Probabilistically Shaped 4-PAM for Short-Reach IM/DD Links with a Peak Power Constraint

TL;DR

This paper introduces a probabilistic shaping method for 4-PAM in short-reach IM/DD optical links, optimizing input distribution under peak power constraints to improve transmission performance.

Contribution

It proposes a novel probabilistic shaping approach tailored for peak power-limited IM/DD links, validated through numerical and experimental analysis.

Findings

Enhanced transmission reach and rate with probabilistic shaping

Validated improvements in both AWGN and fiber channels

Demonstrated rate adaptability and power efficiency

Abstract

Probabilistic shaping for intensity modulation and direct detection (IM/DD) links is discussed and a peak power constraint determined by the limited modulation extinction ratio (ER) of optical modulators is introduced. The input distribution of 4-ary unipolar pulse amplitude modulation (PAM) symbols is optimized for short-reach transmission links without optical amplification nor in-line dispersion compensation. The resulting distribution is symmetric around its mean allowing to use probabilistic amplitude shaping (PAS) to generate symbols that are protected by forward error correction (FEC) and that have the optimal input distribution. The numerical analysis is confirmed experimentally for both an additive white Gaussian noise (AWGN) channel and a fiber channel, showing gains in transmission reach and transmission rate, as well as rate adaptability.