CPM(連續相位調變)訊號由於相位的連續性，具有相當好的頻譜效益，GSM系統即是以高斯波形GMSK脈波來調變載波相位。一般所知對CPM解調方式是採用Viterbi演算法，此乃因為CPM訊號具有狀態轉移的觀念，其狀態轉移可以格子狀圖來表示。Viterbi演算法是以最大相似法則為基本觀念，配合存留路徑(survivor path)的選取來做運算。而其中用來判斷存留路徑的歐氏距離亦可以有不同的變化，這裡是對GMSK調變之CPM訊號的時變相位其正餘弦做取樣，以做為歐氏距離運算的來源。
BCJR演算法是解迴旋編碼的另一種方法，是基於在事後邏輯所發展出來的遞迴運算，這裡我們將它用在解GMSK調變之CPM訊號，並比較它與Viterbi演算法的偵測效益，實驗結果發現BCJR演算法在解GMSK訊號方面的確擁有較好的效果。我們也比較不同的h與L，並且發現在L=3、h=3/4時有最好的正確率。

CPM advantageous in spectral efficiency because of its continuity of the phase in modulation. One of the CPM example is GMSK, which has been applied to the wireless GSM system. The conventional demodulaton og CPM is achieved by Viterbi algorithm. This is because of the state transition structure for the dynamic description of phase of the CPM signal. Furthermore, the state transition can be presented by a trellis diagram, which can be efficiently solved by Viterbi algorithm based upon the strategy of selecting best survivor path to a maximum likelihood criterion. The best survivor path is measured by the Euclidean distance in modulation in this thesis.
Another demodulation method proposed by us is the famous BCJR algorithm. BCJR which is based upon the posteriori probabilities is a alternative method for decoding the convolution code. We compare the BCJR and Viterbi algorithm for the demodulation of the GMSK system. Experiment results demonstrate that BCJR has a better error probability than the Viterbi algorithm. Also, we compare different GMSK system for different overlapping length and modulation index. The best combination of L and h suggested by pur experiments is the case of L=3, and h=3/4.

第一章 引言…...………………………………………………………………1

第二章 Viterbi與BCJR演算法簡介
2.1 Viterbi演算法的簡介………………………………………………...4
2.1.1最大相似序列解碼……………………………………………..4
2.1.2 Viterbi演算法…………………………………………………..6
2.2 BCJR演算法的簡介………………………………………………….9
2.2.1 BCJR演算法的介紹…………………………………………...9
2.2.2 針對解迴旋碼的BCJR演算法………………...…………….13

第三章 連續相位CPM的簡介
3.1 連續相位頻率鍵移CFSK…………………………………………..19
3.2 連續相位調變CPM………………………………………………...21

第四章 運用BCJR與Viterbi解GMSK訊號之方法
4.1 GMSK訊號介紹………………………………………………..…...26
4.2 系統模擬架構與流程……….…….………………………………..27
4.2.1 以SystemView模擬的系統架構………………………….…28
4.2.2 接收端簡易數學模式分析…………………………………...30
4.3 以BCJR及Viterbi為偵測解調之方法……………………………32
4.3.1 以降頻訊號取樣點為偵測訊號……………………………...32
4.3.2 以GMSK為基本波之CPM訊號狀態分析………………...35
4.3.3 應用BCJR與Viterbi偵測CPM訊號……………………….44


第五章 實驗結果與討論
5.1 實驗環境與參數………………………………………………...….47
5.2 實驗結果與討論…………………………………………………....48

第六章 結論………………………………………………………………….64

附錄A. 不同訊號之位元錯誤率與區塊錯誤率表……..…….65

附錄B. 不同部分響應以及調變指數下的CPM訊號
之頻譜圖………….………………………….………………….68

參考文獻……………………………………………………………………...69

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