH carrier, BCCH can be staggered as far as possible. A certain
segment of frequency band should be saved for 4X3 multiplexing
when in frequency hopping. If frequency is enough, BCCH can adopt
5 x 3 or even 6 x 3 multiplexing models to reduce interference among
BCCH.
In large or medium scale cities, different close frequency
multiplexing methods are adopted according to different functions
supported by equipments, such as MRP, 1 x 3, and 1 x 1 frequency
hopping. Meanwhile saving part of frequency points in advance for
micro cellular to establish layered network. Its frequency
multiplexing coefficient is small.
In large or medium scale cities, different frequency multiplexing
methods are adopted according to different functions supported by
equipments. Whether it is needed to establish layered network or not,
it is based actual conditions. Its frequency multiplexing coefficient is
a little bit more than that of large or medium scale cities.
In towns and villages, frequency resource are abundant, then regular
4 x 3 frequency multiplexing method can be adopted.
For stations build on high mountains for the geographic reason,
independent frequency points can be assigned.
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The above are about some principles in frequency planning, while doing
frequency planning needs another important principle, that is, it should be in
accordance with the local actual circumstance. Morphology and base station
of each system are different, so are transmission of radio signal, which
requires us to learn more about the local actual circumstance before we
make frequency planning. We should not be confined in the common
frequency multiplexing methods, for frequency assignment methods is in
accordance with the local actual circumstance. Frequency planning should
satisfy the local situation as far as possible. If possible, we may use some
specific planning tools integrated with e-map to do field-strength prediction.
First, observe that whether the coverage area of each cell is reasonable.
Then make coverage modification or frequency planning revise for those
areas which are not satisfied with the interference requirements (generally
we set the co-frequency interference as 12dB or so and leave 3Db margin
while in prediction). After base station is in operation, it is to judge whether
coverage frequency planning is proper or not by line test and some statistic
data. For the areas with heavy interference, it is to solve the problems by
modifying coverage, revising frequency planning and other methods.
We make frequency planning in a way of geographic slicing, while we
should keep a certain number of frequency points (frequency is enough) or
make frequency band division at slicing border. The choice of border should
avoid hot areas or networking complex areas as far as possible.
Usually, planning should start from the densest area in base station. For
example, first from urban bustling areas, then to suburban base station with
less carrier assignment (usually choose O1/ or S1/1/1 as border). Notice that
there are rivers or l4arger lakes in the urban areas. Interference made by
strong transmission of water surface should be avoided.
As a result of irregularity of actual base station distribution, it is hard to
ensure that the frequency in the same layer carrier can be planned fully
following 4 x 3 or 3 x 3 and other common models. It needs flexible
modification according to actual circumstance.
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Chapter 2 Neighboring Cell Planning
Knowledge point
Know about the principles and methods in neighboring cell planning.
Analyze cases of improper neighboring cell planning.
Neighboring cell planning determines consecutive coverage of a GSM
network and network performance indexes. The principles of neighboring
cell planning are as follows:
(1) Frequencies of the primary cell and neighboring cells must be
different.
(2) The number of neighboring cells must be less than or equal to 32, and
the OMCR can be configured for a maximum of 32 neighboring cells.
To acquire a better neighboring cell planning, the QoS and load of the
system must be taken into consideration. Actually, more neighboring cells,
more system load will bring because of handover. However, moderate
number of neighboring cells reduces call drops because of handover.
When you plan neighboring cells, take the following aspects into
consideration.
If there are a large number of neighboring cells, handover occurs frequently
and thus leads to signaling overload.
If there are fewer neighboring cells, call drop may occur because of
handover failure, or QoS is affected severely.
Generally, in planning neighboring cells, it is determined that cells are
distributed based on cellular structure. Therefore, pay attention to the
following points:
When you configure neighboring cells for microcells in urban areas,
two-layer adjacency is required, as shown below:
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A3
D2 B1
D1
D3
C1 B3
C2
B2
C3
A1
A2
A3
D2 B1
D1
D3
C1 B3
C2
B2
C3
A1
A2
A3
D2 B1
D1
D3
C1 B3
C2
B2
C3
A1
A2 A3
D2 B1
D1
D3
C1 B3
C2
B2
C3
A1
A2
Figure 2-1
When you configure neighboring cells in remote areas such as suburban
areas or the countries, one-layer adjacency is required. This is because in
these areas, network is sparsely distributed and each cell is with wide
coverage. In this case, a long distance is between the first layer and the
second layer. For details, see the following figure:
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43
A3
D2 B1
D1
D3
C1 B3
C2
B2
C3
A1
A2
A3
D2 B1
D1
D3
C1 B3
C2
B2
C3
A1
A2
A3
D2 B1
D1
D3
C1 B3
C2
B2
C3
A1
A2 A3
D2 B1
D1
D3
C1 B3
C2
B2
C3
A1
A2
Figure 2-2
In a dual frequency network, cooperation and setting principles between the
two-layer network should be considered in configuring neighboring cells.
Therefore, the network adjacency should be configured according to
different principles of network sharing.
Generally, it is considered that cells are arranged in order based on cellular
shape. But actually, cells are hardly arranged in order because site selection
is affected by various factors. In this case, configuration should be
performed based on data that is obtained by simulating networking planning.
In addition, if the transmit power of a BTS is very large, the covered edge
zone takes a great proportion. In this case, the adjacency cannot be obtained
based on the geographic position; instead, it must be obtained by on-site
measurement, or configure more adjacencies, as shown below:
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A
B
C
Figure 2-3 Neighboring cell configuration
From the figure, we can see that a cell phone walks along the curve line in
the covered edge zone. Theoretically, the cell phone selects service area A
first, then service area B, and last service area C. But actually, signals of
BTS B cannot size the control position in the curve line because of certain
complex factors in the radio propagation environment. In this case, if cells
of BTS C are not configured as the neighboring cells of sector 1 of BTS A,
the cell phone is always in sector 1 of BTS A, until call drop occurs or the
cell is selected again. To solve this problem, configure sectors 1 and 2 of
BTS A and sector 1 of BTS C as neighboring cells (sense frequency points).
However, you cannot configure all cells as neighboring cells. If all cells are
configured as neighboring cells, unexpected problems may occur, such as
cell reselection and handover.
The following lists improper neighboring cell planning.
One-way neighboring cell
Many neighboring cells
Few neighboring cells
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The following lists problems that may occur because of improper
neighboring cell planning.
Call drop
Handover failure
Frequent handover
Isolated cells
Abnormal inter-cell handover
Unbalanced traffic
Decreased handover precision
Cases
Case 1
1. Fault Symptom
A BTS in the urban area is configured as S333, and the single frequency
GSM900 network is used at local with 1*3 RF frequency hopping mode.
The cutin failure rate in a sector of this BTS is constantly high. That is, cutin
failure rate for this cell from source cell A is about 80%, but indexes such as
call drop rate and failure rate of the voice channel allocation are normal.
2. Fault Analysis
The fault is not caused by hardware fault and interference. This is because
though the cutin failure rate is high, TCH allocation does not fail, which
indicates that MS can occupy TCH channels allocated by BSC. In addition,
severe interference does not exist because no call drop occurs on MS and
voice communication is with good quality. After analysis, it can be
determined that source cell A is far away from this cell with high cutin
failure rate and thus handover requests should be fewer. Therefore, the fault
may be caused by island effect.
3. Fault Locating
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The fault is caused by island effect after checking hardware, transmission
stability, and interference and no fault occurs. Check neighboring cells of
source cell A to check whether cells with the same frequency and same color
codes as cell C exist. It is found that such a cell exists. Locate the fault
further. It is found that a very big square is set up between cell B and cell A,
which makes radio propagation conditions between cell A and cell B better.
MS senses signals, and these signals are ones of cell B, but BSC determines
send Handover Command to cell C. At the same time, the level of cell C
may be very low, which makes handover failure. This is handover failure
caused by island effect.
4. Troubleshooting
Modify the frequency point of cell C and add isolation cell B into the
neighboring cell table of cell A. Then, the fault is rectified.
5. Conclusion
In troubleshooting network faults, pay attention to environment change. For
example, whether radio signal propagation is affected, or whether radio
signals can be propagated better. If these factors are changed, engineering
parameters (such as antenna height, downtilt angle, and directional angle)
and cell parameters need to be adjusted (for example, add, delete, or modify
neighboring cells or frequency). Frequency resources of a GSM network are
limited. Therefore, with expansion of the network scale, island effect is
more likely to be generated, especially on handover. In addition, if
co-channel interference is severe, handover success ratio is severely
affected.
Case 2
1. Fault Symptom
A user at border areas of a province complains that he/she cannot disengage
the roaming signals from another province once receiving these signals, but
the roaming problem does not exist at home. These two provinces are not
neighboring cells.
2. Fault Analysis
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Perform the drive test at local. It is found the following network results:
Figure 2-4
The user usually is located in point P, which is in cell A. Cell A and cell B
are neighboring cells and are home networks for the user. Cell C and cell D
are neighboring cells and are networks for roaming area. In addition, cell A
is not the neighboring cell for cell C and cell D.
Cell D has the same BCCH frequency point as neighboring cell B that is
defined by cell A; therefore, the mobile phone at point P may re-select cell
D and then re-select cell C through cell D. The neighboring cell table of cell
C and cell D does not define the frequency point of cell A. Therefore, the
user resides in the networking of the roaming area. If the user powers off the
mobile phone and then powers it on in cell C, the mobile phone searches
cell C and frequency points of the neighboring cells that are defined by cell
C. This is because the mobile phone keeps the frequency point of the cell
where it is located when being powered off. This leads to the roaming
problem.
3. Troubleshooting
Comment [m14]:
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Define neighboring cells between two provinces. If neighboring cells cannot
be configured, modify the frequency point of cell B to rectify the fault.
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Chapter 3 BSIC Planning
Knowledge point
Know about the definition, value range, and planning principles of BSIC.
1. Definition
In a GSM system, each BTS is allocated with a local color code, which is
called base station identity code (BSIC). In a physical position, if the mobile
phone receives BCCH carrier frequencies of two cells concurrently, and
these two cells are with the same channel ID, the mobile phone
differentiates these two cells by BSIC. In network planning, to decrease
co-channel interference, BCCH carrier frequencies of neighboring cells are
assured with different frequencies. However, in the cellular
telecommunication system, it is possible that BCCH carrier frequencies are
multiplex. For these cells with the same BCCH carrier frequency, make sure
that they have different BSICs, as shown below:
Figure 3-1 BSIC selection
In the figure, BCCH carrier frequencies of cells A, B, C, D, E, and F are
with the same absolute channel ID, and other cells use different channel ID
as the BCCH carrier frequency. Generally, cells A, B, C, D, E, and F are
requested to have different BSICs. When the BSIC resources are insufficient,
consider whether neighboring cells of these cells adopt different BSICs.
Here, take cell E as an example, if BSIC IDs are insufficient, preferentially
consider that cells D and E, B and E, and F and E use different BSICs
respectively, but cells A and E, and C and E use the same BSIC respectively.
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The BSIC consists of the network color code (NCC) and base transceiver
color code (BCC), as shown in figure 3-2. The BSIC is transmitted over the
synchronization channel (SCH) of each cell. It mainly provides the
following functions:
Figure 3-2 BSIC composition
When the mobile phone receives codes from SCH, it is determined
that the mobile phone is synchronized with the cell. To correctly
translate the information of the downstream common signaling
channel, the mobile phone needs to know training sequence code
(TSC) used by the common signaling channel. According to GSM
specifications, TSC has eight fixed formats, which are presented from
ID 0 to ID 7. The TSC SN used by the common signaling channel of
each cell is determined by BCC of the cell. Therefore, one of
functions of BSIC is to notify the mobile phone of the TSC that is
used by the common signaling channel of the cell.
BSIC takes part in random access channel (RACH) translation, so it
can be used to prevent RACH that is transmitted to neighboring cells
by the mobile phone from being incorrectly translated as the access
channel of the cell by the BTS.
When the mobile phone is in use, it must measure the levels of BCCH
carrier frequencies of neighboring cells and then report the results to
the BTS according to specifications of the relevant neighboring cell
table on BCCH. At same time, for each frequency point in the
measurement report, the mobile phone must point out BSIC of the
measured carrier frequency. In specified conditions, that is, two or
BCC
BSIC
NCC
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more cells contained in neighboring cells of a cell adopt the same
BCCH carrier frequency; the BTS differentiates these cells based on
BSIC. This prevents incorrect handover or even handover failure.
When the mobile phone is in use, it must measure signals of
neighboring cells and report the results to the network. In each report,
contents of only six neighboring cells are included. Therefore, take
control for that the mobile phone reports situations of only cells
having handover relationships with the current cell. In BSIC, the
higher three bits (representing NCC), are used for control. In this case,
carrier frequencies can control the mobile phone to report situations
of only neighboring cells in the allowed range of NCC through the
broadcast parameter "allowed NCC".
2. Format
Format of BSIC: NCC-BCC.
Value range of NCC: 0-7.
Value range of BCC: 0-7.
3. Setting and Effect
In many cases, different GSMPLMNs adopt the same frequency, but their
network planning is independent. To ensure that neighboring BTSs that are
with the same frequency point have different BSICs, neighboring
GSMPLMNs adopt different NCCs.
However, situations in China are special. Actually, the GSM network of
China Telecom is an integer and independence GSM network. Though
China Telecom subordinates a majority of local mobile offices, these offices
belong to the same carrier frequency, China Telecom. However, China is a
vast land and it is hard to manage the GSM network in a united manner.
Therefore, the entire GSM network is divided and managed by local mobile
offices (or their equal agencies) of cities and provinces. These local mobile
offices plan their networks independently. To ensure that BTSs with the
same BCCH frequency that are used by border areas of cities and provinces
have different BSICs, NCCs of cities and provinces are coordinated by
China Telecom in a united manner.
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BCC is a component part of BSIC, which is used to identify different BTSs
that are with the same BCCH carrier frequency in the same GSMPLMN.
And the BCC value should meet the requirements described above as much
as possible. In addition, according to GSM specifications, the TSC of the
BCCH carrier frequency in a cell need to be the same as the BCC of the cell.
Usually, vendors assure this consistency.
4. Precautions
Make sure that neighboring cells that are with the same BCCH carrier
frequency have different BSICs. Especially, when two or more cells
contained in neighboring cells of a cell adopt the same BCCH carrier
frequency, these two cells must have different BSICs. Pay attention to
configurations of cells in the border areas of cities and provinces. Otherwise,
inter-cell handover failure may occur.