Improved Metric Sorting for Successive Cancellation List Decoding of Polar Codes

Abstract

This letter focuses on the metric sorter, which is a module that affects the overall latency in the LLR-based successive cancellation list (SCL) decoder of polar codes. We propose two improved two-step sorters, which are achieved by revising the sorters used in the pruned bitonic extractor (PBE), and selecting one of two candidate architectures based on a threshold on $L$ . Moreover, we remove the first stage of the PBE in the second step sorting, which can reduce the latency in the second step and further increase the throughput of the sorting units. We first describe the architectures of the proposed sorters, and then derive the required number of the compare-and-swap units (CASUs) and the stages. The evaluation results indicate that the two proposed improved sorters outperform the state-of-the-art sorters in terms of the CASUs and the stages, which significantly lowers the overall latency and resource consumption. Compared with the PBE, the proposed sorters can reduce 57.8% of stages and 46.3% of CASUs for $L=128$ , and 33.3% of stages and 21.1% of CASUs for $L=8$ .