Frequency Reuse

Telecom Tutorials
Monday, June 03, 2013www.tempustelcosys.com
Frequency Reuse

GSM900
Upward 890 915 MHz
Downward 935 960 MHz
Tx-Rx difference is 45MHz，carrier space is 200KHz。
GSM1800
Upward 1710 1785 MHz
Downward 1805 1880 MHz
Tx-Rx difference is 95MHz，carrier space is 200KHz。

GSM900
GSM900： FU(n) = 890 + 0.2 n MHz
FD(n) = Fu(n) + 45 MHz，1 n 124
E-GSM900：FU(n) = 890 + 0.2 (n-1024)，974 n 1023
FD(n) = Fu(n) + 45 MHz
n is called Absolutely Radio Frequency Channel Number.
GSM1800
Fu(n) = 1710.2 + 0.2 (n-512) MHz
FD(n) = Fl(n) + 95 MHz， 512 n 885
374 channels

Frequency reuse is referred to as the same frequency is reused after
a certain distance in cellular wireless system. Generally, a limited
frequency band is divided into many groups, each containing a few
carriers, which are in turn assigned to adjacent cells

Usually, frequency reuse patterns include:
Ordinary frequency reuse: “4 3”, “3 3” and more close “2 3”
and “1 3”.
Dual frequency reuse: BCCH and TCH adopt different frequency
reuse patterns respectively.
Concentric: the Underlay and Overlay adopt different frequency
reuse patterns respectively.
MRP: different layers adopt different reuse patterns.

“4 3” frequency reuse pattern is that each site is divided into 3
sector. 12 frequencies form a group which are distributed to 4
different sites. Each site owns 3 frequencies.
A3
D2B1
C3
B2D1
D3
A2C1
B3
C2A1
B3
C2A1
A3
A1B1
D1
D3D2
C3
B2A1
C3D2
C3
C1
D2B1C2A1
A2C1
D3

A3
C2B1
B3
A2C1
C3
B2A1
A3
C2B1
B3
A2C1
B3
A1C1
A1
A3A2
C3
B2A1
A3A3
C3
C1
B2A1B2A1
A2C1
B3
“3 3” frequency reuse pattern is that each site is divided into 3 sector. 9 frequencies
form a group which are distributed to 3 different sites. Each site owns 3 frequencies.

A1 B1 C1 A2 B2 C2 A3 B3 C3
1 1 2 3 4 5 6 7 8 9
2 10 11 12 13 14 15 16 17 18
3 19 20 21 22 23 24 25 26 27

 The following equation is used to estimate frequency reuse
distance:
D —— frequency reuse distance
R —— cell radius
N —— frequency reuse factor.
N=9 for “3 3”; N=12 for “4 3”
 For “3/9” frequency reuse, D=5.2R
 For “4/12” frequency reuse, D=6R

• Co-channel Interference C/I：
 C/I refers to the interference of another cell
using the same frequency to the current cell. The
ratio of carrier to interference is called C/I.
 GSM specification regulates that C/I >9dB. In
implementing, it requires C/I>12dB.
• Adjacent channel interference C/A
 C/A refers to interference of adjacent cell to
the current cell. The ratio is called C/A. The GSM
specification regulates that C/A>-9dB.

Where, Pown_cell is the signal strength of current cell; Pi_BCCH is
BCCH signal strength of interfering cell i measured by MS.

 In GSM, presume a trunk can support 30 TCHs, GOS=2%，then the
total traffic will be 22Erl according to Erlang-B table. The total
subscribers that can be supported will be 733 if the traffic of each
subscriber is 30mErl.
TRX TCH （ GOS=2% ）
Rtrunk （ % ）
1 7 2． 94 37
2 14 8． 20 57
3 22 14 ． 90 66
4 30 21 ． 93 72
5 37 28 ． 25 75
6 45 35 ． 61 78
7 53 43 ． 06 80

 Analyze total traffic and traffic distribution feature
within the service area
 Calculate system capacity, the traffic that each cell can
provide and subscriber numbers
 Calculate the total numbers of TRX s required
 Determine the total numbers of cells required
 Determine the TRX configuration mode of each cell.
 Adjust the configuration (optimization) in combination
with
cell planning and frequency planning

• Analogy of telephone set
 Prediction mode: Y5 = r X5
 where, Y5：predicted subscriber number on 5th year
 X5：total numbers of telephone set on 5th year
 r ：percentage of mobile phone to telephone set
• Social survey and city analogy
 Based on social survey, compare with the cities where mobile
network has been established and where the city scale, economy
and customer requirement are similar. Refer to that cities’ subscribers
to predict the potential numbers of mobile subscribers of the city.

Frequency Reuse

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