Performance Limits of Nyquist-WDM and CO-OFDM in High-Speed PM-QPSK Systems

TLDR

The study compares the performance of two high‑spectral‑efficiency PM‑QPSK generation techniques. The authors evaluate coherent optical OFDM and Nyquist WDM, the latter employing optical pulses with an almost rectangular spectrum equal to the baud rate. Both methods show identical sensitivity and spectral efficiency under ideal conditions, yet CO‑OFDM requires a larger receiver bandwidth and faster ADCs, and N‑WDM outperforms CO‑OFDM on long‑haul nonlinear links.

Abstract

We compare by simulation the performance of two techniques for generating polarization-multiplexed quadrature phase-shift keying with a high spectral efficiency (SE). The first is based on coherent optical orthogonal frequency-division multiplexing (CO-OFDM). The second, which we call Nyquist wavelength-division multiplexing (N-WDM), is based on the use of optical pulses having an "almost" rectangular spectrum, with bandwidth ideally equal to the Baud-rate. We show that the two techniques have the same sensitivity and SE under idealized assumptions. However, we found that CO-OFDM requires a much larger receiver bandwidth and proportionally faster speed of the analog-to-digital converters. We also tested CO-OFDM and N-WDM over long-haul nonlinear links and found N-WDM to outperform CO-OFDM in this case, too.