湍流信道無線光通信中的分集接收技術

柯熙政，劉妹

?

湍流信道無線光通信中的分集接收技術

柯熙政，劉妹

摘要：大氣分子的吸收、散射和大氣湍流等因素引起的光強閃爍嚴重影響無線光通信系統的性能，導致激光束能量衰減、信噪比下降，而分集接收技術能有效地克服這種影響。采用開關鍵控（OOK）調制，建立了強湍流模型-K分布模型下無線光通信空間接收分集系統模型，在不同信道參數和接收天線數下，分別對比分析了最大比合并（MRC）、等增益合并（EGC）和選擇合并（SC）的差錯性能。仿真結果表明，分集接收能在很大程度上改善大氣激光通信的性能，具有較強的抗大氣信道衰落能力。三種合并算法中，MRC性能最優，EGC次之，SC性能最差。

關鍵詞：大氣和海洋光學；無線光通信；強湍流信道；K分布模型；分集合并；差錯性能

Performance limits of nyquist-WDM and CO-OFDM in high-speed PM-QPSK systems

G.Bosco; A.Carena; V.Curri; P.Poggiolini; et al.

Abstract:In high-speed coherent optical orthogonal frequency division multiplexing (CO-OFDM) systems,wide range estimation of carrier frequency offset (CFO) and hardware-efficient symbol timing synchronization are essential.The CFO estimation using the Chinese remainder theorem (CRT) was recently proposed to provide wide range estimation.In this paper,the feasibility of the proposed CRT-based CFO estimation method is demonstrated in a real-time CO-OFDM system.We also propose two-step symbol timing synchronization with simplified timing metric for CO-OFDM systems.The real-time experimental results show that the CRT-based CFO algorithm can estimate over a wide range corresponding to the sampling frequency and hardware efficient accurate symbol timing synchronization is achieved by the proposed synchronization method. We experimentally demonstrate a record 400G optical wireless integration system simultaneously delivering 2×112 Gb/s two-channel polarization-division-multiplexing 16-ary quadrature amplitude modulation (PDM-16QAM) signal at 37.5 GHz wireless carrier and 2×108 Gb/s two-channel PDM quadrature phase shift keying (PDM-QPSK) signal at 100 GHz wireless carrier,adopting two millimeter-wave (mm-wave) frequency bands,two orthogonal antenna polarizations,multiple-input multiple-output (MIMO),photonic mm-wave generation and advanced digital signal processing (DSP).In the case of no fiber transmission,the bit error ratios (BERs) for both the 112 Gb/s PDM-16QAM signal after 1.5 m wireless delivery at 37.5 GHz and the 108 Gb/s PDM-QPSK signal after 0.7 m wireless delivery at 100 GHz are below the pre-forward-error-correction (pre-FEC) threshold of 3.8×10-3.To our knowledge,this is the first demonstration of a 400G optical wireless integration system in mm-wave frequency bands and also a capacity record of wireless delivery. We proposed and experimentally demonstrated a novel scheme to generate an optical millimeter-wave and realized wavelength reuse for uplink connection in a radio-over-fiber system.We employed an optical interleaver to separate the spectrum of the doublesideband signals generated by a single-arm intensity external modulator.The separated first-order sideband modes were used to generate optical millimeter-wave with double RF frequency,while the separated optical carrier was reused for uplink connection. We have experimentally demonstrated a novel wavelength reuse scheme for up-link connection in a radio-over-fiber (ROF) system.The dual-mode lightwave generated by optical carrier suppression modulation is separated by an optical interleaver.One of the dual-mode lightwaves is modulated with a baseband data at 2.5 Gb/s.The other lightwave will be used for the optical carrier for the up-link data modulation.There is no additional laser source for the upstream data generation in the base station.By this method,we simplify the configuration of the ROF system and reduce the cost of the system. Optical interconnects are becoming attractive alternatives to electrical wiring in intra-and interchip communication links.Particularly,the integration with silicon complementary metal-oxide semiconductor (CMOS) technology has received considerable interest because of the ability of cost-effective integration of electronics and optics on a single chip(1).Although silicon enables the realization of optical waveguides(2)and passive components(3),the integration of another,optically absorbing,material is required for photodetection.Traditionally,germanium(4)or compound semiconductors(5)are used for this purpose; however,their integration with silicon technology faces major challenges.Recently,graphene(6)emerged as a viable alternative for optoelectronic applications(7),including photodetection(8).Here,we demonstrate an ultra-wideband CMOS-compatible photodetector based on graphene.We achieved a multigigahertz operation over all fibre-optic telecommunication bands beyond the wavelength range of strained germanium photodetectors(9),the responsivity of which is limited by their bandgap.Our work complements the recent demonstration of a CMOS-integrated graphene electro-optical modulator(10),and paves the way for carbon-based optical interconnects. We report the first experimental realization and detailed characterization of thulium doped fiber amplifiers (TDFAs) specifically designed for optical communications providing high gain (>35 dB),noise figure as low as 5 dB,and over 100 nm wide bandwidth around 2 mu m.A maximum saturated output power of 1.2 W was achieved with a slope efficiency of 50%.The gain dynamics of the amplifier were also examined.Our results show that TDFAs are well qualified as high performance amplifiers for possible future telecommunication networks operating around 2 μm. In this paper,we compare the performance of multiple-input-multiple-output (MIMO) techniques applied to indoor optical wireless communications (OWC) assuming line-of-sight (LOS) channel conditions.Specifically,several 4×4 setups with different transmitter spacings and different positions of the receiver array are considered.The following MIMO algorithms are considered:Repetition Coding (RC),Spatial Multiplexing (SMP) and Spatial Modulation (SM).Particularly,we develop a framework to analyticallybook=26,ebook=30approximate the bit error ratios (BERs) of these schemes and verify the theoretical bounds by simulations.The results show that due to diversity gains,RC is robust to various transmitter-receiver alignments.However,as RC does not provide spatial multiplexing gains,it requires large signal constellation sizes to enable high spectral efficiencies.In contrast,SMP enables high data rates by exploiting multiplexing gains.In order to provide these gains,SMP needs sufficiently low channel correlation.SM is a combined MIMO and digital modulation technique.We show that SM is more robust to high channel correlation compared to SMP,while enabling larger spectral efficiency compared to RC.Moreover,we investigate the effect of induced power imbalance between the multiple transmitters.It is found that power imbalance can substantially improve the performance of both SMP and SM as it reduces channel correlation.In this context,we also show that blocking some of the links is an acceptable method to reduce channel correlation.Even though the blocking diminishes the received energy,it outweighs this degradation by providing improved channel conditions for SMP and SM.For example,blocking 4 of the 16 links of the 4×4 setup improves the BER performance of SMP by more than 20 dB,while the effective signal to noise ratio (SNR) is reduced by about 2 dB due to the blocking.Therefore,MIMO techniques can provide gains even under LOS conditions which provide only little channel differences. Relay-assisted free-space optical (FSO) transmission exploits the fact that atmospheric turbulence fading variance is distance dependent and yields significant performance gains by taking advantage of the resulting shorter hops.In this paper,we investigate how to determine optimal relay locations in serial and parallel FSO relaying so as to minimize the outage probability and quantify performance improvements obtained through optimal relay placement.We further present a diversity gain analysis for serial and parallel FSO relaying schemes and quantify their diversity advantages in terms of the number of relays and channel parameters. Carrier-less amplitude and phase (CAP) modulation can be a good candidate for short range optical communications for considerable computational complexity reduction and simple system structure.In this paper,a detailed investigation on the digital filters in CAP modulation system is presented.An adaptive equalizer based on cascaded multi-modulus algorithm (CMMA) is used for the demodulation at the receiver.The impact of digital filter taps on system performance is investigated through comprehensive simulations and a 10 Gb/s CAP16 modulation system is demonstrated experimentally.The BER performance for different length of fiber link is measured.Compared with back-to-back (BTB) transmissions,2 dB and 3.5 dB receiver power penalty are observed at BER of 10-3for 20 km and 40 km fiber link respectively.It clearly demonstrates the feasibility of the CAP16 modulation for the short range transmission systems. Space-division multiplexing (SDM) uses multiplicity of space channels to increase capacity for optical communication.It is applicable for optical communication in both free space and guided waves.This paper focuses on SDM for fiber-optic communication using few-mode fibers or multimode fibers,in particular on the critical challenge of mode crosstalk.Multiple-input-multiple-output (MIMO) equalization methods developed for wireless communication can be applied as an electronic method to equalize mode crosstalk.Optical approaches,including differential modal group delay management,strong mode coupling,and multicore fibers,are necessary to bring the computational complexity for MIMO mode crosstalk equalization to practical levels.Progress in passive devices,such as (de) multiplexers,and active devices,such as amplifiers and switches,which are considered straightforward challenges in comparison with mode crosstalk,are reviewed.Finally,we present the prospects for SDM in optical transmission and networking. Optical wireless communication (OWC) refers to transmission in unguided propagation media through the use of optical carriers,i.e.,visible,infrared (IR),and ultraviolet (UV) bands.In this survey,we focus on outdoor terrestrial OWC links which operate in near IR band.These are widely referred to as free space optical (FSO) communication in the literature.FSO systems are used for high rate communication between two fixed points over distances up to several kilometers.In comparison to radio-frequency (RF) counterparts,FSO links have a very high optical bandwidth available,allowing much higher data rates.They are appealing for a wide range of applications such as metropolitan area network (MAN) extension,local area network (LAN)-to-LAN connectivity,fiber back-up,backhaul for wireless cellular networks,disaster recovery,high definition TV and medical image/video transmission,wireless video surveillance/monitoring,and quantum key distribution among others.Despite the major advantages of FSO technology and variety of its application areas,its widespread use has been hampered by its rather disappointing link reliability particularly in long ranges due to atmospheric turbulence-induced fading and sensitivity to weather conditions.In the last five years or so,there has been a surge of interest in FSO research to address these major technical challenges.Several innovative physical layer concepts,originally introduced in the context of RF systems,such as multiple-input multiple-output communication,cooperative diversity,and adaptive transmission have been recently explored for the design of next generation FSO systems.In this paper,we present an up-to-date survey on FSO communication systems.The first part describes FSO channel models and transmitter/receiver structures.In the second part,we provide details on information theoretical limits of FSO channels and algorithmic-level system design research activities to approach these limits.Specific topics include advances in modulation,channel coding,spatial/cooperative diversity techniques,adaptive transmission,and hybrid RF/FSO systems. This paper investigates the use of a pilot signal in reducing the electrical peak-to-average power ratio (PAPR) of an orthogonal frequency division multiplexing (OFDM) intensitymodulated optical wireless communication system.The phase of the pilot signal is chosen based on the selected mapping (SLM) algorithm while the maximum likelihood criterion is used to estimate the pilot signal at the receiver.Bit error rate (BER) performance of the pilot-assisted optical OFDM system is identical to that of the basic optical OFDM (with no pilot and no PAPR reduction technique implemented) at the desired BER of less than 10-3needed to establish a reliable communication link.The pilot-assisted PAPR reduction technique results in higher reduction in PAPR for high order (M > 4) constellations than the classical SLM.With respect to a basic OFDM system,with no pilot and no PAPR reduction technique implemented,a pilot-assisted M-QAM optical OFDM system is capable of reducing the electrical PAPR by over about 2.5 dB at a modest complementary cumulative distribution function (CCDF) point of 10-4for M = 64.Greater reductions in PAPR are possible at lower values of CCDF with no degradation to the system’s error performance.Clipping the time domain signal at both ends mildly (at 25 times the signal variance level) results in a PAPR reduction of about 6.3 dB at the same CCDF of 10-4but with an error floor of about 3 × 10-5.Although it is possible to attain any desired level of electrical PAPR reduction with signal clipping,this will be at a cost of deterioration in the systems's bit error performance. This paper proposes and analyzes the performance of the multihop free-space optical (FSO) communication links using a heterodyne differential phase-shift keying modulation scheme operating over a turbulence induced fading channel.A novel statistical fading channel model for multihop FSO systems using channel-state-information-assisted and fixed-gain relays is developed incorporating the atmospheric turbulence,pointing errors,and path-loss effects.The closed-form expressions for the moment generating function,probabilitybook=28,ebook=32density function,and cumulative distribution function of the multihop FSO channel are derived using Meijer’s G-function.They are then used to derive the fundamental limits of the outage probability and average symbol error rate.Results confirm the performance loss as a function of the number of hops.Effects of the turbulence strength varying from weak-to-moderate and moderate-to-strong turbulence,geometric loss,and pointing errors are studied.The pointing errors can be mitigated by widening the beam at the expense of the received power level,whereas narrowing the beam can reduce the geometric loss at the cost of increased misalignment effects. In this letter,we experimentally demonstrated a high level quadrature amplitude modulation (QAM) optical orthogonal frequency division multiplexing (OFDM) transmission system utilizing a cost-effective directly modulated laser.This experiment was enabled by intrasymbol frequency-domain averaging-based channel estimation,large fast Fourier transform size to enhance the resistance of intersymbol interference,and the discrete Fourier transform-spread technique to reduce peak-to-average power ratio.A 31.7 Gbit/s 2048QAM OFDM signal can be successfully transmitted over 20 km large effective area fiber under a 20% soft-decision forward-errorcorrection threshold (bit error rate of 2.4 × 10-2). In this letter,a generalized nonlinear Schrodinger-Maxwell-Bloch system is investigated,which can be used to describe the solitons in optical fibers.By virtue of the generalized Darboux transformation,higher-order rogue-wave solutions are derived.Rogue-wave propagation and interaction are analyzed:(1) Complex envelope of the field,q,appears as a bright rogue wave,the measure of the polarization of the resonant medium,p,is a bright-dark rogue wave while the extant of the population inversion,eta,is a dark rogue wave; (2) Group velocity inhomogeneity and the linear and Kerr nonlinearity inhomogeneity affect q,p and eta more than the other parameters do; (3) Character of the interaction between the propagating field and erbium atoms,the gain or loss term and the linear and Kerr nonlinearity inhomogeneous parameter affect the interaction range of the second-order rogue waves. Orbital angular momentum (OAM),which describes the “phase twist” (helical phase pattern) of light beams,has recently gained interest due to its potential applications in many diverse areas.Particularly promising is the use of OAM for optical communications since:(i) coaxially propagating OAM beams with different azimuthal OAM states are mutually orthogonal,(ii) inter-beam crosstalk can be minimized,and (iii) the beams can be efficiently multiplexed and demultiplexed.As a result,multiple OAM states could be used as different carriers for multiplexing and transmitting multiple data streams,thereby potentially increasing the system capacity.In this paper,we review recent progress in OAM beam generation/detection,multiplexing/demultiplexing,and its potential applications in different scenarios including free-space optical communications,fiber-optic communications,and RF communications.Technical challenges and perspectives of OAM beams are also discussed. The prospects for using fiber optical parametric amplifiers (OPAs) in optical communication systems are reviewed.Phase-book=29,ebook=33insensitive amplifiers (PIAs) and phase-sensitive amplifiers (PSAs) are considered.Low-penalty amplification at/or near 1 Tb/s has been achieved,for both wavelength and time-division multiplexed formats.High-quality mid-span spectral inversion has been demonstrated at 0.64 Tb/s,avoiding electronic dispersion compensation.All-optical amplitude regeneration of amplitude-modulated signals has been performed,while PSAs have been used to demonstrate phase regeneration of phase-modulated signals.A PSA with 1.1 dB noise figure has been demonstrated,and preliminary wavelength-division multiplexing experiments have been performed with PSAs.512 Gb/s have been transmitted over 6000 km by periodic phase conjugation.Simulations indicate that PIAs could reach data rate x reach products in excess of 14000 Tb/s × km in realistic wavelength-division multiplexed long-haul networks.Technical challenges remaining to be addressed in order for fiber OPAs to become useful for long-haul communication networks are discussed. A new hybrid modulation scheme (PDM-QPSK-MPPM) for free space optical (FSO) communication is proposed in this paper,which is based on the combination of polarization-division-multiplexed quadrature phase-shift keying (PDM-QPSK) and multi-pulse pulse position modulation (MPPM).We describe the transmitter and receiver block diagram and the decoding scheme of proposed scheme,and derive the Formulary expressions for symbol-error rate (SER) and upper bound of bit-error rate (BER) in Gamma-Gamma turbulence channel.Performance evaluations and comparisons are implemented through detailed analysis.The numerical results indicate that,under turbulence channel with average power constraint,the proposed hybrid PDM-QPSK-MPPM always offer better BER performance and SER performance than ordinary BPSK and MPPM.In addition,the proposed scheme offers higher bandwidth-utilization efficiency than conventional modulation formats. In free space optical communication,photodetectors serve not only as communications receivers but also as position sensitive detectors (PSDs) for pointing,tracking,and stabilization.Typically,two separate detectors are utilized to perform these tasks,but recent advances in the fabrication and development of large-area,low-noise avalanche photodiode (APD) arrays have enabled these devices to be used both as PSDs and as communications receivers.This combined functionality allows for more flexibility and simplicity in optical system design without sacrificing the sensitivity and bandwidth performance of smaller,single-element data receivers.This work presents the development of APD arrays rated for bandwidths beyond 1 GHz with measured carrier ionization ratios of approximately 0.2 at moderate APD gains.We discuss the fabrication and characterization of three types of APD arrays along with their performance as high-speed photodetectors.

來源出版物：Journal of Lightwave Technology,2015,33(11):2294-2302

A 400G optical wireless integration delivery system

Xinying Li; Jianjun Yu; Junwen Zhang; et al.

來源出版物：Optical Express,2013,21(16):18812-18819

A radio-over-fiber system with a novel scheme for millimeter-wave generation and wavelength reuse for up-link connection

Lin Chen; Hong Wen; Shuangchun Wen

來源出版物：Photonics Technology Letters,2006,18(19):2056-2058

A novel radio-over-fiber system with wavelength reuse for upstream data connection

Lin Chen; Yufeng Shao; Xiaoyan Lei; et al.

來源出版物：Photonics Technology Letters,2007,19(6):387-389

CMOS-compatible graphene photodetector covering all optical communication bands

Pospischil,Andreas; Humer,Markus; Furchi,Marco M; et al.

來源出版物：Nature Photonics,2013,7(11):892-896 聯系郵箱：Pospischil,Andreas; thomas.mueller@tuwien.ac.at

Thulium-doped fiber amplifier for optical communications at 2 μm

Li,Z.; Heidt,A.M.; Daniel,J.M.O.; et al.

來源出版物：Optics Express,2013,21(8):9289-9297 聯系郵箱：Li,Z.; zl3g10@orc.soton.ac.uk

Performance comparison of MIMO techniques for optical wireless communications in indoor environments

Fath,Thilo; Haas,Harald

Keywords:MIMO; optical wireless communications; repetition coding; spatial modulation; spatial multiplexing MIMO; optical wireless communications; repetition coding; spatial modulation; spatial multiplexing Free-space optical (FSO) communication; optical wireless communication (OWC); channel modeling; optical modulation; spatial diversity; channel capacity; channel coding; hybrid RF/FSO systems; relay-assisted networks; adaptive transmission Average optical power reduction; optical communications; orthogonal frequency division multiplexing (OFDM); optical wireless; PAPR reduction; pilot-assisted modulation atmospheric turbulence (AT); average symbol error rate (ASER); heterodyne detection; multihop free-space optical (FSO); outage probability (OP); pointing errors optical orthogonal frequency division multiplexing (OFDM); intra-symbol frequency-domain averaging (ISFA); discrete Fourier transform-spread (DFT-spread); large fast fourier transform (FFT) size rogue-wave solutions; generalized darboux transformation; generalized nonlinear Schrodinger-Maxwell-Bloch system; opticalfiber communication parametric amplifiers; fiber optic communication; fiber nonlinearities; phase-sensitive amplification; signal regeneration; phase regeneration; mid-span spectral inversion free space optical communication; bit-error rate; symbol-error rate; PDM-QPSK-MPPM; Gamma-Gamma turbulence channel

來源出版物：IEEE Transactions on Communications,2013,61(2):733-742 聯系郵箱：Fath,Thilo; thilo.fath@ed.ac.uk

Optimal relay placement and diversity analysis of relay-assisted free-space optical communication systems

Kashani,Mohammadreza A.; Safari,Majid; Uysal,Murat

來源出版物：Journal of Optical Communications and Networking,2013,5(1):37-47

Experimental demonstration of 10 Gb/s multi-level carrier-less amplitude and phase modulation for short range optical communication systems

Tao,Li; Wang,Yiguang; Gao,Yuliang; et al.

來源出版物：Optics Express,2013,21(5):6459-6465

Space-division multiplexing:The next frontier in optical communication

Li,Guifang; Bai,Neng; Zhao,Ningbo; et al.

來源出版物：Advances in Optics and Photonics,2014,6(4):413-487 聯系郵箱：Zhao,Ningbo; nbzhao@tju.edu.cn

Survey on free space optical communication:A communication theory perspective

Khalighi,Mohammad Ali; Uysal,Murat

來源出版物：IEEE Communications Surveys and Tutorials,2014,16(4):2231-2258

Pilot-Assisted PAPR reduction technique for optical OFDM communication systems

Popoola,Wasiu O.; Ghassemlooy,Zabih; Stewart,Brian G

來源出版物：Journal of Lightwave Technology,2014,32(7):1374-1382

Multihop free-space optical communications over turbulence channels with pointing errors using heterodyne detection

Tang,Xuan; Wang,Zhaocheng; Xu,Zhengyuan; et al.

來源出版物：Journal of Lightwave Technology,2014,32(15):2597-2604 聯系郵箱：Tang,Xuan; xtang2012@gmail.com

High-level QAM OFDM system using DML for low-cost short reach optical communications

Li,Fan; Li,Xinying; Chen,Lin

來源出版物：IEEE Photonics Technology Letters,2014,26(9):941-944 聯系郵箱：Li,Fan; fanli0809@gmail.com

Rogue waves for the generalized nonlinear Schrodinger-Maxwell-Bloch system in optical-fiber communication

Zuo,Da-Wei; Gao,Yi-Tian; Xue,Long

來源出版物：APPLIED Mathematics Letters,2015,40:78-83 聯系郵箱：Gao,Yi-Tian; gaoyt163@163.com

Optical communications using orbital angular momentum beams

Willner,A.E; Huang,H.; Yan,Y

來源出版物：Advances in Optics and Photonics,2015,7(1):66-106

Fiber optical parametric amplifiers in optical communication systems

Marhic,Michel E.; Andrekson,Peter A.; Petropoulos,Periklis; et al.

來源出版物：Laser & Photonics Reviews,2015,9(1):50-74

Hybrid polarization-division-multiplexed quadrature phase-shift keying and multi-pulse pulse position modulation for free space optical communication

Shi,Wenxiao; Wu,Pengxia; Liu,Wei

來源出版物：Optics Communications,2015,334:63-73

InAlAs/InGaAs avalanche photodiode arrays for free space optical communication

Ferraro,Mike S.; Clark,William R.; Rabinovich,William S.; et al.

來源出版物：Applied Optics,2015,54(31):182-188 聯系郵箱：Ferraro,Mike S.; mike.ferraro@nrl.navy.mil

編輯：衛夏雯

領跑者5000論文

材料科學綜合

來源出版物：光學學報,2015,35(1):01060005 聯系郵箱：柯熙政，xzke@263.net