當無線通訊的技術已經廣泛的應用在我們生活之中，頻譜的使用效率也變得越來越重要。感知無線電技術可以改善頻譜的使用效率，而在感知無線電之中，頻譜的感測是重要的一環。許多研究提出了新的頻譜感測方法，而在這之中能量感測是一個廣為人知且低複雜度的方法。但是能量感測有著需要知道雜訊能量的缺點。而協方差絕對值的方法雖然不需要知道雜訊能量，但也有著沒有給最佳比重分配的缺點。而在正交分頻多工訊號感測中，許多的研究利用前輟循環相關性的特性，做為感測的方法。在本篇論文之中，提出了一個相關性感測的方法，且不需要知道雜訊能量，這個方法是利用正交分頻多工訊號的相關性做出改善。我們分析了正交分頻多工時域訊號在單路徑通道和多路徑通道時相關性的情況，另外我們也分析了在時間不同步下，我們所提出的方法是否能正常運作。模擬結果顯示我們所提出的利用訊號相關性特性分配比重的方法，不但比協方差絕對值方法的表現要來的好，也比利用前輟循環感測的方法來的好。

As wireless communication technology widely used in our life, spectrum utilization efficiency becomes more important. Cognitive radio is a technology to improve the efficiency of spectrum use, spectrum sensing is a fundamental part for cognitive radio. Many research proposed the new method for spectrum sensing. Energy detection is the most common and low complexity method. But, energy detection need perfect knowledge of the noise power. Statistical covariance method does not need knowledge of the noise power, however, it is not optimal weighted method. In the OFDM sensing, many studies use the characteristics of cyclic prefix as the sensing method. In this paper, we proposed a correlation-based method exploiting orthogonal frequency-division multiplexing (OFDM) time domain signal correlation under single path Rayleigh fading channel and multipath Rayleigh fading channel. Furthermore, we analyzed the amount of change in OFDM time domain signal correlation over single path channel and multipath channel and we also considered signal time unsynchronized issue.
Simulation shows our proposed correlation-based method performance under single path Rayleigh fading channel and multipath Rayleigh fading channel. And our proposed correlation-based method performance is better than Statistical covariance method and CP-based detector.

口試委員會審定書 #
中文摘要 1
ABSTRACT 2
誌謝 3
CONTENTS 4
LIST OF FIGURES 6
Chapter 1 Introduction 8
1.1 Background Knowledge 8
Chapter 2 System Model 13
Chapter 3 Correlation-Based OFDM Spectrum Sensing Method 15
3.1 Scenario1 –Single Path Channel Case: 15
3.1.1 Properties of OFDM Time Domain Signal 15
3.1.2 Analysis-Noise Alone Case Autocorrelation 20
3.1.3 Proposed Correlation-Based Method over Single Path Channel 21
3.2 Analysis-Maximum Ratio Combining 24
3.3 Scenario2 -Multipath Channel Case: 31
3.4 Analysis-Time Unsynchronized 34
Chapter 4 Simulation Results 38
4.1 Effect of Guard Band on Autocorrelation 39
4.2 Simulation over Single Path Channel 40
4.3 Simulation over Multipath Channel 45
Chapter 5 Conclusion 49
References 50

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