A Flexible DAML Phase Estimation Algorithm In Coherent Optical Communications

With the progress of the Internet and multimedia services, the demand of data traffic is exponentially growing and it motivates the research of more spectrally efficient systems to better utilize the limited bandwidth of optical fibers. With the recent availability of ADCs, full information of electric field can be preserved, such as amplitude, frequency, phase and polarization. Advanced modulation formats together with coherent detection are shown to be the most promising solution to increase the data rate without increasing required bandwidth. However, one of the main challenges in coherent optical systems is to recover the carrier phase when using a LO to beat with the received signals to retrieve modulated phase information. Therefore, carrier phase estimation techniques will be studied to recover the carrier phase for coherent optical communications in this paper.Firstly, we introduce the basic theory of coherent optical communications, including the development, basic structure and key technologies. Then conventional DAML and adaptive DAML phase estimation algorithms are introduced based on the characteristic of laser phase noise and their limitations are indicated.Secondly, a flexible DAML phase estimation algorithm is proposed which introduces weighted coefficients to recover carrier phase more accurately than conventional DAML. This paper provides a detailed derivation of flexible DAML phase estimation, analyzes weighted coefficients and phase estimation error and explores the inference of block length from the aspects of phase estimation error, BER and complexity.Finally, simulations are performed to analyze the performance of flexible DAML receiver with M-PSK and M-QAM modulation formats. The results show that flexible DAML receiver outperforms the conventional DAML receiver. The optimization of flexible DAML is more obvious especially when the block length is longer. Furthermore, the flexible DAML can eliminate the block length effect of conventional DAML, which relaxes the requirement of laser linewidth.