Sampling frequency offset compensation scheme for single-carrier signals in coherent systems

Transceiver sampling frequency offset (SFO) due to imperfections is one of the major factors restricting the transmission performance of the optical fiber coherent systems, especially when signals with high baud rate and therefore requiring higher sampling frequency of sampling devices like DACs or ADCs. In single-carrier signals transmission system, although adaptive clock recovery methods like Gardener or Godard method can compensate sampling error, when SFO is large enough so that one-frame length samples can accumulate to approximate one sample or above, the clock recovery will directly fail due to an extra sample is added or missed. The research of this issue for single-carrier signal is still rare. In this paper, we propose to use a digital interpolation method for single-carrier QAM signals for SFO compensation. The effectiveness of the proposed method is verified by a 32GBaud 16-QAM dual-polarization 80-km SSMF optical coherent transmission experiment. The experimental results show that with proposed method, up to at least 1000-ppm SFO can be almost compensated to the bit error rate (BER) level as if without SFO. Besides, when using the proposed method, the required optical signal-to-noise ratio (OSNR) can be reduced by about 7 dB to achieve the BER level without SFO.