List Decoding of Arıkan’s PAC Codes
Hanwen Yao, Arman Fazeli, Alexander Vardy
Abstract
Polar coding gives rise to the first explicit family of codes that provably achieve capacity with efficient encoding and decoding for a wide range of channels. However, its performance at short block lengths under standard successive cancellation decoding is far from optimal. A well-known way to improve the performance of polar codes at short block lengths is CRC precoding followed by successive-cancellation list decoding. This approach, along with various refinements thereof, has remained the state of the art in polar coding since it was first introduced in 2011. Last year, Arıkan presented a new polar coding scheme, which he called polarization-adjusted convolutional (PAC) codes. Such PAC codes provide another dramatic improvement in performance as compared to CRC-aided list decoding. These codes are based primarily upon the following main ideas: replacing CRC precoding with convolutional precoding (under appropriate rate profiling) and replacing list decoding by sequential decoding. Arıkan's simulation results show that PAC codes, resulting from the combination of these ideas, are quite close to finite-length lower bounds on the performance of any code under ML decoding.One of our main goals in this paper is to answer the following question: is sequential decoding essential for the superior performance of PAC codes? We show that similar performance can be achieved using list decoding when the list size L is moderately large (say, L ≥ 128). List decoding has distinct advantages over sequential decoding in certain scenarios such as low-SNR regimes or situations where the worst-case complexity/latency is the primary constraint. Another objective is to provide some insights into the remarkable performance of PAC codes. We first observe that both sequential decoding and list decoding of PAC codes closely match ML decoding thereof. We then estimate the number of low weight codewords in PAC codes, and use these estimates to approximate the union bound on their performance under ML decoding. These results indicate that PAC codes are superior to polar codes and Reed-Muller codes, and suggest that the goal of rate-profiling may be to optimize the weight distribution at low weights.