, it includes the most important ones. subjected to interference and may not be able to inform the CR that its receiver is experiencing interference. 2.4b.5.1 Geolocation Regulators, spectrum owners, and developers of CR are working to find robust solutions to the hidden node Geolocation is an important CR enabling technology due problem. Again, a cooperative approach may help to to the wide range of applications that may result from mitigate some of the hidden node problems, but a a radio being aware of its current location and possibly cooperative approach will not necessarily eliminate the being aware of its planned path and destination. hidden node problem. The GPS is a satellite-based system that uses the In addition to knowing the frequency and transmit time difference of arrival (TDoA) to geolocate a receiver. activity properties of a radio transmitter, it may also be The resolution of GPS is approximately 100 m. GPS re- desirable for the radio to be able to recognize the wave- ceivers typically include a one-pulse-per-second signal form properties and determine the type of modulation, that is Kalman filtered as it arrives at each radio from each thereby allowing a radio to request entry into a local satellite, resulting in a high-resolution estimate of propa- network. Many articles have been published on this gation delay from each satellite regardless of position. By topic, as well as a textbook by Azzouz and Nandi [32]. compensating each pulse for the predicted propagation Once the modulation is recognized, then the CR can delay, the GPS receivers estimate time to approximately choose the proper waveform and protocol stack to re- 340 nanoseconds (ns, or 10- seconds) of jitter [31]. quest entry into the local network. In the absence of GPS signals, triangulation approaches may be used to geolocate a radio from cooperative or 2.4b.5.3 Biometrics even non-cooperative emitters. Literature is available to discuss the classical approaches of TDoA, time of arrival A CR can learn the identity of its user(s), en