Constellation Shaping for Bit-Interleaved LDPC Coded APSK

TL;DR

This paper introduces a novel energy-efficient constellation shaping method for LDPC-coded APSK systems, optimizing bit probabilities and decoding strategies to improve performance over AWGN channels.

Contribution

It proposes a new constellation shaping technique combined with degree-distribution optimization and iterative decoding for LDPC-coded APSK, achieving over 1 dB gain.

Findings

Achieves over 1 dB gain in AWGN at 3 bits/symbol.

Optimizes LDPC degree distributions for shaped APSK.

Demonstrates effectiveness of iterative decoding with shaping.

Abstract

An energy-efficient approach is presented for shaping a bit-interleaved low-density parity-check (LDPC) coded amplitude phase-shift keying (APSK) system. A subset of the interleaved bits output by a binary LDPC encoder are passed through a nonlinear shaping encoder whose output is more likely to be a zero than a one. The "shaping" bits are used to select from among a plurality of subconstellations, while the unshaped bits are used to select the symbol within the subconstellation. Because the shaping bits are biased, symbols from lower-energy subconstellations are selected more frequently than those from higher-energy subconstellations. An iterative decoder shares information among the LDPC decoder, APSK demapper, and shaping decoder. Information rates are computed for a discrete set of APSK ring radii and shaping bit probabilities, and the optimal combination of these parameters is identified for the additive white Gaussian noise (AWGN) channel. With the assistance of extrinsic-information transfer (EXIT) charts, the degree distributions of the LDPC code are optimized for use with the shaped APSK constellation. Simulation results show that the combination of shaping, degree-distribution optimization, and iterative decoding can achieve a gain in excess of 1 dB in AWGN at a rate of 3 bits/symbol compared with a system that does not use shaping, uses an unoptimized code from the DVB-S2 standard, and does not iterate between decoder and demodulator.