Optical scintillations and fade statistics for a satellite-communication system

Estimates of the scintillation index, fractional fade time, expected number of fades, and mean duration of fade time associated with a propagating Gaussian-beam wave are developed for uplink and downlink laser satellite-communication channels. Estimates for the spot size of the beam at the satellite or the ground or airborne receiver are also provided. Weak-fluctuation theory based on the log-normal model is applicable for intensity fluctuations near the optical axis of the beam provided that the zenith angle is not too large, generally not exceeding 60°. However, there is an increase in scintillations that occurs with increasing pointing error at any zenith angle, particularly for uplink channels. Large off-axis scintillations are of particular significance because they imply that small pointing errors can cause serious degradation in the communication-channel reliability. Off-axis scintillations increase more rapidly for larger-diameter beams and, in some cases, can lead to a radial saturation effect for pointing errors less than 1 μrad off the optical beam axis.     

Far-Field Pattern Measurement of an Onboard Laser Transmitter by Use of a Space-to-Ground Optical Link

The far-field pattern of an onboard laser transmitter was measured with a transatmospheric optical link with a distance of ~33,000 km between the satellite and the optical ground station. The far-field pattern was acquired with a new method used to analyze statistically downlink irradiance data obtained at the ground station. The statistical tracking and pointing characteristics of the transmitter were taken into account in estimating downlink irradiance, assuming that there were no atmospheric scintillation effects. The peak directive gain of the downlink laser beam was 104.3 dB. The beam width (full width at half-maximum) was 28.5 x 17.5murad. These results were consistent with the results froma laboratory test undertaken before launch of the satellite.     

Atmospheric optical communication with a Gaussian Schell beam

We consider a wireless optical communication link in which the laser source is a Gaussian Schell beam. The effects of atmospheric turbulence strength and degree of source spatial coherence on aperture averaging and average bit error rate are examined. To accomplish this, we have derived analytic expressions for the spatial covariance of irradiance fluctuations and log-intensity variance for a Gaussian beam of any degree of coherence in the weak fluctuation regime. When spatial coherence of the transmitted source beam is reduced, intensity fluctuations (scintillations) decrease, leading to a significant reduction in the bit error rate of the optical communication link. We have also identified an enhanced aperture-averaging effect that occurs in tightly focused coherent Gaussian beams and in collimated and slightly divergent partially coherent beams. The expressions derived provide a useful design tool for selecting the optimal transmitter beam size, receiver aperture size, beam spatial coherence, transmitter focusing, etc., for the anticipated atmospheric channel conditions.     

Effects of time averaging on optical scintillation in a ground-to-satellite atmospheric propagation

Temporal natures for a variance of turbulence-induced log-intensity fluctuations are obtained. The variance of the optical fluctuation is reduced when the optical signals are integrated in a photodetector, and we express the index of reduction (called the time-averaging factor) by using an autocovariance function of the optical fluctuation. The optical fluctuations for a ground-to-satellite path are caused by both atmospheric turbulence and the beam-pointing jitter error of the optical transmitter. The turbulence-induced optical scintillation can be discriminated from the fluctuation that is due to the beam-pointing jitter error. The compared result from the probability density function of the optical signal reveals good agreement. The temporal autocovariance functions of optical scintillation are obtained and used to calculate the time-averaging factor. The analytically expected effects of time averaging are verified by the experimental results. The estimations contribute to the link budget design for the optical tracking channel through atmospheric turbulence.     

Scintillation and beam-wander analysis in an optical ground station-satellite uplink

In an optical communication link between an optical ground station and a geostationary satellite the main problems appear in the uplink and are due to beam wander and to scintillation. Reliable methods for modeling both effects simultaneously are needed to provide an accurate tool with which the robustness of the communication channel can be tested. Numerical tools, especially the split-step method (also referred to as the fast-Fourier-transform beam propagation method), have demonstrated their ability to deal with problems of optical propagation during atmospheric turbulence. However, obtaining statistically significant results with this technique is computationally intensive. We present an analytical-numerical hybrid technique that provides good information on the variance in optical irradiance with an important saving of time and computational resources.     

Bit-error rate for free-space adaptive optics laser communications

An analysis of adaptive optics compensation for atmospheric-turbulence-induced scintillation is presented with the figure of merit being the laser communications bit-error rate. The formulation covers weak, moderate, and strong turbulence; on-off keying; and amplitude-shift keying, over horizontal propagation paths or on a ground-to-space uplink or downlink. The theory shows that under some circumstances the bit-error rate can be improved by a few orders of magnitude with the addition of adaptive optics to compensate for the scintillation. Low-order compensation (less than 40 Zernike modes) appears to be feasible as well as beneficial for reducing the bit-error rate and increasing the throughput of the communication link.     

用闪烁调制理论研究激光大气传输的相位起伏

针对Rytov近似下相位起伏的理论结果与实际情况不符的缺陷,本文提出用Andrews的调制近似理论来研究光波在湍流大气中传输的相位起伏.该方法根据能量守恒的原理,得到了闪烁调制的具体表达式,然后通过功率谱的替换,从闪烁方差得到了上行、下行和水平三种链路的相位起伏的表达式.结果表明,Andrews调制近似其实有很多具体的形式,对于每一种形式,其中的物理量都有不同的物理含义.与Rytov近似下的结果相比,本文的结果将适用于倾斜传输路径和存在闪烁的聚焦和饱和的情况.     

Statistical distribution of the optical intensity obtained using a Gaussian Schell model for space-to-ground link laser communications

Abstract

Based on the characteristics of the laser device and the inevitable error of the processing technique, a laser beam emitted from a communication terminal can be represented by the Gaussian Schell model (GSM). In space-to-ground link laser communications, the optical intensity is affected by the source coherence parameter and the zenith angle. With full consideration of these two parameters, the statistical distribution model of the optical intensity with a GSM laser in both downlink and uplink is derived. The simulation results indicate that increasing the source coherence parameter has an effect on the statistical distribution of the optical intensity; this effect is highly similar to the effect of a larger zenith angle. The optical intensity invariably degrades with increasing source coherence parameter or zenith angle. The results of this work can promote the improvement of the redundancy design of a laser communication receiver system.     

A real-time remote control architecture using mobile communication

This paper presents a new development of mobile communication for application to remote data transmission and control. The concept of G/sup 3/, including global positioning system, global system for mobile (GSM), and geographic information system, is proposed. In this paper, a GSM real-time control system for data uplink and downlink remote terminals is considered. The proposed system can be implemented on a PC-based or a /spl mu/P-based to integrate into G/sup 3/ configuration. This paper presents the system design and implementation with practical data verification. The proposed GSM real-time control system has accomplished a very reliable two-way point-to-point uplink and downlink data transmission within a 0.6-s time delay.     

分层结构分布式天线系统的性能分析

提出了一种分层结构的分布式天线系统(HDAS).在此HDAS中,其中一个远端接入单元(RAU)以大功率发射保证对整个广义小区(GN-cell)的覆盖,而GN-cell中用户密度较大或者链路质量较差的区域配置较小功率的RAU对其进行重复覆盖.在CDMA网络环境中分析了HDAS上下行链路的信干比,其中上行链路采用基于信干比平衡准则的功率控制算法,下行链路采用RAU选择传输方案.链路性能分析表明,上行链路的信干比与GN-cell中RAU数目成正比,且与用户分布和小功率RAU的位置无关;下行链路信干比在RAU选择和RAKE接收结合时与用户分布及RAU位置基本无关.这些结论表明,CDAM HDAS可以较好地解决蜂窝移动通信系统中由于用户非均匀分布造成的扩容以及随之产生的频繁切换问题.     

自适应光学技术在星地激光通信地面站上的应用

综述了自适应光学技术在星地激光通信地面站上应用的最新进展。针对星地链路中湍流效应导致的相干度退化和可用度降低的问题,自适应光学技术成为美国和欧洲等国正在研制的中继卫星至地光通信系统解决上述问题的主导手段。这些项目计划开展的自适应光学技术、白天和夜晚多地面站接收技术和相干通信技术等关键技术验证表明,星地激光通信正向高速相干和全天时高可用度的工程化推进。国内成功进行了多次星地光通信试验,高可用度的相干激光通信技术的验证正在积极开展,自适应光学技术已应用到多个地面站并取得了较好的初步试验效果,相关技术进展与国外水平保持一致。

     

Intensity Fluctuations in a Two-mode Ring Laser

Intensity fluctuations in a two-mode ring laser with zero detuning are derived using a noise amplification rate-equation model. Both approximate and exact forms of the gain saturation are treated. Steady-state distributions of the intensity are derived analytically permitting calculation of the mean and normalized variance. Numerical solutions yield the time-dependent evolution of these quantities from initial input noise. Recently derived mode competition effects, such as a steady state value of } for the normalized variance (rather than zero as in a conventional laser) and negative correlations between the intensity fluctuations of the two modes, appear more simply here and their statistical origin is explained.     

Digital Doppler measurement with spacecraft

Digital and analog phase-locked loop (PLL) receivers were operated in parallel, each tracking the residual carrier from a spacecraft. The PLL tracked the downlink carrier and measured its instantaneous phase. This information, combined with a knowledge of the uplink carrier and the transponder ratio, permitted the computation of a Doppler observable. In this way, two separate Doppler measurements were obtained for one observation window. The two receivers agreed on the magnitude of the Doppler effect to within 1 mHz. There was less jitter on the data from the digital receiver. This was due to its smaller noise bandwidth. The demonstration and its results are described     

Characterization of Frequency Stability: Uncertainty due to the Finite Number of Measurements

We consider the characterization of the frequency stability of signal generators by means of the Allan variance of their fractional frequency fluctuations. As the number m of measurements is always finite, one can only get an estimate of this variance, which is a random function of m. We calculate the variance of the Allan variance for commonly encountered spectral densities of frequency fluctuations and deduce the relative uncertainty in the evaluation of frequency stability, as a function of m. The theory is experimentally checked for white noise of phase and flicker noise of frequency. The main hypothesis of the calculations, i.e., the Gaussian character of frequency fluctuations is verified. The m dependence of the uncertainty (one sigma) in the evaluation of frequency stability agrees with the predicted value.     

The communication system

The paper discusses the salient features of system design considerations for both uplink and downlink, subsystem details and the in-orbit performance of the communication system forAryabhata.     

Development of a spectrometer using a continuous wave distributed feedback quantum cascade laser operating at room temperature for the simultaneous analysis of N2O and CH4 in the Earth's atmosphere

We report on the development and performance of a gas sensor based on a distributed feedback quantum cascade laser operating in continuous wave at room temperature for simultaneous measurement of nitrous oxide (N(2)O) and methane (CH(4)) concentrations at ground level. The concentrations of the gases are determined by a long path infrared diode laser absorption spectroscopy. The long-term stability of the instrument is evaluated using the Allan variance technique. A preliminary evaluation of the instrument performance is realized by in situ measurements of N(2)O and CH(4) concentrations at ground level during 1 day. The sensor has also been applied to study the time response of N(2)O concentrations to a fertilizer addition in a soil sample and for the comparison between various types of soils.     

Influence of atmospheric turbulence on the uplink propagation in an optical time transfer

The time transfer by laser link experiment T2L2 aims for a precision of 60 ps, which could be degraded by the atmospheric turbulence because of the strong variations of the photon number received by the satellite detector, from a measurement to another. The light intensity fluctuations in the satellite plane are estimated for the planned situation for which the beam radius at the atmosphere exit is significantly larger than the coherence length. Such speckle-type fluctuations are experimentally studied.     

Mode structure effects in optical resonance excitation

The dependence of the excitation strength on the longitudinal mode structure of a pulsed, resonant excitation field is examined. As an experimental test case, resonance ionization signal fluctuations are studied with a two-step excitation of strontium. Successive ionization signals, the corresponding pulse energies, and the mode structures of the resonant step laser pulses are recorded. The large signal fluctuations, up to 120%, cannot be explained by the modest (1-3%) pulse-energy fluctuations of either the resonant or the photoionizing field. In the case of weak excitation, the fluctuations in the signal correlate strongly with the intensity in a narrow frequency band around the resonance. In this weak-field region, the experimental correlation curves, i.e., the correlation between the signal and the spectral intensity versus the frequency, agree well with calculations based on a simple linear-response model. With the aid of correlation analysis a resonance can be localized with a single-moderesolution. As the resonant field is increased the correlation between the signal and single-mode intensity diminishes and almost disappears at full saturation. However, also in the saturation region, the correlation technique can be applied to localize a resonance with a resolution much better than that determined by the laser linewidth.     

Performance of a laser Earth-to-satellite link over turbulence and beam wander using the modulated gamma-gamma irradiance distribution

We present an analytical framework for the performance evaluation of laser satellite uplinks over the major probabilistic impairments, i.e., atmospheric turbulence and beam wander. Specifically, we consider a ground-station-to-space laser uplink with a Gaussian beam wave model, and we focus on the particular regime assuming untracked beams where beam wandering takes place. In that regime, the modulated gamma-gamma distribution has been proposed as an effective irradiance model to characterize the combined effect of turbulence and beam wander. First we provide a closed-form expression of the probability density function and deduce the fundamental statistics of the new model. Then we evaluate the performance of the laser system assuming coherent detection for several modulation schemes. An appropriate set of numerical results is presented to verify the accuracy of the derived expressions.     

背景光对星地量子密钥分配量子误码率的影响

量子误码率是量子密钥分配系统的重要参数之一.对于星地量子密钥分配,采用具有泊松分布的高度衰减激光脉冲作为单光子源,建立了由背景光引起的量子误码率理论模型,给出了量子误码率的表达式.针对轨道高度为300km的低轨卫星-地面站间链路,进行了数值仿真分析.研究表明,背景光是限制自由空间量子密钥分配链路距离的主要因素之一,在低轨卫星-地面站间进行量子密钥分配是可行的.在白天,量子误码率的量级为10-3～10-2;在夜晚,满月时为10-4,新月时为10-6～10-5.