LTE-A系統中利用天線選擇之場域內協調式多點傳送

Abstract

眾所周知，蜂巢式無線系統的細胞邊緣使用者會遭受到從干擾細胞來的干擾訊號。LTE/A系統因此提出協調式多點(CoMP)處理來解決此問題。CoMP將相鄰的細胞傳送/接收使用者的訊號聯合起來一起處理，這樣一來我們可以避免干擾或將干擾當作訊號來估計。在CoMP系統裡，基地台與使用者之間的天線可以形成一個很大的多輸入多輸出系統(MIMO)。這種系統的前置編碼需要很大維度的碼書，然而LTE/A並未支援大維度的碼書。在本篇論文中，我們提出一種方式來解決此問題，主要的想法是使用天線選擇技術在傳送端與接收端做天線選擇，這樣的話我們就可使用現有維度較小的碼書，同時也可提升前置編碼的效能以及可減少回授的資訊。我們推導出最大近似接收機基於OFDM系統的最佳選擇準則，結果顯示為在每個子載波中必須執行SVD或QRD，而且選擇的候選量相當大，因此，所需要的計算複雜度非常高。我們因而提出多種不需要矩陣運算的選擇準則以及減少選擇候選量的方法。模擬結果顯示，我們提出來的演算法在細胞邊緣的使用者可以大大地提高連接的品質。

It is known that in cellular systems, the users in cell edge suffer from the interference from interfering cells. In LTE/A, coordinated multipoint (CoMP) processing has been proposed to solve the problem. In CoMP, the transmit/receive signals of users in neighbor cells can be jointly processed such that interference can be either avoided or estimated as desired signals. In CoMP, antennas between involved users and basestations can form a large multiple-input-multiple-output (MIMO) system. Precoding in such system will require a large codebook which is not supported by LTE/A. In this thesis, we propose a method to solve the problem. The idea is to use antenna selection to select transmit and receive antennas such that the existing codebooks can be used while the performance of the precoding can be enhanced and the feedback overhead can be reduced. We derive optimum selection criterion for OFDM-based systems with maximum likelihood receivers. It turns out that the SVD or QRD has to be conducted for each subcarrier and the number of the candidates for selection is large. Thus, the required computational complexity is very high. We then propose selection criterions without matrix operations and methods to reduce the number of selection candidates. Simulations show that the proposed algorithms can greatly enhance the link qualify of the uses in the cell edges.