Information Rates of Approximate Message Passing for Bandlimited Direct-Detection Channels

TL;DR

This paper introduces a GVAMP detector for bandlimited direct-detection channels, achieving near-capacity rates with lower complexity, and demonstrates its effectiveness through fiber-optic channel simulations.

Contribution

It develops a GVAMP-based detection method that approaches channel capacity with reduced computational complexity in fiber-optic communication systems.

Findings

Bipolar modulation operates within 0.26 bits of the coherent capacity.

GVAMP achieves 93 multiplications per information bit for 5 bpcu at 300 gigabaud.

Simulation results confirm power gains and near-capacity performance.

Abstract

The capacity of bandlimited direct-detection channels is challenging to compute or approach due to the receiver non-linearity. A generalized vector approximate message passing (GVAMP) detector is designed to achieve high rates at a reasonable level of complexity. The rates increase by using multi-level coding and successive interference cancellation. The methods are applied to fiber-optic channels with intersymbol interference caused by spectrally efficient pulse shapes, chromatic dispersion, and receiver sampling at twice the baud rate. Bipolar modulation operates within 0.26 bits per channel use (bpcu) of the real-alphabet coherent capacity for optically amplified links, reducing the best-known theoretical gap of 1 bpcu. Remarkably, bipolar modulation achieves 6 dB and 3 dB of power gain over unipolar modulation with and without optical amplification, respectively. Simulations with polar-coded modulation confirm the gains. The GVAMP complexity, measured in multiplications per information bit (mpib), is proportional to the number of iterations and to the logarithm of the block length, and is substantially lower than that of other equalizers. For example, a system with 64-ary bipolar modulation and a root-raised cosine pulse with a 1% roll-off factor was simulated over 4 km of optically amplified standard single-mode fiber in the C-band. The GVAMP receiver requires 93 mpib to achieve 5 bpcu at 300 gigabaud.