Soft-Output PAC Decoder for Uplink Sparse Code Multiple Access System.

In this paper, we investigate short packet transmissions in a Polar Coded Sparse Code Multiple Access (PC-SCMA) system. Since recently introduced by Arikan PolarizationAdjusted Convolutional (PAC) codes have shown remarkable performance at short block lengths with an ability of reaching theoretical limits, in this work we propose a PC-SCMA system based on the soft output PAC decoder. The proposed decoder is a combination of a sequential stack decoder and an enhanced Belief Propagation (BP) algorithm. The numerical analysis shows its ability to outperform the state-of-the-art PC-SCMA system with a Successive Cancellation List (SCL) decoder at short block lengths in terms of the bit error rate with smaller computational complexity. Furthermore, we enhance the SCL-based scheme by row-merging of the polarization kernel and evaluate its performance.