Advanced Coding for Optical Communications

This chapter represents an overview of advanced coding techniques for optical communication. Topics include: codes on graphs, coded modulation, rate-adaptive coded modulation, and turbo equalization. The main objectives of this chapter are: (i) to describe different classes of codes on graphs of interest for optical communications, (ii) to describe how to combine multilevel modulation and channel coding, (iii) to describe how to perform equalization and soft-decoding jointly, and (iv) to demonstrate efficiency of joint de-modulation, decoding, and equalization in dealing with various channel impairments simultaneously. Codes on graphs of interest for next-generation FEC for high-speed optical transport include turbo codes, turbo-product codes and low-density parity-check (LDPC) codes. We describe both binary and nonbinary LDPC codes, their design and decoding algorithms. We also discuss an FPGA implementation of decoders for binary LDPC codes. We demonstrate that an LDPC-coded turbo equalizer is an excellent candidate to simultaneously mitigate uncompensated chromatic dispersion, PMD, fiber nonlinearities and I/. Q-imbalance. For completeness of presentation, we also provide the information capacity study of optical channels with memory.