DSSS/MCM system performance in a Doppler spread channel

There has been a great deal of interest in the use of direct sequence spread spectrum (DS-SS) in combination with multicarrier modulation (MCM). Although the proposed system implementations in the literature vary, all of these systems share the functionality of DS-SS, MCM, and some form of a branch combiner at the receiver. We investigate the performance of such a system transmitting through a Doppler spread channel. In Doppler spread fading, the proposed system performance exhibits an irreducible bit error probability (BEP) even for the single user case. Dramatic performance improvements can be realized with the use of an ideal maximal ratio combiner (MRC). A system utilizing an equal gain combiner (EGC) is analyzed. Finally, we compare the analytic results with the results from several computer simulations.