Analysis of optical coherence multiplexing networks for satellite communication

Free-space optical communication between satellites networked together can enable a high data rate between the satellites. Coherence multiplexing (CM) is an attractive technique for satellite networking due to its ability to cope with the asynchronous nature of communication traffic and the dynamic changes taking place in the satellite constellation. The use of optical radiation for intersatellite links creates very narrow beam divergence angles. Due to the narrow beam divergence angle, the vibration of the pointing system, the movement of the satellite, and the large distance between them the pointing from one satellite to another is a complicated task. The vibration of the pointing system is caused by two stochastic fundamental mechanisms: 1) tracking noises created by the electrooptic tracker and 2) vibrations created by internal satellite mechanical mechanisms and external environments. We derive mathematical models of signal, noise, the approximate signal-to-noise ratio, and the approximate bit-error rates of optical communication satellite networks as functions of the system's parameters, the number of satellites, and the vibration amplitude for frequency-shift keying coherence multiplexing (FSK-CM). Based on these models, we can calculate the negative impact of both the number of satellites and the optical terminal vibration on the system's performance.     

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     

瞄准误差和Gamma-Gamma大气湍流下信号束散角的优化分析

在自由空间激光通信(FSO)系统中,瞄准误差和大气湍流严重影响系统通信链路,是链路性能恶化的两大主要因素.通过对联合信道的理论分析,建立了信道模型,并利用Meijer G函数推导出了直接探测系统和相干探测系统平均信道容量的表达式.通过在Matlab中的仿真结果可知,在给定瞄准误差和大气湍流的条件下,信号光的光束束散角并非越小越好,而是存在最优束散角,该束散角不仅可以保证系统性能良好,而且使得发射系统功率取得最小值,并且采用相干探测系统可以抑制大气湍流对系统的影响,有效提高系统的性能.     

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     

电磁-高斯谢尔模型阵列光束在大气湍流中的传输特性

根据广义惠更斯菲涅尔原理与维格纳分布函数相 结合的方法,导出了电磁高斯谢尔模型阵列光 束(EGSMA Beams)在大气湍流中传输的均方根空间扩展、角扩展以及M²因子的解析式。(分析了其传输特 性与光束宽度、初始相干长度、阵列光束数量、初始偏振度、湍流内尺度以及折射率结构常 数的关系。)研 究结果表明相对M²因子随初始偏振度、初始相干长度和折射率结构常数的减 小,以及阵列光束数量、光束 宽度、和湍流内尺度的增大而减小,此时相对M²因子受大气湍流影响更小。当经过大约5km的传输距离 后,初始相干长度对相对M²因子的影响开始明显加大,且随传输距离增大而 增大。同时研究结果表明当阵 列光束数量增加越多时,相对M²因子越接近1。并且分析得出初始偏振度以及折射率结构 常数对相对M²因子的影响大于相对均方根空间扩展和角扩展。     

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     

Growth of 1‐eV GaNAsSb‐based photovoltaic cell on silicon substrate at different As/Ga beam equivalent pressure ratios

We report the performance of 1‐eV GaNAsSb‐based photovoltaic samples grown on a Si substrate using molecular beam epitaxy at different As/Ga beam equivalent pressure (BEP) ratios. The light current–voltage curve and spectral response of the samples were measured. The sample grown at an As/Ga BEP ratio of 10 showed the highest energy conversion efficiency with an open circuit voltage (V_OC) of 0.529 V and a short circuit current density of 17.0 mA/cm². This measured V_OC is the highest ever reported value in GaNAsSb 1‐eV photovoltaic cell, resulting in the lowest ever reported E_g/q‐V_OC of 0.50 eV. The increase in the As/Ga BEP ratio also resulted in an increase in the bandgap‐voltage offset value (E_g/q‐V_OC) and a decrease in quantum efficiency up to As/Ga BEP ratio of 18. Copyright © 2015 John Wiley & Sons, Ltd.     

Effect of clear atmospheric turbulence on quality of free space optical communications in Yemen

Free space optical (FSO) communication is one of the most recently developed modes of wireless communication.FSO is a technique used to convey data carried by a laser beam through the atmosphere.While FSO offers a broadband service,it requires a line of sight communication between the transmitter and receiver.The atmosphere has effects on the laser beam passing through it.For instance,the quality of data received is affected by the scattering and atmospheric turbulence.The atmospheric turbulence is caused by both temporary and special random fluctuations of the refractive index along the optical propagation path.Clear air turbulence impairs the performance of the FSO due to the fluctuation in the intensity of the laser beam.By referring to the two criteria,namely bit error rate (BER) and signal to noise ratio (SNR),this work includes analysis of the effect of atmospheric turbulence on FSO systems in Yemen by using an appropriate model.     

Effects of pointing errors and channel fading on the performance of free‐space optically‐preamplified PPM systems

In this paper, we study the impact of pointing errors and channel fading on the performance of free‐space, optically preamplified, M ‐ary PPM systems. We consider two types of free‐space optical links: (i) inter‐satellite links and (ii) inter‐building links. For inter‐satellite links, only pointing error is considered. Starting with a Rayleigh model for the pointing error angle, we derive analytically the PDF for the pointing error parameter and for the signal‐to‐noise ratio (SNR) per bit. For inter‐building links, we derive the density function for the SNR per bit that includes the combined effects of pointing errors and channel fading, assuming Rayleigh‐distributed pointing errors. The channel fading models considered in this study for inter‐buildings links are the log‐normal and gamma–gamma models. We provide the error probability as a function of the average SNR per bit for both types of links. To cover systems with and without forward error correction, we compute the average SNR per bit required to achieve a bit error rate of 10^?4 and 10^?9. The corresponding power penalties are computed for different symbol sizes, scintillation indexes, and pointing jitters. Copyright © 2016 John Wiley & Sons, Ltd.     

弱湍流大气传输广义Helmholtz-Gauss光束

在Rvtov近似和采用Tatarskii湍流大气折射率起伏谱条件下,运用实数和复数矩阵元的ABCD矩阵研究了湍流大气中广义Helmholtz-Gauss束的传输规律,研究表明广义Hellmholtz-Gauss束的横向波数与高斯束传输参量q的积在弱湍流大气传输过程中保持不变。导出了湍流大气中传输广义Helmholtz-Gauss束的平均光强。     

Gamma Gamma强海洋湍流和瞄准误差下水下无线光通信系统的性能研究

采用外差式差分相移键控(Differential phase-shift keying,DPSK)调制的水下无线光通信(Underwater wireless optical communication,UWOC)系统经过Gamma Gamma强海洋湍流信道传输,当接收端与发送端之间存在瞄准误差并采用孔径接收方式时,分析了湍流效应和瞄准误差对接收光强的抖动影响,推导了UWOC系统的平均误码率(Bit error rate,BER)和中断概率(Outage probability,OP)的解析表达式。数值模拟研究了不同的瞄准误差、束宽、接收孔径和海洋湍流参数对平均BER和OP性能的影响。结果表明,在相同的束宽和信道环境下,瞄准误差越大,系统性能越差;光束束宽与孔径半径之比越大,接收孔径直径越大,系统性能越好;另外,选择较小的温度和盐度波动对海洋湍流贡献的比值ω和均方温度耗散率χT,以及较大的湍流动能耗散率ε和动力粘度u的海洋湍流环境也有利于获得较好的系统性能。     

10.6μm激光大气通信的自动瞄准与跟踪

激光大气通信双端的自动瞄准与跟踪，是开发激光大气传输信道、赖以传送大容量信息的一个关键问题。本文在分析扰动大气对光束瞄准跟踪所产生的影响的基础上，提出了适于大气激光通信的跟瞄系统结构和设计准则，建立了跟瞄系统模型，合理地设计了数字式瞄准跟踪系统，并给出了光束跟踪系统的实验结果。１０．６μｍ大气通信瞄准问题，国内外还未见有更深入的研究报告。     

Performance analysis of a variable‐gain amplify‐and‐forward relayed mixed RF‐FSO system

In this work, an amplify‐and‐forward variable‐gain relayed mixed RF‐FSO system is studied. The considered dual‐hop system consists of a radio frequency (RF) link followed by a free space optical (FSO) channel. The RF link is affected by short‐term multipath fading and long‐term shadowing effects and is assumed to follow the generalized‐K fading distribution that approximates accurately several important distributions often used to model communication channels. The FSO channel experiences fading caused by atmospheric turbulence that is modeled by the gamma‐gamma distribution characterizing moderate and strong turbulence conditions. The FSO channel also suffers path loss and pointing error induced misalignment fading. The performance of the considered system is analyzed under the collective influence of distribution shaping parameters, pointing errors that result in misalignment fading, atmospheric turbulence, and path loss. The moment‐generating function of the Signal power to noise power ratio measured end‐to‐end for this system is derived. The cumulative distribution function for the Signal power to noise power ratio present between the source and destination receiver is also evaluated. Further, we investigate the error and outage performance and the average channel capacity for this system. The analytical expressions in closed form for the outage probability, symbol and bit error rate considering different modulation schemes and channel capacity are also derived. The mathematical expressions obtained are also demonstrated by numerical plots.     

Antenna resolution as limited by atmospheric turbulence

Spatial variations of the index of refraction of the atmosphere introduce fluctuations in the phase and amplitude of a wave propagating through it. The effect of these fluctuations on the resolution capability of microwave antennas is discussed in this paper. The measure of resolution used in the calculations is the beamwidth, which is defined as the square root of the second moment about the mean of the normalized antenna pattern. This measure simplifies the calculations greatly and results in a simple expression for the beam broadening due to the variation of the refractive index. This broadening is dependent on the distance to the target, the scale of the turbulence, and the variance of the refractive index and is relatively insensitive to the shape of the spatial correlation function of the refractive index. Measurements by others of the nature of the refractive index spatial variation are then used to obtain numerical estimates of the ultimate resolution capability of a microwave antenna as limited by atmospheric turbulence.     

Free space optical system design using particle swarm optimization

The performance of the free‐space optical (FSO) communication system is severely affected by atmospheric properties and misalignment between transmitter‐receiver sections. These effects can be minimized by selecting the precise and proper value of operating parameters. The divergence angle can play a key role in mitigating the negative impact of deviation of an optical beam from its fundamental direction due to different adverse influences. In this paper, we have presented a particle swarm optimization model for determining the optimum divergence angle for designing an optimal FSO link. The FSO transreception quality depends on the medium properties which have a number of detrimental effects like rain, fog, haze, physical obstruction, scattering, and atmospheric turbulence. The proposed work investigate the optimum divergence angle with respect to the maximum deviation angle under different adverse conditions, as mentioned above. Taking into account these effects, the analytical expression of optical link margin and the optimum divergence angle with respect to the maximum deviation angle is derived. The results are analyzed for finding out the most suitable wavelength of the transmitter, transmitted power, link margin, and the link budget under different adverse conditions for reliable communication.     

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.     

Deep‐space communications with a 2‐hop downlink with high link‐availability

We recall and discuss the theory of a 2‐hop downlink in deep‐space communications. The first hop (length d₁) links the deep‐space spacecraft to either a geostationary satellite or to a low Earth orbiting satellite. The second hop (length d₂, with d₂ ? d₁) links the satellite to the Earth receiver through a transparent or regenerative transponder, in slant paths affected by troposphere attenuation. If we adopt a BPSK or QPSK modulation scheme and the same carrier frequencies in the two hops with a transparent transponder, a particular value of the carrier frequency makes the noise‐to‐signal ratio minimum. A better choice is to assign a low carrier frequency (X‐band) to the second hop and a high one (W‐band) to the first hop. A 2‐hop downlink is superior to a 1‐hop downlink with a large power gain proportional to (d₁/d₂)² ? 1 at high microwave frequencies and large troposphere attenuation (high link‐availability). Shannon's capacity theorem provides the same large gain independently of the choice of the carrier frequencies in the two hops, if we use a regenerative transponder. The carrier frequencies of a 2‐hop downlink need not be those reserved to deep‐space communication, because the spacecraft could communicate as if it were transmitting ‘from the Earth’ through a conventional satellite connection. We have applied the theory to a first‐order design in the frequency range 10–100 GHz, slant path elevation angles 30°, 45° and 90° at Gera Lario or Fucino (Italy), and downlinks form Mars and Saturn, although the general findings and methodology are of global applicability. Copyright © 2005 John Wiley & Sons, Ltd.     

Study of Scintillation for Infrared Laser Beam Propagating in Atmospheric Turbulence on Earth-Space Paths

On earth-space paths, by applying a modification of the Rytov method that incorporates spatial frequency filter function under strong atmosphere fluctuation conditions, a tractable model is developed for the scintillation index of infrared laser Gaussian beam wave that is valid under moderate-to-strong irradiance fluctuations. At Infrared band, based on ITU-R C ² _n model, the scintillation indexes for collimation laser beam are predicted by this analytic model. The results agree with theoretic expected scintillation. This scintillation model can be converted into a plane or a sphere wave scintillation index model on earth-space paths, and also reduced to a Gaussian beam wave model on horizontal sight paths of invariable C ² _n.     

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。     

Analysis of Inner-Scale Effect on Atmosphere Scintillation for Infrared Laser Beam Propagating on Earth-Space Paths

On Earth-space paths, based on ITU-R C _n ² (h) model, by means of a modification of the Rytov method that incorporates an amplitude spatial frequency filter function under strong fluctuation conditions, considering atmospheric turbulence inner-scale, an expression is developed for the scintillation index of a visible or a near and middle infrared laser beam wave that is valid under moderate to strong irradiance fluctuations. The quantitative analyses of scintillation index are done by this analytic model for laser Gaussian collimated beam wave at visible or near and middle infrared wave band. The results agree with theoretic analysis scintillation. This scintillation model can be converted into a Gaussian beam wave model with finite inner-scale on horizontal sight paths of invariable C _n ².