LETTER

Key Enabling Techniques and Deployment of 120Gb/s Long-Haul Optical Transmission in Backbone Networks

JI Yuefeng1, CHEN Yufei2, CHEN Xue1, SHI Sheping2, ZHANG Min1, XIA Yan2, GU Rentao1

'State Key Laboratory of Information Photonics and Optical Communications, School of Information and Communication Engineering, Beijing University of Posts and Telecommunications, Beijing 100876, China 2ZTE Corporation, Beijing 100191, China

Abstract: The explosive increase in data traf­fic requires networks to provide higher capac­ity and long-haul transmission capabilities. This paper introduces new results on high-order modulation and efficient Digital Signal Proc­essing algorithms to reduce various transmis­sion limitations in coherent receiving systems. Polarization Division Multiplexed Quadrature Phase Shift Keying (PDM-QPSK) is deployed to reach high bit rates, provides modified dig­ital clock recovery, and allows BER-Aided Con­stant Modulus Algorithm (BA-CMA) equalis­ing. A Soft Decision-Forward Error Correc­tion (SD-FEC) algorithm and a joint scheme with timing recovery and adaptive equaliser are used to achieve better performance. A com­pact coherent transceiver is also developed. Th­ese techniques have been applied in the largest 100 G Optical Transport Network (OTN) de­ployment in the world, the backbone expan­sion project for Phase 3 of the China Educa­tion and Research Network (CERNET), with a total transmission length of 10 000 km.

Key words: optical transmission; high-order modulation; coherence detector; coherent tran­sceiver

I. INTRODUCTION

Recently, the average data traffic growth in backbone networks has exceeded 50% per year, which means the traffic doubles every 2 years.

Furthermore, information from China Telecom reveals that the traffic growth in China appro­aches 100% annually. This situation compels us to consider how to fully utilise the potential of optical transmission technologies [1]. As a result, optical transmission at ultra-high bit ra­tes has been in the limelight recently [2-3]. 120 Gb/s and higher long-distance transmis­sion technology has been explored in the con­tinuous pursuit of higher transmission capacity, longer transmission distance, and lower trans­mission costs per bit. However, compared with the evolution of single wavelength transmis­sion rates from 10 Gb/s to 40 Gb/s, the evolu­tion towards 100 Gb/s faces more stringent ph­ysical limits and additional significant rese­arch and advancement is expected.

Addressing the difficult problems of single wavelength transmission at 120 Gb/s and be­yond, a joint research project between the Bei­jing University of Posts and Telecommunica­tions (BUPT) and ZTE Corporation (ZTE) has been carried out and solutions have been pro­posed and implemented. This paper discusses the key techniques of single-carrier-based 120 Gb/s transmission and introduces a more efficient algorithms for coherent detection and an adaptive coherent transceiver in Section II. With these technical solutions implemented, the deployment of the backbone network is des­cribed in Section III.

158 China Communications · August 2013

II. KEY TECHNIQUES FOR LONG-HAUL OPTICAL TRANSMISSION AT 120 GB/S PER WAVELENGTH

To make the best use of the bandwidth resou­rces at an acceptable cost, higher-order modu­lations (or advanced modulations) and digital coherent detection have attracted much atten­tion. The reason is that higher-order modula­tion formats yield lower symbol rates and thus mitigate the impact of harmful physical effects, such as dispersion, nonlinearity and Polariza­tion Mode Dispersion (PMD). Therefore, high-order modulation, efficient Digital Signal Pro­cessing (DSP) algorithms that reduce various transmission limitations and the use of a cohe­rent transceiver are three key techniques which will be described in this section.

2.1 High-order modulation Higher-order modulation formats yield lower symbol rates and thus mitigate the impact of harmful physical effects. Modulation increases the spectrum efficiency and relieves the limi­tations imposed by the high Analogue-to-Dig-ital Converter (ADC) bandwidth and sampling rate [4].

Multi-carrier modulation, for example Or­thogonal Frequency Division Multiplexing (OFDM), is an alternative to reduce the sym­bol rate while raising the spectral efficiency. However, the generation of multi-carrier signals is complex and expensive, and furthermore OFDM signals are more sensitive to the nonl-inearities of the fibre and the optical modula­tor. Therefore, using only a single carrier tog­ether with higher order modulation and po­larization multiplexing is more practical for 120 Gb/s using 50 GHz channels for ultra-long transmissions. In this case, the symbol rate for PDM-QPSK, PDM-8PSK, PDM-8QAM and PDM-16QAM is 30, 20, 20 and 15 Gbaud, respectively. With single-carrier-based adva­nced modulation formats, DSP algorithms are usually applied after homodyne detection to achieve higher performance. PDM-QPSK is chosen as a suitable modulation format due to its good OSNR (optical SNR) tolerance and

lower nonlinearity penalty to support ultra-long distance transmission at 120 Gb/s in 50 GHz channels. To generate an optical QPSK signal, a compact parallel structure can be utilised wh­ere a π/2-biased dual-parallel Mach-Zehnder Modulator (MZM) or I/Q modulator is empl­oyed [5].

2.2 Algorithms for reduction of transmission limitations Combined PDM-QPSK, coherent detection and DSP algorithms for reduction of limitations represent the most important enabling tech­nique for long-haul transmission at 120 Gb/s. Due to optical coherent detection, the homo-dyne signal contains both amplitude and phase information which allows flexible compensa­tion of limitations in the electrical domain us­ing digital signal processing. We propose an improved DSP-based CMA algorithm and a joint scheme with timing recovery and adap­tive equaliser to overcome transmission limi­tations.

Our DSP algorithm's structure is shown in Figure 1. The mechanisms of some key algo­rithms are described in the following.

One of the limitations that DSP has to deal with is ISI (inter-symbol interference) which often degrades system performance severely. DSP-based channel equalization with butterfly structured Finite-Impulse Response (FIR) fil­ters is one of the techniques used to mitigate the effects of ISI, since adaptive algorithms can be used to compensate for time-varying transmission interference and realise polariza­tion de-multiplexing. The Constant Modulus Algorithm (CMA) is often applied due to its low computational complexity and immunity to phase noise. However, conventional CMA is highly sensitive to the initial FIR tap values, which means that improper initial tap values may lead to poor convergence. As a result, the two polarization components tend to reverse at the equaliser output, giving rise to incorrect polarization de-multiplexing.

Addressing this problem, we designed and implemented a modified scheme of CMA, na­mely BER-Aided CMA (BA-CMA) [6], which

China Communications · August 2013 159

Chromatic dispersion equaliser

Clock recovery

PMD comp.&

polarization demux

Carrier frequency estimation

Carrier phase

estimation

Soft decision &

FEC decoder

Fig.l DSP algorithm structure in PDM-QPSK coherent receiver at 120 Gbls

generates a feedback signal for updating the initial FIR tap values according to BER so that proper polarization de-multiplexing is always assured.

In a PDM-QPSK coherent receiver, clock recovery is an indispensable block. The gen­eral methods of clock recovery in digital co­herent receivers involve hybrid feedback tim­ing recovery, and we propose the use an all-digital timing recovery loop which has two advantages: all digital realisation and simple operation. But an all-digital timing recovery loop encounters a serious problem in that the Timing Error Detector (TED) is seriously af­fected by Chromatic Dispersion (CD). If the residual CD is more than several hundred pi­coseconds per nanometre, which is inevitable after coarse equalization in a practical system, the estimated timing error cannot accurately indicate the trend of the actual timing error. So adaptive equalization is needed before the TED; however, the CMA cannot run prior to the timing recovery loop because its effectiveness depends on the synchronous signal. Therefore, we propose a joint clock/timing recovery and equalization scheme [7]. In the joint scheme, the butterfly structured adaptive equaliser is added between the interpolator and the timing error detector, therefore the dispersion im­pairment of a signal can be compensated be­fore the signal enters the TED and thus the out­put of the synchronous signal from the interpo­lator satisfies the requirements of the equaliser. The mutual cooperation of timing recovery and adaptive equaliser solves the problem of in­compatible prerequisites between them and makes it possible to complete synchronization, equalization, and polarization de-multiplexing synchronously by using the joint scheme.

2.3 120 Gb/s coherent transceiver

Coherent transceivers are expected to be very important building blocks for 120 Gb/s trans­mission systems to achieve the higher capacity and longer fibre reach that coherent technol­ogy offers. Coherent transceivers also provide cost-effective electronic equalization of fibre transmission obstacles, such as CD and PMD, as well as extensive performance monitoring capabilities that ease system operation, admin­istration and maintenance in practice.

To realise a 120 Gb/s coherent transceiver, we have to miniaturise the optics. That is, we must integrate more optical building blocks into a single package. And we have to inte­grate the sophisticated DSP functions to com­pensate for the imperfections from the trans­mitter, fibre and receiver optics. Namely, the optical components and the DSP should be concurrently integrated to guarantee a useful, compact and cost effective 120 Gb/s coherent transceiver.

The proposed 120 Gb/s coherent transce­iver has been designed and implemented as shown in Figure 2 (a) and 2 (b). In the optics section, a PDM-QPSK signal is generated by using a parallel QPSK modulator. We split the CW laser source into two orthogonally po­larised sources, and launch them into two in­dependent QPSK modulators respectively, then recombine the two modulated signals with a polarization beam combiner. In the DSP section, a series of algorithms are integrated to compensate for or mitigate various limitations. These mitigation methods include coarse CD compensation, joint digital clock recovery, BA-CMA for timing and fine CD and PMD self-adaptive compensation and polarization de-multiplexing, frequency offset estimation,

160 China Communications · August 2013

SD-FEC coder

U CU 'i-Li χ 5

|ADÇ H:l

_ V ^ _ «H

90°hybrid & photo

detectors

90°hybrid & photo

detectors

H PBS-sz PBS-—■ LO v < Laser J

Optical receiver

10-10" 10"

x- 10" Dû IQ"1'

IO"1

IO"1

IO"1

IO"1

V No FEC Pre-FEC Port-FEC

11 1 e B codin g g a i n -*

10 12 14 16 18 OSNR/dB

20 22

10"

10" U LU LL

ω IO"4

IO"-

Η^^^^ΐ"Ι""Ι""Ι""Ι"Η ^^^J^™^*^^ ! ^

rs^^p^^'^j rTpUM JJT4JJI

| · DZD Γ - * - 1 6 x 2 2 d B w / o DCM

1 T i l |

, , , , , > , l 11 12

(b)

13 14 15 OSNR/dB

(d)

16 17 18 19

Fig.2 Proposed 120 Gb/s PDM-QPSK transceiver (a) packaging of the transceiver (b) functional structure (e) SD-FEC performance (d) B2B and transmission performance

phase decision and modified SD-FEC. With these functions, the transceiver performs very well, as shown by the measured BER in Fig­ure 2 (c) and 2 (d).

III. THE DEPLOYMENT IN BACKBONE NETWORKS

With the key techniques we have implemented, a series of devices, i.e., the ZXONE8700 se­ries, has been produced. Our technical solu­tions are well represented by the products in this series. They are capable of optical trans­mission at 120 Gb/s per 50 GHz channel and support 96 wavelength channels per fibre. Namely, the maximum capacity over a single fibre is 11.5 Tb/s. In fact, by using the afore­mentioned high-order modulation formats and advanced DSP algorithms, the capacity of the ZXONE8700 series can be expanded further, with 480 Gb/s or even 1.2 Tb/s per channel. Additionally, the chromatic dispersion toler­ance is 50 000 ps/nm and PMD tolerance is 180ps, thus supporting ultra-long electrical repeater spacing.

We have applied our technical solutions in the largest 100 G Optical Transport Network (OTN) deployment in the world, namely, the backbone expansion project for Phase 3 of CERNET.

With our coherent 100 G solution and the ZTE ZXONE8700 series, we have expanded the coverage and transmission capacity of the CERNET backbone and improved its capabil­ity for services access. Moreover, the 120 Gb/s PDM-QPSK signals have been successfully transmitted under an existing 10 Gb/s On-Off Keying (OOK) transmission system. Therefore smooth upgrading has been achieved. The upg­raded national-level 120 Gb/s backbone cov­ers Beijing, Shenyang, Shanghai, Guangzhou, Wuhan, Chengdu, Zhengzhou and Xi'an. The total transmission distance of the 100 G OTN exceeds 10 000 km. As a result, CERNET is now the world's largest national academic com­puter network, connecting more than 1 600 universities and institutes and 20 000 000 us­ers. This project is actually the first large-scale deployment of 120 Gb/s optical transmission

China Communications · August 2013 161

in China. In addition to the technical solutions men­

tioned in Section II, we have implemented gl­obal optimization functions in the system to meet practical engineering requirements. The built-in hybrid transmission capability of 100 Gb/s, 40 Gb/s and 10 Gb/s services allows seamless upgrading of existing systems from 10 Gb/s and 40 Gb/s to 100 Gb/s. Services of various granu­larities are able to be transported transparently with a variety of protection policies and sev­eral energy-saving schemes. As a result, the expanded 120 Gb/s CERNET backbone per­forms well and is stable.

IV. CONCLUSION

This paper introduced new results on three key techniques of ultra-long-haul transmission, i.e., high-order modulation, efficient Digital Signal Processing (DSP) algorithms and high speed coherent transceivers. PDM-QPSK was deplo­yed because of its optimal performance, and a modified CMA algorithm and joint clock re­covery and self-adaptive equalising and polari­zation de-multiplexing method were proposed to achieve better reduction of transmission limi­tations. DSP and integrated optics were com­bined to satisfy the size and cost requirements of coherent transceivers. Benefiting from these techniques, ZXONE8700 equipment has been deployed in the largest 100 G OTN deployment in the world, the backbone expansion project for Phase 3 of CERNET with a total transmis­sion length reaching upwards of 10 000 km.

ACKNOWLEDGEMENT

This research was jointly supported by the National Natural Science Foundation of China under Grant No. 60932004; the National High Technical Research and Development Pro­gram of China (863 Program) under Grants No. 2012AA011301, No. 2012AA011303.

References [1] JI Yuefeng, REN Danping, l i Hui, et al. Analysis

and Experimentation of Key Technologies in

Service-Oriented Optical Internet[J]. Science China Information Sciences, 2011, 54(2): 215-226.

[2] ZHANG Xiaohong, YI Xiaobo, UU Xiang, et al. 100 G Transport Systems: Technology Bench-Mark Testing in China and Evolution to Tera-bit/s Interfaces[J]. China Communications, 2013, 10(4): 19-30.

[3] LI Xinying, YU Jianjun, DONG Ze, et al. WDM Transmission of 108.4-Gbaud PDM-QPSK Sig­nals (40x433.6-Gb/s) over 2 800-km SMF-28 with EDFA-Only[J]. Optic Express, 2012, 20(26): B217-B222.

[4] SEIMETZ M. High-Order Modulation for Op­tical Fiber Transmission[M]. Springer Berlin Heidelberg, 2009.

[5] MENG Xiaobo, UU Yan, SHI Yue, et al. 112 Gbit/s Long-Reach Real-Time Coherent Passive Op­tical Network Downlink Transmission Experi­ment Based on Polarization Multiplexing Qu­adrature Phase Shift Keying Formatti]. Optic Engineering, 2012, 51(4): 82-84.

[6] ZHU Hai, CHEN Xue, ZHOU Weiqin, et al. An Improvement on Constant Modulus Algori­thm for Polarization Demultiplexing in Optical Coherent Receivers!.)]. Optics Communica­tions, 2010, 283(22): 4541-4545.

[7] ZHOU Xian, CHEN Xue, ZHOU Weiqing, et al. All-digital Timing Recovery and Adaptive Eq­ualization for 112-Gbit/s POLMUX-NRZ-DQPSK Optical Coherent Receivers[J]. Journal of Op­tical Communications and Networking, 2010, 2(11): 984-990.

Biographies JI Yuefeng, received his Ph.D. degree from Beijing University of Posts and Telecommunications (BUPT), China. Now, he is a Professor in BUPT. His research interests are primarily in the area of broadband communication networks and optical communica­tions, with emphasis on key theory, realization of technology and applications. Email: jyf@bupt.edu.cn

CHEN Yufei, received his M.S. degree from Beijing Institute of Technology, China. He has been engaged in research and development and management works in optical communication system products in ZTE Corporation, China.

CHEN Xue, received her M.S. degree from Beijing University of Posts and Telecommunications (BUPT), China in 1985. She is now a Professor of the State Key Laboratory of Information Photonics and Optical Communications, BUPT. Her main research interests focus on backbone optical transmission and optical

162 China Communications · August 2013

access networks. Email: xuechen@bupt.edu.cn and photonic networking. Email: mzhang@bupt.edu.cn

SHI Sheping, received his M.S. degree from Beijing University of Posts and Telecommunications, China in 1988. He has been engaged in research and devel­opment works in optical communication system products in ZTE Corporation, China.

ZHANG Min, received his Ph.D. degree from Beijing University of Posts and Telecommunications (BUPT), China in 2003. He is now a Professor of the State Key Laboratory of Information Photonics and Optical Communications, BUPT. His main research interests focus upon advanced optical communication systems

XIA Van, received his M.S. degree from Tianjin Uni­versity, China. He has been engaged in R8tD and management works in optical communication system products in ZTE Corporation, China.

GU Rentao, received his Ph.D. degree from Beijing University of Posts and Telecommunications (BUPT), China. He has been a faculty member of School of Information and Communication Engineering in BUPT since 2010. His current research interests in­clude optical network and intelligent information processing. Email: rentaogu@bupt.edu.cn

China Communications · August 2013 163