3.2.2.1.1 GSM 

networks 

The traffic capacity that a given amount of radio spectrum can support in a 

cellular network is a function of the number of times the same radio channels 

can be re used within a cluster of cells and the amount of traffic carried per 

channel.  This is determined by the carrier to interference ratio the radio 

receiver needs for a minimum quality of baseband signal.  The re use factor 

determines the number of cells in a re use cluster and the smaller the cluster 

size, the greater is the re use of frequencies in a given area.  Finally, the 

smaller the cell size, the greater is the density of traffic that can be supported. 

Thus, given a frequency re use factor, an amount of radio spectrum and 

definition of the cell area; a theoretical measure of spectrum utilisation can be 

calculated and used in benchmarking the spectral utilisation of an actual 

network.  However, in a practical network there are variations as radio 

propagation is highly variable and ideal geometry may not be achievable.  

But, the theoretical benchmark is a useful performance bound when 

considering relative performance of networks. 

The cellular network deployment is bound by the worst case Carrier to 

Interference ratio. The detailed derivation of C/I is given in Annexure C.  

The worst case C/I for K (cluster size) = 3, 7, 9, 12 is given below. 

K 

3 

7 

9 

12 

C/I (dB) 

14.5 20.4 

22.2 

24.2 

2 Interferers 

120 Degree   

Sectored cell 

C/I (dB) 

2.6 11.6  14.0  16.7 

6 Interferers   

Omni Antenna 

The minimum C/I for GSM is 9dB, to which a margin needs to be added to 

allow for irregular terrain and imperfect site locations. Section 4.4 Annexure 2 

of ITU R recommendations M.1037 on  Digital  Cellular  Land  Mobile  

Telecommunication  Systems   states 

 Co channel protection ratio down to C

/I   9 dB is acceptable by the system 

and yields a possible reuse corresponding to a 9 cell cluster (3 cell reuse 

patterns with three sectors per cell).  

The table above shows two cases: 

Sectored antenna (120  sectored cell) with two first tier co 

channel interferers 

Omni Antenna cell with 6 first tier co channel interferers 

The minimum value of k for an acceptable C/I can be considered to be greater 

than 9 dB: k=7 for 2 interferers; k=12 for 6 interferers. In a real situation the 

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