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(gè)不同的波長(zhǎng)，但是實(shí)際應(yīng)用環(huán)境下，由于毫米波信號(hào)短距作用的特性，需要部署大量的基站，這時(shí)如何降低基站的成本，同時(shí)適應(yīng)波分復(fù)用(WDM)-RoF系統(tǒng)是設(shè)計(jì)中需要重視的問(wèn)題。在光網(wǎng)絡(luò)中，波長(zhǎng)是一個(gè)重要的資源，如果能實(shí)現(xiàn)波長(zhǎng)的重復(fù)利用則將大大降低資源消耗，所以當(dāng)前波分復(fù)用無(wú)源光網(wǎng)絡(luò)(WDM-PON)是光網(wǎng)絡(luò)中發(fā)展的重要趨勢(shì)。目前在毫米波RoF系統(tǒng)中實(shí)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(hào)，同時(shí)保留光載波，下行基帶數(shù)據(jù)采用強(qiáng)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(hào)用調(diào)制器或者半導(dǎo)體光放大器調(diào)制上光作為上行信號(hào)上傳。

中心站采用外調(diào)制法實(shí)現(xiàn)光外差信號(hào)，下行基帶數(shù)據(jù)的上載方式采用DPSK相位調(diào)制?；咎?，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(duì)于上行的基帶數(shù)據(jù)則可利用鋰酸鈮調(diào)制器采用強(qiáng)度調(diào)制上光，中心站只需要進(jìn)行強(qiáng)度檢測(cè)即可。

5、結(jié)束語(yǔ)

由于在傳輸帶寬和無(wú)線接入方面的巨大優(yōu)勢(shì)，毫米波RoF系統(tǒng)是下一代接入網(wǎng)的一個(gè)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(shí)現(xiàn)系統(tǒng)的“成本—資源”考慮，本文針對(duì)毫米波光產(chǎn)生、下行信號(hào)上變頻和傳輸性能、上行信號(hào)下變頻、雙向鏈路中的波長(zhǎng)重用等方面進(jìn)行了毫米波RoF系統(tǒng)結(jié)構(gòu)優(yōu)化方面的討論。相信隨著未來(lái)相應(yīng)結(jié)構(gòu)優(yōu)化研究的深入，毫米波RoF系統(tǒng)的實(shí)用化將指日可待。

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