Analysis of ML-decoding of linear block codes using orthogonal signalling over Rayleigh-fading multiple-access channel

We discuss soft decision decoding of a DS-SSMA scheme with orthogonal signalling. The asymptotic efficiency of the discussed system is 0.209 for hard decision decoding (HD) and 0.531 information bits per channel chip for error/erasure decoding with the ratio threshold test (RTT) respectively. We derive a surprisingly easy and optimal decoding rule and give an upper bound for the decoding error rate of soft decision ML-decoding. Applying the optimal decoding rule, a suboptimal decoding algorithm is suggested that combines the ideas of error/erasure decoding and Chase (1972) algorithm. Finally we propose a modified encoding such that the efficiency of the system is doubled.