An Efficient Fixed-Complexity Sphere Decoder with Quantized Soft Outputs

In practical multiple-input multiple-output bit-interleaved-coded-modulation (MIMO-BICM) systems, demodulators have to deliver finite word-length (quantized) log-likelihood-ratios (LLRs). In this letter, we propose an efficient modification of the fixed-complexity sphere decoder for MIMO-BICM systems working with quantized LLRs. Our approach reduces the complexity of previously proposed schemes via pruning strategies that exploit the clipping and quantization of LLRs. Numerical results confirm that our scheme achieves a significant complexity reduction (by 37% for the case of 2 bits per LLR and by 31% for the case of 3 bits per LLR) with negligible degradation in bit error rate performance.