於時間相關通道下之雙模式有限回授傳送波束形成技術

Abstract

在新一代無線通訊中，有限回授前置編碼（Limited Feedback Precoding）為一用以提升系統可靠度的技術，其特色在由接收機回傳最佳前置編碼的編號給發射機，且發射機不需要任何的通道狀態資訊。在本論文中，吾人主要探討在多輸入單輸出（Multiple-Input Single-Output, MISO）系統中的傳送波束形成技術（Transmit Beamforming）；傳統的作法，在固定平均回授率的情況下，單一模式有限回授系統（只有一個編碼簿）在時變通道下並不夠強健。假設接收端有完整的通道狀態資訊、無錯誤的回授通道，和一階馬可夫多輸入單輸出通道模型，吾人提出一個雙模式有限回授系統和基於符元錯誤率的模式選擇準則，在連續的兩個單位時間內，考慮兩種傳送波束形成模式：（1）在每個獨立的單位時間內，用較小容量的編碼簿個別作波束形成；（2）在兩個單位時間內，用較大容量的編碼簿聯合作波束形成。利用連續兩個單位時間內的時間相關係數和完整的通道狀態資訊，吾人提出的模式選擇準則，可以選擇較佳的模式傳輸。由模擬結果可以看出雙模式有限回授前置編碼系統效能優於單模式有限回授前置編碼系統。

Limited feedback precoding is a technique used to improve system reliability in modern wireless communications. The feature of limited feedback precoding is that receiver feeds back the index of the optimal precoder to the transmitter. The transmitter does not need to have any channel state information (CSI). In this thesis, we focus on the transmit beamforming scheme in multiple-input single-output (MISO) systems. In conventional approach, under a fixed average feedback rate, the single-mode limited feedback system (has only one codebook) is not robust enough in time-varying channels. Perfect channel knowledge known at the receiver, error-free feedback channel, and first-order Markov MISO channel model are assumed in this thesis. We propose a dual-mode limited feedback system with a symbol-error-rate (SER) based mode selection criterion. Two transmit beamforming modes over two consecutive time slots are considered: (1) separate beamforming for the individual time slot with a small-size codebook, and (2) joint beamforming for both slots with a large-size codebook. By exploiting the temporal-correlation coefficient of two consecutive time slots and perfect channel knowledge, the better mode can be selected by using the mode selection criterion. Simulation results show that the performance of the dual-mode limited feedback system is better than that of the single-mode limited feedback system.