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.