Analog and Digitized Radio-over-Fiber

Radio-over-Fiber (RoF) techniques aim to simplify as much as possible the configuration and the cost of the radio terminations of wireless networks. Both wireless LANs (WLAN) and cellular networks can potentially benefit from RoF. In wireless systems, the most costly equipment is the radio oscillators today necessary at each antenna site (called remote access unit or RAU in this chapter). The preprocessing of the radio signal achieved in the electrical domain at each RAU is particularly costly for the operators, both in terms of CAPEX (capital expenditure) and OPEX (operational expenditure). RoF enables a shift of the most sophisticated radio equipment from the RAUs to a central remote site. In the context of public cellular networks, such a central site typically corresponds to a mobile switching center (MSC) with permanent staff. The basic idea of RoF consists in transporting onto standard monomode fibers (SMF) radio frequencies from an MSC to the multiple RAUs under its supervision and vice versa. At the end of the 1990s, the first experimentations of RoF technologies have demonstrated the economic interest of this approach. The fact that the most costly and intelligent equipment originally located at each RAU could be co-located at a same site favors their potential mutualization. This site corresponds typically to a MSC or to a Radio Network Controller (RNC) in the case of public cellular networks. In the case of a private environment, it corresponds to a private branch exchange (PABX) or to an IP router. The benefit of RoF may be considerably increased if point-to-multipoint fiber infrastructures may be deployed between each RNC and its attached RAUs. For the sake of simplicity and coherence with the other chapters of this book, we shall refer in the remaining of this chapter to the term "Central Site" (CS) instead of MSC or RNC.