Carrier recovery for satellite-to-ground coherent laser communication systems using double feedback loop and Viterbi-Viterbi feedforward cascade structure

High dynamic and wide range frequency offset in satellite-to-ground coherent optical communication system caused by Doppler frequency shift, transceiver laser frequency offset and laser frequency stabilization technology based on perturbation is one of the important differences compared with terrestrial fiber-optic coherent transmission. The design of a reliable frequency offset acquisition and fine carrier tracking algorithm is critical to ground optical stations. Two-stage feedback loop and Viterbi-Viterbi (V-V) feedforward cascaded carrier recovery algorithm is presented in this work. Pre-decision-based angle differential estimator (PADE) is adopted as the residual frequency offset (RFO) estimator in the first stage to estimate the high dynamic and wide range of frequency offset. Then V-V based RFO estimator in the second stage is employed to accurately compensate the residual frequency. V-V algorithm is used to compensate phase offset, which share the same logic resource with V-V based RFO estimator. A real-time 10-Gbps data rate PM-QPSK experiment based on FPGA is performed to evaluate the algorithm performance. The proposed cascaded algorithm exhibits excellent performance in terms of dynamic tracking speed and normalized mean-square-error (MSE) of the FOE.