Abstract--By using cognitive radio technology, opportunistic spectrum access has the potential to solve the underused spectrum problem. In this paper, we employ class-based call admission control to admit primary users with the employment of cognitive scheme. By first introducing a specific cognitive radio scheme, we analyze the secondary user's capacity and the collision probability. Employing this scheme, we build two call admission control models to admit primary users for three classes of service, namely, handoff voice calls, new voice calls and data calls. In the two models, we analyze the secondary user's capacity and the blocking probability experienced by voice calls. We show that they achieve improved blocking probability and throughput by exploiting the cognitive radio scheme. Further, we show a tradeoff between the collision probability and the blocking/throughput in the call admission control.