Equalizer complexity of mode-division multiplexed coherent receivers

One of the possible solutions is to use space division multiplexing in order to overcome the capacity crunch. Few mode fibers (FMF) have attracted a lot of attention in the recent years, however still a lot of research is required to enable transmission over FMF. One of the problems with the realization of FMF transmission is that these fibers have a modal dispersion which results in high number of equalization taps which might make the FMFs impractical. The complexities of common equalizer schemes are analyzed in this paper in terms of complex multiplications per bit. It is found that training symbol based equalizers have significantly lower complexity compared to blind approaches for long-haul transmission. Among the training symbol based schemes, OFDM requires the lowest complexity for crosstalk compensation in a mode-division multiplexed receiver. The main challenge for training symbol based approaches is the increased data rate due to extra overhead required for cyclic prefix and training symbols. In order to achieve 2000 km transmission, the modal dispersion must be below 6 ps/km when the OFDM specific overhead is limited to 10%. This shows that for few mode transmission systems the reduction of modal delay is essential to enable long-haul performance.