Prediction model for the diurnal behavior of the troposphericscintillation variance

Tropospheric scintillation is caused by variations of the refractive index due to turbulence. The only meteorological input parameter for two common current scintillation models by Karasawa et al. (1988) and by the ITU-R is the monthly average of the wet part of the refractivity N_wet at ground level, which is not directly associated with turbulence. The diurnal correlation between N_wet and scintillation variance is very weak. Because clouds and cloud formation are closely associated with the turbulence, quantitative cloud parameters were looked for. Cloud type information based on edited synoptic cloud reports are available from the common database of CDIAC and NCAR. Both diurnal and seasonal variations between scintillation variance and average amount of Cumulus type clouds are well correlated. Using this cloud information together with N_wet, a new method for tropospheric scintillation variance predicting also the diurnal variations is introduced. This model is derived and tested using scintillation measurements at four sites in different climates in Finland, United Kingdom, Japan, and Texas     

Prediction of tropospheric scintillation on satellite links fromradiosonde data

On the basis of radiosonde data, a new method is proposed for predicting tropospheric scintillation effects on slant paths. It stems from a rigorous statistical development and consists of two steps. First, statistical features of tropospheric turbulence responsible for scintillation are extracted from the analysis of a large amount of radiosonde ascents. Second, long-term scintillation statistics are inferred from these turbulence characteristics, using the theory of propagation through a turbulent medium. The method is applied to a complete year of radiosonde data measured in Belgium and the predicted scintillation results are compared with measurements carried out on the same year near to the meteorological station. An agreement better than with any other usual prediction method is found. The method yields very accurate predictions of scintillation annual statistics and also adequately represents the seasonal and monthly variability of scintillation. Unlike the current prediction models, the proposed radiosonde-based method does not rest on empirical relationships derived from particular propagation experiments and could, therefore, be applied more widely     

Frequency dependence of amplitude scintillation

In the prediction models of tropospheric scintillation on Earth-satellite paths from Karasawa, Yamada, and Allnutt and ITU-R, the frequency dependence of scintillation is expressed as a power law with a different exponent for each model. This is verified using a collection of measurement results from different satellite links in Europe, the US, and Japan at frequencies from 4 to 50 GHz and elevation angles from 2.5 to 52°. It shows that the exponent of the power law varies widely among the results from the different sites. Possible explanations of this are: (1) the frequency dependence of scintillation due to cloudy turbulence is different from that due to clear-sky turbulence and this kind of scintillation may be present to different extents in the various databases due to climatic differences and different clear-sky selection procedures or (2) angle-of-arrival fluctuations due to turbulence have a different frequency dependence and this effect may have some impact on the measured scintillation at some of the sites     

基于子空间的无线光通信副载波盲均衡算法研究

文中基于Gamma-Gamma光强起伏分布大气湍流信道模型,对无线光副载波相移键控调制系统进行了子空间盲均衡算法的研究。对比分析了经过大气湍流信道盲均衡前后的副载波调制信号星座图聚敛性,给出了均衡后不同光强起伏方差下的误码率曲线,当R2=0.1,信噪比为20 dB时,误码率从4.710-1降低到1.5610-3,均衡后误码率明显改善。同时采用两种典型实测天气数据(阴、中雨)模拟大气信道进行了子空间盲均衡实验,均衡后调制信号星座图聚敛性与相位可识别性明显优于均衡前。仿真结果表明,子空间盲均衡算法对大气湍流信道下的副载波调制信号有良好的均衡效果。     

Scintillation effects on total fade distributions forEarth-satellite links

Scintillation effects on the cumulative fade distributions for Earth-satellite links are considered. Two combination methods for scintillation and attenuation distributions are compared using filtered and unfiltered 11.2 GHz beacon measurements from Austin, TX, on a 5.8° elevation path. A combination method based on the assumption of independent scintillation and attenuation phenomena is proposed. Attenuation and total fade distributions at 19.8 and 29.7 GHz from Kirkkonummi, Finland, on a 12.7° elevation path, are presented for a summer period. The effects of attenuation and scintillation are also calculated using meteorological data from several radiosonde stations. The calculations show that the relative significance of the scintillation is smaller at frequencies from 20 to 50 GHz than at 12 GHz     

Prediction of tropospheric amplitude scintillation on a satellitelink

Semi-empirical models are presented for annual and worst-month distributions of scintillation fades, enhancements, peak-to-peak amplitude excursions, and intensity. The models utilize the well-known theoretical scaling of scintillation with signal frequency, antenna diameter, and path-elevation angle, and its empirical dependence on the wet term of atmospheric refractivity. Dependence on time percentage is obtained by detailed regression analysis of experimental scintillation data obtained at Sparsholt, UK (51.5850° N, 1.5033° W) over a one-year period using the Olympus satellite 19.7704 GHz beacon viewed at a nominal elevation of 28.74°. Results are compared where possible with the ITU-R and Moulsley-Vilar scintillation models and with Olympus measurements in Germany     

Ka频段卫星信道的衰减特性

在Ka及以上高频段卫星通信系统中,其链路信号的衰减很严重,主要是大气、降雨、闪烁对卫星信道的影响。本文主要研究了雨衰和对流层闪烁产生衰减的机理与特性,提出雨衰采用DAH模型,闪烁采用Van de Kamp模型。然后根据实测气象数据,计算我国各参考站点的雨衰和闪烁值。为抗衰减对策的提出提供理论依据,为工程设计提供具体的参考数据。     

Performance of APD-based, PPM free-space optical communication systems in atmospheric turbulence

In this paper, we characterize the performance of a direct-detection, avalanche photodiode-based free-space optical (FSO) communication system in terms of the overall bit-error rate. The system of interest uses pulse-position modulation (PPM) and is subjected to scintillation due to optical turbulence. Two scenarios are considered. In one case, a weak turbulence (clear-air) scenario is considered, for which the received signal intensity may be modeled as a log-normal random process. In the other case, we consider a negative exponentially distributed received signal intensity. To arrive at the desired results, it is assumed that the system uses a binary PPM (BPPM) modulation scheme. Furthermore, it is assumed that the receiver thermal noise is nonnegligible, and that the average signal intensity is large enough to justify a Gaussian approximation at the receiver. Union bound is used to assess the performance of M-ary PPM systems using the results of the BPPM scenario. Numerical results are presented for the BPPM case to shed light on the impact of turbulence on the overall performance.     

Review and comparison of tropospheric scintillation prediction models for satellite communications

An overview of the origin and characteristics of tropospheric scintillation is presented and a measurement database against which scintillation models are to be tested is described. Maximum likelihood log‐normal and gamma distributions are compared with the measured distribution of scintillation intensity. Eleven statistical models of monthly mean scintillation intensity are briefly reviewed and their predictions compared with measurements. RMS error, correlation, percentage error bias, RMS percentage error and percentage error skew are used in a comprehensive comparison of these models. In the context of our measurements, the ITU‐R model has the best overall performance. Significant difference in the relative performance of the models is apparent when these results are compared with those from a similar study using data measured in Italy. Copyright © 2006 John Wiley & Sons, Ltd.     

Seasonal and diurnal variations of total attenuation measured with the ITALSAT satellite at Spino d'Adda at 18.7, 39.6 and 49.5 GHz

The ITALSAT satellite experiment started in January 1991 and ended in January 2001, permitting an extensive program of propagation measurements at 18.7, 39.6 and 49.5 GHz. In these frequency bands, up‐ and down‐links experience severe signal attenuation due to meteorological effects such as those due to gas (oxygen and water vapour), clouds, turbulence and, especially, rain. The propagation measurement campaigns aim mainly at assessing and at modelling the appropriate fade margin compensating propagation attenuation in the design of satellite communication systems. This margin depends significantly on the season and on the time of the day, due to variations of meteorological conditions. This paper reports the results obtained from copolar signal measurements carried out at the Earth station of Spino d'Adda, near Milano (North Italy), at the three frequencies during 7 years (from 1994 to 2000). The measured cumulative distribution functions of total attenuation are compared to ITU‐R models' prediction. Moreover the statistics conditioned to single months of the year, seasons and 4 h contiguous periods of the day are also shown. These statistics can be useful for telecommunication systems whose service quality and design must be matched to the season of the year or the time of the day. Copyright © 2004 John Wiley & Sons, Ltd.     

一种多光束发射和接收FSO系统的信道建模方法

在单光束发射和接收无线光通信(FSO)系统中,目前人们普遍接受的是光强闪烁在强湍流下服从负指数分布,在弱湍流下服从对数正态分布,而在强弱湍流下都适用的Gamma-Gamma分布则是研究的热点。在这几种分布的基础上,将它们推广到多光束发射和接收FSO系统,分别推导出了基于对数正态分布、负指数分布以及Gamma-Gamma分布的多光束发射和接收FSO系统的光强闪烁概率密度分布函数,建立起多光束发射和接收系统光强闪烁信道模型,并对基于负指数分布的多光束信道模型与通信距离、激光波长、接收孔径以及收发天线数目等参数的相互关系进行了具体的仿真分析。这对多光束发射和接收FSO系统的理论分析和系统设计具有一定的指导意义。     

Observed effects of cloud and wind on the intensity and spectrum of scintillation

This paper examines the observed effects of wind and cloud presence on the intensity and power spectrum of tropospheric scintillation, using propagation data recorded during a three-month period (May-July 1997) at Sparsholt from the ITALSAT satellite beacons at 39.59 and 49.49 GHz. The results show strong correlation between corner frequency and transverse wind speed, a weaker correlation with overall wind speed and a negligible correlation between scintillation intensity and transverse wind speed. Furthermore, it was found that the presence of cumulus clouds led on average to increased scintillation levels and a reduced correlation of the corner frequency with wind speed.     

基于闪烁法的大气湍流廓线测量技术

为了研究基于闪烁效应测量湍流强度廓线技术,以SCIDAR、闪烁计阵列和多孔径闪烁测量为基础,介绍了其理论基础、技术设备、发展现状及其优缺点.根据多孔径闪烁测量的基本原理和Hufnagel-Valley 5/7湍流廓线模型,采用理论模拟方法,利用奇异值分解法反演得到了湍流强度廓线随高度的分布.从反演结果可以看出:除含有100%噪声的反演结果外,其余结果的量级均为10~(-15)～10~(-17),与Hufnagel-Valley 5/7模型基本一致.说明奇异值分解法可以用于数据的处理,并且基本满足需要.最后,针对现有MASS的缺点提出了改进方案.     

Modeling Ka-band scintillation as a fractal process

We propose a model that describes the signal fading process due to scintillation in the presence of rain. We analyzed a data set of uplink (30 GHz) and downlink (20 GHz) attenuation values averaged over 1 s intervals. The data are samples relative to ten significant events, for a total of 180 000 s recorded at the Spino d'Adda (North of Italy) station using the Olympus satellite. Our analysis is based on the fact that the plot of attenuation versus time recalls the behavior of a self-similar process. We then make various considerations, and propose, a fractional Brownian motion model for the scintillation process. We describe the model in detail, with pictures showing the apparent self-similarity of the measured data. We then show that the Hurst parameter of the process is a simple function of the rain fade. We describe a method for producing random data that interpolate the measured samples, while preserving some of their interesting statistical properties. This method can be used for simulating fade countermeasure systems. As a possible application of the model, we show how to optimize fade measurement times for fade countermeasure systems     

测量大气折射率结构常数的大口径激光闪烁仪

基于光闪烁的孔径平均效应,研制了测量大气折射率结构常数的大口径激光闪烁仪(LALS)。该仪器在一定程度上实现了闪烁饱和现象的抑制。仪器采用对称式结构布局,同时采用调制解调方法提高信噪比,并以嵌入式控制单元完成对数据的采集、处理、显示和存储。与传统仪器设备的测量结果相比,在1000m的水平传输路径上两者的线性拟合系数为1.01,相关系数为0.92;进一步分析表明,大气湍流内尺度会对闪烁方差的测量产生影响,从而导致大气折射率结构常数的测量结果略显离散。     

基于孔径平滑效应的无线光通信接收孔径的设计

基于激光大气湍流传输理论,讨论了光强闪烁对无线光通信接收性能的影响。从孔径平滑效应出发．提出了一种结合信噪比、误码率、孔径平滑3方面来考虑接收孔径尺寸的设计方法。该设计方法克服了传统只考虑孔径平滑效应来设计接收孔径不准确的缺点,从而为设计接收孔径尺寸提供理论依据。用MATLAB对参考文献中的实验数据进行数值模拟,结果表明：在此实验数据下BER〈10^-6时,当SNR0为30dB时,最优接收孔径为4cm,当SMR0为35dB时,最优接收孔径为3cm。     

Effects of light propagation in middle intensity atmospheric turbulence

The purpose of this report is to present an experimental study of the effects of light propagation through atmospheric turbulence.Free space optical communication is a line-of-sight technology that transmits a modulated beam of visible light through the atmosphere for broadband communication.The fundamental limitations of free space optical communications arise from the environment through which it propagates.However these systems are vulnerable to atmospheric turbulence, such as attenuation and scintillation, Scintillation is due to the air index variation under the temperature effects.These factors cause an attenuated receiver signal and lead to higher bit error rate (BER).An experiment of laser propagation was carried out to characterize the light intensity through turbulent air in the laboratory environment.The experimental results agree with the calculation based on Rytov for the case of weak to intermediate turbulence.Also, we show the characteristics of irradiance scintillation, intensity distribution and atmospheric turbulence strength.By means of laboratory simulated turbulence, the turbulence box is constructed with the following measurements: 0.5 m wide, 2m long and 0.5m high.The simulation box consists of three electric heaters and is well described for understanding the experimental set up.The fans and heaters are used to increase the homogeneity of turbulence and to create different scintillation indices.The received intensity scintillation and atmosphere turbulence strength were obtained and the variation of refractive index, with its corresponding structure parameter, is calculated from the experimental results.     

Observed frequency scaling of amplitude scintillation at 20, 40, and 50 GHz

This paper presents an analysis of instantaneous frequency scaling of scintillation using propagation data recorded during a three month period (May-July 1997) at Sparsholt UK from the ITALSAT satellite beacons at frequencies 18.7, 39.6, and 49.5 GHz. Variations in the height of turbulence within reasonable limits were found to have a negligible effect on the scaling ratios. Furthermore, the exponent in the power law dependence of scintillation intensity on signal frequency was found to be on average 27% smaller than the theoretical value of 7/12 and to exhibit a slight diurnal effect. It is shown that this behavior can be partly accounted for by receiver thermal noise contribution to the measured signal variance. Ascribing the minimum observed short-term variance in each beacon to thermal noise and excluding this contribution yielded a higher exponent, which was nevertheless 15% below the theoretical value.     

空间光通信的时间平滑实验研究

为了给空间光通信的瞄准捕获跟踪链路的设计提供参考,实验研究了强湍流区的大气闪烁时间平滑效应,并考虑大气闪烁和瞄准抖动误差,分析了空间光通信的跟瞄链路的强度起伏统计规律;在此基础上,分析了时间平滑对链路衰落起伏的改善作用.实验研究结果表明,随着曝光时间的增加,强湍流区的大气闪烁起伏幅度减小,当曝光时间达到几十毫秒时,强湍流区的大气闪烁起伏服从对数正态分布;在确保空间光通信系统的链路跟踪中断概率小于10-3以下,采用时间平滑可以使所需的衰落冗余减少15dB～20dB.     

大气Gamma-Gamma湍流信道中采用圆偏振位移键控的信道容量分析

在自由空间光通信(FSO)中,由系统设计以及大气湍流所引起的光强闪烁和相位起伏会造成圆偏振控制误差,这严重影响圆偏振位移键控(CPolSK)通信系统的性能。因此,基于Gamma-Gamma大气信道模型,在较全面考虑光强起伏、相位噪声和圆偏振控制误差三个参数的基础上,建立了CPolSK系统信道的数学统计模型,推导了平均信道容量的闭合表达式,仿真研究了误码率在连续相位噪声下的性能以及三个参数对信道容量的影响。结果表明,光强起伏的增强和相位噪声的增大均会导致CPolSK系统性能的劣化,而提高平均信噪比在一定程度上能够改善系统性能。特别地,当圆偏振控制误差大于8时,CPolSK系统的平均信道容量显著下降。