Laser satellite communication network-vibration effect and possiblesolutions

A number of serious consortiums develop satellite communication networks. The objective of these communication projects is to service personal communication users almost everywhere on Earth. The intersatellite links in those projects use microwave radiation as the carrier. Free-space optical communication between satellites networked together can make possible high-speed communication between different places on Earth. Some advantages of an optical communication system over a microwave communication system in free space are: (1) smaller size and weight, (2) less transmitter power, (3) larger bandwidth, and (4) higher immunity to interference. The pointing from one satellite to another is a complicated problem due to the large distance between the satellite, the narrow beam divergence angle, and vibration of the pointing system. Such vibration of the transmitted beam in the receiver plane decreases the average received signal, which increases the bit error rate. We review: (1) the present status of satellite networks, (2) developing efforts of optical satellite communication around the world, (3) performance results of vibration effects on different kinds of optical communication satellite networks, and (4) seven approaches to overcome the problems caused by transmitter pointing vibration     

自由空间光通信中的振动抑制研究

卫星联网可以实现高速率的自由空间光通信;;但是由于卫星间的长距离、窄光束的发散角和瞄准系统的抖动使得从一颗卫星到另一颗卫星的瞄准变成很复杂的问题像这样接受平面内发射的光束的抖动会降低平均接收功率增加误码率针对上述问题;;介绍了7种抑振方法     

Performance limitation of laser satellite communication due to vibrations and atmospheric turbulence: down‐link scenario

In this paper, we analyse the effects of vibrations and the atmosphere on the performance of a broadband laser inter‐satellite link (BLISL) which was studied within the framework of the BLISL joint Israeli–German applied research project. The use of optical radiation as a carrier between satellites and in satellite‐to‐ground links enables transmission using very narrow beam divergence angles. Due to the narrow beam divergence angle and the large distance between the satellite and the ground station or airplane the pointing is a complicated process. Further complication results from vibration of the pointing system caused by two fundamental mechanisms of a stochastic nature: (1) tracking noise created by the electro‐optic tracker and (2) vibrations caused by internal satellite mechanical mechanisms. Additionally an inhomogeneity in the temperature and pressure of the atmosphere leads to variations of the refractive index along the transmission path. These variations of refractive index as well as pointing vibrations can cause fluctuations in the intensity and the phase of the received signal leading to an increase in link error probability. In this paper, we develop a bit error probability (BEP) model that takes into account both pointing vibrations and turbulence‐induced log amplitude fluctuations (i.e. signal intensity fading) in a regime in which the receiver aperture D₀ is smaller than the turbulence coherence diameter d₀. Our results indicate that BLISL can achieve a BEP of 10^?9 and data rate of 1Gbps with normalized pointing vibration of G_T*σ_θ²=0.05 and turbulence of σ_X=0.3. Copyright © 2003 John Wiley & Sons, Ltd.     

The SILEX optical interorbit link experiment

The European Space Agency (ESA) has instigated an in-orbit demonstration project known as SILEX (semiconductor laser intersatellite link experiment) using a pre-operational link between the French SPOT-4 low earth orbit satellite and the ESA Advanced Relay and Technology Mission Satellite (ARTEMIS). Such an optical communication system brings some new and challenging requirements. In particular, the communication beam, which has a divergence of around 6 μrad, must achieve and maintain a very demanding pointing accuracy in the presence of both host satellite vibration and relative satellite motion. The authors provide an overview of the SILEX project and describe a novel pointing mechanism     

ATP跟瞄精度与最佳信号光发射角的研究

激光星间通信中捕获、跟踪、瞄准(ATP)技术是保障通信正常进行的关键一环，在完成捕获进入跟踪状态下，ATP的跟瞄精度是一个重要指标。然而．在实际系统中，ATP的跟瞄精度受到很多因素的制约．只能达到一定的精度。在这种条件下．信号光束发散角的选取对通信系统的整体性能影响很大。提出了影响光束发散角的三种因素，并进一步分析了在给定跟瞄精度的条件下．信号光束的发散角并不是越小越好而是存在一个最佳发散角．该发散角可以在满足通信系统指标的情况下．对发射端的功率需求最小。     

一种自由空间光通信光路的设计与分析

提出了一种用于自由空间光通信的光学系统的设计方法,在这种设计方法中通过卡塞格伦天线实现对激光束的准直,通过偏振分光镜和γ/2波片的巧妙使用来减少杂散光和反射光对信号光的干扰,通过对光学系统的自动跟踪控制消除了通信终端的失准问题。分析和实验表明,基于该光学系统的通信终端激光发散角达到0.4 mrad,对准误差小于6μrad,可实现2 km的自由空间光通信。     

无人机空中OFDM光通信链路性能研究

基于无人机空中光正交频分复用(OFDM)通信模型链路的误码性能理论和仿真研究,综合考虑服从对数正态分布的大气湍流模型和服从瑞利分布的指向误差模型对通信系统的影响,采用高斯-厄米特积分方法得出了通信系统链路误码率的闭合表达式。在此基础上,仿真分析了发射端光束发散角、符号速率、子载波数和接收孔径大小与系统误码率的关系。仿真结果表明,在弱湍流条件下,发射端激光存在可使链路误码性能达到最优的光束发射角,湍流强度的改变对最优光束发散角的影响不大。光OFDM系统能够满足无人机间通信速率为吉比特级的光通信要求,并且通信时接收端的孔径尺寸不宜增大,该结果对通信链路的优化设计有一定的参考意义。     

对准误差对前置光放大卫星激光通信系统性能的影响

发射机及接收机的对准误差都会引起前置光放大卫星激光通信系统信号的衰落,在同时考虑发射机、接收机对准误差的条件下优化系统性能非常重要。将接收机对准误差引起的空间光耦合损耗用一个高斯函数近似,并同时考虑发射机对准误差引起的对准损耗,推导出了接收光功率概率密度的近似解析表达式,应用该概率密度函数,建立了基于平均误码率原则的前置光放大卫星激光通信系统的优化模型。仿真结果表明,在给定平均误码率要求及对准误差一定时,存在一个最佳发射光束宽度、接收天线直径及空间光耦合参数,使所需的发射功率最小,采用更大的接收天线并不能降低对发射功率的要求。     

振动对空间光-光纤耦合效率影响及补偿实验研究

利用现有成熟光纤通信技术是目前卫星光通信技术的重要发展方向,但需首先解决空间光-光纤耦合这一关键问题。文中对卫星平台振动对空间光-光纤耦合效率的影响进行了理论分析;在此基础上针对卫星光通信终端,建立以CCD 为探测器,高速偏转镜为补偿器件的卫星平台微振补偿系统,并进行了实验研究。实验结果表明:所采用的基于反馈控制技术的主动补偿系统能有效地抑制低频振动。在振动频率介于1～50 Hz,振动幅度介于75～300 rad 之间时,补偿系统对耦合效率的最大改善率可达54.73%,验证了补偿方案的可行性及有效性,为在卫星光通信系统中应用各种光纤通信技术打下基础。     

一种新型实用的天线控制器的设计

随着卫星通讯技术的不断发展,卫星的数量不断增加,对卫星天线伺服设备的要求也越来越高,为了保证系统的不间断通讯,同时避免对空间微波环境产生不必要的干扰,高可靠的天线控制器起到了重要的作用。特别是大口径的卫星天线,由于其波束宽度窄,对天线的角度控制要求更高。介绍了一种基于系统级单片机的实用型大口径卫星通讯地面站天线控制器的基本设计,阐述了该系统的设计特点,并对主要部分的软硬件实现方式进行了说明。     

Stepped-reflector antenna for dual-band multiple beam Satellite communications payloads

A novel stepped-reflector antenna (SRA) suitable for dual-band multiple beam satellite payloads is introduced in this paper. The SRA system produces "flat-top" radiation patterns for receive beams and highly efficient Gaussian patterns for transmit beams over a geographic coverage region as seen by the geo-synchronous satellite. It combines the reflector improvements through the use of SRA with the feed horn advancements through the use of dual-band "high-efficiency horns" in order to realize an efficient multiple beam antenna (MBA) system supporting both downlink transmit and uplink receive signals of communication satellites. It is shown that the SRA provides a congruent set of spot beams on ground for both transmit and receive frequencies with the benefits of significantly improved edge-of-coverage gain, improved co-polar isolation among beams that re-use the same frequency channels, receive beam patterns that are less sensitive to satellite pointing error, and reduced number of reflector antennas when compared to conventional MBAs.     

星间光通信振动对系统性能的影响——直接检测PPM调制方式

星间光通信中振动是影响系统性能的重要因素,即使其中一个卫星的瞄准系统的很小幅度的振动也可以引起整个网络的性能急剧恶化。分析了直接检测脉位调制(PPM)下振动对网络性能的影响。     

星间光通信振动对系统性能的影响--直接检测PPM调制方式

星间光通信中振动是影响系统性能的重要因素，即使其中一个卫星的瞄准系统的很小幅度的振动也可以引起整个网络的性能急剧恶化。分析了直接检测脉位调制（PPM）下振动对网络性能的影响。     

Pointing, acquisition and tracking for optical space communications

Free-space links between satellites provide a means of introducing increased connectivity into a satellite system. This can result in enhanced traffic capacity and extended coverage for telecommunication systems and real-time relay of instrument data to the ground for Earth observation missions. Optical links offer the potential advantages over microwave links of lower mass and power and reduced size so that they can be accommodated more easily on the satellite. However the narrow optical beamwidths involved result in stringent pointing, acquisition and tracking (PAT) requirements. This paper describes a concept for a compact PAT system which employs a passive antivibration mount to filter out high-frequency disturbances from the satellite. This is under development as part of the ESA-funded Small Optical User Terminal (SOUT) programme. The system concept, hardware implementation, and results of breadboarding critical items are presented     

Optimization of urban optical wireless communication systems

Urban optical wireless communication systems are considered a "last mile" technology. An optical wireless communication system uses the atmosphere as a propagation medium. In order to provide line-of-sight (LOS), the transceivers are placed on high-rise buildings. However, dynamic wind loads, thermal expansion, and weak earthquakes cause buildings to sway. These sways require the designer to increase the transmitter beam divergence angle so as to maintain LOS between the transmitter and the receiver. It is clear that an overly wide divergence angle increases the required laser power, and, as a result, terminal cost and complexity increase. On the other hand, an overly narrow beam divergence angle may result in cutoff in communication when there is building sway. In this paper, we derive a mathematical model to minimize transmitter power and optimize transmitter gain (divergence angle) as a function of the building-sway statistics, the communication system parameters, and the required bit-error probability (BEP). Reduction in laser power could improve overall system performances and cost. For example, for BEP of 10/sup -9/, we can attain at least a 4-dB reduction of the required transmitter power in comparison to a system with both half and twice the optimum beam divergence angle.     

Closer spacing of geostationary satellites through adaptive nulling at the ground terminal

The results of a preliminary study suggest that interference to the downlink of one geostationary satellite from that of another satellite in the adjacent orbital slot can be substantially reduced by the addition of a simple adaptive nulling system to the ground terminal antenna. Alternatively, the nulling system allows the satellites to be spaced significantly closer together with no increase in interference. Significant improvement is obtained with simple open-loop antenna pointing. The improvement is greater, however, if the adaptive antenna automatically tracks the direction of the desired source. For a dish antenna with a single receiving feed horn, the extra hardware required to provide the adaptive capability consists of two more receiving feed horns, a variable weighting device for each of the feed lines (now three in number) and the adaptive control electronics. An example shows that the minimum spacing between one satellite and an interfering satellite, consistent with certain assumed operating requirements, could be reduced from the2.25degneeded for a simple single beam antenna to1.35deg, for the adaptive antenna using open loop pointing, to as little as0.7deg, for the adaptive antenna using automatic pointing. The adaptive hulling also provides essentially the same protection against downlink interference from the two other satellites occupying the slots on both sides of the satellite serving the terminal. The three degrees of freedom of the postulated nulling system, however, cannot cope with three or more nearby interfering satellites.     

星间光通信链路优化设计

建立了一种空间光通信链路模型,对光通信链路的总体性能进行了仿真计算,并研究了在一定误码率、一定发射功率下优化通信光束发散角与径向偏角方差的关系。     

大气光通信PAT子系统指标初步设计分析

分析了跟踪角速度，功率，跟踪精度等因素对大气激光通信系统中瞄准，捕获和跟踪子系统参数选择的影响，给出了波束发散角，扫描角范围，接收视场等主要参数的选择原则，对地面终端光通信系统设计背景提出了瞄准，捕获和跟踪了系统的相应指标。     

User Capacity of a Demand-Assigned Satellite Communication System with a Hybrid TD/FDMA Up-Link and a TDM Down-Link

The advent of processing satellites which perform demodulation and remodulation of received signals has made it practical to consider satellite communication systems in which the up-link and down-link multiplexing/multiple-access schemes are different. This paper considers a satellite communication system in which channels are demand-assigned, and in which a hybrid time-division/frequencydivision multiple-access approach is employed for the up-link and pure time-division multiplexing is employed for the down-link. This scheme has significant advantage for systems which may be subject to electronic jamming. The user capacity is analyzed in general, and numerical results are given for some specific parameters of interest. Depending on the number of ground terminals in the network, it is found that the user capacity of the system can be limited by either the terminal's or the satellite's channel capacity.     

XY-2号卫星激光通信载荷PAT在轨测试

针对低轨小卫星星座的通信需求，设计了基于双棱镜和四象限雪崩光电二极管(QAPD)结构收发同轴的激光通信载荷，该方案是无信标光体制，具有体积小、轻量化和大视场的特点。文中针对双棱镜结构，给出了双棱镜输入输出光线的计算模型，在此基础上，提出了星间指向、捕获和跟踪的实现方式，并在XY-2号卫星上进行了在轨测试和验证，进行指向测试时，更新了指向偏移量，标定了QAPD跟踪点，并进行了双向建链测试。进行了15次双向建链测试表明，该激光通信载荷捕获时间小于20 s，捕获成功率达到100%，捕获后双星建链时间优于2 s，建链测试成功率达到了93%，建链后跟踪精度RMS值小于30 μrad。