Low-Latency Successive-Cancellation List Decoders for Polar Codes with Multi-bit Decision

Polar codes, as the first provable capacity-achieving error-correcting codes, have received much attention in recent years. However, the decoding performance of polar codes with traditional successive-cancellation (SC) algorithm cannot match that of the low-density parity-check (LDPC) or turbo codes. Because SC list (SCL) decoding algorithm can significantly improve the error-correcting performance of polar codes, design of SCL decoders is important for polar codes to be deployed in practical applications. However, because the prior latency reduction approaches for SC decoders are not applicable for SCL decoders, these list decoders suffer from the long latency bottleneck. In this paper, we propose a multi-bit-decision approach that can significantly reduce latency of SCL decoders. First, we present a reformulated SCL algorithm that can perform intermediate decoding of 2 bits together. The proposed approach, referred as 2-bit reformulated SCL (2b-rSCL) algorithm, can reduce the latency of SCL decoder from (3n-2) to (2n-2) clock cycles without any performance loss. Then, we extend the idea of 2-bit-decision to general case, and propose a general decoding scheme that can perform intermediate decoding of any 2K bits simultaneously. This general approach, referred as 2K-bit reformulated SCL (2Kb-rSCL) algorithm, can reduce the overall decoding latency to as short as n/2K-2-2 cycles. Furthermore, based on the proposed algorithms, VLSI architectures for 2b-rSCL and 4b-rSCL decoders are synthesized. Compared with a prior SCL decoder, the proposed (1024, 512) 2b-rSCL and 4b-rSCL decoders can achieve 21% and 60% reduction in latency, 1.66 times and 2.77 times increase in coded throughput with list size 2, and 2.11 times and 3.23 times increase in coded throughput with list size 4, respectively.