在單路RoF系統(中心站、基站各一)中，下行鏈路和上行鏈路可以分別占有兩個不同的波長，但是實際應用環境下，由于毫米波信號短距作用的特性，需要部署大量的基站，這時如何降低基站的成本，同時適應波分復用(WDM)-RoF系統是設計中需要重視的問題。在光網絡中，波長是一個重要的資源，如果能實現波長的重復利用則將大大降低資源消耗，所以當前波分復用無源光網絡(WDM-PON)是光網絡中發展的重要趨勢。目前在毫米波RoF系統中實現WDM-PON有幾種可行方案。

中心站采用外調制法發送下行信號，同時保留光載波，下行基帶數據采用強度調制。在基站中則將光載波利用光濾波器濾出，然后將下變頻的用戶上行基帶信號用調制器或者半導體光放大器調制上光作為上行信號上傳。

中心站采用外調制法實現光外差信號，下行基帶數據的上載方式采用DPSK相位調制。基站處，發送的下行數據可用1比特馬赫-曾德干涉儀解調，而對于上行的基帶數據則可利用鋰酸鈮調制器采用強度調制上光，中心站只需要進行強度檢測即可。

5、結束語

由于在傳輸帶寬和無線接入方面的巨大優勢，毫米波RoF系統是下一代接入網的一個很有潛力的解決方案。鑒于目前毫米波頻段器件的昂貴以及實現系統的“成本—資源”考慮，本文針對毫米波光產生、下行信號上變頻和傳輸性能、上行信號下變頻、雙向鏈路中的波長重用等方面進行了毫米波RoF系統結構優化方面的討論。相信隨著未來相應結構優化研究的深入，毫米波RoF系統的實用化將指日可待。

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