Modified Fincke-Pohst Algorithm for Low-Complexity Iterative Decoding over 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 employing spacetime codes, 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 decoding) algorithm [l] to estimate the maximum a posteriori (MAP) probability of the received symbol sequence. The new algorithm (FP-MAP) solves a nonlinear integer least-squares problem and, over a wide range of rates and SNRs, has polynomial-time (often cubic) expected complexity. The FP-MAP algorithm provides soft detection information for the soft channel decoder. The soft decoder’s output is then fed back to the FP-MAP, and iterated on. The performance of the FP-MAP algorithm on a multiple antenna system employing turbo code is demonstrated.