正交頻域多工 (orthogonal frequency division multiplexing，簡稱OFDM) 近年來是一個非常熱門的研究主題且有很廣泛的應用。在正交分頻多工系統中，接收器的射頻模組採用直接轉換架構，則是近年來流行採用的方式。直接轉換架構的好處在於成本較低且功率消耗較少，但是其最主要的缺點是會造成I/Q失衡。在本篇論文中，我們針對OFDM系統中採用直接轉換接收器所產生的I/Q失衡現象加以估測並補償。在先前提出的I/Q失衡補償方法中大部分都未考慮未知的多路徑通道 (multipath channel)，在這篇論文中，我們考慮有可加性白色高斯雜訊 (additive white Gaussian noise，簡稱AWGN) 的多路徑衰減通道，而這個通道在一個封包(packet)之內是不變的。我們提出了三種I/Q失衡的補償方法：最大可能性估測法、最小平方估測法、直覺式估測法。我們透過電腦模擬來比較這三種方法的表現，並考慮時序偏移 (timing offset) 及頻率偏移 (frequency offset) 的影響。模擬結果顯示盲蔽最大可能性估測法有很好的表現，並且對時序偏移及頻率偏移有很好的抵抗能力。最小平方估測法及直覺式估測法在訊號對雜訊的比值 (signal-to-noise ratio，簡稱SNR) 較高時有較好的表現，對時序偏移也有很好的抵抗能力，但是在頻率偏移存在的條件下表示會較差。且在不同的振幅不平衡 (gain imbalance) 及相位不平衡 (phase imbalance) 的情況下，我們提出的盲蔽最大可能性估測法、最小平方估測法及直覺式估測法都有很好的表現。

Orthogonal frequency division multiplexing (OFDM) is a popular research topic and has been employed in various applications in recent years. In OFDM systems, it is attractive to employ the direct conversion architecture in the radio frequency module of the receivers for the propose of low cost and low power consumption. But the main problem caused by using the direct conversion architecture is I/Q imbalance. In this thesis, we estimate and compensate the I/Q imbalance generated by the direct conversion receiver in OFDM systems. In previously proposed methods for I/Q imbalance compensation, the unknown multipath channel is usually not considered. In this thesis, the channels are considered to be multipath fading channels with additive white Gaussian noise (AWGN), and the channels are assumed quasi-static during an entire packet. Three methods for I/Q imbalance compensation are proposed: the maximum-likelihood (ML) estimation method, the least-square (LS) estimation method, and a heuristic method. Computer simulations are conducted for comparisons of the performance of three methods. Timing offset and frequency offset effects on performance are also considered. We find that the blind ML estimation method generally performs well and is robust against timing offset and frequency offset.

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