Concatenated Coded Modulation for Optical Communications Systems

The use of trellis-coded modulation (TCM) in combination with an outer block code is considered for next-generation 100-Gb/s optical transmission systems. Two block codes are employed as an outer code: a 16 times interleaved byte-oriented (255,239) Reed Solomon (RS) code and a code consisting of two interleaved extended three-error correcting Bose Chaudhuri Hocquenghem (BCH) (1020,988) codes. Simulations show that soft-decision decoding of a selected TCM inner code in combination with hard-decision decoding of the outer RS code achieves a net coding gain (NCG) of 8.42 dB at a bit-error rate of 10-13. When the concatenated code based on the two interleaved BCH codes is used as the outer code, the NCG is 9.7 dB. The impact of quantization on the performance of the concatenated TCM scheme with the two interleaved BCH outer codes is evaluated, and it is shown that 4-bit quantization is sufficient to approach the "infinite precision" performance to within 0.15 dB.