Carrier recovery for M-QAM signals based on a block estimation process with Kalman filter

A novel carrier recovery scheme for demodulating optical M-ary quadrature amplitude modulation (M-QAM) signals is proposed and demonstrated. The proposed scheme treats a certain number of consecutive symbols as a processing block for which linear evolution of the carrier phases in time is assumed. The Kalman filter algorithm is employed to simultaneously estimate the carrier-frequency offset and carrier phases of the symbols in each block from the observation result. Consequently, an optimal carrier recovery operation with minimum mean squared error can be obtained, and large phase errors due to optical noise and large carrier-frequency offsets can be tolerated. We experimentally demonstrate the proposed scheme in demodulating optical 16- and 64-QAM signals, confirming its stable operation for carrier-frequency offsets even larger than 10% of the symbol rate of the signal. We also demonstrate 160-km transmission of a single-channel, single-polarization 64-QAM signal by using the proposed scheme in the demodulation process.