Soft sphere decoder for an iterative receiver in time-varying MIMO channels

Iterative multi-user detection based on linear minimum mean square error (LMMSE) filtering in time-variant MIMO systems involves high computational complexity. Sphere decoding after an initial interference cancelation step allows for substantial complexity reduction if the sphere decoder exploits the reduced rank time-variant channel subspace. However, a performance loss in terms of bit error rate (BER) versus signal to noise ratio (SNR) is incurred mainly due to the hard decision supplied by the sphere decoder. Building on past work of the authors we demonstrate in this paper that a soft output max-log sphere decoder can exploit the reduced rank time-variant channel subspace as well. We show that the proposed sphere decoder solution provides a computationally more efficient approach, with maintained quality in terms of bit error rate compared to an iterative LMMSE multi-user detector.