  In this paper, we consider the problem of iterative detection and decoding
(IDD) for multi-antenna systems using low-density parity-check (LDPC) codes.
The proposed IDD system consists of a soft-input soft-output parallel
interference (PIC) cancellation scheme with linear minimum mean-square error
(MMSE) receive filters and two novel belief propagation (BP) decoding
algorithms. The proposed BP algorithms exploit the knowledge of short cycles in
the graph structure and the reweighting factors derived from the hypergraph's
expansion. Simulation results show that when used to perform IDD for
multi-antenna systems both proposed BP decoding algorithms can consistently
outperform existing BP techniques with a small number of decoding iterations.
