Interference Margin for CDMA
In determining RF coverage in CDMA systems, the effect of interference generated from the serving cell as well as the neighboring cells must be considered, this is in contrast to the RF coverage analysis for AMPS cells where interference mainly affects the frequency assignment but not the coverage.
The interference margin is dependent upon the amount of loading assumed in the system. Different cell deployment strategies can be modeled by varying the interference margin. CDMA cell deployments could be based on loading individual frequencies one by one, until they achieve the target load (for instance, a 6 dB noise rise). An alternative deployment could utilize more CDMA radio carriers, initially operating at a reduced load, to further extend the range of the cells (for instance, 3 dB noise rise) while trading off capacity (exploiting any immediate spectrum available). This 3 dB system rise improvement would result in approximately 30% fewer CDMA cell sites at system turn-on.
The following equation can be used as a first pass approximation to the amount of interference margin one should add to the link budget to account for loading the CDMA system with users.
NoiseRise = 10 log [1/(1-X)]
Where X is the maximum allowed number of users, specified as a fraction of pole capacity. For example, a capacity cell site has X equal to seventy-five percent (75%). Noise rise varies as a function of propagation, environment, load, user distribution, etc.