高性能多用戶平行傳輸多重天線架構之研究

Abstract

隨著對高速資料量的不斷需求，多輸入多輸出多重天線技術在現今與未來的無線通訊系統變的極為重要。而經由多重天線技術得到的巨大容量增益將能透過點對多點的傳輸模式更進一步增加。這種點對多點的傳輸能同時支援多組個人化的資料服務，並傳送給多位使用者。在本篇論文中，我們研究透過在不同維度上使用多重天線技術，進而實現個人化的平行傳輸。

在第一個部份中，我們利用多重天線技術在空間維度上，實現個人化的平行傳輸給多位使用者，此技術稱為多重天線廣播系統。首先，我們比較了使用傳送端波束為基礎以及使用接收端波束為基礎的多重天線廣播系統。我們發現利用通道回授資訊作使用者選取的強制歸零接收端波束多重天線廣播系統對於回授資訊不確定性的容忍度較強。而使用通道回授資訊來計算天線波束權重的傳送端波束多重天線廣播系統則會嚴重受到回授資訊不確定性的影響。接著，我們透過對傳送端波束多重天線廣播系統的分析，介紹使用多用戶排程來增加涵蓋範圍與連線品質。多天線系統將不需要使用額外的傳送功率就可以達到涵蓋範圍與連線品質的增益。最後，我們評估了通道資訊估測錯誤對於強制歸零接收端波束多重天線廣播系統的影響。透過分析，我們發現錯誤的估測資訊將使得系統容量會有一個上限值且不隨著訊號對雜訊比的增加而提升。此結果完全不同於回授資訊錯誤所造成的影響。此外，在連線品質效能上，錯誤的估測資訊會造成涵蓋範圍縮減及增加連線中斷的機率。

在第二個部份中，我們使用網路多重天線系統在多細胞中進行個人化平行傳輸。此系統為多重天線廣播系統在多細胞環境下的應用。結合基地台間地理位置的關係與頻帶的分割，我們使用多重天線在多個細胞內進行合作式的傳輸。在此部份提出了一個結合細胞扇區化、部分頻率重複使用之三細胞合作為基礎的網路多重天線系統。此架構用來降低多細胞系統中嚴重的細胞間干擾問題。從系統架構與佈局的觀點，我們提供的三細胞合作為基礎之網路多重天線系統不僅可以有效對抗細胞間干擾，也能降低多個基地台合作時的複雜度。

在第三個部份中，我們結合多重天線技術於正交分頻多工系統，並透過頻帶使用之排程與分配達到多使用者的個人化平行傳輸。在此部分的設計目標為補償空間多工多重天線系統所缺乏的連線品質可靠度。首先，我們探討如何透過多用戶維度與多頻帶維度來增加多重天線正交分頻多工系統的涵蓋範圍。透過分析與模擬，我們發現要有效率的延展多重天線正交分頻多工系統涵蓋範圍的關鍵在於設計同時考慮多用戶與多頻帶兩個維度的排程。接下來，我們設計一排程機制可以動態地根據給定的容量比例做資源分配。此機制包含兩步驟，低複雜度的子通道分配與功率分配。透過模擬，即使在各使用者擁有不同的通道衰減效應下，所提出的方法可達到使用者間的預設容量比例。另外，所提供的機制也可以改善使用者的連線品質。

總結來說，在這篇論文中我們探討了三種用來實現個人化平行傳輸的多重天線系統：(1) 多重天線廣播系統；(2) 網路多重天線系統；(3) 多重天線正交分頻多工系統。透過在多用戶、多頻帶、以及基地台間地理位置關係等不同維度的多樣性，我們提升了系統連線品質、涵蓋範圍、以及系統容量之效能。

With the increasing demand for high-speed data rates, multiple-input multiple-output (MIMO) antenna techniques become extremely important in current and future wireless communication systems. The huge capacity gain offered by MIMO antenna techniques can be further exploited in point-to-multiple transmissions, which can support personalized data services to multiple users concurrently. In this dissertation, we investigate MIMO antenna techniques to realize personalized parallel transmissions in different domains.

In the first part, we utilize MIMO antenna techniques in the spatial domain to carry out personalized parallel transmissions among multiple users, named as the MIMO broadcast systems. At first, we quantitatively compare the MIMO broadcast systems with transmit and receive beamforming techniques. We find thatutilizing feedback channel state information (CSI) for user selection in the receive zero-forcing (ZF) MIMO broadcast systems is more robust to feedback channel variations compared with utilizing feedback CSI for calculating antenna beamforming weights in the transmit MIMO broadcast systems. Next, we provide analytic formulas for the transmit MIMO broadcast systems to illustrate how multiuser scheduling can function as a link diversity compensation and soft coverage enhancement technique to improve the deficient diversity. Finally, we evaluate the effects of channel estimation errors on the receive ZF MIMO broadcast systems. Our analysis indicates the sum rate affected by estimated channel errors will be bounded by a value as signal-to-noise ratio (SNR) increases. This result is different from the effects of feedback errors which only causes certain sum-rate degradation. In addition, channel estimation errors will also cause shrinkage on reliable coverage and zero link diversity order on outage performance.

In the second part, an extended application of the MIMO broadcast systems, named as the network MIMO systems, transfers personalized data transmissions from single-cell scenario to multi-cell environment. In the network MIMO systems, we utilize the MIMO antenna techniques to coordinate parallel transmissions among multiple cells in geographically separated spatial domain. We propose a three-cell network MIMO architecture combined with sectorization and fractional frequency reuse (FFR) to reduce inter-cell interference in a multi-cellular system. From the aspects of architecture and deployment, the proposed FFR-based 3-cell network MIMO architecture can not only effectively overcome the inter-cell interference, but can relieve the burden of executing complex multi-base stations joint processing for a huge number of cells.

In the third part, we apply MIMO antenna techniques to broadband orthogonal frequency division multiplexing (OFDM) system and achieve parallel transmissions among users in the frequency domain by scheduling and allocating personalized resource. Our design focuses on compensating the drawback of degraded link reliability in the diversity-deficient spatial multiplexing MIMO-OFDM systems. First, we execute scheduling from the perspective of exploiting multiuser diversity and frequency diversity to extend the coverage region of the MIMO-OFDM systems. Our analysis and simulations indicate that the key of coverage enhancement is to jointly utilize multiuser and frequency diversity. Next, we design a scheduler to adaptively assign resource among users under a predetermined proportional rate constraint. Our method including two stages: a low-complexity subchannel allocation algorithm at first and a computational efficient power allocation method later. Simulation results show that the proposed algorithm can achieve the capacity to the algorithm with the maximal-rate scheduling but provides better link reliability. Additionally, the proposed two-stage method can meet the predetermined rate requirements well even if service users are under different large-scale power decay conditions.

In summary, we investigate three kinds of MIMO antenna techniques to realize personalized parallel transmissions: (1) MIMO broadcast systems; (2) network MIMO systems; and (3) MIMO-OFDM systems. We utilize diversity in various domains including multiuser, frequency, and geographic location of base stations to enhance system performance in terms of link reliability, achievable reliable coverage, and sum-rate capacity.