An efficient MMSE-based demodulator for MIMO bit-interleaved coded modulation

In bit-interleaved coded modulation (BICM) systems employing maximum-likelihood decoding, a demodulator (demapper) calculates a log-likelihood ratio (LLR) for each coded bit, which is then used as a bit metric for Viterbi decoding. In the MIMO case, the computational complexity of LLR calculation tends to be excessively high, even if the log-sum approximation is used. Thus, there is a strong demand for efficient suboptimum MIMO-BICM demodulation algorithms with near-optimum performance. We propose an efficient MIMO-BICM demodulator that is derived by means of a Gaussian approximation for the post-detection interference. Our derivation results in an MMSE equalizer followed by per-layer LLR calculation (i.e., LLRs are calculated separately for each layer). The novel demodulator can be interpreted as an MMSE analogue of a recently proposed ZF-equalization based demodulator, as well as an extension of ZF-equalization based demodulation to correlated post-detection interference. Because it performs per-layer LLR calculation, it has the same (low) computational complexity as the ZF-equalization based demodulator. Simulation results demonstrate that the performance of our demodulator is close to that of LLR calculation using all layers jointly, and significantly better than that of the ZF-equalization based demodulator.