Contents 1. Introduction
Wireless services are taking a steadily increasing share of the telecommunications market. The end users not only benefit from its main virtue, mobility, but are also demanding ever larger bandwidth. Larger wireless capacity per user requires the reduction of the wireless cell size, i.e. establishing pico-cells, which is enabled by increasing the radio microwave frequency. The wireless LAN IEEE 802.11g WiFi standard offers up to 54 Mbit/s in the 2.5 GHz range; the IEEE 802.16 WiMAX standard offers up to 100 Mbit/s in the range of 10 to 66 GHz; IEEE 802.15.3 UWB operates at frequencies up to 60 GHz, offering short-range capacity up to 480 Mbit/s. Smaller radio cells imply that ever more antennas are needed to cover a certain area. Such an area may encompass the rooms in a residential home, but may also be a hospital, an office building, an airport lounge, a conference site, etc. When needing so many antenna sites, it becomes economically attractive to locate the microwave signal generation and modulation not at every antenna, but centrally in a cabinet from where optical fibers with their inherent low losses and wide bandwidth can nicely bring the signal to the antennas. The antennas then only have to do the simple optical-to-electrical conversion, and to emit and receive the wireless signal. Centralizing the sophisticated signal handling eases maintenance and upgrading. The signal handling may include techniques for multiple-input-multiple-output antenna schemes, smart beam forming antennas, mobility and connection handovers, feeding multiple radio standards to an antenna, reconfiguration of services to antennas, etc. Hence radio-over-fiber (RoF) technologies can bring many advantages in operating, maintaining and upgrading wireless networks.
