A novel high precision adaptive equalizer in digital coherent optical receivers

• The linear channel distortion can be compensated roughly in frequency-domain.
• The signals with large error can be picked out in frequency-domain by error measurement stage.
• The signals with large error can be equalized afresh in time-domain with high precision.
• The computation complexity of proposed method is lower than traditional CMA.
• The equalization performance of proposed method is better than traditional CMA.

A novel high precision adaptive equalization method is introduced and applied to dynamic equalization for quadrature phase shift keying (QPSK) coherent optical communication system in this paper. A frequency-domain constant modulus algorithm (CMA) method is used to equalize the received signal roughly. Then, some non-ideal output signals will be picked out through the error measurement, and they will be equalized accurately further in a fixed time-domain CMA equalizer. This high precision equalization method can decrease the equalization error, then it can reduce the bit error ratio (BER) of coherent communication system. Simulation results show that there is a 6% decrease for computation complexity by proposed scheme when compared with time-domain CMA. Furthermore, compared with time-domain CMA and frequency-domain CMA, about 2 dB and 2.2 dB in OSNR improvement can be obtained by proposed scheme at the BER value of 1e−3, respectively.