Towards closing the capacity gap on multiple antenna channels

In recent years, soft iterative decoding techniques have been shown to greatly improve the bit error rate performance of various communication systems. For multiple antenna systems, however, it is not clear what is the best way to obtain the soft-information required of the iterative scheme with low complexity. In this paper, we propose a modification of the Fincke-Pohst (sphere decoder) algorithm to estimate the MAP probability of the received symbol sequence. The new algorithm solves a nonlinear integer leasts-quares problem and, over a wide range of rates and SNRs, has polynomial-time (often cubic) complexity. The performance of the algorithm, combined with convolutional, turbo, and LDPC codes is demonstrated on several multiple antenna channels.