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The exact error probability of quadriphase shift keying (QPSK) signals with homodyne crosstalk is derived analytically. With respect to the signal, homodyne crosstalk may be from the same, the worst, or random polarization. Valid for many interferers, Gaussian model overestimates the SNR penalty for homodyne crosstalk from a single interferer. For 1-dB SNR penalty, QPSK signal may tolerate up to a crosstalk ratio of -16.3 dB from a single interferer from the worst polarization alignment.