In order to increase the transmittable data rate and to enlarge the transmission distance, the spacing between WDM channels has to be decreased while the optical transparent length must be increased both giving rise to interchannel crosstalk induced by fiber nonlinearities like cross-phase modulation (XPM). Thus, the development of modulation techniques being robust towards these effects is necessary. Recently, phase shift keying (PSK) techniques have attracted remarkable interest. For PSK-techniques, the optical power as a function of time is approximately constant (for nonreturn- to-zero (NRZ) signaling) or periodic (for return-to-zero (RZ) signaling). This is an advantageous property for the reduction of nonlinear phase modulation (PM) induced by the effect of XPM. On the other hand, since for PSKtechniques the information is carried by the phase of the optical carrier, the sensitivity to the nonlinear PM is high. In our contribution, we present an analytical model for the XPM-induced PM. With the help of this model and the visualization of XPM in the complex plane, we prove that the differential self-homodyne implementation of PSK is robust towards the nonlinear PM while PSK-techniques using a local oscillator in the receiver are extremely sensitive.