In cognitive OFDMA networks, spectrum sensing is highly needed to accurately observe the spectrum environment, so as to avoid harmful interference to licensed users. However, to save energy, selfish users may not be willing to perform sensing. In order to encourage cognitive users to sense the multiple channels as well as guarantee fairness among the sensing users, this paper presents a joint PHY-MAC framework for cognitive OFDMA systems. In this framework, a higher access priority in the MAC layer will be rewarded to sensing users and the scheduling decision is made based on both the channel quality and sensing contribution of each user. The throughput for the cognitive system and the fairness for each cognitive user are analyzed under the joint framework. Simulation results will then show that the proposed protocol can achieve fairness and efficiency for cognitive users.