相干光通信系统中QPSK调制解调实验研究
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摘要
为了使相干接收机输出的两路信号相位正交,正确地解调信号,采用施密特正交化算法对两路信号进行了理论分析和实验验证,经正交失衡算法补偿后,星座图的性能得到改善;通过松尾环实现载频的跟踪、捕获以达到载波恢复,实现了基带信号正确解调。结果表明,经施密特正交化算法补偿后,旋转的星座图得到了修正,星座点间的欧几里德距离不相等问题得到修正,基带信号畸变得到了改善;当频偏在-300kHz～300kHz范围内,松尾环可以实现载频的跟踪、捕获,基带信号实现正确解调。该方案实现复杂度低,切实可行,适用于相干光通信系统的研究。
In order to make the received two signals orthogonal to each other and correctly demodulate the signal, Schmitt orthogonalization algorithm was used to theoretically analyze and experimentally verify the two signals. The performance of constellation was improved after orthogonal imbalance compensation. The carrier frequency was tracked and captured by the loose tail ring to achieve carrier recovery, and the baseband signal was demodulated correctly. The experimental results show that, after compensation by Schmidt orthogonalization algorithm, the rotated constellation is corrected and the problem of unequal Euclidean distance between constellation points is improved. The baseband signal distortion becomes corrected. When the frequency offset is in the range of-300 kHz～300 kHz, the loose tail loop can track and capture carrier frequency and baseband signal can be correctly demodulated. The scheme is low complexity and practical, and is suitable for the study of coherent optical communication systems.
In order to make the received two signals orthogonal to each other and correctly demodulate the signal, Schmitt orthogonalization algorithm was used to theoretically analyze and experimentally verify the two signals. The performance of constellation was improved after orthogonal imbalance compensation. The carrier frequency was tracked and captured by the loose tail ring to achieve carrier recovery, and the baseband signal was demodulated correctly. The experimental results show that, after compensation by Schmidt orthogonalization algorithm, the rotated constellation is corrected and the problem of unequal Euclidean distance between constellation points is improved. The baseband signal distortion becomes corrected. When the frequency offset is in the range of-300 kHz～300 kHz, the loose tail loop can track and capture carrier frequency and baseband signal can be correctly demodulated. The scheme is low complexity and practical, and is suitable for the study of coherent optical communication systems.
引文
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[2] YAN S,WENG X,GAO Y,et al.Generation of square or hexagonal 16-QAM signals using a dual-drive IQ modulator driven by binary signals[J].Optics Express,2012,20(27):29023-29034.
[3] SUN H,WU K T,ROBERTS K.Real-time measurements of a 40Gb/s coherent system[J].Optics Express,2008,16(2):873-879.
[4] CHUNG H S,SUN H C,KIM K,et al.Effects of carrier phase estimation on front-end IQ mismatch compensation in DP-QPSK coherent receiver[C]//Optoelectronics and Communications Conference Held Jointly with 2013 International Conference on Photonics in Switching.New York,USA:IEEE,2013:TuPR_2.
[5] SAVORY S J.Digital coherent optical receivers:algorithms and subsystems[J].IEEE Journal of Selected Topics in Quantum Electronics,2010,16(5):1164-1179.
[6] WANG X J,LEIBLE B,WANG W H,et al.Joint IQ imbalance compensation and channel estimation in coherent optical OFDM systems[C]//2016 10th International Conference on Signal Processing and Communication Systems.New York,USA:IEEE,2016:16653749.
[7] FATADIN I,SAVORY S J,IVES D.Compensation of quadrature imbalance in an optical QPSK coherent receiver[J].IEEE Photonics Technology Letters,2008,20(20):1733-1735.
[8] SUN H C,CHUNG H S,KIM K.Impact of quadrature imbalance in optical coherent QPSK receiver[J].IEEE Photonics Technology Lett-ers,2009,21(11):709-711.
[9] PETROU C S,VGENIS A,ROUDAS I,et al.Quadrature imbalance compensation for PDM QPSK coherent optical systems[J].IEEE Photonics Technology Letters,2009,21(24):1876-1878.
[10] FARUK M S,KIKUCHI K.Compensation for in-phase/quadrature imbalance in coherent-receiver front end for optical quadrature am-plitude modulation[J].IEEE Photonics Journal,2013,5(2):7800110.
[11] NGUYEN T H,GOMEZ-AGIS F,GAY M,et al.IQ imbalance compensation based on maximum SNR estimation in coherent QPSK systems[C]//2014 16th International Conference on Transparent Optical Networks.New York,USA:IEEE,2014:14526430.
[12] FARUK M S,SAVORY S J.Digital signal processing for coherent transceivers employing multilevel formats[J].Journal of Lightwave Technology,2017,35(5):2-13.
[13] MAGARINI M,BARLETTA L,SPALVIERI A,et al.Pilot-symbols-aided carrier-phase recovery for 100G PM-QPSK digital cohe-rent receivers[J].IEEE Photonics Technology Letters,2012,24(9):739-741.
[14] FLUDGER C R S,BOSCO G,BILAL S M,et al.Multistage carrier phase estimation algorithms for phase noise mitigation in 64-quadrature amplitude modulation optical systems[J].Journal of Lightwave Technology,2014,32(17):2973-2980.
[15] PAJOVIC M,MILLAR D,KOIKEAKINO T,et al.Multi-pilot aided carrier phase estimation for single carrier coherent systems[C]//Signal Processing in Photonic Communications.Washington DC,USA:The Optical Society of America,2015:3-4.
[16] HOSSAIN M J,FARUK M S.An efficient scheme for receiver-side quadrature imbalance compensation in coherent optical receivers[C]// 2012 International Conference on Fiber Optics and Photonics.New York,USA:IEEE,2012:13597286.
[17] FANG Y,GU Q.Angle statistical blind compensation method for IQ imbalance of QPSK signal [J].System Simulation Technology,2012,8(2):87-92(in Chinese).
[18] CHANG Q,BI C K,ZHANG Q Sh.The principle and implementation of Costas ring in DSSSK system[J].Microcomputer Information,2006,22(35):241-242(in Chinese).
[19] ZHANG X.Digital baseband signal processing algorithm for spread spectrum communication and its VLSI implementation [M].Beijing:Science Press,2004:113-115(in Chinese).
[20] DU Y.MATLAB and FPGA implementation of digital communication synchronization technology[M].Beijing:Publishing House of Electronics Industry,2015:96-218(in Chinese).