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The idea to allow unlicensed users to utilize licensed bands whenever it would not cause any interference is one of the fundaments of cognitive radio. In this paper we propose some allocation channel strategies to provide more QoS for unlicensed users. We assume that one channel is dedicated for unlicensed users that can eventually use licensed users channels if idle. The first allocation channel strategy reconsiders, in case of unavailable licensed users's channels, the request of unlicensed users to be processed by the dedicated channel. The second strategy doesn't reconsider it and simply reject it. QoS performances of the two patterns are evaluated and discussed using analytical models and simulations. For the first communication strategy, we based our model on the work of Habachi and Hayel which we enhance. We proceed to some corrections of mathematical and formulations errors. Analytical and simulation results show that the performances offered by the primary channels to secondary user are better than performances offered by the dedicated channel, however, the performance offered by the primary channels are negatively impacted by the cost of sensing.