ties should be expected of aware, adaptive, and CRs. to a new country, the policy may be reloaded to comply in Constraint by policy and use of inference engine to the new jurisdiction. Also, if a band is available for use make policy-constrained decisions. during a certain period but not during another, a machine- The first examples of CRs were modeled in the Defense readable policy can realize that behavior. Advanced Research Projects Agency (DARPA) NeXt The language being used to describe a CR is based on the Generation (XG) radio development program. These assumption of a smart agent model, with the following radios sense the spectrum environment, identify an capabilities: 2 unoccupied portion, rendezvous multiple radios in the Sensors creating awareness in the environment. unoccupied band, communicate in that band, and vacate Actuators enabling interaction with the environment. the band if a legacy signal re-enters that band. These Memory and a model of the environment. behaviors are modified as the radio system learns more Learning and modeling of specific beneficial adaptations. about the environment; the radios are constrained by regulatory policies that are machine interpretable. The Specific performance goals. first demonstrations of these systems took place late in Autonomy. 2004 and in 2005 [30]. smart agent also has the ability to not use some or all of the listed capabilities. Cognitive radio: The technologies required CHAPTER 2.4b 2.4b.4 Comparison of radio are required to create an accurate location estimate. The capabilities and properties circular error probability (CEP) characterizes the esti- mate accuracy. Table 2.4b-4 summarizes the properties for the classes of advanced radios described in the previous sections. 2.4b.5.2 Spectrum awareness/frequency Classes of radios have "fuzzy boundaries," and the com- occupancy parison shown in the table is broad. There are certain examples of radios that fall outside the suggestions in the A radio that is aware of spectrum occupancy may exploit table. A 