How to Construct Polar Codes

TL;DR

This paper introduces efficient methods for constructing polar codes by approximating complex bit-channels with manageable ones, enabling practical code design close to channel capacity.

Contribution

It presents two novel approximation algorithms for polar code construction that are both computationally efficient and theoretically sound.

Findings

Approximation methods are highly accurate in practice.

Construction algorithms run in linear time and space for large code lengths.

Polar codes can be constructed close to capacity with manageable complexity.

Abstract

A method for efficiently constructing polar codes is presented and analyzed. Although polar codes are explicitly defined, straightforward construction is intractable since the resulting polar bit-channels have an output alphabet that grows exponentially with he code length. Thus the core problem that needs to be solved is that of faithfully approximating a bit-channel with an intractably large alphabet by another channel having a manageable alphabet size. We devise two approximation methods which "sandwich" the original bit-channel between a degraded and an upgraded version thereof. Both approximations can be efficiently computed, and turn out to be extremely close in practice. We also provide theoretical analysis of our construction algorithms, proving that for any fixed $\epsilon > 0$ and all sufficiently large code lengths $n$, polar codes whose rate is within $\epsilon$ of channel capacity can be constructed in time and space that are both linear in $n$.