Long-Haul Optically Uncompensated IMDD Transmission With MLSE Using the M-Method

Recent experimental evidence with off-line processing has shown that intensity-modulation direct-dectection receivers using maximum-likelihood sequence estimation (MLSE) can successfully operate over a very long distance without any optical dispersion compensation. One hurdle towards the actual exploitation of the technology is the required complexity of the processor, which grows exponentially with distance. In this letter, we investigate the use of a complexity-reduction algorithm originally proposed for nonoptical communications, called the M-method. We show by simulation that over 400 and 700 km of G.652 fiber, at 10.7 Gb/s, the M-method allows us to operate with virtually no excess penalty with respect to conventional MLSE, while keeping track of only 32 and 128 trellis states, respectively, out of optimum full-fledged trellises requiring 512 and 8192 states, respectively. We investigate the robustness of the algorithm versus limited analog-digital resolution and confirm its effectiveness by testing it over experimental data at 10 Gb/s over 1040 km.