毫米波光载无线系统的结构优化

从自由振荡的5 GHz提高到了超过20 GHz。如果将纵模间距为毫米波频率的光相干双纵模信号从中心站发送到基站并注入锁定DFB激光器，同时将基站接收到的天线信号调制该DFB激光器，则注入纵模的调制后的边带将落在另一个纵模的基带，由于两个纵模是相干的，于是直接进行光滤波则可将中频或者基带信号获得。这样就可以在不需要在高速光电二极管和混频器的条件下进行毫米波信号的下变频。

4、波分复用毫米波RoF系统中的波长重用

在单路RoF系统(中心站、基站各一)中，下行链路和上行链路可以分别占有两个不同的波长，但是实际应用环境下，由于毫米波信号短距作用的特性，需要部署大量的基站，这时如何降低基站的成本，同时适应波分复用(WDM)-RoF系统是设计中需要重视的问题。在光网络中，波长是一个重要的资源，如果能实现波长的重复利用则将大大降低资源消耗，所以当前波分复用无源光网络(WDM-PON)是光网络中发展的重要趋势。目前在毫米波RoF系统中实现WDM-PON有几种可行方案。

中心站采用外调制法发送下行信号，同时保留光载波，下行基带数据采用强度调制。在基站中则将光载波利用光滤波器滤出，然后将下变频的用户上行基带信号用调制器或者半导体光放大器调制上光作为上行信号上传。

中心站采用外调制法实现光外差信号，下行基带数据的上载方式采用DPSK相位调制。基站处，发送的下行数据可用1比特马赫-曾德干涉仪解调，而对于上行的基带数据则可利用锂酸铌调制器采用强度调制上光，中心站只需要进行强度检测即可。

5、结束语

由于在传输带宽和无线接入方面的巨大优势，毫米波RoF系统是下一代接入网的一个很有潜力的解决方案。鉴于目前毫米波频段器件的昂贵以及实现系统的"成本—资源"考虑，本文针对毫米波光产生、下行信号上变频和传输性能、上行信号下变频、双向链路中的波长重用等方面进行了毫米波RoF系统结构优化方面的讨论。相信随着未来相应结构优化研究的深入，毫米波RoF系统的实用化将指日可待。

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