用于FTN系统的二阶导频辅助载波相位恢复算法

Two-Stage Pilot-Aided Carrier Phase Recovery Algorithm for FTN System

针对超奈奎斯特系统引入的码间干扰使载波相位估计难度增大的问题,提出了一种二阶电域导频辅助载波相位估计方案,将导频辅助相位噪声补偿模块与基于Viterbi的最大似然相位估计模块级联,牺牲约1.7%的带宽开销,可以有效克服超奈奎斯特系统引入的码间干扰,对激光器线宽导致的相位噪声进行准确估计。在调制格式为16进制正交幅度调制的超奈奎斯特系统中,经仿真验证可知:该算法在加速因子不超过Mazo极限时可以有效跟踪相位噪声;在加速因子低至0.833、误码率为2×10^(-2)、光信噪比代价为1 dB的条件下,该算法能容忍的最大线宽乘积为5×10^(-4);相较于一阶导频辅助载波相位恢复算法,该算法具有更大的线宽容忍度,所需的光信噪比也更小。

Objective The largescale commercialization of 5G networks promotes the development of fiber optical communication,cloud computing,and Internet of Things technologies.To realize the optical communication systems with high capacity and high spectrum efficiency,higherorder modulation formats are required.However,the higherorder modulation formats require high signaltonoise ratios(SNR)to ensure communication quality,which will limit the transmission distance.Within the same amount of time,the fasterthanNyquist(FTN)technology can transmit more signals than the Nyquist system with the same modulation format.Thus,this technology becomes a key technology for the nextgeneration optical communication networks with the advantages of high spectral efficiency and large capacity.The intersymbol interference is artificially introduced into the FTN system to make the symbol interval between two adjacent pulses much smaller than the corresponding Nyquist symbol period.Thus,highspeed digital signal processing(DSP)unit,which can equalize and compensate the signal impairments efficiently in electrical domain,is a key module in FTN systems.It can improve the signal quality and support the optical fiber communication systems with ultrahigh capacity.Pilotaided carrier phase recovery(PACPR)is an important DSP algorithm for optical receiver,which is used to compensate the frequency deviation between the lasers on both sides of the transceiver and the phase noise generated by the laser linewidth.In present study,we report a twostage electricdomain pilotaided carrier phase estimation algorithm,named PAViterbiML,in which the PACPR algorithm is combined with the Viterbibased maximum likelihood(ML)estimation algorithm.The simulation results verify that the proposed twostage PAViterbiML algorithm can effectively track the phase noise when the Mazo limit is not exceeded in the FTN16QAM system.Methods The proposed PAViterbiML,which combines the PACPR algorithm with the Viterbibased maximum likelihood estimation algorithm,can effectively overcome the intersymbol interference(ISI)introduced by FTN technology.The first stage of the algorithm can estimate and compensate most of the phase noise in FTN16QAM system.In order to compensate the residual phase deviation,the ML phase estimation is used as the second stage of the phase recovery algorithm to obtain a more refined phase estimate value.However,the ML phase estimation will also fail under the influence of the ISI introduced by FTN system,so the Viterbi algorithm is cascaded with it to remove the influence of ISI,and the estimated value of the phase noise close to the real value is obtained.During the simulation,the pilotsignalratio(PSR)and the bandwidth of the lowpass filter(BLPF),two important parameters of the PAViterbiML algorithm,are optimized first to achieve the optimal system performance.Then,the performance of the proposed algorithm for tracking system phase noise within the Mazo limit is shown.Finally,the maximum linewidth tolerance of the algorithm is determined at the threshold of the bit error rate.Results and Discussions In the dualcarrier PDM16QAM FTN system,theΔf·Ts is set to 1×10^(-4).With the decrease of accelerating factor namedα,the artificially introduced ISI will become more serious.The PAViterbiML algorithm can overcome the effect of ISI introduced by FTN whenαdoes not exceed the Mazo limit,and effectively estimate the phase noise caused by the laser linewidth(Fig.7).Finally,the performance of the proposed algorithm and the traditional PACPR is compared.When theΔf·Ts is small,the OSNR penalties of both algorithms are almost the same.As theΔf·Ts increases,the OSNR penalty of the traditional PACPR is relatively higher(Fig.9).This means that the linewidth tolerance of the proposed PAViterbiML algorithm is larger for the same OSNR penalty,and the OSNR penalty of the proposed algorithm is less for the sameΔf·Ts.In addition,with the decrease ofα,the performance advantage of the PAViterbiML algorithm is more significant.Whenα=0.850,Δf·Ts is 1×10^(-4),the OSNR penalty of the PAViterbiML algorithm is 0.02 dB lower than that of the PACPR algorithm.Whenα=0.833,Δf·Ts is 1×10^(-4),the OSNR penalty of the PAViterbiML algorithm is 0.28 dB lower than that of the PACPR algorithm.Compared with the traditional algorithm,the performance advantage of the PAViterbiML algorithm will become larger when the smaller value ofαand the larger value ofΔf·Ts are taken(Table 1).Conclusions In the present study,a twostage electricdomain pilotaided carrier phase estimation algorithm,named PAViterbiML,is proposed,in which the PACPR algorithm is combined with the Viterbibased maximum likelihood estimation algorithm.The PAViterbiML algorithm will occupy about 1.7%of the bandwidth,which can effectively overcome the ISI introduced by the FTN technology.The simulation is taken to verify that the proposed twostage PAViterbiML algorithm can effectively track the phase noise when the Mazo limit is not exceeded in the FTN16QAM system.The simulation results show that the OSNR penalty requirements of the proposed algorithm are smaller than those of the traditional PACPR when the linewidth tolerance values are the same,which means the performance of the linewidth tolerance of the proposed algorithm is better.The maximum linewidth tolerance value is defined when the BER and the OSNR penalty are equal to 2×10^(-2) and 1 dB,respectively.When the accelerating factorαis as low as 0.833,the maximum linewidth tolerance value is about 5×10^(-4) for the PAViterbiML algorithm,and the corresponding value for the traditional PACPR algorithm is about 1×10^(-4).