Interleavers and BCH Codes for Coherent DQPSK Systems With Laser Phase Noise

The relatively high phase noise of coherent optical systems poses unique challenges for forward error correction (FEC). In this letter, we propose a novel semianalytical method for selecting combinations of interleaver lengths and binary Bose-Chaudhuri-Hocquenghem (BCH) codes that meet a target post-FEC bit error rate (BER). Our method requires only short pre-FEC simulations, based on which we design interleavers and codes analytically. It is applicable to pre-FEC BER ~10^-3, and any post-FEC BER. In addition, we show that there is a tradeoff between code overhead and interleaver delay. Finally, for a target of 10^-5, numerical simulations show that interleaver-code combinations selected using our method have post-FEC BER around 2× target. The target BER is achieved with 0.1 dB extra signal-to-noise ratio.

I. Introduction

Forward error correction (FEC) is crucial for coherent optical systems with multi-level modulation. Traditionally, coding theory focuses on additive white Gaussian noise (AWGN) channels with independent identically distributed (i.i.d.) errors [1]–[3]. However, some communication systems have non-i.i.d. errors. This affects the choice of FEC codes. For example, wireless systems use codes that correct burst errors from fading. In the case of coherent optical systems, transmitter and local oscillator (LO) lasers have relatively high phase noise (PN). Algorithms for estimating and compensating PN result in non-zero probability of cycle slips [4], [5]. We consider codes specifically for such systems. Recently, several approaches have been proposed. In [6]–[8], the authors consider low-density parity-check (LDPC) codes. In [9], we consider binary Bose-Chaudhuri-Hocquenghem (BCH) and Reed-Solomon (RS) codes. In [10], we improve the method for dimensioning binary BCH codes in [9] by using a bivariate distribution. However, the codes selected using the method in [10] have high overhead, which reduces system throughput.