Low-Complexity Factor Graph Receivers for Spectrally Efficient MIMO-IDMA

Interleave-division multiple access (IDMA) has recently been introduced as an attractive alternative to CDMA. IDMA employs user-specific interleavers combined with low-rate channel coding for user separation. In this paper, we consider a MIMO-IDMA system with increased spectral efficiency due to the use of higher-order symbol constellations. Based on a factor graph framework and the sum-product algorithm, we develop an iterative turbo multiuser receiver. Gaussian approximations for certain messages propagated through the factor graph lead to a complexity that scales only linearly with the number of users. To further reduce complexity, we introduce a selective message update scheme. Numerical simulations demonstrate the performance of the proposed receiver algorithms.