Steady-state performance of the adaptive MLD receiver in dual-polarized 16-QAM systems with crosstalk

The adaptive maximum-likelihood detector (MLD) is used to combat the deleterious effects of depolarization crosstalk in dual-channel M-QAM (quadrature-amplitude modulation) systems. The authors use a truncated union upper bound to calculate the probability of symbol error (P_es) for dual-channel 16-QAM. The frequency of occurrence of P_es versus crosstalk phase shift, with crosstalk level as a parameter, and a 3-D plot of P_es as a function of the two crosstalk phase shifts, is presented. Assuming uniform distributions for the phase shifts, the average probability of symbol error <P_es> is determined, and the performance of the MLD is compared to that of the diagonalizer canceller. It is shown that the MLD gains about 1.5 dB improvement in signal-to-noise ratio above that of the diagonalizer for crosstalk values above -10 dB, and that it actually uses the crosstalk to decrease the <P_es> below the zero crosstalk value for crosstalk levels less than -10 dB