Low-Complexity Capacity Achieving Two-Stage Demodulation/Decoding for Random Matrix Channels

Iterative processing for linear matrix channels, aka turbo equalization, turbo demodulation, or turbo CDMA, has traditionally been studied as the concatenation of conventional error control codes with the linear (matrix) channel. However, in several situations, such as CDMA, multiple-input multiple- output channels, OFDM, and intersymbol-interference channels, the channel itself either contains inherent signal redundancy or such redundancy can readily be introduced at the transmitter, for example, the direct-spread signature sequences of CDMA form inherent repetition codes. For such systems, iterative demodulation of the linear channel exploiting this redundancy using simple iterative cancellation demodulators, followed by conventional feed-forward error control decoding provides a low-complexity, but extremely efficient decoding alternative. It is shown that this two-stage demodulator/decoder, which outperforms more complex turbo CDMA methods for equal power modes (users), can support an arbitrary number of modes if an unequal power distribution is adopted, and that the capacity of the Gaussian multiple access channel can be approached to at least within less than one bit everywhere.