Development of Radio on Free Space Optics System for Ubiquitous Wireless

Radio on Fiber (RoF) with layer 1 routing capability can realize a cost efiective universal platform for future ubiquitous wireless services. The layer 1 routing concept can be realize by not only RoF but also RoFSO (Radio on Free Space Optics) or RoR (Radio on Radio) networks, which provide a free space for heterogeneous wireless services in Free Space Optics or millimeter wave radio. Development of a new advanced RoFSO system has been started. This paper describes its concept and features, and furthermore, discusses about its role in future ubiquitous wireless. In ubiquitous network society, users want an environment to access any communication services at any time, any place, and any situations. In order to realize the ubiquitous networks, a combination of IP network and broadband heterogeneous wireless access services will play an important role. In current wireless networks, however, various operators independently overlaid their own radio base stations and networks. This leads redundant equipments and investments on infrastructures, and prevents the quick start of a new wireless service and employing microcellular architecture. These problems are revealed especially in in-building, underground at urban areas, and rural areas where broadband flber-infrastructures have not yet been constructed due to their high cost and a low population. Radio on Fiber (RoF) technologies can realize a cost efiective universal platforms for future ubiquitous wireless services. Furthermore, RoF networks can be extended to Virtual Radio Free Space Network with layer 1 routing realized by not only RoF but also RoFSO (Radio on Free Space Optics) or RoR (Radio on Radio) networks, which can provide a free space for heterogeneous wireless services in Free Space Optics or millimeter wave radio. This paper describes the concept and features of a new advanced RoFSO system development, and furthermore, discusses software deflnable radio networks (1,2) from a viewpoint of its role in future ubiquitous wireless. 2. LAYER 1 ROUTING WITH RADIO ON FIBER, RADIO, AND FREE SPACE OPTICS Radio on Fiber (RoF) links shown in Fig. 1 have a function of transmitting radio signals to remote stations with keeping their radio formats. Consequently, RoF link becomes a hopeful candidate of a common platform for various wireless access networks. When RoF equips photonic routing functions, any radio signal can be forwarded to its destination control station. We call such RoF networks \Virtual Radio Free Space Network (VRFSN)" (3). By using RoF, architecture for radio access zones easily employs micro or pico cellular systems. A RBS receiving or transmitting radio signals in each radio zone, equips only O/E and O/E converters. The RBS requires neither the modulation functions nor demodulation functions of radio signals. The radio signals converted into optical signals are transmitted via a RoF link with the beneflt of its low transmission loss and broadband. Therefore, RoF links can be independent of radio signal formats and can provide universality for various types of radio access methods. This means that VRFSNs are very ∞exible to the modiflcation of radio signal formats, the opening of new radio services, or the accommodation of difierent types of radio signal formats. VRFSNs with layer 1 routing are required for ubiquitous wireless especially in radio dead zones at private or public spaces such as in-house, in-building, and underground at urban areas, and at rural areas where broadband flber-infrastructures have not yet been constructed due to their high cost and a low population. The layer 1 routing concept shown in Fig. 2 can be also extended to