Performance comparison for NRZ, RZ, and CSRZ modulation formats in RS-DBS Nyquist WDM system

Non-return-to-zero (NRZ), carrier-suppressed return-to-zero (CSRZ), and 33% return-to-zero (RZ) are the three most commonly used modulation formats in the current fiber-optical communication system. In this paper, we investigate and comprehensively compare their performance using the recently proposed receiver side duobinary shaping Nyquist wavelength division multiplexing (RS-DBS Nyquist WDM) technique, which is believed to be one of the promising candidates in the next high-capacity, high-data-rate, long-distance optical transmission systems with its cost-effective capability. It is found that, in a scenario of 3 × 112 Gbps polarization division multiplexing quadrature phase-shift keying (PDM-QPSK) RS-DBS Nyquist WDM transmission, for back-to-back (B2B) transmission, NRZ can obtain almost the same performance as CSRZ and 33% RZ at the optimum bandwidth of 0.82 × symbol rate for optical filters at the transmitter side. However, under narrower bandwidth optical filtering, CSRZ and 33% RZ signals have advantages over NRZ signal for requiring 0.5 and 1 dB less optical signal-to-noise ratio at BER = 1 × 10^-3 with a 0.6 × symbol rate bandwidth optical filter. Moreover, using the optimum optical filter, after a 1600 km single-mode fiber (SMF) transmission, the CSRZ and 33% RZ signals can achieve 0.55 and 0.7 dB Q factor improvements over NRZ signals, by taking fiber nonlinearity into account. The RZ format is more ideal for the RS-DBS Nyquist WDM system with long-haul SMF transmissions.