基于对流层散射通信传输损耗预计方法的研究

对流层散射通信是一种超视距无线通信方式,其传输速度、传输质量都与其他通信方式相比具有绝对的优势,在军事领域中通信传输方面占据着重要的地位.本文主要针对ITU-R617和NBS-101两种预计方式对流层散射通信传输损耗进行分析,明确不同应用模式下对流层散射通信传输损耗的实际情况,为对流层散射通信传输的研究提供了可靠的研究依据.     

一种超远程对流层散射链路传输损耗修正预测方法

超远程散射通信应用需求日益增加，然而通过实测验证，目前的传输损耗预测方法在超远程散射链路预测方面存在较大偏差.文中基于ITU-R P.617-3（China-ZHANG）方法针对超远程散射传播特点进行分析，给出了一种适用于超远程散射链路的传输损耗修正预测方法.通过多条实际链路测试验证，证明了该修正方法的有效性和准确性，为超远程散射通信系统设计提供了可靠的参考依据.     

高仰角对流层散射电波传播损耗的一种预计方法

偏远山区应急通信需通过无线传输手段解决。由于山峰等障碍物对无线电波的阻挡,使得微波、超短波等通信手段应用受到限制。对流层散射具有跨越一定障碍物的通信能力,有可能用于山区中通信。传统散射使用条件是基于低仰角,本文主要讨论了高仰角、近距离情况对流层电波传播损耗,通过大量实际线路测量和理论分析,验证了散射通信用于跨越近距离障碍物的可行性,在现有散射传输损耗预计模式的基础上给出了一种与实测数据较为吻合的高仰角散射传播损耗预计方法。     

散射通信系统工作频段的选择

散射通信中常用的频段有L、S、C、X和Ku频段,针对散射通信的频率选择问题,从链路损耗入手分别对5个频段进行了分析。根据不同应用场景,利用ITU-R617方法对不同频段、不同距离的散射通信的传输损耗进行了计算仿真,结果表明:在近距离、基本传输损耗较小的应用条件下,宜使用频率较高的C、X或者Ku频段,这样有利于设备的小型化;而在远距离、基本传输损耗很大的应用条件下,频率较低的L或者S频段才是更合适的选择,才能使得远距离散射通信更加实用。     

近距离散射跨山通信传播损耗的简便算法

近距离散射跨山通信存在山峰绕射和对流层散射2种电波传播模式,需要按照不同的传播方式计算传播损耗。无论是绕射还是散射,现有的算法都不能快速预计出传播损耗。为了便于在实际应用中对传播损耗做出快速预计,结合实测数据,给出了近距离跨山通信中散射传播损耗的简便工程算法。介绍了山峰绕射传播损耗的计算方法,提出了绕射参数的工程算法,并给出了近距离散射跨山通信站址选择的建议,为选址提供理论参考。     

对流层散射超视距信道传输损耗快慢衰落特性研究

对流层散射通信是一种地面微波超视距传播的重要手段。针对现有对流层散射传输损耗预测模型无法描述大气环境等因素随机变化问题,该文基于电场强度的快慢衰落特性,首次开展了传输损耗的快慢衰落特性研究,建立了传输损耗分布模型,并结合ITU-R P.617-3给出了该分布待定参数的计算方法。选取了国际电信联盟公布的部分散射链路试验数据,借助正态分布的坐标图纸,验证了该分布模型的有效性,结果表明传播损耗慢衰落特性服从正态分布,可为下一步计算散射链路误码率奠定基础。此外,基于分布模型还提出一种传输损耗预测方法,并利用试验数据验证了所提方法具有较好的精度,克服了现有方法无法计算任意概率传输损耗的问题。     

对流层散射最坏月转换模式研究

针对目前的最坏月预测模型无法有效反映对流层散射传输损耗的最坏月时间概率分布随地域和气象气候条件变化的特点,结合ITU-R P.617-3和ITU-R P.841-4建议书给出的最坏月预测模型,提出了一种对流层散射传输损耗最坏月时间概率预测方法。通过合理的数学推演和数值优化,该方法得到了对流层散射最坏月与年平均传输损耗的时间概率转换与地面折射率Ns的对应关系。该方法不仅解决了上述2种ITU模型的缺点,而且更加简单实用,适用范围更广。随后,为了验证该方法的准确性和全球适用性,利用该方法模拟了全球对流层散射数据库中各实验链路典型概率(50%、90%、99%)下的最坏月传输损耗,并与实测值和ITU-R P.841模型进行了对比验证。研究结果对对流层散射传播的发展和应用有一定的参考价值。     

基于不同散射相函数的紫外光传输特性研究

为了研究三种不同散射相函数下紫外光大气传输特性,文章采用非直视紫外光通信的单次散射简化模型,对系统接收机接收到的能量及紫外光传输路径损耗随传输距离和散射角的变化情况进行分析.结果表明:三种相函数下,都表现为随传输距离的增大,接收能量减小,路径损耗增大,但三者之间存在一定的差异,对于紫外光通信系统,其传输性能的影响以前向散射为主,后向散射作用很小;随着散射角增大,后向散射作用相对明显;三种散射相函数下,紫外光通信链路的最佳散射角不同,但均随发射仰角的增大或接收仰角的增大最佳散射角增大,而接收能量减小.     

散射通信在山区的应用研究

结合山区地形的特点,针对山区的远距离越障通信需求,分析了散射信号在山区地形下的电波传播模式,证明了散射通信的理论可行性。为检验散射通信的实际通信效能,开展了散射通信试验。通过对试验结果的分析,评估散射通信的应用效果,证明了散射通信能够满足在山区地形的通信保障需求。提出了采用速率自适应方法对山区通信手段进行改进,以提高通信效能。     

基于对流层散射信道的时间同步技术研究

针对对流层散射时间同步系统高损耗、信道延迟估计难度大以及多径效应明显等问题,在介绍分布式系统时间同步方案的基础上,结合对流层散射通信传输距离远、抗干扰能力强以及越障能力强的优点,对基于对流层散射通信的分布式时间同步系统进行了简要介绍,分析了主要误差来源,给出了分集接收结合双向时间比对的时间同步解决方案;对接收电平、相干带宽等信道参数的变化进行了仿真分析,结果表明,该解决方案可有效提高基于对流层散射通信的分布式时间同步系统的时间同步精度,具有可行性。     

高仰角对流层散射传播损耗的改进预测方法

鉴于ITU-R P.617建议只可用来计算低仰角而不适合于高仰角情形下的对流层散射传播损耗,提出了一种适用于高仰角对流层散射传播损耗的改进预测方法.分析了不适用于高仰角情形的原因,发现散射体离收发连线的高度H可能小于离地高度h,而通常的物理含义下H > h.因此根据散射几何模型,替换了原有的散射体高度表达式,在近似条件下令两个散射体高度相等,将新的散射体高度表达式代入计算.链路预测结果表明:改进的简单预测方法极大地改善了原有预测结果,更加接近实验结果,且与适合于高仰角的复杂美国国家标准局（National Bureau of Standards,NBS）方法预测结果基本一致.     

Worldwide techniques for predicting the multipath fading distribution on terrestrial LOS links: comparison with regional techniques

A revised worldwide method has been recommended by the Radiocommunications Sector of the International Telecommunication Union (ITU-R) in Recommendation ITU-R 530-8 for predicting the deep-fading distribution due to multipath propagation in the average worst month on VHF-EHF terrestrial line-of-sight links. This paper presents detailed testing results for the revised method on a 239-link database from 22 countries around the world in comparison with results for the leading regional methods still frequently used by some link designers for worldwide applications (viz., the Barnett-Vigants (1972, 1970) method of the United States and the Morita (1970) method of Japan). An evaluation of the form of the prediction equation is given for the revised and previous ITU-R methods and several regional methods. The revised ITU-R method is shown to perform significantly better than the other methods for both overland and coastal/overwater links.     

Worldwide techniques for predicting the multipath fadingdistribution on terrestrial LOS links: background and results of tests

In 1988, the International Telecommunication Union (ITU) adopted new methods for predicting the deep-fading distribution due to multipath propagation for the average worst month on ultra high-frequency (UHF)-super high frequency (SHF) terrestrial line-of-sight (LOS) links. Employing refractivity gradient statistics available from world maps, the methods were recommended for application in all regions of the world. One method for the deep-fading range did not utilize detailed path profile information and was designed for preliminary planning or licensing purposes. A second method, which did employ the path profile was intended for more detailed design purposes. A multipath fading data base comprising some 246 links (including 34 over water) in 23 countries of the world has now been assembled for testing and revising these methods. Revisions to the geoclimatic model used in both methods and an associated method for predicting the shallow-fading distribution have been adopted by Study Group 3 of the new radiocommunications sector of the ITU (ITU-R). This paper summarizes the basis of the original and revised versions of the deep-fading prediction methods and presents the results of the most extensive tests to date. Some discussion of potential future revisions is also provided     

雷达仙波的抑制方法研究

仙波在特殊的气候条件下出现,会导致雷达终端显示器出现成片的慢速运动“目标冶,从而严重影响雷达的性能。文中结合某中重频雷达的实测数据,分析了雷达实测数据中仙波的特征,根据仙波的多普勒频率特征提出了分别基于自适应动目标显示、自适应动目标检测(MTD)处理的仙波抑制方法。由于仙波的多普勒频率未知,提出一种在MTD处理基础上对检测结果所在多普勒通道进行简单判断的仙波抑制方法。实测数据处理结果表明,三种方法都对仙波具有良好的抑制作用。     

A study of the relationship between multipath distortion and wavenumber spectrum of refractive index in radio links

The study reported here is concerned with relating the multipath and frequency selective fading characteristics of radio links caused by scatter phenomena to the statistical spatial characteristics of the refractive index. Two specific antenna configurations are studied in detail: the LOS (line-of-sight) case where the two antennas are pointed concentrically and the OTH (over-the-horizon) case where the antennas point along a great circle path. For each of these antenna configurations two scatter situations are considered: isotropic scatter with the propagation medium controlling the focusing of energy and locally isotropic scatter with narrow-beam antennas. In all cases single scattering theory is used and narrow-band signal transmission is assumed. It is found that when multipath is of any importance in causing signal distortion for a LOS link, the scatter portion of the channel may be modeled as a continuum of uncorrelated scatterers. Also OTH links are found to be characterizable in the same way. Single integrals and specific analytic and numerical examples are given relating frequency correlation functions and delay power spectra to the wavenumber spectra of assumed refractive index fluctuations. It is shown that a measurement of the frequency correlation function or delay power spectrum can yield an estimate of the wavenumber spectrum of the refractive index fluctuations.     

Wideband CDMA system for personal radio communications

This article describes a wideband spread code-division multiple access (W-CDMA) system for high-capacity and high-quality personal radio communication. This system has been authorized as an EIA/TIA Interim Standard IS-665, T1P1 Trial Use Standard J-STD-015, and ITU-R Recommendation M 1073. The system uses wideband spreading to accomplish good interference immunity, high-quality speech, and high-speed data transmission. The system uses coherent detection (CD) and an interference canceller system (ICS) to enhance the capacity. The CD and ICS use continuous pilot signals in the forward/reverse links to estimate the propagation path parameters. PN and Hadamard sequences are used as the spreading code for minimal mutual interference between traffic and pilot/sync/paging channels. A robust 32 kb/s waveform speech coder, ITU-T COM101+, has been developed to achieve toll-quality speech in the radio environment. This system provides up to 128 voice channels per cell and data transmission up to 64 kb/s by 5 MHz spreading. Unification of low- and high-mobility applications and reduced complexity of system and hardware configurations are accomplished     

Electromagnetic Waves Attenuation due to Rain: A Prediction Model for Terrestrial or L.O.S SHF and EHF Radio Communication Links

Because of the interest raised for SHF and EHF radio communications, the attenuation of electromagnetic waves by rain will always constitute a major concern for telecommunication engineers and scientists. The rain attenuation prediction models exposed in literature calculate the attenuation related to a given rain rate or else to a given percentage of time. The new model proposed in this paper, predicts with a good accuracy the percentage of time for which any given rain attenuation will be exceeded on terrestrial SHF, EHF radiowaves links, provided the rain rate R₀₀₁ (mm/h) that represents rain rate value exceeded for 0.01% of time in the locality of interest is available. R₀₀₁ (mm/h) data being available for most of the localities across the world in ITU-R data base, we may conclude that this new model proposed here, can be broadly and successfully used.