Low-Latency Software Polar Encoders and Decoders for Short Blocklengths

Abstract: This paper presents our low-latency Polar code encoders and decoders developed for the 2025 International Symposium on Topics in Coding (ISTC 2025) contest, which challenges participants to implement the fastest possible channel code encoders and decoders in terms of average and maximum latency on a CPU target. Our solution is based on Polar codes with an Adaptive Successive Cancellation List (ASCL) decoder. We introduce a novel ASCL unrolled decoder generator. We conduct an extensive exploration of the design space, including code construction, CRC selection, and list size, to identify optimal trade-offs between signal-to-noise ratio and decoding time across various operating points. The considered operating points are frame error rates of 10^{-3} and 10^{-5}, information bit lengths of 64, 128, 256, and 512, and code rates of 1/4, 1/2, and 4/5. We also propose an optimized bit-packed encoder. All implementations of the encoders and decoders, along with the code construction and the unrolled decoders generator, are released as open source in the AFF3CT toolbox.