A New Optimal Hybrid Spectrum Access in Cognitive Radio: Overlay-Underlay Mode

In this paper, we propose a hybrid cognitive radio (CR) system where underlay and overlay CR approaches are combined under SIRN constraints. This new access type allows to optimize the spectrum sensing time with throughput improvement. Our proposed access type is based on a merger of these two modes depending on the number of secondary users (SU) in the Underlay Mode and on the access probabilities for these two modes. The number of all users in the Underlay access mode should not exceed a certain number of users. When a new SU arrives and if the number of SUs in Underlay mode is already equal to the predifined max number, the SU decides to switch to the Overlay mode to access the canal using this method. The SU states will be modeled with a Markov chain, the aim of this work is attempting to show that this new method allows to increase the throughput of the SU with energy optimization. The exact outage probability of secondary system is also derived for this model. Simulation results show that the performance, especiallay the throughput can be significantly increased by the proposed hybrid system.