在本論文中，我們介紹一個訊雜比（Signal-to-Noise Ratio，SNR）估測器並加以分析此估測器於同步非理想下之效能變化。此訊雜比估測器主要利用最大期望值（Expectation-Maximization，EM）演算法完成之，並且可應用於適應性調變（Adaptive Modulation）技術之中。若接收端同步機制未達理想狀態時，則OFDM系統會產生同步誤差，此同步誤差分為時序誤差以及頻率誤差。不論時序誤差或是頻率誤差，均使訊雜比估測器產生估測偏移量，此偏移量將直接影響適應性調變之效能。我們分別探討兩種同步誤差對估測器效能之影響程度，藉由模擬結果驗證理論分析之正確性。模擬結果顯示同步誤差量越大，估測器之效能越降低；如果時序誤差經過疊帶運算之後，將更加衰減估測器之效能，簡單來說，在OFDM系統產生同步誤差下，如果疊帶越多次，則估測器的準確性越低，對適應性調變影響甚鉅。

In this thesis, we introduce a signal-to-noise ratio (SNR) estimator and analyze the performance degradation of the SNR estimator due to the synchronization error in the orthogonal frequency division multiplexing (OFDM) systems. This SNR estimator through the expectation-maximization (EM) algorithm is used in the adaptive modulation. When the synchronization is imperfectly done, the synchronization error reduces the performance of the OFDM systems and the accuracy of the SNR estimator. We investigate the estimation offset of the SNR estimator with the synchronization error. Simulation results demonstrate that the theoretical analyses are correct. In addition, the simulation results show that the more synchronization errors cause the more estimation errors of the SNR estimator. And the estimation errors are not decreased by the iteration of the EM algorithm.

目錄
圖目錄...III
第一章 導論...1
1.1研究動機...1
1.2論文架構...2
第二章 正交分頻多工之簡介...3
2.1 OFDM之系統架構與特性...3
2.1.1簡介傳統分頻多工調變技術...4
2.1.2 簡介正交分頻多工調變技術..6
2.1.2.1 頻寬使用效率...8
2.1.2.2 正交性...9
2.1.2.3 循環字首...10
2.2 OFDM系統於非理想同步下之探討...11
2.2.2.1 超前時序誤差...11
2.2.2.2 落後時序誤差...13
2.2.2.3 頻率誤差...14
2.3 適應性調變...15
第三章 EM-訊雜比估測器之介紹...18
3.1 最大期望值演算法...18
3.2 訊號估測...19
3.3 雜訊變異數估測...21
3.4 訊雜比估測...23
第四章 EM-訊雜比估測器之效能分析...25
4.1 超前時序誤差...25
4.2 落後時序誤差...29
4.3 頻率誤差...33
第五章 模擬...37
第六章 結論與未來展望...43
附錄A...44
附錄B...46
參考文...47

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