Iterative Receivers for Downlink MIMO-SCMA: Message Passing and Distributed Cooperative Detection

TL;DR

This paper introduces low-complexity iterative receivers for downlink MIMO-SCMA systems using factor graphs, belief propagation, and expectation propagation, along with distributed cooperative detection schemes to enhance performance over fading channels.

Contribution

It proposes a novel convexified BP-EP receiver with guaranteed convergence and two distributed cooperative detection schemes exploiting diversity gain.

Findings

Conv-BP-EP outperforms existing methods in simulations.

Distributed schemes improve bit error rate performance.

Bregman ADMM outperforms belief consensus in noisy links.

Abstract

The rapid development of the mobile communications requires ever higher spectral efficiency. The non-orthogonal multiple access (NOMA) has emerged as a promising technology to further increase the access efficiency of wireless networks. Amongst several NOMA schemes, the sparse code multiple access (SCMA) has been shown to be able to achieve better performance. In this paper, we consider a downlink MIMO-SCMA system over frequency selective fading channels. For optimal detection, the complexity increases exponentially with the product of the number of users, the number of antennas and the channel length. To tackle this challenge, we propose near optimal low-complexity iterative receivers based on factor graph. By introducing auxiliary variables, a stretched factor graph is constructed and a hybrid belief propagation (BP) and expectation propagation (EP) receiver, named as `Stretch-BP-EP', is proposed. Considering the convergence problem of BP algorithm on loopy factor graph, we convexify the Bethe free energy and propose a convergence-guaranteed BP-EP receiver, named as `Conv-BP-EP'. We further consider cooperative network and propose two distributed cooperative detection schemes to exploit the diversity gain, namely, belief consensus-based algorithm and Bregman alternative direction method of multipliers (ADMM)-based method. Simulation results verify the superior performance of the proposed Conv-BP-EP receiver compared with other methods. The two proposed distributed cooperative detection schemes can improve the bit error rate performance by exploiting the diversity gain. Moreover, Bregman ADMM method outperforms the belief consensus-based algorithm in noisy inter-user links.