在單路RoF系統(tǒng)(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(huán)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(tǒng)是設計中需要重視的問題。在光網(wǎng)絡中，波長是一個重要的資源，如果能實現(xiàn)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(wǎng)絡(WDM-PON)是光網(wǎng)絡中發(fā)展的重要趨勢。目前在毫米波RoF系統(tǒng)中實現(xiàn)WDM-PON有幾種可行方案。

中心站采用外調(diào)制法發(fā)送下行信號，同時保留光載波，下行基帶數(shù)據(jù)采用強度調(diào)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(diào)制器或者半導體光放大器調(diào)制上光作為上行信號上傳。

中心站采用外調(diào)制法實現(xiàn)光外差信號，下行基帶數(shù)據(jù)的上載方式采用DPSK相位調(diào)制。基站處，發(fā)送的下行數(shù)據(jù)可用1比特馬赫-曾德干涉儀解調(diào)，而對于上行的基帶數(shù)據(jù)則可利用鋰酸鈮調(diào)制器采用強度調(diào)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(yōu)勢，毫米波RoF系統(tǒng)是下一代接入網(wǎng)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(xiàn)系統(tǒng)的“成本—資源”考慮，本文針對毫米波光產(chǎn)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(tǒng)結構優(yōu)化方面的討論。相信隨著未來相應結構優(yōu)化研究的深入，毫米波RoF系統(tǒng)的實用化將指日可待。

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