Truncated power control in code division multiple access communications

We analyze the performance of truncated power control in a code division multiple access communication system. This power control scheme compensates for fading above a certain cutoff fade depth: below the cutoff level an outage is declared. We assume a channel with fast Rayleigh fading and slow log-normal shadowing, where truncated power control is applied to the shadowing and a RAKE receiver combines the Rayleigh fading multipath components separated by more than a chip time. We investigate the power gain as well as the capacity gain of this power control scheme relative to conventional power control as a function of outage probability. We find that truncated power control significantly increases capacity relative to conventional power control, since less power is wasted compensating for deep fading conditions. It is also found that truncated power control is most effective for channels with large power fluctuations or large background noise. We then apply truncated power control to multimedia systems, where different traffic types have different performance requirements (e.g. outage probability, bit error rate, information rate, etc.) For these systems truncated power control can significantly reduce the required transmit power for each traffic type. We conclude by examining the effects of estimation error. For a log-normally distributed estimation error we find that capacity is reduced by a factor of a e^{-(M+1)σ(e)(2)b(2)/2}, where σ_e is the standard deviation of the error, M is the number of paths in the RAKE receiver, and b = ln 10/10.