Asymptotically Efficient Phase Recovery For QAM Communication Systems

We introduce a new blind phase offset estimator for general quadrature amplitude modulated (QAM) signals. The estimator is based on the computation of a suitable phase distribution that we call "signature". The signature is defined as the phase-dependent distribution of the received signal magnitude after the application of a nonlinear transformation. The signature of a QAM signal is constituted by a discrete number of pulses and it has good autocorrelation properties in the sense of maximum/side-lobe ratio. Since the effect of a phase offset is a cyclic shift of the signature, the phase offset can be estimated by searching for the maximum of the cyclic cross-correlation between the zero-phase signature of the expected constellation, and the signature calculated on the received signal. The resulting estimator is characterized by a low computational complexity and does not need gain control. The comparison shows that the presented estimator is asymptotical efficient and outperforms existing estimators for medium to high values of SNR, especially for complex constellations