Enabling Technologies for High Spectral-efficiency Coherent Optical Communication Networks
Description
More details
Other editions
Additional editions
Persons
Content
List of Contributors xv
Preface xvii
1 Introduction 1 Xiang Zhou and Chongjin Xie
1.1 High-Capacity Fiber Transmission Technology Evolution, 1
1.2 Fundamentals of Coherent Transmission Technology, 4
1.2.1 Concept of Coherent Detection, 4
1.2.2 Digital Signal Processing, 5
1.2.3 Key Devices, 7
1.3 Outline of this Book, 8
References, 9
2 Multidimensional Optimized Optical Modulation Formats 13 Magnus Karlsson and Erik Agrell
2.1 Introduction, 13
2.2 Fundamentals of Digital Modulation, 15
2.2.1 System Models, 15
2.2.2 Channel Models, 17
2.2.3 Constellations and Their Performance Metrics, 18
2.3 Modulation Formats and Their Ideal Performance, 20
2.3.1 Format Optimizations and Comparisons, 21
2.3.2 Optimized Formats in Nonlinear Channels, 30
2.4 Combinations of Coding and Modulation, 31
2.4.1 Soft-Decision Decoding, 31
2.4.2 Hard-Decision Decoding, 37
2.4.3 Iterative Decoding, 39
2.5 Experimental Work, 40
2.5.1 Transmitter Realizations and Transmission Experiments, 40
2.5.2 Receiver Realizations and Digital Signal Processing, 45
2.5.3 Formats Overview, 49
2.5.4 Symbol Detection, 50
2.5.5 Realizing Dimensions, 51
2.6 Summary and Conclusions, 54
References, 56
3 Advances in Detection and Error Correction for Coherent Optical Communications: Regular, Irregular, and Spatially Coupled LDPC Code Designs 65 Laurent Schmalen, Stephan ten Brink, and Andreas Leven
3.1 Introduction, 65
3.2 Differential Coding for Optical Communications, 67
3.2.1 Higher-Order Modulation Formats, 67
3.2.2 The Phase-Slip Channel Model, 69
3.2.3 Differential Coding and Decoding, 71
3.2.4 Maximum a Posteriori Differential Decoding, 78
3.2.5 Achievable Rates of the Differentially Coded Phase-Slip
Channel, 81
3.3 LDPC-Coded Differential Modulation, 83
3.3.1 Low-Density Parity-Check (LDPC) Codes, 85
3.3.2 Code Design for Iterative Differential Decoding, 91
3.3.3 Higher-Order Modulation Formats with V < Q, 100
3.4 Coded Differential Modulation with Spatially Coupled LDPC Codes, 101
3.4.1 Protograph-Based Spatially Coupled LDPC Codes, 102
3.4.2 Spatially Coupled LDPC Codes with Iterative Demodulation, 105
3.4.3 Windowed Differential Decoding of SC-LDPC Codes, 108
3.4.4 Design of Protograph-Based SC-LDPC Codes for
Differential-Coded Modulation, 108
3.5 Conclusions, 112
Appendix: LDPC-Coded Differential Modulation-Decoding Algorithms, 112
Differential Decoding, 114
LDPC Decoding, 115
References, 117
4 Spectrally Efficient Multiplexing: Nyquist-WDM 123 Gabriella Bosco
4.1 Introduction, 123
4.2 Nyquist Signaling Schemes, 125
4.2.1 Ideal Nyquist-WDM (¿f = Rs), 126
> Rs), 128
4.2.3 Super-Nyquist-WDM (¿f < Rs), 130
4.3 Detection of a Nyquist-WDM Signal, 134
4.4 Practical Nyquist-WDM Transmitter Implementations, 137
4.4.1 Optical Nyquist-WDM, 139
4.4.2 Digital Nyquist-WDM, 141
4.5 Nyquist-WDM Transmission, 146
4.5.1 Optical Nyquist-WDM Transmission Experiments, 148
4.5.2 Digital Nyquist-WDM Transmission Experiments, 148
4.6 Conclusions, 149
References, 150
5 Spectrally Efficient Multiplexing - OFDM 157 An Li, Di Che, Qian Hu, Xi Chen, and William Shieh 5.1 OFDM Basics, 158
5.2 Coherent Optical OFDM (CO-OFDM), 161
5.2.1 Principle of CO-OFDM, 161
5.3 Direct-Detection Optical OFDM (DDO-OFDM), 169
5.3.1 Linearly Mapped DDO-OFDM, 169
5.3.2 Nonlinearly Mapped DDO-OFDM (NLM-DDO-OFDM), 173
5.4 Self-Coherent Optical OFDM, 174
5.4.1 Single-Ended Photodetector-Based SCOH, 175
5.4.2 Balanced Receiver-Based SCOH, 177
5.4.3 Stokes Vector Direct Detection, 177
5.5 Discrete Fourier Transform Spread OFDM System (DFT-S OFDM), 180
5.5.1 Principle of DFT-S OFDM, 180
5.5.2 Unique-Word-Assisted DFT-S OFDM (UW-DFT-S OFDM), 182
5.6 OFDM-Based Superchannel Transmissions, 183
5.6.1 No-Guard-Interval CO-OFDM (NGI-CO-OFDM) Superchannel, 184
5.6.2 Reduced-Guard-Interval CO-OFDM (RGI-CO-OFDM) Superchannel, 186
5.6.3 DFT-S OFDM Superchannel, 188
5.7 Summary, 193
References, 194
6 Polarization and Nonlinear Impairments in Fiber Communication Systems 201 Chongjin Xie
6.1 Introduction, 201
6.2 Polarization of Light, 202
6.3 PMD and PDL in Optical Communication Systems, 206
6.3.1 PMD, 206
6.3.2 PDL, 208
6.4 Modeling of Nonlinear Effects in Optical Fibers, 209
6.5 Coherent Optical Communication Systems and Signal Equalization, 211
6.5.1 Coherent Optical Communication Systems, 211
6.5.2 Signal Equalization, 213
6.6 PMD and PDL Impairments in Coherent Systems, 215
6.6.1 PMD Impairment, 216
6.6.2 PDL Impairment, 222
6.7 Nonlinear Impairments in Coherent Systems, 228
6.7.1 System Model, 229
6.7.2 Homogeneous PDM-QPSK System, 230
6.7.3 Hybrid PDM-QPSK and 10-Gb/s OOK System, 233
6.7.4 Homogeneous PDM-16QAM System, 234
6.8 Summary, 240
References, 241
7 Analytical Modeling of the Impact of Fiber Non-Linear Propagation on Coherent Systems and Networks 247 Pierluigi Poggiolini, Yanchao Jiang, Andrea Carena, and Fabrizio Forghieri
7.1 Why are Analytical Models Important?, 247
7.1.1 What Do Professionals Need?, 247
7.2 Background, 248
7.2.1 Modeling Approximations, 249
7.3 Introducing the GN-EGN Model Class, 260
7.3.1 Getting to the GN Model, 260
7.3.2 Towards the EGN Model, 265
7.4 Model Selection Guide, 269
7.4.1 From Model to System Performance, 269
7.4.2 Point-to-Point Links, 270
7.4.3 The Complete EGN Model, 272
7.4.4 Case Study: Determining the Optimum System Symbol Rate, 286
7.4.5 NLI Modeling for Dynamically Reconfigurable Networks, 289
7.5 Conclusion, 294
Acknowledgements, 295
Appendix, 295
A.1 The White-Noise Approximation, 295
A.1 BER Formulas for the Most Common QAM Systems, 295
A.2 The Link Function ¿¿¿¿, 296
A.3 The EGN Model Formulas for the X2-X4 and M1-M3 Islands, 297
A.4 Outline of GN-EGN Model Derivation, 299
A.5 List of Acronyms, 303
References, 305
8 Digital Equalization in Coherent Optical Transmission Systems 311 Seb Savory
8.1 Introduction, 311
8.2 Primer on the Mathematics of Least Squares FIR Filters, 312
8.2.1 Finite Impulse Response Filters, 313
8.2.2 Differentiation with Respect to a Complex Vector, 314
8.2.3 Least Squares Tap Weights, 314
8.2.4 Application to Stochastic Gradient Algorithms, 316
8.2.5 Application to Wiener Filter, 317
8.2.6 Other Filtering Techniques and Design Methodologies, 318
8.3 Equalization of Chromatic Dispersion, 318
8.3.1 Nature of Chromatic Dispersion, 318
8.3.2 Modeling of Chromatic Dispersion in an Optical Fiber, 318
8.3.3 Truncated Impulse Response, 319
8.3.4 Band-Limited Impulse Response, 320
8.3.5 Least Squares FIR Filter Design, 321
8.3.6 Example Performance of the Chromatic Dispersion Compensating Filter, 321
8.4 Equalization of Polarization-Mode Dispersion, 323
8.4.1 Modeling of PMD, 324
8.4.2 Obtaining the Inverse Jones Matrix of the Channel, 325
8.4.3 Constant Modulus Update Algorithm, 325
8.4.4 Decision-Directed Equalizer Update Algorithm, 326
8.4.5 Radially Directed Equalizer Update Algorithm, 327
8.4.6 Parallel Realization of the FIR Filter, 327
8.4.7 Generalized 4 × 4 Equalizer for Mitigation of Frequency or Polarization-Dependent Loss and Receiver Skew, 328
8.4.8 Example Application to Fast Blind Equalization of PMD, 328
8.5 Concluding Remarks and Future Research Directions, 329
Acknowledgments, 330
References, 330
9 Nonlinear Compensation for Digital Coherent Transmission 333 Guifang Li
9.1 Introduction, 333
9.2 Digital Backward Propagation (DBP), 334
9.2.1 How DBP Works, 334
9.2.2 Experimental Demonstration of DBP, 335
9.2.3 Computational Complexity of DBP, 336
9.3 Reducing DBP Complexity for Dispersion-Unmanaged WDM Transmission, 339
9.4 DBP for Dispersion-Managed WDM Transmission, 342
9.5 DBP for Polarization-Multiplexed Transmission, 349
9.6 Future Research, 350
References, 351
10 Timing Synchronization in Coherent Optical Transmission Systems 355 Han Sun and Kuang-Tsan Wu
10.1 Introduction, 355
10.2 Overall System Environment, 357
10.3 Jitter Penalty and Jitter Sources in a Coherent System, 359
10.3.1 VCO Jitter, 359
10.3.2 Detector Jitter Definitions and Method of Numerical Evaluation, 361
10.3.3 Laser FM Noise- and Dispersion-Induced Jitter, 363
10.3.4 Coherent System Tolerance to Untracked Jitter, 366
10.4 Digital Phase Detectors, 368
10.4.1 Frequency-Domain Phase Detector, 369
10.4.2 Equivalence to the Squaring Phase Detector, 371
10.4.3 Equivalence to Godard's Maximum Sampled Power Criterion, 373
10.4.4 Equivalence to Gardner's Phase Detector, 374
10.4.5 Second Class of Phase Detectors, 377
10.4.6 Jitter Performance of the Phase Detectors, 378
10.4.7 Phase Detectors for Nyquist Signals, 380
10.5 The Chromatic Dispersion Problem, 383
10.6 The Polarization-Mode Dispersion Problem, 386
10.7 Timing Synchronization for Coherent Optical OFDM, 390
10.8 Future Research, 391
References, 392
11 Carrier Recovery in Coherent Optical Communication Systems 395 Xiang Zhou
11.1 Introduction, 395
11.2 Optimal Carrier Recovery, 397
11.2.1 MAP-Based Frequency and Phase Estimator, 397
11.2.2 Cramér-Rao Lower Bound, 398
11.3 Hardware-Efficient Phase Recovery Algorithms, 399
11.3.1 Decision-Directed Phase-Locked Loop (PLL), 399
11.3.2 Mth-Power-Based Feedforward Algorithms, 401
11.3.3 Blind Phase Search (BPS) Feedforward Algorithms, 405
11.3.4 Multistage Carrier Phase Recovery Algorithms, 408
11.4 Hardware-Efficient Frequency Recovery Algorithms, 416
11.4.1 Coarse Auto-Frequency Control (ACF), 416
11.4.2 Mth-Power-Based Fine FO Estimation Algorithms, 418
11.4.3 Blind Frequency Search (BFS)-Based Fine FO Estimation Algorithm, 421
11.4.4 Training-Initiated Fine FO Estimation Algorithm, 423
11.5 Equalizer-Phase Noise Interaction and its Mitigation, 424
11.6 Carrier Recovery in Coherent OFDM Systems, 429
11.7 Conclusions and Future Research Directions, 430
References, 431
12 Real-Time Implementation of High-Speed Digital Coherent Transceivers 435 Timo Pfau
12.1 Algorithm Constraints, 435
12.1.1 Power Constraint and Hardware Optimization, 436
12.1.2 Parallel Processing Constraint, 438
12.1.3 Feedback Latency Constraint, 440
12.2 Hardware Implementation of Digital Coherent Receivers, 442
References, 446
13 Photonic Integration 447 Po Dong and Sethumadhavan Chandrasekhar
13.1 Introduction, 447
13.2 Overview of Photonic Integration Technologies, 449
13.3 Transmitters, 451
13.3.1 Dual-Polarization Transmitter Circuits, 451
13.3.2 High-Speed Modulators, 452
13.3.3 PLC Hybrid I/Q Modulator, 455
13.3.4 InP Monolithic I/Q Modulator, 455
13.3.5 Silicon Monolithic I/Q Modulator, 457
13.4 Receivers, 459
13.4.1 Polarization Diversity Receiver Circuits, 459
13.4.2 PLC Hybrid Receivers, 461
13.4.3 InP Monolithic Receivers, 462
13.4.4 Silicon Monolithic Receivers, 462
13.4.5 Coherent Receiver with 120° Optical Hybrids, 465
13.5 Conclusions, 467
Acknowledgments, 467
References, 467
14 Optical Performance Monitoring for Fiber-Optic Communication Networks 473 Faisal N. Khan, Zhenhua Dong, Chao Lu, and Alan Pak Tao Lau
14.1 Introduction, 473
14.1.1 OPM and Their Roles in Optical Networks, 474
14.1.2 Network Functionalities Enabled by OPM, 475
14.1.3 Network Parameters Requiring OPM, 477
14.1.4 Desirable Features of OPM Techniques, 480
14.2 OPM Techniques For Direct Detection Systems, 482
14.2.1 OPM Requirements for Direct Detection Optical Networks, 482
14.2.2 Overview of OPM Techniques for Existing Direct Detection Systems, 483
14.2.3 Electronic DSP-Based Multi-Impairment Monitoring Techniques for Direct Detection Systems, 485
14.2.4 Bit Rate and Modulation Format Identification Techniques for Direct Detection Systems, 488
14.2.5 Commercially Available OPM Devices for Direct Detection Systems, 489
14.2.6 Applications of OPM in Deployed Fiber-Optic Networks, 489
14.3 OPM For Coherent Detection Systems, 490
14.3.1 Non-Data-Aided OSNR Monitoring for Digital Coherent Receivers, 491
14.3.2 Data-Aided (Pilot Symbols Based) OSNR Monitoring for Digital Coherent Receivers, 494
14.3.3 OPM at the Intermediate Network Nodes Using Low-Cost Structures, 495
14.3.4 OSNR Monitoring in the Presence of Fiber Nonlinearity, 496
14.4 Integrating OPM Functionalities in Networking, 499
14.5 Conclusions and Outlook, 499
Acknowledgments, 500
References, 500
15 Rate-Adaptable Optical Transmission and Elastic Optical Networks 507 Patricia Layec, Annalisa Morea, Yvan Pointurier, and Jean-Christophe Antona
15.1 Introduction, 507
15.1.1 History of Elastic Optical Networks, 509
15.2 Key Building Blocks, 511
15.2.1 Optical Cross-Connect, 512
15.2.2 Elastic Transponder, 513
15.2.3 Elastic Aggregation, 515
15.2.4 Performance Prediction, 516
15.2.5 Resource Allocation Tools, 520
15.2.6 Control Plane for Flexible Optical Networks, 524
15.3 Practical Considerations for Elastic WDM Transmission, 527
15.3.1 Flexible Transponder Architecture, 527
15.3.2 Example of a Real-Time Energy-Proportional Prototype, 529
15.4 Opportunities for Elastic Technologies in Core Networks, 530
15.4.1 More Cost-Efficient Networks, 531
15.4.2 More Energy Efficient Network, 532
15.4.3 Filtering Issues and Superchannel Solution, 532
15.5 Long Term Opportunities, 534
15.5.1 Burst Mode Elasticity, 534
15.5.2 Elastic Passive Optical Networks, 536
15.5.3 Metro and Datacenter Networks, 537
15.6 Conclusions, 539
Acknowledgments, 539
References, 539
16 Space-Division Multiplexing and MIMO Processing 547 Roland Ryf and Nicolas K. Fontaine
16.1 Space-Division Multiplexing in Optical Fibers, 547
16.2 Optical Fibers for SDM Transmission, 548
16.3 Optical Transmission in SDM Fibers with Low Crosstalk, 551
16.3.1 Digital Signal Processing Techniques for SDM Fibers with Low Crosstalk, 552
16.4 MIMO-Based Optical Transmission in SDM Fibers, 553
16.5 Impulse Response in SDM Fibers with Mode Coupling, 558
16.5.1 Multimode Fibers with no Mode Coupling, 561
16.5.2 Multimode Fibers with Weak Coupling, 561
16.5.3 Multimode Fibers with Strong Mode Coupling, 565
16.5.4 Multimode Fibers: Scaling to Large Number of Modes, 566
16.6 MIMO-Based SDM Transmission Results, 566
16.6.1 Digital Signal Processing for MIMO Transmission, 567
16.7 Optical Components for SDM Transmission, 568
16.7.1 Characterization of SDM Systems and Components, 570
16.7.2 Swept Wavelength Interferometry for Fibers with Multiple Spatial Paths, 571
16.7.3 Spatial Multiplexers, 576
16.7.4 Photonic Lanterns, 578
16.7.5 Spatial Diversity for SDM Components and Component sharing, 582
16.7.6 Wavelength-Selective Switches for SDM, 583
16.7.7 SDM Fiber Amplifiers, 590
16.8 Conclusion, 593
Acknowledgments, 593
References, 594
Index 609
CHAPTER 1
INTRODUCTION
Xiang Zhou1 and Chongjin Xie2
1Platform advanced technology, Google Inc, Mountain View, CA, USA
2R&D Lab, Ali Infrastructure Service, Alibaba Group, Santa Clara, CA, USA
1.1 HIGH-CAPACITY FIBER TRANSMISSION TECHNOLOGY EVOLUTION
Since the first demonstration of an optical fiber transmission system in 1977 [1], the demands for higher capacity and longer reach have always been the dominant driver behind the evolution of this new communication technology. In less than four decades, single-fiber transmission capacity has increased by more than five orders of magnitude, from the early 45 Mb/s, using direct modulation and direct detection [2], to more than 8.8 Tb/s by using the digital coherent optical transmission technology [3]. In the meantime, optical transmission reach has increased from only a few kilometers to more than 10,000 km [4]. Such dramatic growth in capacity and reach has been enabled by a series of major breakthroughs in device, subsystem, and system techniques, including lasers, modulators, fibers, optical amplifiers, and photodetectors, as well as various modulation, coding, and channel impairment management methods.
The first generation of optical fiber communications was developed during the late 1970s, operating near 0.8 µm using GaAs semiconductor lasers [2] and multimode fibers (MMF). Although the total capacity of the first commercial system was only running at 45 Mb/s, with an optical reach or repeater spacing of 10 km, this capacity is now much greater than that of comparable coax systems (assuming identical reach or repeater spacing).
With breakthroughs in InGaAsP semiconductor lasers/photodetectors and single-mode fiber manufacturing technologies, the second generation shifted the wavelength to 1.3 µm by taking advantage of the low attenuation (<1 dB/km) and low dispersion of single-mode fibers. A laboratory experiment in 1981 demonstrated transmission at 2 Gb/s over 44 km of single-mode fiber [5]. By 1987, second-generation optical fiber communication systems, operating at bit rates of up to 1.7 Gb/s with a repeater spacing of about 50 km, were commercially available.
The optical transmission reach of second-generation fiber communication systems was limited by fiber losses at the operating wavelength of 1.3 µm (typically 0.5 dB/km). Losses of silica fibers approached minimum near 1.55 µm. Indeed, a 0.2-dB/km loss was realized in 1979 in this spectral region [6]. However, the introduction of third-generation systems operating at 1.55 µm was delayed by large fiber dispersion near 1.55 µm. Conventional InGaAsP semiconductor lasers (with Fabry-Perot type resonators) could not be used because of pulse spreading occurring as a result of simultaneous oscillation in several longitudinal modes. Two methods were developed to overcome the dispersion problem: (i) a dispersion-shifted fiber was designed to minimize the dispersion near 1.55 µm and (ii) a single longitudinal mode laser, that is the widely used distributed feedback (DFB) laser, was developed to limit the spectral width. By using these two methods together, bit rates up to 4 Gb/s over distances in excess of 100 km were successfully demonstrated in 1985 [7]. Third-generation fiber communication systems operating at 2.5 Gb/s became available commercially in 1990 with a typical optical reach of 60-70 km. Such systems are capable of operating at a bit rate of up to 10 Gb/s [8].
To further increase optical transmission reach and reduce the number of costly optical-electrical-optical (O-E-O) repeaters for long distance transmission, efforts were focused on coherent optical transmission technology during the late 1980s. The purpose was to improve optical receiver sensitivity by using a local oscillator (LO) to amplify the received optical signal. The potential benefits of coherent transmission technology were demonstrated in many system experiments [9]. However, commercial introduction of such systems was postponed with the advent of erbium-doped fiber amplifiers (EDFAs) in 1989. The fourth generation of fiber communication systems makes use of optical amplification for increasing O-E-O repeater spacing and of wavelength-division multiplexing (WDM) for increasing total capacity. The advent of the WDM technique in combination with EDFAs started a revolution that resulted in doubling of the system capacity every 6 months or so and led to optical communication systems operating at >1 Tb/s by 2001. In most WDM systems, fiber losses are compensated for by spacing EDFAs 60-80 km apart. EDFAs were developed after 1985 and became available commercially by 1990. A 1991 experiment showed the possibility of data transmission over 21,000 km at 2.5 Gb/s, and over 14,300 km at 5 Gb/s, using a recirculating-loop configuration [10]. This performance proved that an amplifier-based, all-optical, submarine transmission system was feasible for intercontinental communications. By 1996, not only had transmission over 11,300 km at a bit rate of 5 Gb/s been demonstrated by using actual submarine cables [11], but commercial trans-Atlantic and trans-Pacific cable systems also became available. Since then, a large number of submarine fiber communication systems have been deployed worldwide.
In the late 1990s and early 2000s, several efforts were made to further increase single-fiber capacity. The first effort focused on increasing system capacity by transmitting more and more channels through WDM. This was mainly achieved by reducing channel bandwidth through (i) better control of the laser wavelength stability and (ii) development of dense wavelength multiplexing and demultiplexing devices. At the same time, new kinds of amplification schemes had also been explored, as the conventional EDFA wavelength window, known as the C band, only covers the wavelength range of 1.53-1.57 µm. The amplifier bandwidth was extended on both the long- and short-wavelength sides, resulting in the L and S bands, respectively. The Raman amplification technique, which can be used to amplify signals in all S, C, and L wavelength bands, had also been intensely investigated. The second effort attempted to increase the bit rate of each channel within the WDM signal. Starting in 2000, many experiments used channels operating at 40 Gb/s. Such systems require high-performance optical modulator as well as extremely careful management of fiber chromatic dispersion (CD), polarization-mode dispersion (PMD) and fiber nonlinearity [12]. To better manage fiber CD, dispersion compensating fiber (DCF) has been developed and various dispersion management methods have also been explored to better manage fiber nonlinearity. These efforts led in 2000 to a 3.28-Tb/s experiment in which 82 channels, each operating at 40 Gb/s, were transmitted over 3000 km. Within a year, the system capacity was increased to nearly 11 Tb/s (273 WDM channels, each operating at 40 Gb/s) but the transmission distance was limited to 117 km [13]. In another record experiment, 300 channels, each operating at 11.6 Gb/s, were transmitted over 7380 km [14]. Commercial terrestrial systems with the capacity of 1.6 Tb/s were available by the end of 2000.
Until early 2000s, all the commercial optical transmission systems used the same direct modulation and direct detection on/off keying non-return-to-zero (NRZ) modulation format. The impressive fiber capacity growth was mainly achieved by advancement in photonics technologies, although forward error correction (FEC) coding also played a significant role in extending the reach for 10 Gb/s per channel WDM systems. Starting from 40 Gb/s per channel WDM systems, it became evident that more spectrally efficient modulation formats were needed to further increase the fiber capacity to meet the ever-growing bandwidth demands.
High spectral-efficiency (SE) modulation formats can effectively increase the aggregate capacity without resorting to expanding the optical bandwidth, which is largely limited by optical amplifier bandwidth. Using high-SE modulation formats also help reduce transceiver speed requirements. Furthermore, high-SE systems are generally more tolerant of fiber CD and PMD, since they use smaller bandwidths for the same bit rate. CD and PMD tolerance are particularly attractive for high-bit-rate transmission, since dispersion tolerance is reduced by a factor of 4 for a factor-of-2 increase in bit-per-symbol [15].
Early efforts in achieving high SE used direct detection. The first widely investigated modulation format with SE > 1 bit/symbol was the optical differential quaternary phase-shift keying (DQPSK) with differential detection. This is a constant intensity modulation format, which can transmit 2 bits/symbol, corresponding to a theoretical SE of 2 bits/s/Hz [16, 17]. This modulation format also exhibits excellent fiber nonlinearity tolerance due to the nature of constant intensity. To go beyond 2 bit/s/Hz, polarization-division multiplexing (PDM) has been suggested to further increase SE in combination with DQPSK [18]. However, as the state of polarization of the light wave is not preserved during transmission, dynamic polarization control is required at the receiver to recover the transmitted signals.
The need for higher SE and the advancement in digital signal processing (DSP) eventually revives coherent optical communication. The concept of digital coherent communication was proposed by several research groups around 2004-2005 [19-22]. Quickly, this technology was recognized as the best technology for 40 Gb/s, 100 Gb/s and beyond WDM transmission systems, mostly due to the...
