平流层多波长自由空间光通信系统

以近地面大气作为传输介质的大气激光通信(FSO)受地面情况和复杂的气象条件影响很大,传输的信号质量会受到很大干扰。本文介绍以底部平流层大气作为远距离激光通信信道选取平台的使用,以及多路多波长激光传输在此平台下的应用。这种技术可以增大传输距离并降低大气不稳定性对通信链路的影响。本文以模拟试验结果为例分析了系统效果和可行性,试验模拟了大气湍流效应对光路的影响。     

FSO通信系统接收性能的仿真研究

通过对自由空间光通信(FSO)系统组成原理的分析,利用OptiSystem光通信软件设计了FSO光传输链路仿真模型.在IM/DD调制方式下,结合眼图分析仪和误码率分析仪,对光链路接收性能进行分析,得到了很低的误码率,验证了无线光信号在较为恶劣天气下传输具有切实可行性.     

基于GPON业务的FSO传输系统

主要探讨了3G网络时代下基于GPON业务的无线激光传输设备(FSO)的经济性,比较分析了GPON 业务在FSO系统中传输的技术优势.重点描述了突发式通信机制如何联合FSO设备实现通信传输,并对其实现方案进行了阐述.     

基于FSO系统的多进制LT码性能分析

为克服空间光通信(FSO)链路受大气湍流效应的影响,将LT码应用到FSO系统中.多进制LT码具有较高的传输效率.提出一种新型度分布函数来改善多进制LT码的译码性能.计算不同进制LT码的译码失败率,并模拟强湍流信道,对采用不同编码方案的FSO性能进行仿真.结果表明:采用新型度分布的LT码能够降低译码失败率,且能有效改善FSO系统在强湍流情况下的性能.     

微角锥棱镜阵列在逆向调制激光通信中的应用

基于逆向调制反射器(MRR)的自由空间光(FSO)技术是将传统FSO链路中一个终端的激光发射器和跟瞄系统替换成MRR而构成的一个非对称的FSO系统。MRR主要由光调制器和无源逆向反射器构成。针对未来超高速信息传输与小型化链路的需求,研究使用微角锥棱镜阵列(MCCRAs)来替代MRR中的无源逆向反射器,研究MCCRAs的波前补偿特性在MRR FSO系统中的应用。对MCCRAs的波前补偿原理进行了理论分析,通过实验分析验证了MCCRAs的波前补偿特性。在同等实验条件下,实验结果表明,MCCRAs的波前补偿特性在抵抗大气湍流对激光传输造成的影响上有着良好的效果表现,可提升1～2个数量级的BER性能。对基于MCCRAs的MRR FSO系统进行整体评估,探讨国内MRR FSO技术面临的技术瓶颈并对未来工作提出进一步建议。     

短距离FSO系统研究

自由空间光通信(FSO)系统性能受激光产品的安全标准、激光的大气传输特性、激光波长及背景噪声等诸多因素限制，在分析计算上述因素影响的基础上给出了短距离FSO系统的一些设计原则和参数。     

大气湍流和指向误差下混合RF/FSO航空通信系统性能分析

基于解码-转发中继方式,研究了混合射频/自由空间光(RF/FSO)航空通信系统性能。采用副载波多进制相移键控调制方式,建立了孔径平均作用下混合RF/激光通信系统模型,其中FSO通信链路采用指数型Weibull大气湍流信道的同时考虑指向误差影响,射频通信链路为Nakagami-m衰落信道。利用Meijer′s G函数推导得到航空激光通信系统平均误码率和中断概率的闭合表达式。在不同湍流强度和接收孔径大小条件下,对比分析了中断概率及误码率性能。仿真结果表明,与弱湍流条件相比,中等湍流强度条件下指向误差对系统中断性能影响较小;孔径平均效应可有效改善混合RF/FSO传输系统的性能;FSO信道对混合系统性能起主要作用。     

同频干扰下混合MUD-RF/FSO航空中继通信系统性能分析

为了建立同频干扰条件下的混合射频/自由空间光(Radio Frequency/Free Space optical,RF/FSO)航空中继通信系统链路精确模型,提出多用户分集(Multi-User Diversity,MUD)的航空平台RF/FSO混合链路性能分析方法.以独立且同分布的Nakagami-m衰落信道表征射频(Radio Frequency,RF)用户信号和同频干扰(Co-Channel Interfer-ence,CCI)信号信道;构建了指数Weibull分布大气湍流与Rayleigh分布指向误差的FSO信道传输模型;在电光转换中继节点采用解码转发协议降低噪声累积.基于系统端到端等价信噪比累积分布函数,获得了RF/FSO混合链路中断概率及平均误码率表达式.研究结果表明,采用相干二进制相移键控调制时,系统误码性能最优;可通过增加用户数的方式来减弱干扰信号的影响.     

基于LDPC编码的双工逆向调制自由空间光通信系统

在逆向调制(modulating retro-reflector,MR R)自由空间光通信(free-space optical,FSO)系统中, 激光信号在传输过程中受到双程大气湍流的严重影响,导致系统误码率性能恶化。本文提出 采用低密度奇 偶校验编码(low density parity check,LDPC)方法来降低MRR FSO通信系统的误码率, 并且尝试使用微角 锥棱镜阵列(microcorner cube retroreflector arrays,MCCRA)来改善MRR FSO信道的传 输稳定性。本项研究 中搭建了MRR FSO通信实验平台,进行了基于LDPC编码的频分双工MRR FSO通信实验。实验结 果表明 使用LDPC编码的MRR FSO系统增益约为3dB,使用微角锥棱镜阵列作为MRR FSO反射器件能够 通过补 偿大气湍流引起的相位畸变来提升系统性能。结果还证明了LDPC编码相较于MCCRA,更能够 提升MRR FSO系统的误码率性能。由此可见,将LDPC编码方式与微角锥棱镜阵列结合运用于MRR FSO系 统能获得更加优化的系统性能。     

基于SC分集的混合FSO/RF航空通信系统性能分析

研究了基于选择合并(SC)分集的混合激光/射频(FSO/RF)航空通信系统性能,该系统设计简单且在发射端无需信道状态信息(CSI)。激光链路服从平均孔径效应下的Exponentiated Weibull大气湍流分布模型,射频采用Nakagami-m衰落信道模型,推导求出混合FSO/RF航空通信系统接收端信噪比的累积分布函数,进一步利用Meijer′s G函数推导了混合FSO/RF通信系统中断概率以及平均误码率的闭合表达式,通过仿真对比分析了不同湍流强度、调制方式及分集通信方案对系统中断、误码性能的影响。仿真结果表明,混合FSO/RF航空通信系统有效地利用了FSO及RF链路在不同大气信道条件下的互补性质,具有更优的中断和误码性能;采用BPSK调制能有效降低大气湍流对混合FSO/RF航空通信系统的影响;信噪比高时,并行传输、分集接收的通信方案对系统误码性能的改善要优于低阶调制对误码性能的改善。     

Experiment on Regional Broadband Network Using Free-Space-Optical Communication Systems

The transmission of a free-space optical (FSO) beam through the atmosphere is influenced by several factors: chief among them are weather conditions such as fog, rain, and snow. In the design of FSO communication systems, it is necessary to determine the weather dependence of the system and derive a relation between the weather parameters and performance of the FSO communication systems. From our experiments, we have established that the link-operating rate, which is an important parameter for overall FSO-communication-system performance, cannot be estimated using the cumulative-distribution probability of the visibility data. A technique of estimating short-term FSO-link performance characteristics is required. Using this approach, a method for approximating an FSO link-operating rate is proposed for the western Kanto area in Japan, where the link-performance deterioration is mostly a result of rain. The method is based on the analysis of short-term recorded rain intensity and visibility data. The estimated FSO link-operating rate is shown to have a good conformance with the experimental measured result.     

基于激光雷达观测数据的无线光通信系统性能研究

在分析大气衰减效应对无线光(FSO)通信性能影响 时,获取及时准确的能见度数据是关键。首先,分析了 目前获取能见度方法的不足之处,提出由激光雷达(LADAR)反演得到的数据可以更好地满足 FSO通信性能分析的 需求;其次,在介绍了利用LADAR观测数据反演水平能见度方法的基础上,以烟台1月18日典型天气为 例,获取了全天的能见度数据,并结合80%的相对湿度阈值对雾霾的转化过程进行分析 ;最后,结合 实测数据,通过设置适当的通信系统参数,对该日天气条件下的误码率(BE R)进行了实时分析,并得出雾比霾更 容易引起通信系统性能的降低。本文所得结论,将为建立实际的FSO通信链路提供 指导。     

Performance analysis of weather-dependent satellite–terrestrial network with rate adaptation hybrid free-space optical and radio frequency link

Due to the rapid development of satellite laser communication technology, free-space optical (FSO) links present a promising alternative to traditional radio frequency (RF) links. In this paper, taking the influence of weather factors into consideration, we investigate the performance of the hybrid FSO/RF links where the feeder link operates in the FSO band and the user link operates in the hybrid FSO/RF band. Specifically, the FSO feeder link is modeled by the gamma–gamma distribution in the presence of beam wander and pointing error, and the detection method adopts either the intensity modulation with direct intensity (IM/DD) or heterodyne detection. The RF user link is assumed to follow the shadowed Rician model. In addition, in order to improve the transmission rate of the link under the time-varying satellite–terrestrial channel, a rate adaptation scheme is proposed. The performance of the system under study is evaluated in terms of the outage probability, average bit error rate (BER), and average transmission rate. Our results provide some important insights, for example, (1) due to the constraints of the feeder link and weather factors, there is an upper limit on the outage performance and bit error rate of the hybrid link; (2) the adaptive transmission strategy can significantly improve the transmission rate of the link compared with traditional design.     

Aggregation defragmentation algorithm based on channel density and minimum gap in elastic optical networks

Free-space optics (FSO) is a data relaying technology, which requires a direct line of sight between the transmitter and the receiver units for reliable transmission. FSO communication links have many merits such as high modulation bandwidth, high data transmission rates, low cost, and easy installation process. The performance of FSO link is affected by certain external parameters such as absorption, scintillation, and atmospheric attenuation due to different weather conditions. This paper reports the designing and simulative comparison of two wavelength division multiplexing-based FSO links under rain and snow weather conditions. The proposed system reports successful transmission of \(32\times 10\) Gbps of data along a link distance of 16.5 and 1.07 km under rain and snow weather conditions, respectively, with acceptable performance levels (\(Q\sim \) 6 dB and \(\hbox {BER} \le 10^{-9}\)).     

基于双模turbo码的自由空间光通信系统特性分析

在自由空间光通信(FSO)系统中,大气信道的信噪比(SNR)特性对系统的通信性能影响很大,提出了双模turbo码的编码方案.从研究大气对自由空间光通信系统的影响出发,在分析了大气信道特性后建立了大气信道信噪比和能见度的关系,详细阐述了双模turbo码的编码、译码原理.应用建立的大气信道的信噪比关系,仿真分析了大气信道的信噪比和大气能见度的关系和两种编码条件下自由空间光通信系统的误码率(BER).结果表明,对于一个实用的无线光通信系统,在要求系统的误码率低于10-5时,基于双模turbo码的系统所要求的信道信噪比比基于普通turbo码的系统的要求低1 dB.因此在相同的信噪比条件下,相对于普通turbo码,双模turbo码能有效降低系统的误码率.     

Hybrid fiber-to-the-x and free space optics for high bandwidth access networks

The authors demonstrate an implementation of hybrid fiber-to-the-x (FTTx) and free space optical (FSO) technologies as another alternative solution to bring light-speed performance to the residential users. A simulation experiment was performed with a 20 km single-mode fiber spanning from the Central Office to 1:32 splitter, before the signal propagated through the air to FSO receivers which were individually equipped with an Optical Network Unit (ONU). A downstream performance of the video signal transmitted at 1550 nm wavelength with a bit rate of 1 Gbps was studied under different weather conditions. The results show that for an acceptable bit error rate of 10⁻⁹, 32 ONUs are supportable at a combined FSO: FTTx distance ratio of 20:4.8, 20:2, and 20:1.4 km during clear weather, average and heavy rain, respectively.     

Performance Characterization of Spatial Diversity Based Optical Wireless Communication over Atmospheric Turbulence Channels

Optical wireless communication is regarded as the next-generation high-speed technology. It has demonstrated its capability to deliver data faster than any other state-of-the-art wireless communication technique. This technology has drawn attention as a means of implementing reliable high capacity outdoor systems that cannot be implemented by conventional fiber optics. It has emerged recently as an efficient solution to match the larger bandwidth and high data rates requirement of the upcoming wireless communication systems. However, although FSO (free space optics) system has many appealing features, it has rather disappointing performance for long links due to the degrading effects of atmospheric turbulence-induced fading. In the presence of such type of performance impairments, the received signal exhibits random intensity fluctuations, which increase the BER (bit error-rate), where the severe weather conditions can have a detrimental impact on the performance, which may result in an inadequate availability. The MIMO wireless optical procedure, in which the spatial dimensions are used to improve the reliability and spectral efficiency of point-to-point links, provides a promising approach to mitigate turbulence effects due to its powerful performance enhancing capabilities.In this paper, we investigate a terrestrial atmospheric FSO communication system operating under the influence of strong atmospheric turbulences. Additionally, the MIMO technique with equal gain combining (EGC) is used in this work to enhance the data rate of the proposed system. Atmospheric turbulence impacts are modeled as a lognormal channel with due regard for geometric losses. With the use of NRZ line coding, an FSO highly sensitive receiver using either avalanche photodetector (APD) or PIN is designed and simulated for best system performance. The preference is achieved by using Bessel and Gaussian filters. It has been found that APD receiver using Gaussian filter is suitable for long-range links with APD gain value of 3. Also, the selection of APD gain is critical to the system performance. In addition, the optimal value of APD gain required for best system performance decreases as the size of the MIMO technique increases. The achievable performance improvements including received power levels, BER and Q-factor are also discussed. The results show that the system with optical amplifier at the transmitter gives an optimum performance. In addition the system performance is enhanced in most weather conditions by using an amplified 2×2 MIMO-FSO system with booster amplifier.     

Performance analysis of mode division multiplexing based free space optics system incorporating on–off keying and polarization shift keying under dynamic environmental conditions

In this paper, we propose a novel mode division multiplexing (MDM) based FSO transmission system incorporating polarization shift keying (PolSK) to enhance the information carrying capacity of the system. Using numerical simulations, we report the transmission of two independent 40 Gbps information signals using distinct Laguerre Gaussian modes up to an FSO transmission reach of 90 km under the influence of clear environmental conditions using the proposed system. Further, the influence of different environmental conditions such as rain, haze, and fog on the performance of the proposed link using bit error rate as performance metrics has also been investigated in this paper. Also, we report a comparative analysis of PolSK and on–off keying modulation formats in the proposed MDM-FSO link under the same environmental conditions. The simulation results show that under different weather conditions, PolSK based MDM-FSO transmission system demonstrates better performance.

     

一种大气激光通信信道测试系统的设计

介绍了一种大气激光通信信道测试系统的设计方案,并使用该系统在桂林进行了长期的大气信道测试。通过对测试数据的处理分析,给出了天气状况与FSO设备通信质量的关系。     

Adaptive hybrid free space optical/radio frequency communication system

Wireless communication has achieved lot of attention and the demand is continually increasing day by day. Radio frequency (RF) is highly attracted by various wireless communication applications. The RF spectrum is already very crowded and the rapid increase in the use of wireless services has led the problems of RF spectrum exhaustion and eventually RF spectrum deficit. Free space optical (FSO) communication is a viable technology with a plenty of bandwidth, license-free spectrum and interference free link. On the other hand, FSO channel is severely corrupted by atmospheric turbulence and non-predictive weather scenarios. We suggest a hybrid FSO/RF communication system in our previous research, which can mitigate the issues of the individual links. In this research, we investigate the performance of the proposed adaptive system for reliable data transmission. We develop modulation and power adaptive schemes for maximizing the mutual information. The proposed adaptive system is compared with non-adaptive system, which gives 2.75 dB gain for the joint power and 0.75 dB gain for the separate power constraint.