UHF頻帶電波在微細胞都會區環境之傳播機制

Abstract

在本論文中，我們分別提出環境適應式(site-specific)與合併式兩種電波散射模型來研究UHF頻帶電波在微細胞都會區環境傳播之平均衰損與衰落等特性。 首先，我們發展一種三維環境適應式電波散射模型來預估電波在微細胞都會區環境之傳播平均衰損值。由於都會區建物牆壁之結構與材質大多隨戶而異，所以用「平均建物材質」與「尺寸無限大平板」建立街道牆面模型，忽略各戶之差異性，可能會低估傳播衰減。環境適應式電波散射模型結合嵌片式牆壁模型，利用極化散射矩陣評估嵌片式牆壁之散射場，並同時考慮直接波與地面反射波之貢獻，其預估結果將比傳統式射線追跡模型能更準確地預估電波之傳播平均衰減值。此預估電波傳播模型之優異效能已用量測資料驗證。嵌片式牆壁模型將使用不同介電係數值與不同尺寸之嵌片來描述街道兩旁之牆面。 其次，我們也另外發展一種結合環境適應式電波散射模型與統計式散射模型之合併模型，能準確地預估都會區微細胞通信波道中收信強度之統計特性。在此模式中，環境適應式電波散射模型是用來預估決定性場強，統計式散射模型則是用來預估隨機散射場，這些隨機散射場強主要是由一些位於接收機附近隨機之散射體所造成。在研究中，發現隨機散射場對於區域性之信號衰落分佈影響甚大，並且發現此隨機散射場強度與傳播距離成對數線性遞減變化關係，而此遞減趨勢在三個截然不同的量測地區均相似。另外在數值模擬分析中，也發現發射天線高度之變化對收信小尺度衰落特性有顯著之影響，而相形之下，牆壁嵌片尺寸或街道寬度之變化對其影響則較輕微。 此外，根據1.8GHz都會區微細胞之電波傳播量測資料，在本論文中提出一種有關Rician參數之隨機分佈模型，發現此Rician參數之機率密度函數為一對數常態分佈型式。

This thesis presents a site-specific and a statistically scattering models to investigate the characteristics of UHF radio propagation in microcellular urban environments. First, a novel 3-D site-specific scattering model is developed to evaluate the average path loss of a microcellular radio channel in an urban environment. The analytical scattering model combined with a patched-wall model predicts the median path loss more accurately than the conventional analytical ray-tracing model in the cases studied. Comparing the path loss with the measured one at 1.8-GHz demonstrates the effectiveness of the scattering model. The scattering model includes three major propagation modes: (1) a direct-path wave; (2) a ground-reflected wave; and (3) the scattered field from the walls aligned along a street. The proposed model, with a polarization scattering matrix associated with the patched-wall model, aptly describes the third mode, which is usually neglected or oversimplified. Second, an analytical hybrid scattering model, combining a site-specific scattering model and a statistically scattering model, is developed to evaluate the statistical characteristics of received power for microcellular radio channel. The former model yields a deterministic prediction of the received power. The later model describes the scattered field due to the randomly positioned scatterers around the receiver. From this study, it is found that the randomly scattered field gives a significant effect on localized fading distribution. The mean power of the randomly scattered field is linearly dependent on logarithm of propagation distance and the decay factors at three different sites are similar. From the simulation results, it is also found that changing transmitting antenna height may yield a significant effect on the small-scale fading characteristics. However, varying the patch size or street width may have little effect on the fading distribution. In addition, a model of the random distribution of the Rician factor is suggested from the measurement of 1.8 GHz radio propagation in outdoor urban microcells. It is found that the probability density function (pdf) of the Rician factor for low tier systems in urban environments follows a lognormal distribution.