多天線行動通訊系統先進傳輸技術研究

Abstract

本論文旨在對新一代多天線行動通訊傳輸技術進行研究。首先，在第二章中，針對實體層群播波束形成(physical-layer beamforming)技術進行研究與分析，實體層群播波束形成技術係考量一具備多天線的基地台將資料以群播的方式傳送給一群特定的單天線終端用戶，在這樣的系統中，系統可達到的最大傳輸速率受限於信號品質最差的終端用戶，因此如何找到最佳的波束形成向量(beamforming vector)以最大化系統可傳輸速率是問題的核心，這個問題在現存的文獻中已證明是NP問題(NP-hard)；我們根據最佳化理論建立模型進行最佳性分析，經由推導及歸納其KKT (Karush-Kuhn-Tucker)充分條件，提出創新性的演算法TUAA (Two-User Approximation Algorithm)，其效能較目前文獻已知技術深具進步性，可在較快速的時間內取得較佳的波束形成向量及群播傳輸速率。 接下來，在第三章中，考量基地台具備巨量天線的情境下，針對實體層群播波束形成技術，提出另一更快速之演算法GaEq (Gain-Equalization Algorithm)。巨量天線的使用已被視為提升下一代通訊系統容量與效能重要的技術手段之一，而GaEq演算法在巨量天線的系統上，以低運算複雜度，達到了接近群播容量上限(multicast capacity bound)的傳輸速率。 此外，在第四章中，針對具多通道傳輸特性之系統，有別於慣用之單一通道編碼(single codeword, SCW)或多通道編碼(multiple codeword, MCW)架構，提出單一編碼多碼率控制技術(single codeword multiple code rate)，可根據通道品質調適編碼率，其複雜度相近於傳統的單一通道編碼系統，但效能相較單一通道編碼系統顯著提升。在第五章中，針對廣播信號的傳輸，結合階層式調變(hierarchical modulation)與時空編碼(space-tome code)技術，提出一個新型的時空編碼調變技術，單一信號源即可提供多型態的解調變模式，以同時滿足具不同通道條件或硬體配備的所有終端用戶，讓終端用戶可以根據其自身天線數及接收信號強度做最適當的解調選擇。最後，在第六章中，在行動廣播系統中，提出一個著重於降低終端用戶耗電的設計，運用通道編碼技術在實體層提供可調適接收(scalable reception)的傳送方式。當終端用戶處於較佳的信號品質環境，在取得相同資料量的前提下，終端用戶可漸次增加接收編碼後的信號直到正確解碼為止，根據模擬結果終端用戶可隨信號強度增加而降低平均的接取時間，進而達到省電的功能。

This dissertation proposes advanced transmission techniques for multiple-antenna systems in mobile cellular systems. Firstly, in Chapter 2, we investigate and analyze the problem of transmit beamforming for physical-layer multicasting, where a multiple-antenna base station multicasts common information to a pre-specified group of single-antenna users. The optimal beamforming vector is derived in closed form for the two-user case and an iterative algorithm, called two-user approximation algorithm (TUAA), is developed to search for the optimal beamforming vector for the case of more than two users. The performance of the proposed method is evaluated through extensive computer simulations. As compared to the state-of-the-art methods in the literature, the proposed method can achieve a much higher multicast rate and provide an attractive tradeoff between performance and complexity. Secondly, in Chapter 3, the problem of transmit beamforming for physical-layer multicasting is reinvestigated under the scenario that the base station is equipped with a large-scale antenna system which has been regarded widely as one of the most promising technologies to increase the system capacity and user throughput for the future cellular networks. With massive antennas at the base station, a new gain-equalization algorithm (GaEq) is proposed to further improve the performance. From simulation results, the proposed method can approach closely to the multicast capacity bound and outperform the existing methods in the aspects of performance and/or complexity. Thirdly, in Chapter 4, a new strategy of codewords to layers mapping in MIMO system is proposed. The proposed multi-rate control method provides a link performance lying between the conventional single codeword and multiple codewords schemes but with com-plexity and overhead close to single codeword schemes. Fourthly, in Chapter 5, for broadcasting services, a MIMO scheme combining the hierarchical modulation and space-time coding is proposed, offering multiple reception modes simultaneously to accommodate users with different SNR conditions and antenna configurations. Finally, in Chapter 6, a scalable reception mechanism in the physical layer is proposed, enabling mobile devices to reduce power consumption while subscribing broadcasting services. Simulation results show that the averaged reception time of mobile devices under good channel conditions is reduced dramatically comparing to the conventional schemes.