Offset DPSK and a comparison of conventional and symmetric DPSK with noise correlation and power imbalance

The bit-error-rate performance of binary DPSK with an arbitrary offset of the phase-change vector is derived in the presence of noise correlation and power imbalance, and the special cases of conventional DPSK (0 phase-change offset) and symmetric DPSK (pi/2 phase-change offset) are compared. Precise conditions are given for optimum offsets, and it is shown that conventional DPSK gives the optimum performance of any offset scheme and outperforms symmetric DPSK by an order of magnitude and more in error rate when the noise correlation becomes significant. It is also found that performance improvement over conventional DPSK can be realized, provided the noise correlation is known, by making the bit-decision regions in phase space unequal, thereby demonstrating that the MAP estimator for DPSK without correlation is no longer optimum when the noise is correlated.