中繼器支援之多輸入多輸出系統在直視環境下的通道容量增益及覆蓋範圍分析

Abstract

多輸入多輸出系統(MIMO)最多能夠提供隨「使用天線數目」線性增加的通道容量(channel capacity)增益。然而在直視環境(line-of-sight)此種缺乏散射的環境下沒有辦法有如此優秀的增益表現。文獻上曾提出利用人為製造的反射物，如中繼器(repeater)等，讓接收端能收到從四面八方入射的信號，可以讓直視環境下依然能有線性增加的通道容量增益。 在本文中，我們首先討論在固定佈建有限個中繼器之後，能享受到線性增加通道容量增益的涵蓋範圍。我們的分析結果指出在適當參數之下，在相當廣的區域範圍裡使用者皆能享受到MIMO系統所能提供的高線性通道容量增益。在文中我們以數學方程式表示能有增益的覆蓋區域範圍；此範圍會與基地台、中繼器位置、陣列天線距離、陣列天線轉向等有關。 由於通訊使用者具有移動的特性，線性通道容量增益倍率(capacity scaling factor)亦隨其位置與天線陣列轉向而變化。在本文中我們進一步討論天線陣列的幾何形狀及陣列轉向對線性通道容量增益倍率的影響。我們推導出在有兩個中繼器的環境下，若天線陣列的轉向不定，能夠有兩倍通道容量增益的機率為何，並發現不論在行動端的天線陣列幾何形狀為何，此機率在傳輸涵蓋範圍裡有固定形狀的幾何分佈。我們的結果指出，利用中繼器輔助，在很大的覆蓋範圍裡行動端都能有線性增加的通道容量增益。此外，當陣列天線的轉向不定時，此增益的有效機率可藉由使用非線性的天線陣列來提升。

Multiple-input multiple-output (MIMO) technique provides capacity improvement which scales linearly with the number of antennas. However, such improvement cannot be realized in line-of-sight (LOS) environments due to lack of scattering. It had been proposed that, in LOS environments, if a mobile station can receive signal from multiple repeaters, the artificial multipath could render an equivalent high-rank end-to-end MIMO channel. In this dissertation, we first investigate whether fixed-location repeaters can induce significant MIMO capacity gain throughout the coverage of a cell in a LOS environment. Our results indicate that, for a wide range of repeater locations and antenna parameters, the majority of the coverage area of a cell can support a high-order capacity scaling factor. We further show that for a capacity scaling factor of 2, the corresponding coverage consists of a small number of simply-connected areas. We present the expressions for the contour of these areas as a function of base station and repeater locations, antenna spacings, and antenna orientations. Next we investigate the effect of antenna geometry and orientation on the capacity scaling behavior. A MIMO system aided by two repeaters in a LOS environment is considered. For a particular MS location, the supporting MIMO gain can be quite different depending on the array orientation. Due to the mobile nature, the MS antenna array may not usually orient to a suitable direction to enable the MIMO gain. In light of this concern, we derive the formulation of the two-eigenmode availability, defined to be the probability of a mobile location enjoying two useful spatial degrees of freedom given a random antenna orientation. Regardless of the antenna geometry, locations of the same eigenmode availability must constitute one or more horseshoe curves. Our result shows that the repeaters can provide MIMO gain with a high probability over a large area.