We have analyzed the impact of digital and optical signal processing algorithms, that is, Volterra equalization (VE), digital backpropagation (BP), and optical phase conjugation with nonlinearity module (OPC-NM), in next generation 10？Gbit/s (also referred to as XG) DP-QPSK long haul WDM (fixed-grid) passive optical network (PON) without midspan repeaters over 120？km standard single mode fiber (SMF) link for downstream signals. Due to the compensation of optical Kerr effects, the sensitivity penalty is improved by 2？dB by implementing BP algorithm, 1.5？dB by VE algorithm, and 2.69？dB by OPC-NM. Moreover, with the implementation of NL equalization technique, we are able to get the transmission distance of 126.6？km SMF for the 1？:？1024 split ratio at 5？GHz channel spacing in the nonlinear region. 1. Introduction Due to the increasing demand of bandwidth and capacity requirements from enterprises and households, the data rates of broadband access network will be required over 10？Gbit/s for each customer. Several passive optical network (PON) architectures have been proposed, that is, G-PON, E-PON, TDM-PON, and so forth, in order to remove the capacity bottleneck. Recently, the 10？Gbit/s long haul wavelength-division multiplexed- (WDM-) PON system has been demonstrated with coherent detection; this configuration represents a significant improvement with a receiver sensitivity of ？45？dBm (25 photons/bit) [1]. With the implementation of advanced modulation formats, that is, QPSK, QAM, and so forth, and multiplexing techniques, that is, dual-polarization and so forth, the system performance is limited due to fiber linear and nonlinear effects [2]. These effects are very much dominant at higher signal launch powers and in WDM systems with narrow channel spacing [3]. Rosenkranz and von Hoyningen-Huene presented the results of nonlinearity compensation in access networks; however, the results are limited to the transmitters where optical field is derived from the modulation current with the directly modulated laser (DML) rate equations; thus modulation nonlinearity and chirp are included [4] and are detected by direct detection method. In this paper, we have numerically analysed the transmission characteristics and the nonlinear equalization techniques by employing BP, VE, and OPC-NM in 10？Gbit/s DP-QPSK long haul WDM-PON transmission with coherent receivers, for downstream signals. Furthermore; the impact of nonlinear equalization algorithm on the transmission distance and split ratio factor is investigated. 2. Nonlinear Equalization Methods In this section, we
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