equired to mitigate such problems as not interfere with legacy systems. We assume, however, hidden node or deeply fading RF channels for CR access to that it periodically transmits a beacon signal, and we spectrum on a non-interference basis. assume that this beacon includes reference time, next frequency-hop(s), and a description of frequencies in use 2.4b.6.3 The rendezvous problem within the local region. Furthermore, we assume that the infrastructure beacon is followed by an interval in which The difficulty for the CR network is the radios locating CRs request an available time-frequency slot and asso- each other and getting the network started. Before any ciate a net name to that slot, as well as to their location transmission can occur, all radios must survey the spec- and transmit power, followed by a response from the trum to determine where the available "holes" (frequency infrastructure recommending which frequency to use reuse opportunities) are located. However, each receiver- and when to check back. Subsequent requests by other sensor will perceive the spectrum slightly differently net members can be directed to the proper time- because each will see different objects shadowing differ- frequency slot, and the geographic distribution of net ent transmitters, and will either see each transmitter with members can be tracked, allowing the infrastructure to a different signal strength or will not see some assess interference profiles. Remote CR Local CR Other CR Legacy (Talk with) (Talk with) (No Communications) Users Spectrum Usage Sensor Report Sensor Report Sensor Report Sensor Report Request Communication Decision Regarding Decision Regarding Band to Use to Use Rendezvous Protocol Communications Spectrum Usage Sensor Report Move Band Communications Figure 2.4b-11 Dynamic spectrum access sequence diagram (assumes a control channel). A pair of CR communicating on an unoccupied channel must vacate when interference potential is detected. CHAPTER 2.4b Cognitive radio: The technologies required Unaided ren