Signature sequence adaptation for DS-CDMA with multipath

Joint transmitter-receiver adaptation is studied for the reverse link of a direct sequence-code division multiple access system with short signature sequences. The signature for a particular user is computed at the receiver and transmitted back to the transmitter via a feedback channel. A reduced-rank transmitter adaptation scheme is presented in which the signature is constrained to lie in a lower dimensional subspace. This allows a tradeoff between system performance and the number of estimated parameters. Analytical and simulation results show that adaptation of relatively few transmitter coefficients can lead to significant performance improvements. Adaptive algorithms are derived for estimating the transmitter coefficients in the presence of multipath. We consider both collective optimization, in which the users adapt together to improve a global system performance criterion, and individual optimization, in which the signature for a particular user is adapted to optimize individual performance. Numerical results are presented, which show that both individual and collective joint transmitter-receiver adaptation can effectively preequalize the channel and avoid interference at high loads.