Log-likelihood ratio clipping in MIMO-BICM systems: Information geometric analysis and impact on system capacity

The clipping of log-likelihood ratios (LLRs) in soft demodulators for multiple-input multiple-output (MIMO) systems with bit-interleaved coded modulation (BICM) was recently observed to allow for enormous complexity savings. In this paper we first provide an information-geometric interpretation of LLR clipping as information projection onto a log-convex manifold. Then we study the system capacity of MIMO-BICM systems that use LLR clipping. Our results show that strong LLR clipping is possible without significant capacity loss. We finally propose an LLR transformation scheme which is necessary for approaching capacity in case of strong clipping. The usefulness of this LLR transformation is illustrated by numerical simulations for MIMO-BICM systems employing low-density parity check (LDPC) codes.