A simplified quadrature amplitude modulation (QAM) feedforward carrier recovery algorithm is proposed. By introducing a new two-stage estimation structure and sliding-block approach, the required computational effort can be significantly reduced with a tolerable performance deterioration compared with the original algorithm. In order to choose the size of the sliding block properly, an analytical model is proposed that takes the optical SNR (OSNR) and the laser-linewidth-times symbol-rate product into account. The model reveals different influence of OSNR and the laser-linewidth-times symbol-rate product on the performance of any carrier recovery algorithm. The performance of the simplified algorithm is evaluated by Monte Carlo simulations for square 16-, 64-, and 256-QAM. The simulation results show that the computational effort for each symbol can be significantly reduced at the cost of a 11% reduction of laser linewidth tolerance.
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