Evaluation and compensation of interchannel interference effects in a 16-QAM Nyquist-WDM system with LMS equalization

Nyquist-WDM systems have been proposed as a promising solution in multicarrier transmissions to be a technology which could face the elastic networks challenges. In the spectrum, Nyquist-WDM requires an ideal rectangle, characteristic that is impossible in practice. With a channel spacing near or equal to baud rate, the roll-off factor of the pulse shaping filter has influence in the channel spreading, generating spectral overlapping between adjacent channels. This phenomenon is called Interchannel Interference (ICI). This research evaluates the impact of the ICI effects in a 5 × 32 Gbaud Nyquist-WDM system, with 16-QAM modulation. The impact of the Roll-off and channel spacing value is analyzed for distances up to 5 km. Error Vector Magnitude in constellation and bit error rate are estimated according to channel spacing penalty, roll-off factor, and transmission distance variation. ICI effects minimization using LMS algorithm in the digital coherent receiver is proposed. LMS adapts the coefficients of the adaptive filter making equalization by a training sequence, superchannels signals are received in independent coherent receivers in a joint equalization among channels. The performance of the systems is improved with LMS equalization, reducing the log(BER) from -1 to -11 for 5 km in a channel spacing equal to the symbol rate, and from ~2 to ~20 for a channel spacing penalty of 1.5 GHz, with roll-off factor Nyquist filter of 0.15, which is feasible in practice in the pulse shaping filter at the transmitter.