3.4 Normal Frequency Reuse Technology
3.4.1 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: f_{reuse}=N_{ARFCN }/_{ }N_{TRX }
Here N_{ARFCN }is the total number of the available channel numbers, and N_{TRX }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:
f_{reuse}= n x m
In actual planning, however, the allocated number of channel numbers will be greater than n x m, so the actual f_{reuse} is usually greater than n x m. Therefore, the smaller the f_{reuse}, 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.
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 intrafrequency spacing is great, so it can meet GSM system's requirement on the intrafrequency 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 antiinterference 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.
Normal 4 x 3 frequency reuse pattern.
3.4.2 10MHz Bandwidth 4 x 3 Frequency Reuse
Hereunder are several assumptions:
l The available bandwidth is 10MHz.
l The channel number is 45–94.
l 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 Table:
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 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 intrafrequency 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.
3.4.3 19MHz Bandwidth 4 x 3 Frequency Reuse
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 Table:
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.
3.4.4 6MHz Bandwidth 4 x 3 Frequency Reuse
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:
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.
3.4.5 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:
l 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.
l Utilize new frequency resources. For example, you can employ the 1800MHz band to establish a DSC 1800MHz network.
l 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.
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