Improving soft FEC performance for higher-order modulations via optimized bit channel mappings.

Christian Häger,Alex Alvarado,Fredrik Brännström,Alexandre Graell I Amat,Erik Agrell

doi:10.1364/oe.22.014544

Abstract

Soft forward error correction with higher-order modulations is often implemented in practice via the pragmatic bit-interleaved coded modulation paradigm, where a single binary code is mapped to a nonbinary modulation. In this paper, we study the optimization of the mapping of the coded bits to the modulation bits for a polarization-multiplexed fiber-optical system without optical inline dispersion compensation. Our focus is on protograph-based low-density parity-check (LDPC) codes which allow for an efficient hardware implementation, suitable for high-speed optical communications. The optimization is applied to the AR4JA protograph family, and further extended to protograph-based spatially coupled LDPC codes assuming a windowed decoder. Full field simulations via the split-step Fourier method are used to verify the analysis. The results show performance gains of up to 0.25 dB, which translate into a possible extension of the transmission reach by roughly up to 8%, without significantly increasing the system complexity.

Highlights

There is currently a large interest in developing practical coded modulation (CM) schemes that can achieve high spectral efficiency close to the ultimate capacity limits of optical fibers [1]
We address the bit mapper optimization for a bit-interleaved coded modulation (BICM) system based on low-density paritycheck (LDPC) codes in the context of long-haul fiber-optical communications
In order to show the flexibility of the technique, we consider four different scenarios, combining both modulation formats with one code based on the AR4JA protographs and one SCLDPC code, where the lifting factor is M = 3000 in all cases

Summary

Introduction

There is currently a large interest in developing practical coded modulation (CM) schemes that can achieve high spectral efficiency close to the ultimate capacity limits of optical fibers [1]. For a BICM system, a helpful abstraction is to think about transmitting data using a single forward error correction (FEC) encoder over a set of parallel binary-input channels, or bit channels, with different qualities. This is due to the fact that bits are not protected throughout the signal constellation. We use the term “bit mapper” to denote the device that performs the bit channel mapping. We remark that other terms, e.g., “bit interleaver” or “mapping device”, are frequently used in the literature

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Results

Conclusion