Blind carrier recovery for circular QAM using nonlinear least-squares estimation

In this work, a family of blind feedforward nonlinear least-squares (NLS) estimators for joint estimation of the carrier phase and frequency offset of circular quadrature amplitude modulated (QAM) transmissions is introduced. Well-known synchronization techniques are adapted for the circular QAM schemes, and a constellation-dependent optimal matched nonlinear estimator is derived to minimize the asymptotic variance. A computationally efficient approximation to the optimal nonlinearity is introduced and its performance assessed. Finally, computer simulations for various circular QAM transmissions are carried out to show that the proposed estimation technique is competitive with conventional monomial estimators. From the simulation results, conclusions are drawn about selecting the best synchronization method for implementation in actual communication systems.