CARRIER PHASE RECOVERY FOR HIGH SPEED COHERENT OPTICAL COMMUNICATION SYSTEMS BASED ON DIGITAL SIGNAL PROCESSING XU ZHUORAN (B.Eng.), National University of Singapore, Singapore A THESIS SUBMITTED FOR THE DEGREE OF DOCTOR OF PHILOSOPHY DEPARTMENT OF ELECTRICAL AND COMPUTER ENGINEERING NATIONAL UNIVERSITY OF SINGAPORE 2014 ii DECLARATION I hereby declare that this thesis is my original work and it has been written by me in its entirety. I have duly acknowledged all the sources of information which have been used in the thesis. This thesis has also not been submitted for any degree in any university previously. XU ZHUORAN January 28, 2015 iii iv Acknowledgement First of all, I would like to express my deepest gratitude and most sincere appreciation to my supervisors Dr. Changyuan Yu and Prof. Pooi-Yuen Kam for their valuable guidance and kind support throughout my Ph.D study. They enlightened me by sharing their knowledge and experience in research. This thesis would not been possible without their help and encouragement. I very appreciate for my thesis committee’s effort and time to review my thesis. Besides, I wish to thank my seniors Dr. Zhang Shaoliang, Dr. Zhang Banghong and Dr. Shao Xuguang with whom I gain a lot of knowledge in research. My thanks also go to my colleagues in NUS for providing friendly environment and exchanging insightful ideas. Last but not least, I would like to thank my parents and relatives who are always there to support me. I am indebted for their love. v ACKNOWLEDGEMENT vi Contents Acknowledgement v Summary xi List of Tables xv List of Figures xxi List of Abbreviations xxv Introduction 1.1 Review of Coherent Optical Communication . . . . . . . . . . . 1.2 Motivation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.3 Contribution of the Thesis . . . . . . . . . . . . . . . . . . . . 1.4 Outline of the Thesis . . . . . . . . . . . . . . . . . . . . . . . 11 Background 2.1 15 Advanced Modulation Formats . . . . . . . . . . . . . . . . . . 16 2.1.1 Signal Model . . . . . . . . . . . . . . . . . . . . . . . 16 2.1.2 Generation Methods . . . . . . . . . . . . . . . . . . . 17 vii CONTENTS 2.2 2.3 2.4 2.2.1 Linear Channel Impairments . . . . . . . . . . . . . . . 21 2.2.2 Fiber Nonlinearity . . . . . . . . . . . . . . . . . . . . 24 Coherent Receiver . . . . . . . . . . . . . . . . . . . . . . . . . 27 2.3.1 Coherent Detection . . . . . . . . . . . . . . . . . . . . 27 2.3.2 Signal-to-Noise Ratio 2.3.3 Laser Phase Noise . . . . . . . . . . . . . . . . . . . . 32 2.3.4 DSP Algorithms in Coherent Receivers . . . . . . . . . 34 . . . . . . . . . . . . . . . . . . 30 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39 Trellis-Based Maximum Likelihood Sequence Detection 3.1 3.2 Transmission Links . . . . . . . . . . . . . . . . . . . . . . . . 21 41 The Principle of Maximum Likelihood Sequence Detection . . . 42 3.1.1 Signal Model . . . . . . . . . . . . . . . . . . . . . . . 43 3.1.2 Conditional PDF of Carrier Phase . . . . . . . . . . . . 45 3.1.3 Decision Metric of Viterbi Algorithm . . . . . . . . . . 47 The Performance of MLSD in M PSK and M -QAM . . . . . . . 51 3.2.1 Performance of MLSD in Linear Phase Noise Channel . 51 3.2.2 Performance of MLSD in Long Haul Transmission System 64 3.3 Analysis of Phase Estimation Error . . . . . . . . . . . . . . . . 70 3.4 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75 Adaptive MLSD Algorithm 77 4.1 The Principle of Adaptive MLSD . . . . . . . . . . . . . . . . . 78 4.2 Simulation Performance of Adaptive MLSD . . . . . . . . . . . 83 4.2.1 Performance of Adaptive MLSD in Linear Phase Noise Channel . . . . . . . . . . . . . . . . . . . . . . . . . . 83 viii CONTENTS 4.2.2 4.3 4.4 Performance of Adaptive MLSD in Long Haul System . 88 Experiment . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92 4.3.1 Setup . . . . . . . . . . . . . . . . . . . . . . . . . . . 92 4.3.2 Experimental results . . . . . . . . . . . . . . . . . . . 95 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100 Pilot Assissted MLSD Algorithm 103 5.1 The Principle of PA MLSD . . . . . . . . . . . . . . . . . . . . 104 5.2 Simulation Study of PA MLSD . . . . . . . . . . . . . . . . . . 107 5.3 Experiment . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112 5.4 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116 Conclusion and Future Work 119 6.1 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . 119 6.2 Future work . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121 6.2.1 Joint Estimation . . . . . . . . . . . . . . . . . . . . . 121 6.2.2 Sequence Detection in OFDM . . . . . . . . . . . . . . 122 6.2.3 Multi-channel System . . . . . . . . . . . . . . . . . . 122 References 139 Publication List 141 ix CONTENTS x REFERENCES [20] D.-S, Ly-Gaynon, S.Tsukamoto, K.Katoh, and K.Kikuchi, “Coherent detection of optical quadrature phase-shift keying signals with carrier phase estimation,” J. 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Yu, Adaptive Maximum Likelihood Sequence Detection for QPSK Coherent Optical Communication System, IEEE Photon. Technol. Lett., vol. 26, no. 6, pp. 583-586, 2014. 2. Z. Xu, C. Yu, and P. Y. Kam, Trellis-Based Maximum Likelihood Sequence Detection in Coherent Optical Communication Systems, J. Lightw. Technol.,submitted. Conference Papers 1. Z. Xu, H. Zheng, C. Yu, and P.Y. Kam, Performance of pilot-assisted maximum-likelihood sequence detection in a rotated-8QAM coherent optical system, OptoElectronics and Communications Conference (OECC) 2014, Paper TH10E-1, pp. 1-3, Melbourne, Australia, July 6-10, 2014. 2. Z. Xu, P. Y. Kam and C. Yu, Performance of Pilot-Assisted Maximum Likelihood Sequence Detection for QAM Signals, Proc. Signal Process- 141 PUBLICATION LIST ing in Photonic Communications (SPPCom), Paper SW1C.2, pp. 1-3, San Diego, CA, USA, July 13-17, 2014. 3. C. Yu, P. Y. Kam, and Z. Xu, Carrier recovery in coherent receiver of optical fiber communication system with laser phase noise, International Photonics and OptoElectronics Meetings (POEM), Paper OTh3B.1, pp. 1-3, Wuhan, China, June 18-21, 2014. 4. Z. Xu, B. Zhang, C. Yu, and P. Y. Kam, Adaptive Maximum Likelihood Sequence Detection in 100-Gb/s Coherent Optical Communication Systems, Conference on Optical Fiber Communication (OFC) 2013, Paper JTh2A.46, pp. 1-3, Anaheim, CA, USA, March 17-21, 2013. 5. Z. Xu, C. Yu, and P. Y. Kam, Performance of Adaptive Maximum Likelihood Sequence Detection with Nonlinear Phase Noise, OptoElectronics and Communications Conference (OECC) 2013, Paper TuPR-16, pp. 1-2, Kyoto, Japan, June 30-July 4, 2013. 6. Z. Xu, S. Zhang, P. Y. Kam, and C. Yu, On the performance of decisionaided maximum likelihood and its adaptive phase estimation with nonlinear phase noise, OptoElectronics and Communications Conference (OECC) 2011, Paper 6B3-3, pp. 1-2, Kaohsiung, Taiwan, July 4-8, 2011. 7. X. Li, Z. Xu, W. D. Zhong, A. Alphones, and C. Yu, Independent component analysis based modified constant modulus algorithm in coherent optical receiver, OptoElectronics and Communications Conference (OECC) 2014, Paper TH10E-3, pp. 1-3, Melbourne, Australia, July 6-10, 2014. 142 PUBLICATION LIST 8. Y. Yu, C. F. Loh, Z. Xu, and C. Yu, OSNR monitoring for PDM RZDQPSK system by low bandwidth sampling technique, Asia-Pacific Conference on Communications (APCC) 2013, Paper V-2.1, pp. 1-2, Bali Dynasty Resort, Bali, Indonesia, August 29-31, 2013. 143 [...]... lightwave systems that employ optical fibers for information transmission The carrier frequencies for optical communication systems are in the range of 100 THz which is the visible or near-infrared region of the electromagnetic spectrum [1] Thus the available bandwidth for information transmission is much higher than in microwave systems To meet the demand of fast increasing data traffic, fiber-optic communication. .. of coherent optical communication is given, followed by the motivation and outline of this thesis 1.1 Review of Coherent Optical Communication The coherent optical communication systems draw considerable attention and were intensively investigated in 1980s The LO in the receiver dramatically im- 2 1.1 Review of Coherent Optical Communication proves the receiver sensitivity and transmission distance... communication systems can be divided into two categories based on their detection method One is referred to as intensity modulation with direct detection (IM/DD) The information is modulated on the intensity of an optical carrier After transmission through fiber link, the incident signal is converted 1 INTRODUCTION directly to electrical domain using photo detectors The capability of the direct detection system... high- speed analog-todigital converters (ADCs) have revived the coherent detection systems The full information of the electromagnetic field can be preserved at the receiver by sampling the received signal into digital waveforms Thus the amplitude, phase and polarization information of the signal can be modulated simultaneously to increase the transmission capacity Advanced modulation formats such as M ary... communication systems have been widely deployed since 1980 for metropolitan and trans-ocean communications Especially in the past two decades, the amount of data traffic on the backbone networks has been growing exponentially at about 30 to 60% per year [2] The rapid development of cloud computing also requires high speed data communication within data-centers and high performance computers [3] The optical communication. .. be utilized in the receiver to perform all-electronic chromatic dispersion (CD) and polarization-mode dispersion (PMD) compensation, frequency offset and phase noise mitigation, polarization de-multiplexing and so on [17–21] Therefore, the bulky optical components are replaced by compact and cheap DSP circuits in the receiver Single carrier 100-Gbit/s coherent optical systems are commercially available... single -carrier- modulated signals that are seamlessly multiplexed under the coherent optical orthogonal frequencydivision multiplexing (OFDM) conditions [25, 26] Later the concept of superchannel is generalized to the optical signals that are modulated and multiplexed together with high spectral efficiency at a common originating site, transmitted through common optical link and received at a common destination... this method heavily depends on nonlinear operations such as rectangular to polar/polar to rectangular transformations, M th power operation and phase unwrapping [20] In addition, the M th power scheme is first proposed for M -PSK signals, thus extra modifications are required for non-constant-amplitude modulation formats [36, 37] To extend the M th-power scheme to M -ary QAM signals, we can use a subgroup... the coherent receiver is reduced from O(L · M ) to O(M ), where L is the window size for MLSD Simulations for back-to-back and transmission systems are conducted to compare the performance for adaptive and non-adaptive MLSD algorithms It is observed that the adaptive version achieves optimal performance as MLSD with optimum window size For QAM signals, the adaptive MLSD experiences constellation penalty... promising solution to increase the data rate without increasing required bandwidth However, one of the challenges in coherent optical systems is to recover the carrier phase, which is perturbed by laser phase noise and nonlinear phase noise In this thesis, we will study carrier recovery algorithms to compensate for phase noise impairments for coherent optical systems Firstly, a trellis -based maximum . CARRIER PHASE RECOVERY FOR HIGH SPEED COHERENT OPTICAL COMMUNICATION SYSTEMS BASED ON DIGITAL SIGNAL PROCESSING XU ZHUORAN (B.Eng.), National University of Singapore,. factor and is continuously updated based on received symbols. It is shown that the adaptive MLSD outperforms the non-adaptive version especially for MPSK signals. For QAM modulation format, although. of high- speed analog-to -digital converters (ADCs), full information of electric field can be preserved, such as amplitude, phase and polarization. Advanced modulation formats together with coherent detection