MIMO-OFDM 與 OFDM-CDMA 系統之頻率估計

Abstract

正交分頻多工(Orthogonal Frequency Division Multiplexing, OFDM)技術近年來頗受到重視，其應用範圍也越來越廣。OFDM通訊系統之設計必需要考慮到載波頻率偏移(CFO)補償這個重要的課題，因載波頻率偏移會破壞次載波的正交性且將大大的降低系統性能。在這篇論文裡，我們首先證明摩斯氏(Moose)和余氏(Yu)的最大可能頻率估計法可推廣到多輸出輸入正交分頻多工(MIMO-OFDM)系統的頻率估計，並有相當突出的性能表現。由於正交分頻多工分碼多重近接(OFDM-CDMA)很有可能成為下一代寬頻移動通訊的傳輸標準，而其頻率同步的問題尚少人探討，我們便接著研究這個頻率同步問題。我們考慮了非同步的上傳鏈路，其中每個用戶傳送的信號之時軸互不同步(timing asynchronous)，受到不同的通道衰褪並產生相異的頻率偏移。針對這樣的傳輸環境，頻率同步器的設計得要同時考慮多用戶干擾(multiple user interference, MUI)。我們先是應用多用戶偵測理論的最小輸出能量(minimum output energy, MOE)的法則，提出一種頻率估計法。這種方法需先計算相對每一個可能頻率偏移的最小輸出能量，接著再找出整個頻率不確定範圍內那一個頻率的MOE最大。因為這樣的搜尋複雜度高，於是我們進一步結合了陣列信號理論中尋向(directional finding)的慨念，將原先的搜尋極大的最小輸出能量轉換成解多項式根的問題。我們提出的方法簡化了頻率同步演算法的複雜度且提高其性能表現。

For orthogonal frequency division multiplexing (OFDM) based systems, a carrier frequency offset (CFO) existed between the transmitter and receiver will destroy the orthogonality of the subcarrier and degrades the performance. In this thesis, we extend both Moose's and Yu's maximum likelihood CFO estimation algorithm for multiple transmit and multiple receive MIMO-OFDM systems. OFDM Code-Divison Multiple-Access (OFDM-CDMA) have been one of the candidates for the next generation of mobile broadband communication. In an uplink, each user's transmitted signal suffers from asynchronous transmission and independent frequency selective channel and the different frequency offsets of each active user. We present a CFO estimation algorithm to be used in a asynchronous uplink and frequency selective channels for OFDM-CDMA systems. The algorithm estimates the CFO of the desired user and eliminates the multiuser interference (MUI). The CFO estimate is obtained by searching the value which maximizes the output energy of the minimum output energy (MOE) criterion. However, the CFO estimate requires an exhaustive search over the entire uncertainty range. We convert the min/max search into polynomial rooting problem.