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The deployment of third-generation (3G) mobile systems is accelerating due to the explosion of high-speed wireless multimedia communications and mobile Internet services. 3G services in Japan are entering their fourth year of commercial service and therefore are reaching the stage of maturity. ROF (Radio over fiber) technology enables the transmission of radio signals directly over optical fiber with low propagation loss and long-reach feeding instead of a coaxial cable, and allows mobile service providers to deploy base stations with more freedom and fewer restrictions at lower cost in 3G systems. The ROF approach makes it possible to centralize large base station facilities, and easier to add new cell sites, upgrade technology and accommodate multi-service signals within a single system for outdoor applications. In addition to conventional indoor applications for an instance gap filler system, a mobile Centrex service has consistently driven the demand for interconnecting and distributing radio signals from base station to remote antenna units. In this presentation, we review the recent demonstrations and applications in the mobile communication system relevant ROF technologies. We also discuss the system design issues caused by system deployments and optical fiber characteristics, and present research and development topics and challenges for future radio signal transport systems in mobile networks.