Increased CDMA system capacity through adaptive cochannel interference regeneration and cancellation

Abstract

Asynchronous code division multiple access (CDMA) systems using digital matched filtering (DMF) reception techniques suffer from poor multiple access spectral efficiency. This is due entirely to the lower bound on the mean square crosscorrelation levels which exist between the orthogonal codes allocated to each system subscriber. This interference can be estimated through channel measurement, and it is then possible to regenerate and subsequently cancel crosscorrelation components from individual interfering channels, thereafter increasing the effective signal-to-interference ratio on the desired channel for a given subscriber activity level. This paper suggests a technique to exploit this ability in order to gain an increase in the number of multiple access channels available within a particular bandwidth. Theoretical analysis of the proposed cancellation scheme shows an upper bound on the spectral efficiency approaching 130% or 1.3 normalised channels per hertz for successive cascaded cancellation stages. Simulation results for CDMA systems show that, for the traditional DMF receiver without dynamic cancellation, the CDMA efficiency is approximately 10% for a bit error rate of 10−4. Analysis is presented for single and two stage cancellation receivers showing 40% and 50% capacity, respectively, for the same error rates derived from a receiver whose complexity is linear in the number of supported active subscribers.