【作者】 弓树宏；

【导师】 黄际英；

【作者基本信息】 西安电子科技大学 ， 无线电物理， 2008， 博士

【摘要】 电磁波在对流层晴空大气及降雨、降雪、沙尘等气象环境中的传输效应对无线电电子系统有重要的影响,电磁波在对流层中的传播与散射特性研究,是一项既取得了一定的理论和实验成果同时也面临着新挑战的课题。随着无线电电子技术的发展,电波传播工作者开始考虑一些新问题,例如:未来的通信频段和对信号的传输质量要求将会不断提高,那么电磁波在对流层中传输、散射效应的计算及预报模型的精度能否适应新频段、新技术和新精度的要求;空间环境控制和人工改变技术是空间环境军用和民用的重要方面,能否通过某种人为干预手段,改善无线电电子系统通信效果,拓展其在军事、民用等方面的应用,开发新的通信技术;如何结合随机介质中的波传播理论,研究和评估信号在对流层中的传输、散射效应对新的通信技术的影响机理及影响程度,如何对抗这些影响。所以,研究电磁波在对流层中的传输、散射特性,对提高无线电电子系统的性能和发展无线通信技术有重要的理论意义和实用价值,本文就电磁波在对流层中的传输、散射的若干问题展开研究,主要工作及研究成果如下:研究了声波干扰改变晴空大气折射指数起伏特性的机理,推导出了大气折射指数起伏特性与声波参数的关系,得到了声波干扰下单位体积散射截面的计算公式,且做出计算、仿真和讨论。结合降雨、沙尘等离散随机介质的介电特性研究状况,分析了部分现有模型的特点,提出一个改进的计算离散随机介质等效介电常数的计算模型,以降雨环境为例进行了计算、仿真和讨论,并且对模型进行了分析和评估。利用本文得到的等效介电常数计算模型,提出了修正计算雨致特征衰减的ITU-R模型中参数的方法,该方法可以结合特定地区降雨的物理特征,更精确地考虑雨致特征衰减ITU-R计算模型参数受雨滴尺寸、分布和温度等影响,提高对雨衰的计算精度。以Weibull谱为例做了计算、仿真和分析,得出了Weibull谱分布情况下,对ITU-R模型参数的修正结果,利用修正结果计算了雨致特征衰减,并且与实测结果做了比较。拓展了现有文献中利用非一分钟降雨率预报雨衰的建模方法,提出了利用10分钟累积降雨率预报降雨衰减的建模方法。以我国部分地区的10分钟降雨率测量结果和鑫诺I号等卫星参数及雨衰测量结果为例,建立了利用10分钟降雨率预报雨衰的模型,对模型做出评价和分析。分析了降雨和雨衰时间序列的特点,以雨衰时间序列为基础,建立了预报雨衰短期、实时、动态特性的ARIMA预报模型。利用实验测量的雨衰时间序列的频谱特性,对文中预报模型得到的降雨衰减实时、动态序列进行了检验,并且对本文得到的模型进行了评估和分析。以圆柱目标和降雨环境为例研究了随机离散介质环境中目标的信杂比,研究结果表明,信杂比最优化与收、发天线极化状态密切相关,而且雨滴粒子的形状对最优信杂比对应的极化状态影响明显,所以实际应用中需要根据降雨环境物理特征,计算目标回波信号信杂比与收发天线极化状态关系,以便及时调整天线最佳极化状态,使得接收信号信杂比最优化。本文的讨论方法对沙尘和降雪等环境也适用。分析了多径传输环境下接收端信号特点,以及研究接收信号包络概率密度函数的必要性。用随机介质中的波传播理论,研究、分析了降雨、降雪等恶劣气象环境对多径信道包络概率密度的影响机理,分析了在降雨、降雪等环境中多径传输模式可能发生的变化。以降雨环境和全向天线为例仿真、计算了降雨对多径信道包络概率密度的影响。对本文的仿真结果做出了分析,结果表明本文处理问题的方法和研究结果,对于高增益、窄波束天线情况也适用。研究了对流层大气及其中沉降粒子对通信系统噪声的影响机理及重要性。以本文研究结果为基础,仿真了ITU推荐各雨区的降雨特征衰减的年时间统计分布,利用本文得到的雨衰预报模型,以鑫诺I号等卫星参数为例,仿真了我国部分地区降雨导致的噪声温度年时间百分比分布,以及由于降雨影响导致系统G/T下降的年时间百分比分布。研究结果表明,降雨对噪声温度的贡献不可忽略,在具体的通信系统需要结合特定地区降雨、降雪等气象环境的物理特征,精确分析衰减和散射特性,评估它们对系统噪声的影响。探讨了恶劣气象环境如何影响MIMO系统,分析了降雨、降雪等对流层气象环境可能对MIMO系统的影响。从理论角度探讨了降雨、降雪等对流层气象环境对MIMO系统信道容量的影响,以降雨环境和特定的天线结构为例,仿真了降雨衰减对MIMO系统信道容量的影响,结果表明降雨衰减对MIMO系统信道容量的影响明显。因此,对MIMO通信系统的设计过程中有必要考虑对流层气象环境的影响因素。研究了声波干扰下对流层散射通信系统的散射损耗,将声波干扰下的散射损耗与没有声波干扰情况下散射损耗进行了对比,并且对研究结果做出分析、讨论。研究结果表明,声波干扰可以有效减小对流层散射通信散射损耗,声波干扰可以有效改善散射通信系统的性能和稳定性,声波干扰对流层对扩大散射通信系统应用领域可望有广阔的应用前景。更多还原

【Abstract】 The most important impacts on wireless communication systems is from troposphere, clear atmosphere and some meteorologic environments, for example rain, snow, dust-storm and so on. The study on some problems for radio wave propagating and scattering in troposphere is a project, about which some theoretical and experimental results have been given, but there are some new challenges. Some new problems are presented by pursuers on radio wave propagating with the developing of radio-technology, For example: If the precision of calculating and predicting models can satisfy the new request for new frequency, new communication technology, and new precision in future more higher frequency and more higher desire of signal quality; It is important application for military affairs and civilian affairs to control and remodel space environment artificially. if some man-made disturbing can improve the communication quality of radio system, extend the applying fields for military affairs and civilian affairs, exploit new radio communication technology; How to investigate and evaluate the mechanism of the impacts induced by radio wave propagating and scattering in troposphere, how to investigate and evaluate the influencing degree and how to mitigate the effects, using the theory of radio wave propagating through random media.It has the theory and practicality significance for improving performance of communication systems and developing communication technologies to study the propagating and scattering characters of radio wave passing through troposphere. Some problems about propagating and scattering for radio wave through troposphere are studied in this thesis. The main investigated contents and results are following:The mechanism about disturbing the index of clear atmosphere by acoustic wave is investigated. The formulae calculating the scattering cross section of unit volume under acoustic wave disturbing are deduced. The relation between the fluctuant characteristics of refraction index of clear atmosphere and the parameters of acoustic wave is found. Some simulation and discussion are given. The status about the permittivity of meteorologic environments, for example rain, snow, dust-storm and so on. are investigated. The specialties of existing models are analyzed. A improved model for studying the equivalent permittivity of meteorologic environments is given. Some calculating, simulating and discussing are given, rain as an example.Using the model of calculating equivalent permittivity, the method for modifying the parameters of ITU-R model for calculating the specific attenuation induced by rain is discussed by detailedly analyzing the exiting model. The method can improve the precision of evaluating rain attenuation by take the effects of raindrop shape, size distribution, average temperature etc. into account. Some calculating and analyzing are given, and the parameters of ITU-R model for calculating rain-induced specific attenuation are modified under Weibull size distribution. Rain-induced specific attenuation is calculated using the modified results, and that is compared with measured data in Xi’an.The models for predicting rain-induced attenuation are investigated in detail. The method of modeling predicting rain-induced attenuation using no one minute rainfall rate is investigated. A new viewpoint of predicting rain-induced attenuation using 10-minute rain rate is discussed. Based on the rain-induced attenuation of the satellites of Xinnuo-No.1 etc. measured in some areas in China, and the ten minutes rainfall rate measured in Haikou etc., the steps and processes of modeling are researched. The predicting results using the model given in this thesis are evaluated using measured data.Based on the in-depth investigating the statistical peculiarities of rain-induced attenuation time series, the time series of rain-induced attenuation are simulated. An ARIMA model for forecasting the dynamic specialties of rain-induced attenuation is modeled. The spectrum of predicting results using the model given in this thesis accords with that of measured rain-induced attenuation time series. The model given in this thesis is analyzed and evaluated.The SCR(Signal-to-Clutter-Ratio) of a cylinder target in rain is discussed. The results show that the effect of shape of raindrop on SCR changing with polarization state is obvious. So, it is necessary for adjusting polarization state of antenna that SCR need be calculated taking the characters of rainfall in certain area, for example the shape of raindrop, into account. The method used in this thesis is available for snow, dust etc.The characters of received signal in multi-path channel are discussed. The necessary to study the PDFs (probability density functions) in multipath channels is analyzed. The mechanism of weather phenomena impacting on the PDFs in multipath channels, rain, snow etc. for instance, is investigated and analyzed by the theory of radio wave propagating in random media. Some computing and simulating about the effect are made under the assuming of rain environment and isotropy antenna. The simulating results are analyzed. The way used in this thesis is valid for antenna with high gain and narrow beam. The mechanism of producing noise to communication systems by troposphere is investigated. The importance of the effects is analyzed. Based on the introducing for atmospheric noise temperature and antenna noise temperature, the contribution of rain and other sedimentation particles to antenna noise temperature is discussed. The annual CDFs (Cumulative Distribution Functions) of specific attenuation for every zone ITU recommended are simulated based some results in this thesis. The annual CDFs of noise temperature and the reduction of G/T induced by rain in some areas in China are discussed by the prediction model for rain attenuation under the link parameters of the satellites of Xinnuo-No.1 etc. The results show that the contribution of rain to noise temperature can not be neglected. It is necessary to invested in detail the effects of weather phenomena on antenna noise temperature, for instance rain, snow, dust-storm and so on.The possible impacts induced by troposphere weather phenomena, rain, snow, dust-storm etc. for instance, on MIMO communication systems are theoretically introduction. And, it is theoretically analyzed that how troposphere weather phenomena, rain, snow, dust-storm etc. for instance, impact on MIMO communication systems. The effect of attenuation induced by rain or snow etc. on MIMO channel capacity are theoretically discussed, which is computed and simulated under rain environment and given antenna. The results show that it is necessary to take the impacts induced by troposphere weather phenomena, rain, snow, dust-storm etc. for instance, on MIMO communication systems into account.The theory for calculating scattering loss in troposphere scattering communication is analyzed. Scattering loss under acoustic wave disturbing is simulated, which is discussed and compared with that under no acoustic wave disturbing. The results show that the performance and stability of troposphere scattering communication system can be improved by acoustic wave disturbing, which has the theory and practicality significance for extending the applying fields of troposphere scattering communication.更多还原