The optimal detector for coded multiple antenna systems is too complex to be implemented. We thus consider number of decouplers: the linear zero-forcing equalizer, the linear minimum mean-squared error equalizer (LMMSE), the successive soft/hard interference canceller, and the parallel soft/hard interference canceller. The constrained capacities of these decouplers are analyzed. It is shown that the capacity of the LMMSE is larger than those of the other decouplers although the general belief is that the interference cancellation schemes are better than linear filters. It is also shown that the LMMSE maximizes each sub-channel capacity. We discuss the likelihood value derivation from the decouplers, which can be used for decoding error correction codes. The performance of parallel transmission of turbo-coded symbols is given to support the constrained capacity analysis.
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