近年來，正交分頻多工技術受到很大的關注，由於它具有高資料傳輸速率、有效對抗頻率選擇性衰落、窄頻干擾等優點。然而，正交分頻多工系統仍然具有一些問題，例如：載波頻率偏移、相位雜訊與高峰均值功率比。上述的任一問題，都有可能對系統效能造成莫大的影響。為了提高正交分頻多工系統的效能，時序偏移與載波頻率偏移估測技術是非常重要及必須的。先前文獻中曾提到，最大近似時序與載波頻率偏移估測技術利用循環前置碼來估測正交分頻多工系統之時序與載波頻率偏移量。本文將利用Pisarenko harmonic decomposition (PHD) 估測法來估測正交分頻多工系統之載波頻率偏移量。PHD 估測器的最大優點是能夠估測分數載波頻率偏移量與整數載波頻率偏移量，而最大近似估測器只能估測分數載波頻率偏移量。最後將由一些模擬結果來比較PHD 估測器與最大近似估測器的效能。

In recent years, orthogonal frequency division multiplexing (OFDM) has received substantial interest due to its high data transmission capability, and its robustness against the effects of frequency-selective fading and narrowband interference. However, OFDM systems still have some problems, such as carrier frequency offset, phase noise and large peak-to-average power ratio (PAPR). Any of these can result in severe system performance degradation. In order to enhance system performance, the symbol timing error and carrier frequency offset estimation in OFDM systems are necessary and crucial. Previously, maximum likelihood (ML) estimation of time and carrier frequency offsets in OFDM systems using cyclic prefix (CP) has been presented. In this thesis, a Pisarenko harmonic decomposition (PHD) method is employed to estimate the carrier frequency offset in OFDM systems. The main advantage of the PHD estimator is its ability of not only estimating a fractional carrier frequency offset but also an integral carrier frequency offset with no assistance from accurate timing synchronization, meanwhile the ML estimator can only estimate the fractional carrier frequency offset with assistance from ML timing synchronization. Some comparative simulation results are given to illustrate the performance of PHD estimator and ML estimator.

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