多重天線技術於超寬頻/車對車/中繼通道之分析與設計

Abstract

在近年來，多重天線系統在無線通訊領域中是一個很熱門的研究課題。然而，如何有效的使用多重天線技術取決於如何精確的捕捉無線通道的特性。在這篇論文中，我們進行了在超寬頻，車對車，以及中繼通道下，使用多重天線的無線通訊系統的分析與設計。

在第一個部份中，我們分析了在IEEE 802.15.3a和802.15.4a超寬頻通道模型下，考慮遮蔽效應並使用耙式接收器的位元錯誤率。接著，我們展示了在超寬頻通道中，使用多重傳送與接收天線以及脈衝位置調變的訊號-雜訊比的分析表示式。最後，我們轉向於設計一個在使用多重天線的高度頻率選擇性區塊衰減通道下，使得位元錯誤率為最小的空間-時間-頻率碼。我們的結果定量的指出在IEEE 802.15.3a和802.15.4a超寬頻通道下，遮蔽效應和耙式接收器的手指數目對於位元錯誤率的影響。再者，傳送天線可以用來降低超寬頻接收器的複雜度，因為在一個超寬頻系統中，耙式接收器的手指數目可以非常的高。因為在超寬頻系統中傳送的功率非常的低，我們建議採用多重接收天線改善涵蓋的範圍。最後，和其他的多頻帶超寬頻多重天線系統的空間-時間-頻率碼比較，我們的編碼在位元錯誤率為10^(-4)時，分別有1和8dB的編碼增益。

在第二個部份中，我們推導了車對車萊式衰減通道的自我相關函數、幅度穿越率、以及平均衰落時間。然後，我們建議了一個弦波加總的多重天線車對車通道模擬方法，可以用來描述空間和時間上通道的相關性和萊氏衰減的效應。我們也考察了多重天線的通道容量經歷衰減的頻繁程度以及和萊氏因子的關係。我們證明了我們所提出的使用弦波加總的雙環加上可目視元件的近似模型比單環模型更接近理論值，而且只需要稍微增加一些計算量。更進一步的說，我們發現了對於一個具有固定數目的散射體的多重天線系統，增加天線的數目並不能使得容量呈現線性的增加。當萊氏因子增加時，每一根天線的容量會減少。我們也發現了全部的通道容量和散射環境的豐富程度是有關係的。

在第三個部份中，我們考慮了在中繼通道下，使用解碼轉送結合網路編碼的合作式通訊系統。我們推導了合作式網路編碼協定的中斷機率和分集-多工權衡。我們的結果顯示，中繼節點不但能夠提供合作式的分集增益，也可以提供合作式的多工增益。

總而言之，在這篇論文中我們解決了三個重要，具有挑戰性，而且有趣的問題：(1) 使用多重天線的超寬頻系統下的效能分析和空間-時間-頻率碼的設計；(2) 使用多重天線的車對車隨意萊氏通道下的通道模擬模型的建立，自我相關函數、幅度穿越率、平均衰落時間、以及容量的分析；(3) 在中繼通道下，合作式網路編碼的中斷機率分析，以及分集-多工權衡分析。

Multiple-input multiple-output (MIMO) systems are hot research topics recent years. However, how to apply MIMO antenna techniques effectively is related to how to accurately capture the characteristics of wireless channels. In this dissertation, we perform analysis and design for MIMO wireless systems in ultra-wideband (UWB), mobile-to-mobile channels, and relay channels.

In the first part, we analyze the bit error rate (BER) performance in the IEEE 802.15.3a and 802.15.4a UWB channel models with Rake receiver and shadowing effects. Next, we present an analytical expression for the signal-to-noise ratio (SNR) of the pulse position modulated (PPM) signal in an UWB channel with multiple transmit and receive antennas. Finally, we turn to design BER-minimized space-time-frequency (STF) codes for MIMO highly frequency-selective block-fading channels. Our results quantitatively indicate the effect of shadowing and Rake finger numbers on the BER performance in the IEEE 802.15.3a and 802.15.4a UWB channels. Moreover, we suggest to utilize transmit antennas to reduce the UWB receiver's complexity since the number of fingers of a Rake receiver in the UWB system can be very high. Furthermore, due to low transmit power in the UWB system, we suggest to adopt multiple receive antennas to improve the performance from the view point of coverage extension. Finally, compared with other STF codes for multiband UWB-MIMO communication systems, our code has about 1 and 8 dB coding gain at BER=10^(-4), respectively.

In the second part, we derive the autocorrelation function (ACF), level crossing rate (LCR), and average fade duration (AFD) of the mobile-to-mobile Rician fading channel. We suggest a sum-of-sinusoid MIMO mobile-to-mobile channel simulation method, which can characterize the spatial/temporal channel correlation and Rician fading effect. We examine how often the MIMO capacity experiences the fades and relate this to the Rician factor. It is proved that the proposed sum-of-sinusoids approximation developed from the double-ring with a LOS component model can approach the theoretical value more closely than the single-ring model at a slightly higher cost of computation loads. Furthermore, we find that for MIMO systems with constant number of scatterers, increasing number of antennas cannot linearly increase the capacity. The capacity per antenna is decreased as Rician factor increases. We also find that the total channel capacity is related to the richness of the scattering environment.

In the third part, we consider a relay channel and explore a decode-and-forward (DF) cooperative communications system combined with the network coding. We derive the outage probability and diversity-multiplexing tradeoff (DMT) for the proposed cooperative network coding (CNC) protocol. Our results show that the relay nodes not only can provide cooperative diversity gain, but also cooperative multiplexing gain.

In summary, we have solved three important, challenging, and interesting problems in this dissertation: (1) performance analysis and STF codes design in MIMO-UWB systems; (2) channel simulation model, ACF, LCR, AFD, and capacity analysis for MIMO mobile-to-mobile ad hoc Rician channels; and (3) analysis of outage probability as well as DMT for cooperative network coding in relay channels.