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This paper addresses transmission aspects on very high aggregate wavelength-division multiplexing based passive optical networks (WDM-PON). The ultradense WDM-PON system evaluated in this paper transports high-order modulation formats such as M-ary phase-shift keying (PSK) and M-ary quadrature amplitude modulation (QAM) in which coherent detection is performed at the optical network unit (ONU), after transmission over 25 km, 60 km, and 100 km of standard single-mode fiber (SSMF). The first part of this work covers the impact on the system's performance of the most relevant fiber nonlinearities such as self-phase modulation (SPM), cross-phase modulation (XPM) and four-wave mixing (FWM), and their interplay between transmission distance and modulation format. Using a Volterra series method allows estimating the error vector magnitude (EVM) of the received constellation related to different fiber nonlinearities. In a 32 × 625 Mbaud system spaced by 3 GHz (0.025 nm), the FWM to XPM ratio (F/X) varied from 25 up to 30 dB for phase-modulated signals (1.25 Gb/s-QPSK and 1.875 Gb/s-8PSK). On the other hand, this ratio ranged from 2.6 to 7.4 dB for amplitude-modulated signals (2.5 Gb/s-16QAM, 3.75 Gb/s-64QAM and 5 Gb/s-256QAM). After applying a frequency-allocation scheme at both transmitter and local oscillator lasers, some of the FWM crosstalk is mitigated by 2.4 to 3.5 dB. This EVM reduction confirms that increasing the transmission distance and the order of the constellation, the system's performance becomes limited by both interchannel FWM and XPM.