A Very Fast Joint Detection for Polar-Coded SCMA

Abstract

We propose a very fast-convergence joint iterative detection and decoding (JIDD) scheme for channel-coded sparse code multiple access (SCMA). In the conventional JIDD, all users’ channel decoding iterations are performed in parallel after all users’ variable nodes in the SCMA factor graph are updated. The proposed JIDD scheme, however, slices all channel decoding iterations into per-user channel decoding, and inserts them deeply into the factor graph. By doing this, message enhancement by channel decoding immediately propagates to the connected users’ messages in the factor graph, even within one message passing algorithm iteration, while maintaining the same total computational complexity per JIDD iteration. Numerical results confirm that in various link scenarios, the proposed scheme requires only two or three iterations for BER convergence, while the conventional JIDD scheme requires more than six iterations. In downlink scenarios, the proposed scheme achieves an even faster convergence rate. Moreover, in a uplink scenario with perfect power control, the converged BER of the proposed scheme is quite a bit lower than the conventional scheme, and the proposed scheme requires only two iterations to get the same BER level as the conventional scheme. Consequently, thanks to fast convergence, the proposed scheme dramatically reduces the overall computational complexity for achieving BER convergence. In addition, the fast convergence rate compensates for the multi-user detection latency issue, which is inherent in sequential algorithms, and the issue is further overcome by employing the group-wise sequential version of the proposed scheme.

Keywords

• Decoding
• iterative detection
• joint detection
• polar code
• SCMA