應用於MIMO-OFDM系統之前置編碼搜尋與時域通道資訊回傳

Abstract

前置編碼(Precoding)，為一項能夠有效提升MIMO-OFDM系統傳輸效能之技術。在實際的系統中，前置編碼矩陣會先選定，並且只回傳該選定矩陣之編號。然而，候選之矩陣數量可能很多，因此需要高度計算量來搜尋出最佳的前置編碼矩陣。在本篇論文中，為了解決此問題，我們首先提出一個低複雜度的前置編碼搜尋演算法。與完全搜尋(exhaustive search)比較，此方法可以降低80%左右的搜尋複雜度，並且其效能損失是在可以接受的範圍之內。此外，在MIMO-OFDM的系統中，通道資訊(Channel state information, CSI)常常能起到很大的作用。然而，這些必須從接收端回傳的CSI，通常需要相當多的資料量。在本篇論文的第二部份，我們提出一個時域通道資訊回傳方法來降低回傳資料量。在某些通道情況下，此方法只需要少量的回傳資料。最後，針對時變的通道環境，我們提出一個機制，將誤差脈衝編碼調變(Differential Pulse Code Modulation, DPCM)應用於時域通道資訊回傳方法。由此，回傳的資料量可以更加地減低。

Precoding is an effective technique improving the performance of MIMO-OFDM systems. In practical systems, the precoding matrices are pre-determined and only the index of the selected matrix is fed back. Since the number of precoding matrices may be large, the search for the optimum precoder requires high computational complexity. In this thesis, we first propose a low-complexity precoder searching algorithm to solve the problem. Compared to the exhaustive search, the proposed searching method can reduce about 80% searching complexity with acceptable performance loss. Channel state information (CSI) is useful in MIMO-OFDM communication systems. However, the information has to be fed back from the receiver and this will requires a large amount of data. In the second part of the thesis, we propose a time domain CSI feedback scheme to lower the feedback data bits. Under some channel conditions, the proposed method only requires a small amount of feedback data. Finally, for time-varying channels, we propose to use a differential pulse code modulation (DPCM) scheme in our time domain CSI feedback method such that the required feedback data can be further reduced.