1747-M2（A-B）
1747-M2
在無線信道中，由于反射、散射和衍射等的存在，除了直射波之外，還會有信號通過不同的路徑沿不同的方向到達接收端，從而使信號產生時延擴展；若接收終端處于移動狀態，各條信號支路還會具有不同的多普勒頻移，從而使信號產生頻率域彌散。典型的無線移動信道二維沖擊響應可表示如下［參考1］。
上式中，αk、 k、fdk和τk分別為第k條支路的歸一化幅度（衰減因子）、初始相位、多普勒頻移和傳播時延，τk》0，，各條支路相互獨立，如圖1所示。設發射信號為x（t）ej2πfct，x（t）為基帶信號，fc為載波頻率，則接收信號y（t）為發射信號與信道沖擊響應的線性卷積。
信道模擬器的設計與實現
從上可知，要實現信號通過多徑信道的模擬，需要對信號進行時延、（多普勒）頻移和衰減，并將各支路信號相加。因此多徑信道模擬器需要包含延時器、多普勒波產生器、衰減器、乘法器和混合器等單元。對射頻信號進行處理時，衰減較容易實現，而延時和乘法運算則實現難度大且精度不易保證。而數字電路在這方面卻有其獨到之處。因此，信道模擬器通常對信號進行頻譜下搬移到基帶，然后對基帶信號進行采樣，在數字域對信號進行信道畸變處理，之后再經過上變頻恢復成射頻信號，如圖2所示。
圖2中，晶體振蕩器產生的正弦波通過頻率合成得到與輸入信號載波頻率基本一致的本地載波，混頻器對信號進行上下變頻（正交解調和調制）。由于上下變頻用同樣的本地載頻，故可以保證輸出信號與輸入信號不存在頻率偏差。模數轉換器對信號進行采樣，因為數字部分實現容易，通常以幾倍的Nyquist速率進行采樣。虛線框內部分則完成對數字基帶信號進行通過信道的運算，也就是下一節所要討論的內容。
一個好的信道模擬器，其信道沖擊響應的時間分辨率一定要足夠高。信號的大延時在數字電路中通常用存儲器（RAM或FIFO）來實現，而存儲器的讀寫是與時鐘同步的，所以信道沖擊響應的時間分辨率與數字信號處理單元的時鐘周期相等。為提高時間分辨率，需要對輸入信號進行上采樣處理。上采樣后的信號在信道模塊與信道沖擊響應進行卷積得到經過信道畸變的高采樣率信號，運算結果再經下采樣濾波以適當的采樣率輸出，數模轉換電路將其恢復成模擬信號。
為方便信道參數設置，可通過單片機對頻率合成器和數字電路進行配置。通過RS232串口電纜連接單片機和計算機，可獲得良好的界面，通過計算機對信道模擬器進行監控。
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