Low complexity piecewise linear LLR calculation for MIMO-BICM systems

Log-likelihood ratios (LLRs) are efficient metrics used in the decoding of modern channel codes such as low-density parity-check and turbo codes. LLR calculation, however, can be a complex task since LLRs are usually complicated nonlinear functions of the channel output. In multi-input multioutput (MIMO) channels, the complexity of the calculation increases exponentially with the number of antennas. In this paper, an LLR approximation method is proposed for MIMO-BICM systems. In particular, piecewise linear approximation functions are introduced and an accuracy measure is provided to optimize the parameters. Simulations verify that the performance of the optimized approximate LLRs is better than the existing approximation method and quite close to that of true LLRs.