 usually used for voice com- frequencies considered to be unoccupied. Preferably, munications, a microphone often exists in the system. The node B listens about three times as long as a probe captured signal is encoded with a VoCoder (voice coder) pulse. The search time can be dramatically reduced if and transmitted. The source radio can authenticate the node B is able to listen to many simultaneous fre- user (from a copy of the data) and add the known identity quencies (as it might with an OFDM receiver). Node to the data stream. At the destination end, decoded B responds with a probe response when node A is voice can be analyzed for the purposes of authentication, detected. and the result may be correlated with the sent identity. Case 2: Node A selects the five largest unoccupied Other sensors may be added to a CR for the pur- frequency blocks, and rotates probe pulses in each poses of user authentication. A fingerprint scanner in block. Similarly, node B scans all of the unoccupied a push-to-talk (PTT) button is not intrusive. Automatic frequency blocks that it perceives, prioritized by the fingerprint correlation software techniques are available size of the unoccupied frequency block, and with and scalable in terms of reliability versus processing load a longer dwell time. required. Additionally, cell phones have been equipped with digital cameras. This fingerprint sensor coupled Both of these cases are similar, but minor differences in with facial recognition software may be used to authen- first acquisition and robustness may be significant. ticate a user. Again, the reliability is scalable with pro- After node B hears a probe and responds with a probe cessor demands. acknowledge, the nodes will exchange each node's Figure 2.4b-12 shows a context diagram for a CR's perception of locally available frequencies. Each node authentication application. The detailed learning associ- will "logical OR" active frequencies, leaving a frequency ated with adding a user to an access list through a "third- list that neit