Analysis of MC-DS-CDMA systems in doubly spread channels

Kondo and Milstein introduced and analyzed the performance of a system that combined multicarrier modulation (MCM) with direct-sequence spread spectrum (DSSS) in late 1993. Their system and similar systems are called MC-DS-CDMA systems. The dramatic performance improvement of this system (as compared to a single carrier system operating in a flat fading channel) is due, in part, to the use of coherent modulation/demodulation and perfect knowledge of the complex channel gains for each of the independent channels associated with every subcarrier. In this paper we investigate the performance degradation associated with a more easily realized system. Our system employs differentially coherent modulation/demodulation with an equal gain combiner (EGC) at the receiver. We investigate the performance of this system operating in a doubly spread (Doppler and delay spread) channel. In a doubly spread channel, any system which does not account for the rapid variations in channel phase will exhibit an irreducible bit error probability. This phenomenon exists even for the single user case. A system utilizing an EGC is analyzed for the single user case and in the presence of multiple access interference (MAI). The MC-DS-CDMA single user system, with E_b/N₀=18.5 dB, can achieve a BEP of 10^-6 in a doubly spread channel (f_dT=0.08 and a channel delay spread, τ₀=1 νs), while a single carrier DPSK system operating at the same E_b/N₀ and under the same channel conditions would achieve a BEP of 0.037. The multi-user system with 50 active users would achieve a BEP of 0.008 as compared to a BEP of 0.07 for a single carrier system