臺灣博碩士論文加值系統

在無線通訊系統裡，對於多組不同的訊號可以透過不同的載波頻率同時傳送，
而傳統數位通訊接收器在接收不同特性射頻訊號時，必須更改硬體設計，
因此增加了硬體電路的設計成本。軟體無線電概念之接收機，降低了硬體設計時所需之成本，
且整體數位化之解調流程，有利於後續接收機能以軟體的方式實現。
數位正交調解技術為實現軟體無線電概念之接收機架構，
此解調方法是將訊號以符合某種特定取樣頻率取樣，
取樣後的訊號再透過一些規律的運算式得到正確的解調。若取樣時採用帶通取樣法則，
將能大幅度地降低取樣頻率，減輕硬體電路在設計時的困難，而其電路設計較具彈性靈活的優點，
非常適用於軟體無線電。
在本論文中，我們嘗試解決因在多頻帶數位正交解調器中都卜勒效應或本地震盪器的不穩定及不匹配所引起的載波頻率偏移，
造成訊號在正交取樣時，因取樣頻率的偏移，使得解調後的訊號位元錯誤率性能嚴重變差。
我們所提出的方法是透過選擇適當的符元時序同步與估測因載波頻率偏移所產生的平均相位旋轉量，
再利用載波回復架構，使得偏移的訊號相位得以補償回來。我們將以電腦模擬驗證所提出方法的正確性。

In wireless communication systems, different bands of communication signals are transmitted with different carrier frequencies. To cope with different communication standards, conventional receivers often need to modify their hardware designs. The concept of the software radio is to reduce the cost of the hardware modification by implementing most of the receiver functions with software. Digital quadrature demodulation technique is one way to implement the concept of the software radio. This method first samples the modulated signal using a sampling frequency which satisfies certain conditions, then acquire the demodulated signal from the sampled signal via some very simple rules. If a bandpass sampling frequency is adopted, we can substantially lower the sampling frequency. This in turn reduces the cost and the complexity of the receiver. In this paper, we try to solve the problem of the carrier frequency offset caused by Doppler effects or local oscillator mismatch in multiband digital quadrature demodulators. When frequency offset exist, the bit error rate of a multiband digital quadrature demodulator can increase significantly. We propose a method to achieve the symbol-timing and carrier recovery by using symbol-timing and carrier frequency offset estimators together with a carrier recovery loop. The goodness of the proposed method is demonstrated by computer simulation.

第一章 緒論
1.1 軟體無線電
1.2 研究動機與目的
第二章 正交解調器之符元同步與載波回覆
2.1 正交解調器原理說明
2.1.1 QPSK調變器
2.1.2 正交解調器
2.2 符元時序與載波頻率偏移估測法
2.3 載波回復架構
第三章 多組射頻訊號之數位正交解調技術
3.1 單組射頻訊號之數位正交解調技術
3.2 兩組射頻訊號之數位正交解調技術
3.2.1 解調流程原理說明
3.2.2 避免膺頻現象之正交取樣條件
3.2.3 適用的取樣頻率之決定
3.3 雙頻帶之濾波器組實現法
第四章 濾波器組因頻率偏移產生相位誤差之探討
4.1 造成QPSK調變系統載波頻率偏移因素
4.2 本論文的系統描述與訊號模型
4.3 符元時序與估測頻率偏移所產生的相位誤差
4.4 利用載波回復架構修正相位誤差
第五章 模擬實驗結果
5.1 單頻帶正交解調技術之系統效能分析
5.2 雙頻帶濾波器組解調技術之系統效能分析
5.3 考慮有載波頻率偏移時之雙頻帶濾波器組解調器之系統效能分析
第六章 結論

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