Chapter 6 - Coding for Optical OFDM Systems

Abstract

This chapter describes different forward error correction (FEC) schemes suitable for use in optical orthogonal frequency-division multiplexing (OFDM) systems. The schemes were classified into two categories—standard block codes and iteratively decodable codes. Given the fact that convolutional codes are of low rate, one prefers the use of block codes. One may use the puncturing to increase the code at the expense of BER performance degradation. Nevertheless, the convolutional codes are weak unless they are used in concatenation with RS codes. Two classes of standard block codes are cyclic codes and BCH codes. The cyclic codes are suitable for use in error detection, whereas the BCH codes can be used for error correction. RS codes, which are an important subclass of BCH codes, are standardized for use in optical communications. The coded OFDM was considered in the context of long-haul optical communication systems and 100 Gb/s Ethernet. The low-density parity-check (LDPC) codes are used as channel code because of their channel capacity-achieving capabilities. The OFDM is an excellent chromatic dispersion and polarization mode dispersion (PMD) compensation candidate. Based on pilot-aided channel estimation, it is capable of compensating for differential group delay (DGD) of even 1600 ps for thermal noise limited channel, when the channel state information is known on the receiver side. The similar channel estimation can be used to compensate for the common phase error due to self-phase modulation. However, the ICI due to four-wave mixing (FWM) among subcarriers cannot be eliminated by pilot-aided channel estimation, and it remains an important performance degradation factor to be addressed in the future.