centralized servers (in the USA these would be managed by the FCC), which would respond with the information about available free TV channels and guard bands in the area of the BS. Other proposals would allow local spectrum sensing only, where the BS would decide by itself which channels are available for communication. A combination of these two approaches is also envisioned. Devices which would operate in the TV white space band (TVWS) would be mainly of two types: Fixed and Personal/Portable. Fixed devices would have geolocation capability with an embedded GPS device. Fixed devices also communicate with the central database to identify other transmitters in the area operating in TVWS. Other measures suggested by the FCC and IEEE to avoid interference include dynamic spectrum sensing and dynamic power control. Overview of the WRAN topology The initial drafts of the 802.22 standard specify that the network should operate in a point to multipoint basis (P2MP). The system will be formed by base stations (BS) and customer-premises equipment (CPE). The CPEs will be attached to a BS via a wireless link. The BSs will control the medium access for all the CPEs attached to it. One key feature of the WRAN Base Stations is that they will be capable of performing a cognitive sensing. This is that the CPEs will be sensing the spectrum and will be sending periodic reports to the BS informing it about what they sense. The BS, with the information gathered, will evaluate whether a change is necessary in the channel used, or on the contrary, if it should stay transmitting and receiving in the same one. An approach to the PHY layer The PHY layer must be able to adapt to different conditions and also needs to be flexible for jumping from channel to channel without errors in transmission or losing clients (CPEs). This flexibility is also required for being able to dynamically adjust the bandwidth, modulation and coding schemes. OFDMA will be the modulation scheme for transmission in up and downlinks. With OFDMA it will be possible to achieve this fast adaptation needed for the BS's and CPEs. By using just one TV channel (a TV channel has a bandwidth of 6 MHz; in some countries they can be of 7 or 8 MHz) the approximate maximum bit rate is 19 Mbit/s at a 30 km distance. The speed and distance achieved is not enough