【作者】 王仲园；

【导师】 金荣洪；

【作者基本信息】 上海交通大学 ， 电磁场与微波技术， 2009， 博士

【摘要】 模拟电视信号功率历来采用同步脉冲到达时刻传输信号的视频峰值功率测量值,模拟电视的服务场强按F(50,50)(50%地点概率50%时间概率)曲线预测。模拟电视的最小可接收信号功率以天线端噪声系数与接收系统载噪比(C/N)之和为基准,其中美国联邦通信委员推荐的UHF频道天线端噪声系数值为7。数字电视发射机的输出信号类似于高斯噪声,数字电视信号功率是采用以往定义无线电噪音功率的均方根值。在美国数字电视服务场强按F(50,90)曲线规划,中国行标GY/T 237-2008规定移动数字电视按99%的地点概率要求进行覆盖。覆盖时间与地点概率提升意味着信号场的增强。交通线路上的接收天线端噪声系数值大、噪声成分(加性高斯白噪声(AWGN)与干扰脉冲)也发生了变化。DTTB的C/N、载干比(C/I)及移动环境下的信道衰落规律都不同于模拟电视。因此,模拟电视覆盖预测模型难以准确预测移动数字电视的服务现状,车载移动电视接收效果不尽如人意的现象时有发生。然而,有关交通线路上数字电视信道噪声与干扰特征以及信号路径增益特征的文献很少,故本文重点研究UHF频段都市移动数字电视信道的噪声模型和信号路径增益模型。本文给出了交通线路上某UHF频道的平均噪声功率测量系统、方法与结果,提出了采用遗传算法回归分析该数字信道噪声系数概率累积分布曲线的方法,不仅获得了城市内三类交通线路信道噪声三参数weibull分布的三元组合模型,而且列出了噪声系数的90分位数及其K倍标准差值,据此建立了以噪声系数90分位数与C/I之和为基准的干扰信道最小可接收信号功率模型。移动数字电视测量结果表明,以噪声系数50分位数与C/N之和为基准的白噪声信道最小可接收信号功率模型的覆盖率约40%,干扰信道最小可接收信号功率模型的覆盖率约86%。本文给出了UHF频段某数字电视频道信号场强的固定与移动测量系统、方法及结果,提出了采用以物理光学、几何光学、几何绕射理论及一致性绕射理论为基础的三维多径射线跟踪方法,研究移动数字电视在城市复杂交通环境下信道特征的。运用双射线模型揭示了天线高度及其与高架桥桥面中心轴距离的变化与桥下阴影区域的形成、范围大小及构成改变的关系。考虑到四方面环境因素(直射、地面反射、桥面透射和桥劈绕射),建立的六射线跟踪模型计入了复介电常数桥面的高阶透射波与桥劈绕射波。模型计算结果反映了水平与垂直极化波在桥下覆盖直视区与阴影区的路径增益规律,并通过比较说明移动接收的阴影区路径增益规律可涵盖直视区。通过实测数据与Okumura曲线、标准GY/T 237及六射线跟踪模型预测值相比较,提出了修正的双径模型能够预测桥上道路的路径增益,修正的Okumura能够预测江面航线的路径增益,修正的六射线模型更符合桥下道路的路径增益规律;忽略了路边建筑物阻挡影响的六射线模型,在附加反映建筑物透射损耗的十几dB后,能够精确预测桥下道路上的传播路径增益。更多还原

【Abstract】 The power of an analog TV signal is always measured as visual peak power, that is, the power level reaches while the synchronizing pulses are being transmitted. For analogy stations, the F(50, 50) curves predict minimum field strength for 50% of the locations and 50% of the time. The minimum signal power for demodulating analogy TV is based on the sum of the noise factor at the end of receiving antenna and carrier-to-noise ratio (C/N) of the receivers. (Noise factor is 7dB recommended by Federal Communications Commission (FCC)in UHF band.) The output signal of digital TV transmitters resembles a Gaussian noise signal. The only way to define the power of a digital TV signal is a root mean square value, which has been applied to define the power of radio noise. In the U.S., F (50, 90) defines digital television service; Chinese professional standard GY/T 237-2008 defines a mobile digital TV service with location availability of 99%. Increasing the location and time availability percentages requires additional signal margin. On the transportation courses, the noise factor at the end of receiving antenna is big; and noise components (WGN and impulsive interference) vary as well. The C/N and C/I (carrier-to- interference ratio) of digital television terrestrial broadcasting (DTTB) receivers are different from those of the analogy TV receivers. The law of path gain of DTTB on the transportation courses is also different from that of the analogy TV. Therefore, there occurs ocassional failure when these statistical curves for analog TV coverage are applied to predict service margin of mobile digital TV. However, the study on the characteristics of noise and impulsive interference in DTTB channel and signal path gain on transportation courses are few. Therefore, this paper focuses on models of noise, impulsive interference and signal path gain of mobile digital TV channel on urban transportation course in UHF band.The measurement system, procedure and surveying data for average noise power of a DTTB channel in UHF band on public transportation courses were introduced. The application of genetic algorithm regression analysis to analyze the cumulative distribution curves of the noise factors in digital channel was presented. As a result, not only three-component finite mixture distribution of three-parameter Weibull cumulative distribution functions of noise factors but also the K times standard deviation of the noise factors’upper decile under three typical communication environments were obtained. A coverage model of minimun signal power for decoding based on the noise factors’upper decile plus C/I (assumed impulsive interference channel) was suggested, in contrast with that based on the noise factors’50 deciles plus C/N (assumed AWGN channel). Through the trials of mobile reception of digital television, the results showed that the acceptable percentage of the model based on interference statistical distributions was about 86 %, and that based on AWGN noise is about 40%.The measurement system, procedure and surveying data for field strength of a DTTB channel in UHF band on public transportation courses were introduced. Three-dimension multi-ray tracing models based on physical optics (PO), geometrical optics (GO) and geometrical theories of diffraction (GTD) and especially the uniform theory of diffraction (UTD) were presented to analyze transmission characteristics of a mobile digital TV on urban complex communication environment. A 3-dimensional two-ray tracing model revealed the relations between the formation, range, and components of the shade on the road under viaduct and the varying level of antenna and the horizontal. Taken the four environmental factors (direct projection, ground reflection, transmission as well as bridge corner diffraction )into consideration, a six-path ray tracing model including the high-order transmission wave for viaduct deck with complex dielectric constant and the diffraction wave from the viaduct corner was created. Fading condition in LOS region and shadow region under viaduct was illustrated by means of calculating propagation path gain of different polarization waves through different paths. Having compared the range of path gain in the two regions above, it was revealed that path gain model in shaded region could cover that in LOS region. Having compared the test data with Okumura, Chinese professional standard GY/T 237 and the six-ray tracing model, this paper has come to the following conclusions. A two-ray model (corrected) could predict the path gain of the courses on viaduct. An Okumura curve (corrected) could predict the path gain of the course in the river. A six-ray tracing model in addition to an over ten dB error correction factor to compensate for obstacle loss of roadside buildings could predict the path gain of the courses under viaduct accurately.更多还原