Characterization of interchannel interference effects in multicarrier 32-Gbaud QPSK/16QAM Nyquist systems

Multi-carrier optical transmission with ideal spectrum occupancy per-carrier equal to data baud rate (Nyquist criteria) with overall spectral efficiency higher than 2 bit/s/Hz is expected to replace currently deployed single-carrier 100 Gb/s DP-QPSK systems. However, ideal Nyquist shaping per carrier is not easily achievable due to the bandwidth resolution in optical filtering and the high computational complexity in digital filtering. This paper investigates and characterizes in terms of bit-error-rate (BER) performance degradation, the impact of spectral overlapping between adjacent optical carriers known as inter-channel interference (ICI), under variations of the following system parameters: roll-off factor of the digital Nyquist filter, channel spacing, linewidth laser and transmission distance. ICI is numerically studied in a 3×32-Gbaud 384-Gb/s scenario with QPSK and 16-QAM modulation formats. Results show a high impact in the non-ideal spectral channel generation due to the roll-off factor. Channel spacing of an ideal 32 GHz spectrum separation has a high impact in BER, 4 decades of BER difference compared with a 34 GHz separation. Besides, a dependency between ICI and transmitted sequences per channel has been identified. This bit sequences influence, can lead a new bit-probability-awareness codification technique to mitigate ICI effects in future optical scenarios.