Iterative decoding and despreading improves CDMA-systems using M-ary orthogonal modulation and FEC

Iterative decoding is applied to code division multiple access (CDMA) systems with two-stage serial concatenated channel coding. For a direct sequence (DS) CDMA system like in the uplink of the standard IS-95(A) using M-ary orthogonal modulation (inner code) and an outer convolutional code with interleaving we give the optimum symbol-by-symbol maximum a posteriori (MAP) decoding rules for the inner block code in a coherent and noncoherent receiver design. Further suggestions for suboptimum MAP decoders with lower complexity are made. The inner decoding rule is extended to use a priori information for the systematic bits, which is delivered from the outer decoding stage. These algorithms are essential for iterative decoding of the system. Simulation results show a decoding gain of about 0.6 dB for a bit error rate (BER) of 10^-3 by replacing the maximum likelihood (ML) decoder for the inner code by a MAP decoder. A total gain of about 1.2 dB is achieved by the use of iterative decoding after only three to five iterations