Research On Phase Noise Estimation In Flexible Coherent Optical OFDM/OQAM Communication System

With the explosion of new internet technologies,such as big data,internet of things,flexibility is gradually gaining attention in optical networks.Flexible optical networks(FON)are meant to adjust bandwidth,transmission route and modulation format according to practical demand,while assuring reliable communication.Therefore,FON has advantages of effective cost and high efficiency.As a classic multi carrier scheme,orthogonal frequency division multiplexing/offset quadrature amplitude modulation(OFDM/OQAM)system has the potential to support fine-granularly allocation of time-frequency resources,thus suitable for FON.In practical coherent optical OFDM/OQAM(CO-OFDM/OQAM)system,phase noise(PN)induced by laser,fiber can severely degrade the communication performance.Therefore,phase noise estimation(PNE)is important for CO-OFDM/OQAM to maintain reliable communication.The main topic of this paper is PNE of CO-OFDM/OQAM in the context of FON.Firstly,the principle of OFDM/OQAM system is discussed.And a frequency-domain signal model under combined impact of phase noise,residual frequency offset and channel distortion for OFDM/OQAM is deduced.Secondly,two PNE schemes,adaptive extended Kalman filter(AEKF)and pilot-aided Maximum a Posterior method(PMAP),are proposed.Principles of some classic methods are analyzed,including modified blind phase search(MBPS)and modified extended Kalman filter(MEKF).Based on MEKF,a new blind PNE method,AEKF is proposed.The advantage of AEKF is that,its implementation complexity is positively proportional to laser linewidth.AEKF can provide high linewidth tolerance while maintain relatively low complexity.Based on blind Maximum a Posterior estimator,a new pilot-aided PNE method,PMAP,is proposed.The advantages of PMAP are low complexity and simple pilot structure with low spectral efficiency loss.Thirdly,a 20 Gbaud 16 OQAM CO-OFDM/OQAM simulation system is built through Matlab R2017 and VPI Photonics 9.9.On the simulation platform,some blind methods and pilot aided methods are tested and compared on the basis of flexibility and complexity.Flexibility is evaluated through the robustness of PNE schemes against subcarrier allocation,fiber transmission distance,laser linewidth and residual frequency offset.Complexity is evaluated through the implementation time and arithmetic operators of PNE schemes.Numerical results show that,AEKF can adjust implementation complexity according to laser linewidth.At laser linewidth lower than 100 KHz,AEKF's complexity is about 2/5 of MBPS's complexity,also lower than MEKF's complexity.PMAP has very low implementation complexity at the cost of 1.56% spectral loss.Besides,PMAP has a strong robustness to residual frequency offset.Its tolerance of frequency offset is about 1.2 MHz.The research in this paper has important practical significance and theoretical value for development of next generation flexible optical networks.