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    2012年 第10卷 第1期    刊出日期:2012-03-25
    The whole issue of ZTE Communications March 2012, Vol. 10 No. 1
    2012, 10(1):  0. 
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    Guest Editorial of 100G and Beyond: Trends in Ultrahigh-Speed Communications (Part I)
    Gee-Kung Chang, Jianjun Yu, Xiang Wang
    2012, 10(1):  1-2. 
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    Fiber optics underpins the communication infrastructure of today’s information society. Rapid progress in advanced modulation formats, high-gain coding, optical amplification, coherent detection with digital signal processing, and new types of transmission fibers have significantly affected optical communications. Increasing transmission capacity and bit rate per channel is the trend for optical transmission systems and networks. Commercial transmission capacity has increased more than one hundred thousand times since the first optical transmission system was deployed in the 1980s. Spectral efficiency of a single channel has increased from 0.025 b/s/Hz to 2 b/s/Hz. Bit rate per channel for commercial products has increased from 155 Mb/s to more than 100 Gb/s. Larger capacity is driven by the proliferation of broadband FTTH access networks, broadband wireless communications, and high-speed data communication systems in data centers and high performance computing. High bit rate per channel simplifies the management of complex optical networks. Although 100G is just the beginning of the commercial manufacturing and deployment stage, major optical networking research groups have been focusing on standards and technologies beyond 100G. The challenge of generating 400 Gb/s and 1 Tb/s per channel and transmitting at these speeds is one of the hottest topics in recent conferences on optical communications. Many forward-looking solutions have been proposed, and experiments have been carried out to achieve these high bit rates.

    Globally, many research groups have been developing novel enabling technologies for meeting the requirements of high capacity and high bit rate operation using spectrally efficient multiplexing and modulation formats. These advanced techniques include single-carrier polarization multiplexing QPSK (which is currently used in 100G commercial products), multicarrier optical orthogonal frequency division multiplexing, multicore or multimode spatial multiplexing, and coherent detection based on digital signal processing. For high-speed optical signal transmission, a traditional transponder with direct modulation and detection has a simple, low-cost architecture. However, the transmission distance at high bit rate is limited by the rigid requirements of high optical signal-to-noise ratio, polarization mode dispersion, and optical/electrical filtering effects. Coherent detection based on digital signal processing is becoming the trend for optical signal receivers because it can lift these limitations. The change from direct detection to coherent detection is revolutionary. Receiver architecture, transmission fiber and distance, and network management will be completely reshaped from previous direct-detection systems.

    This special issue includes comprehensive reviews and original technical contributions that cover the rapid advances and broad scope of technologies in optical fiber communications. The invited papers of Part I of this issue come from service providers, telecommunication equipment manufacturers, and top universities and research institutes. After peer review, eleven papers were selected for this special issue. We hope it serves as a timely and high-quality networking forum for scientists and engineers.

    The first two papers come from service providers. In the first paper,“High Spectral Efficiency 400G Transmission,”Dr. Xiang Zhou from AT&T labs gives an overview of the generation and transmission of 450 Gb/s wavelength-division multiplexed channels over the standard 50 GHz ITU-T grid at a net spectral efficiency of 8.4 b/s/Hz. In the second paper, “Direct-Detection Optical OFDM Superchannel for Transmitting at Greater Than 200 Gb/s,”Dr. Peng Wei Ren et al. from KDD&I propose and experimentally demonstrate a direct-detection optical orthogonal-frequency-division-multiplexing (OFDM) superchannel and optical multiband receiving method to support a data rate higher than 200 Gb/s and to support longer distance for direct-detection systems.

    Papers 3-9 come from universities that are renowned for research on optical transmission. In the third paper,“Spatial Mode Division Multiplexing for High-Speed Optical Coherent Detection Systems,”Professor William Shieh from the University of Melbourne proposes using spatial mode division multiplexing to increase transmission capacity. In the fourth paper,“Exploiting the Faster-Than-Nyquist Concept in Wavelength-Division Multiplexing Systems by Duobinary Shaping,”Dr. Jianqiang Li from Chalmers University of Technology presents a novel algorithm at the coherent receiver that is based on digital signal processing and is designed to tolerate strong filtering effects. In the fifth paper, “Super Receiver Design for Superchannel-Coherent Optical Systems,”Dr. Cheng Liu from Georgia Institute of Technology presents a novel super-receiver architecture for Nyquist-WDM superchannel coherent systems. This receiver detects and demodulates multiple WDM channels simultaneously and performs better than conventional coherent receivers in Nyquist-WDM systems. In the sixth paper,“Design of Silicon-Based High-Speed Plasmonic Modulator,”Professor Yikai Su from Shanghai Jiao Tong University proposes a silicon-based high-speed plasmonic modulator. This modulator is based on a double-layer structure with a 16 um long metal-dielectric-metal plasmonic waveguide at the upper layer and two silicon single-mode waveguides at the bottom layer. In the seventh paper,“Key Technology in Optical OFDM-PON,”Professor Xiangjun Xin from Beijing University of Posts and Telecommunications proposes a novel optical access network based on OFDM. In the eighth paper,“Compensation of Nonlinear Effects in Coherent Detection Optical Transmission Systems,”Professor Fan Zhang from Beijing University reviews two kinds of nonlinear compensation methods: digital backward propagation, and nonlinear electrical equalizer based on the time-domain Volterra series. The last paper comes from a telecommunication equipment manufacturer. In“Performance Assessment of 1 Tb/s Nyquist-WDMPM-RZ-QPSK Superchannel Transmission over 1000 km SMF-28 with MAP Equalization,”Dr. Ze Dong from ZTE (USA) evaluates the transmission performance of a 1 Tb/s (10 × 112 Gb/s) Nyquist-WDM PM-RZ-QPSK superchannel over a widely deployed SMF-28 fiber with and without MAP equalization.

    We thank all authors for their valuable contributions and all reviewers for their timely and constructive feedback on submitted papers. We hope the contents of this issue are informative and useful for all readers.
    High Spectral Efficiency 400G Transmission
    Xiang Zhou
    2012, 10(1):  3-9. 
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    This paper gives an overview of the generation and transmission of 450 Gb/s wavelength-division multiplexed (WDM) channels over the standard 50 GHz ITU-T grid at a net spectral efficiency (SE) of 8.4 b/s/Hz. The use of nearly ideal Nyquist pulse shaping, spectrally-efficient high-order modulation format, distributed Raman amplification, distributed compensation of ROADM filtering effects, coherent equalization, and high-coding gain forward error correction (FEC) code may enable future 400G systems to operate over the standard 50 GHz grid optical network.
    Greater than 200 Gb/s Transmission Using Direct-Detection Optical OFDM Superchannel
    Wei-Ren Peng, Itsuro Morita, Hidenori Takahashi, and Takehiro Tsuritani
    2012, 10(1):  10-17. 
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    In this paper, we propose direct-detection optical orthogonal frequency division multiplexing superchannel (DDO-OFDM-S) and optical multiband receiving method (OMBR) to support a greater than 200 Gb/s data rate and longer distance for direct-detection systems. For the new OMBR, we discuss the optimum carrier-to-sideband power ratio (CSPR) in the cases of back-to-back and post transmission. We derive the analytical form for CSPR and theoretically verify it. A low overhead training method for estimating I/Q imbalance is also introduced in order to improve performance and maintain high system throughput. The experiment results show that these proposals enable an unprecedented data rate of 214 Gb/s (190 Gb/s without overhead) per wavelength over an unprecedented distance of 720 km SSMF in greater than 100 Gb/s DDO-OFDM systems.
    Spatial Mode-Division Multiplexing for High-Speed Optical Coherent Detection Systems
    William Shieh, An Li, Abdullah Al Amin, Xi Chen, Simin Chen, and Guanjun Gao
    2012, 10(1):  18-22. 
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    Spatial mode-division multiplexing is emerging as a potential solution to further increasing optical fiber capacity and spectral efficiency. We report a dual-mode, dual-polarization transmission method based on mode-selective excitation and detection over a two-mode fiber. In particular, we present 107 Gbit/s coherent optical OFDM (CO-OFDM) transmission over a 4.5 km two-mode fiber using LP01 and LP11 modes in which mode separation is performed optically.
    Exploiting the Faster-Than-Nyquist Concept in Wavelength-Division Multiplexing Systems Using Duobinary Shaping
    Jianqiang Li, Ekawit Tipsuwannakul, Magnus Karlsson, and Peter A. Andrekson
    2012, 10(1):  23-29. 
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    This paper begins with Nyquist wavelength-division multiplexing (WDM) and then introduces faster-than-Nyquist. In faster-than-Nyquist, a certain amount of inter-symbol interference (ISI) is accepted, which violates the fundamental principle of Nyquist WDM. This results in much-relaxed transceiver bandwidth and simpler spectral design. However, in faster-than-Nyquist, implementation complexity is shifted from the transmitter side to the receiver side. Therefore, successful application of faster-than-Nyquist depends on innovation in the receiver structure. In this paper, we discuss the guidelines for implementing suboptimum, low-complexity receivers based on faster-than-Nyquist. We suggest that duobinary shaping is a good technique for trading off achievable spectral efficiency, detection performance, and implementation complexity and might be preferable to Nyquist WDM. Experiments are conducted to verify robustness of the proposed technique.
    Super-Receiver Design for Superchannel Coherent Optical Systems
    Cheng Liu, Jie Pan, Thomas Detwiler, Andrew Stark, Yu-Ting Hsueh, Gee-Kung Chang, and Stephen E. Ralph
    2012, 10(1):  30-33. 
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    In this paper, we propose a novel super-receiver architecture for Nyquist-wavelength-division-multiplexing (WDM) superchannel optical coherent systems. As opposed to a conventional coherent receiver, where each subchannel is demodulated independently, the proposed super-receiver jointly detects and demodulates multiple subchannels simultaneously. By taking advantage of information from side channels that use joint DSP to cancel interchannel interference (ICI), the proposed super-receiver performs much better than a conventional receiver. This architecture also has the potential to compensate for cross-channel impairments caused by linear and nonlinear effects. We examine the proposed architecture through experiment and simulation. OSNR is improved by more than 5 dB after 1280 km fiber transmission with narrow channel spacing.
    Design of a Silicon-Based High-Speed Plasmonic Modulator
    Mu Xu, Jiayang Wu, Tao Wang, and Yikai Su
    2012, 10(1):  34-39. 
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    In this paper, we propose a silicon-based high-speed plasmonic modulator. The modulator has a double-layer structure with a 16 μm long metal-dielectric-metal plasmonic waveguide at the upper layer and two silicon single-mode waveguides at the bottom layer. The upper-layer plasmonic waveguide acts as a phase shifter and has a dielectric slot that is 30 nm wide. Two taper structures that have gradually varied widths are introduced at the bottom layer to convert the photonic mode into plasmonic-slot mode with improved coupling efficiency. For a modulator with two 1 μm-long mode couplers, simulation shows that there is an insertion loss of less than 11 dB and a half-wave voltage of 3.65 V. The modulation bandwidth of the proposed modulator can be more than 100 GHz without the carrier effect being a limiting factor in silicon. The fabrication process is also discussed, and the proposed design is shown to be feasible with a hybrid of CMOS and polymer technology.
    The Key Technology in Optical OFDM-PON
    Xiangjun Xin
    2012, 10(1):  40-44. 
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    In this paper, a novel optical access network based on orthogonal frequency division multiplexing (OFDM) is proposed. An OFDM-based passive optical network (PON) uses multicarriers to carry different information that is transmitted to different optical network units (ONUs). In this paper, system performance is analyzed for OFDM-PON with different linewidths of the lightwave source, different optical signal-to-noise ratio (OSNR), different access distances, and different modulated formats. Colorlessness in the OFDM-PON is also analyzed. Finally, a 40 Gb/s baseband OFDM-PON with two carriers and achieve error-free performance over 25 km fiber transmission is proposed.
    Compensating for Nonlinear Effects in Coherent-Detection Optical Transmission Systems
    Fan Zhang
    2012, 10(1):  45-49. 
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    Fiber nonlinearity is one of the most important limiters of capacity in coherent optical communications. In this paper, we review two nonlinear compensation methods: digital backward propagation (BP) and nonlinear electrical equalizer (NLEE) based on the time-domain Volterra series. These compensation algorithms are implemented in a single-channel 50 Gb/s coherent optical single-carrier frequency-division multiplexed (CO-SCFDM) system transmitting over 10 × 80 km of standard single-mode fiber (SSMF).
    1 Tb/s Nyquist-WDM PM-RZ-QPSK Superchannel Transmission over 1000 km SMF-28 with MAP Equalization
    Ze Dong, Jianjun Yu, and Hung-Chang Chien
    2012, 10(1):  50-53. 
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    In this paper, we evaluate transmission in a 1 Tb/s (10 × 112 Gb/s) Nyquist-WDM PM-RZ-QPSK superchannel over a widely-deployed SMF-28 fiber with and without maximum a-posteriori (MAP) equalization. Over 1000 km can be reached with BER below the HD FEC limit and with a spectral efficiency of 4 b/s/Hz.
    Hardware Architecture of Polyphase Filter Banks Performing Embedded Resampling for Software-Defined Radio Front-Ends
    Mehmood Awan, Yannick Le Moullec, Peter Koch, and Fred Harris
    2012, 10(1):  54-62. 
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    In this paper, we describe resource-efficient hardware architectures for software-defined radio (SDR) front-ends. These architectures are made efficient by using a polyphase channelizer that performs arbitrary sample rate changes, frequency selection, and bandwidth control. We discuss area, time, and power optimization for field programmable gate array (FPGA) based architectures in anM -path polyphase filter bank with modifiedN -path polyphase filter. Such systems allow resampling by arbitrary ratios while simultaneously performing baseband aliasing from center frequencies at Nyquist zones that are not multiples of the output sample rate. A non-maximally decimated polyphase filter bank, where the number of data loads is not equal to the number ofM subfilters, processesM subfilters in a time period that is either less than or greater than theM data-load’s time period. We present a load-process architecture (LPA) and a runtime architecture (RA) (based on serial polyphase structure) which have different scheduling. In LPA,N subfilters are loaded, and thenM subfilters are processed at a clock rate that is a multiple of the input data rate. This is necessary to meet the output time constraint of the down-sampled data. In RA,M subfilters processes are efficiently scheduled withinN data-load time while simultaneously loadingN subfilters. This requires reduced clock rates compared with LPA, and potentially less power is consumed. A polyphase filter bank that uses different resampling factors for maximally decimated, under-decimated, over-decimated, and combined up- and down-sampled scenarios is used as a case study, and an analysis of area, time, and power for their FPGA architectures is given. For resource-optimized SDR front-ends, RA is superior for reducing operating clock rates and dynamic power consumption. RA is also superior for reducing area resources, except when indices are pre-stored in LUTs.
    A Histogram-Based Static Error Correction Technique for Flash ADCs: Implementation
    J Jacob Wikner, Armin Jalili, Sayed Masoud Sayedi, and Rasoul Dehghani
    2012, 10(1):  63-70. 
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    In this paper, we focus on practical issues in implementing a calibration technique for medium-resolution, high-speed flash analog-to-digital converters (ADCs). In [1], we theoretically describ the calibration technique and perform a behavioral-level simulation to test its functionality [1]. In this work, we discuss some issues in transistor-level implementation. The predominant factors that contribute to static errors such as reference generator mismatch and track-and-hold (T/H) gain error can be treated as input-referred offsets of each comparator. Using the proposed calibration technique, these errors can be calibrated with minimal detriment to the dynamic performance of the converter. We simulate a transistor-level implementation of a 5-bit, 1 GHz ADC in a 1.2 V, 65 nm CMOS process. The results show that DNL can be improved from 2.5 LSB to below 0.7 LSB after calibration, and INL can be improved from 1.6 LSB to below 0.6 LSB after calibration.