Wideband underwater acoustic CDMA: adaptive multichannel receiver design

Two receivers that utilize multichannel (array) processing of asynchronous wideband direct-sequence CDMA signals are proposed: the symbol decision feedback (SDF) receiver and the chip hypothesis feedback (CHF) receiver. In the SDF configuration, receiver parameters are adapted at the symbol rate. This limits its applicability to the channels whose time variation is slow compared to the symbol rate, which may not be the case in a mobile underwater acoustic system. In the CHF configuration, the receiver parameters are adapted at the chip rate, and decision-feedback equalization is accomplished at chip resolution. At the price of increased computational complexity (linearly proportional to the modulation level) this receiver provides improved performance for systems where time variation cannot be neglected with respect to the symbol rate. Performance of the two receivers was demonstrated in a four-user scenario, using experimental data obtained over a 2 km shallow water channel. At the chip rate of 19.2 kilochips per second with QPSK modulation, excellent results were achieved at an aggregate data rate of up to 10 kbps.