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It is shown that the phase distribution of the field and the detection scheme strongly affect the strength of the nonlinear impairments in phase-modulated transmission systems. It is found that precompensation is always a useful tool for minimizing the nonlinear impairments in differential phase-shift keying, where the mechanism for minimization is the reduction of the in-phase component of the nonlinear displacement. For differential quadrature phase-shift keying (DQPSK), the nonlinear impairments are instead minimized by a dispersion profile that maximizes the correlation of phase fluctuations of two consecutive pulses. When the number of interacting pulses is large, the performance of a DQPSK system is only weakly dependent on the dispersion profile.