  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.
