Performance of adaptive MIMO filter and adaptive interference canceller for SDM transmission at sub-symbol rate sampling

We investigated the performance of an adaptive multiple-input multiple-output (MIMO) filter with non-integer fractional sampling at sub-symbol rate sampling for long-haul space-division multiplexed (SDM) transmission over coupled-core multi-core fibers. We evaluated a frequency-domain adaptive 8×8 filter operating at a fractional sampling through a numerical simulation of long-haul SDM transmission of 32-Gbaud polarization-division multiplexed quadrature phase shift keying (PDM-QPSK) signals over four-coupled-core fiber (4-CCF) under a sampling timing offset. The fractional sampling frequency-domain adaptive MIMO filter maintained a nearly constant pre-forward error correction (FEC) Q factor across a sampling timing offset range of one symbol period after 100×100-km transmission, even when the sampling rate was reduced to 7/8× of the symbol rate, demonstrating excellent tolerance to timing offset. To mitigate the performance penalty due to intersymbol interference (ISI) at sub-symbol rate sampling, we also examined an adaptive interference canceller that generates interference replicas on the basis of tentative decision results of signals and an adaptive MIMO filter. With 15/16× sampling, the penalty of the pre-FEC Q from the case with 2× oversampling after 100×100 km transmission was reduced by 1 dB when the adaptive interference canceller was used after the frequency-domain adaptive MIMO filter. We also confirmed the compensation of the sampling timing offset by the fractional sampling frequency-domain adaptive MIMO filter at sub-symbol rate sampling and the mitigation of ISI by the adaptive interference canceller in a transmission experiment with 32-Gbaud PDM-QPSK signals over 4-CCFs in a recirculating loop configuration after 6240 km.