FEC codes for optical transmission systems at 40 Gbit/s

This paper describes the performances of different forward error correction (FEC) schemes for optical transmission systems at 40 Gbit/s. In order to limit the optical transmission impairments, only low redundancy schemes (10.6% maximum) have been considered. The schemes are based on two cascaded Reed-Solomon (RS) codes separated by an interleaver. Their performances have been evaluated for a wide variety of cases, under non-iterative and iterative hard decoding, for different interleaver depths and redundancy distribution between inner and outer codes. Two different practical design rules have been found in the two cases. The FEC parameters were chosen as a trade-off between hardware complexity and performances: in any case significant net coding gains (greater than or equal to 7 dB at 1e-11 BER) have been obtained. In more details, section 1 is an introduction to the purpose of this work. The second section describes the FEC schemes, and related constraints and parameters. The third section deals with the analytical evaluation of the performance of non-iterative decoding schemes. Section 4 presents the simulation results obtained for iterative decoding schemes and a design rule for the best choice of the interleaver depth in relation to the RS parameters. Finally, the last section resumes the conclusions of the present paper.