多重輸入多重輸出正交分頻多工空間分割存取下鏈系統在通道估計錯誤下之共同波束設計

Abstract

近年來，多重輸入多重輸出正交分頻多工空間分割多重存取技術已成為現今寬頻無線網路系統的主要方法之一，在此論文中，我們將著重此技術在多用戶下鏈系統中的波束設計及應用。 過去，已有不少的文獻研究收發機共同波束設計在多重輸入多重輸出系統的應用，然而在這些研究都考慮單一用戶系統，此外，他們也都假設在有完美的通道估計來設計。然而，這個假設在實際通訊系統中是不存在的，故在此論文中，我們將針對多用戶系統，在通道估計有誤差的情況下，提供一個收發機共同波束設計方法能有效抵抗通道錯誤所造成的系統效能損失。 收發器共同波束技術能將原本空間上互相干擾的多重輸入多重輸出系統轉換成一個子通道間互相獨立不干擾的等效系統。而在我們所提出的多重輸入多重輸出正交分頻多工空間分割多重存取下鏈系統中，藉由在傳送端利用零空間技巧，我們能將多用戶系統轉換為一個用戶間互相獨立的系統，如此便可將文獻中的單一用戶收發機共同波束技術應用在每個獨立用戶上。此外，在有多路徑的頻率選擇通道下，我們將採用正交分頻多工技術應用在上述系統中。 進一步的，先前所提到的設計方法皆假設在有完美的通道估計條件下，然而在實際系統上通道估計通常會有誤差，此誤差將造成顯著的系統效能損失，此損失可歸咎於因為不完美零空間矩陣所造成多用戶間互相的干擾，以及因為不完美收發機共同波束設計所造成每個用戶本身資料串之間的干擾。在此論文中，我們將考慮多用戶多重輸入多重輸出正交分頻多工空間分割多重存取下鏈系統在通道估計有誤差的情況下，提供一個較能抵抗因為通道錯誤所造成系統效能損失的收發機共同波束設計。最後，我們利用電腦模擬來評估比較系統效能。

The techniques of MIMO-OFDM SDMA have recently considered one of the candidates for the broad-band multiple-access wireless systems. In this thesis, we focus on the research of beamforming in the applications of multi-user MIMO-OFDM SDMA downlink wireless system. Many researchers have used the joint Tx/Rx beamforming approach in the MIMO systems. However, they only considered the single user system. In addition, their methods are proposed under the perfect channel estimation assumption, which is no true in the real world. In this thesis, we will aim at multi-user MIMO SDMA systems to propose a MMSE beamforming algorithm, which has better immunity to the channel estimation errors than the previously published methods. The joint Tx/Rx beamforming technique can transform a mutually cross-coupled MIMO transmission system into a set of independent eigen subchannels so that subchannel signal can be decoded without the interference from other antennas. In our proposed MIMO-OFDM SDMA downlink system, by using null-space technique in the base station (Tx side), the multi-user MIMO downlink system can be simplified into several independent single user MIMO subsystems. The MMSE beamforming algorithm is further applied to each single user MIMO subsystem achieving the spatial diversity gain. In addition, to combat the frequency-selective channel caused by multi-paths, we focus the aforementioned problems in the MIMO-OFDM systems. However, all the previously proposed methods are derived under the assumption of perfect channel estimation. But in real wireless systems, the channel estimate always contains estimation errors which may cause significant performance degradation when these methods are used. The performance loss can be imputed to the multi-user interference induced by the imperfect null-space matrix and the inter-stream interference caused by imperfect Tx/Rx beamforming. In this thesis, without the assumption of perfect channel estimation, we will consider the joint Tx/Rx beamforming design for multi-user MIMO-OFDM SDMA downlink system which is more robust to channel estimation error than the previous methods. Finally, the performance of the proposed schemes will be evaluated by computer simulations.