Low Complexity Decoding of BICM STBC

It was shown earlier that the combination of bit interleaved coded modulation (BICM) with space time block codes (STBC) for OFDM systems leads to the maximum diversity order in space and frequency. In this paper we present low complexity decoding of BICM-STBC systems. First, the maximum likelihood (ML) bit metric calculations are simplified from a distance on the complex plane to a distance on the real line. Furthermore, we present log-likelihood-ratio (LLR) approximations to maximum likelihood (ML) decoding metrics to achieve easier to implement soft decision bit metrics with the same high performance. The LLR bit metrics are specifically calculated for M-ary QAM signal constellations used in IEEE 802.1la/g networks. Simulation results also show that while achieving a substantial improvement in decoder complexity, the proposed bit metrics achieve the same high performance.