Recursive least squares estimation of QAM and QPSK signals in the presence of frequency offset

Adaptive frequency and phase control methods for differential/quasi-coherent phase detection based on recursive least squares (RLS) estimation in the presence of frequency offset are discussed. An RLS estimator for differential detection is presented and its convergence rate is derived. An RLS estimator is proposed for quasi-coherent detection that can accurately demodulate in the presence of frequency offset while achieving fast convergence. It was verified experimentally that the estimators for quadrature amplitude modulation (QAM) quasi-coherent detection and π/4 quadrature phase shift keying (QPSK) differential detection had good performance in the presence of frequency offset, and that the RLS estimator attained convergence in 10 symbols