Abstract
Affine frequency division multiplexing (AFDM) is a promising chirp-assisted
multicarrier waveform for future high-mobility communications. This paper is
devoted to enhanced receiver design for multiple input and multiple output AFDM
(MIMO-AFDM) systems. Firstly, we introduce a unified variational inference (VI)
approach to approximate the target posterior distribution, under which the
belief propagation (BP) and expectation propagation (EP)-based algorithms are
derived. As both VI-based detection and low-density parity-check (LDPC)
decoding can be expressed by bipartite graphs in MIMO-AFDM systems, we
construct a joint sparse graph (JSG) by merging the graphs of these two for
low-complexity receiver design. Then, based on this graph model, we present the
detailed message propagation of the proposed JSG. Additionally, we propose an
enhanced JSG (E-JSG) receiver based on the linear constellation encoding model.
The proposed E-JSG eliminates the need for interleavers, de-interleavers, and
log-likelihood ratio transformations, thus leading to concurrent detection and
decoding over the integrated sparse graph. To further reduce detection
complexity, we introduce a sparse channel method by approaximating multiple
graph edges with insignificant channel coefficients into a single edge on the
VI graph. Simulation results show the superiority of the proposed receivers in
terms of computational complexity, detection and decoding latency, and error
rate performance compared to the conventional ones.