從網路觀點對天線技術於多用戶排程及分時雙工/分碼多工無線系統之研究

Abstract

應用天線技術所帶來的高容量增益，雖然可以讓使用者在不增加頻寬的環境下，提高無線通道的傳輸速率，但在實際應用該技術的同時，卻也可能面臨兩項主要的議題－硬體實現複雜度，以及可能的鏈路可靠度降低。針對上述兩項議題，傳統研究多採用實體層技術的解決方案，然而在本論文中，則嘗試從網路觀點來提供另一種解決問題的方法－開發並利用網路中已經存在的資源(例如用戶分集)，幾乎不需要額外增加成本，就可以提供更多解決問題的維度。在本論文中，除了印證利用網路觀點所得到的用戶分集，可以解決應用天線技術過程可能面臨的兩項議題外，更重要的是，若應用這整個網路觀點的設計概念，於其他通信系統的設計上，將有機會增加該系統的運作效能。

首先，我們考慮以分集為基礎(diversity-based)的天線技術在多用戶排程系統的應用，在這裡我們推導出一個可以同時整合用戶維度、天線維度、以及通道特性維度的公式，利用這個系統容量公式，可以說明多用戶排程技術的特性，以及解釋排程技術與天線分集之間的交互作用。分析結果顯示用戶分集是類似天線系統中的選擇分集，因此如果在一個多用戶通信系統中已經存在了很大的用戶分集增益，這時若使用以分集為基礎的天線技術，該技術所帶來的額外好處將會受限。另外，我們也說明了在多用戶排程系統中，若使用時空塊狀編碼(space time block code)天線技術，甚至可能導致系統容量損失。 其次，我們考慮以多工為基礎(multiplexing-based)的天線技術在多用戶排程系統的應用。近來的研究顯示，使用以多工為基礎的天線技術，可能會犧牲使用以分集為基礎的天線技術所帶來的好處－鏈路穩定度，因此，我們提出了一個「最弱子通道優先」(strongest-weakest-normalized- subchannel-first, SWNSF)排程演算法，淬取出多用戶系統中的用戶分集來補償天線分集的不足，也就可以解決上述鏈路穩定度不足的缺點。接著，我們嘗試在一個使用SWNSF排程演算法的多用戶排程系統中，使用一個簡單的零強制(zero-forcing)接收器，當用戶數趨近無窮大時，我們證明零強制接收器可以是一個最佳接收器，換句話說，應用以多工為基礎的天線技術於多用戶排程系統時，可以用低成本的方式，同時增加系統容量與鏈路穩定度。 最後，我們考慮波束合成天線技術在分時雙工/分碼多工系統的應用， 我們建議，使用波束合成天線技術，來解決基地站與基地站之間的強干擾問題，利用鄰近不同基地台之間的合作，我們介紹一個同時上鏈路接收與下鏈路發射的波束合成機制，來降低在分時雙工/分碼多工系統中出現的基地站對基地站之間的強干擾，分析結果顯示，利用我們建議的波束合成機制，可以在低成本的狀態下，有效消除基地站對基地站之間的強干擾問題。

The huge capacity gain offered by the multiple-input multiple-output (MIMO) antenna technique can achieve higher data rates on wireless channels without sacrificing bandwidth efficiency. The primary challenge to apply the MIMO technique lies in the implementation complexity and the possible side effect of reliability performance degradation. Traditional efforts to resolve these issues associated with the MIMO technique are mostly based on the physical layer treatment. Differently, this dissertation presents a network perspective approach to revisit the MIMO technique with an aim to provide an alternative settlement. The main advantage of the network perspective approach is a full exploitation of existing dimensions in the network, e.g. the inherent multiuser diversity, with a negligible cost. More important than the results presented in this dissertation, however, is the hope that the network perspective methodology here will provide an innovative and flexible design paradigm for wireless systems.

First, the category of fading mitigation based (or diversity-based) MIMO antenna schemes is investigated for the multiuser scheduling system. A unified capacity formula connecting multiple domains of users, antennas and fading characteristics is derived. The analytical capacity formula is powerful in extracting the essence of multiuser scheduling and interpreting the interplay of antenna diversity and multiuser scheduling. Our analysis indicates that the scheduling technique intrinsically delivers multiuser diversity with an analogy of selection diversity in the multiple antenna system. As a result, we show that the additional capacity gain with the use of fading mitigation based antenna techniques is somewhat limited because user population has contributed a large order of selection diversity in the multiuser scheduling system. It is also demonstrated that employing the space time block code (STBC) methods for the multiuser scheduling system may even cause a capacity loss due to the reduced amount of fading gain but without the supplement of array gain.

Second, the category of throughput enhancement based (or multiplexing-based) MIMO antenna schemes is studied for the multiuser scheduling system. Motivated by a recent result of diversity-multiplexing tradeoff in the MIMO system, we propose using the multiuser diversity to replenish the diversity-deficient spatial multiplexing MIMO system. Particularly, we develop a novel strongest-weakest-normalized-subchannel-first (SWNSF) scheduling algorithm, requiring only scalar feedback, to enhance the degraded reliability performance of the MIMO system. Our analysis and results indicate that the SWNSF scheduling can significantly increase the coverage of the multiuser MIMO system while improving the system capacity. Furthermore, we consider a simple spatial multiplexing MIMO system with the zero-forcing receiver. Somewhat surprisingly, from a multiuser scheduling network perspective, we prove that the zero-forcing receiver can be asymptotically optimal in the multiuser scheduling system as the number of users increases to infinity. In other words, the marriage of multiplexing-based MIMO antenna schemes and multiuser scheduling techniques can achieve an elegant cross-layer synergy of providing further capacity and reliability performance improvements in a cost-effective manner.

Finally, the type of interference suppression based antenna schemes (or antenna beamforming) is investigated for the time division duplex/code division multiple access (TDD/CDMA) system. We propose using antenna beamforming techniques to resolve the opposite direction interference problem in the TDD/CDMA system. Through exploiting the multiple antennas of adjacent base stations from the network point of view, we introduce a simultaneous downlink transmit beamforming and uplink receive beamforming scheme to alleviate the impact of the opposite direction interference. Our analysis and results demonstrate that the proposed antenna beamforming scheme can effectively suppress the opposite direction interference with an economical implementation cost.