A Mixed Spectrum Sharing Strategy for Cognitive Radio Systems

This paper deals with a power allocation optimization problem in a Cognitive Radio (CR) system composed of opportunistic unlicensed users called secondary users (SUs) and primary users (PUs) owning the spectrum licence. When the SU is allowed to transmit, these constraints guarantee: a maximum average transmit power and the protection of the PUs by maximum average interference levels at the PUs receivers. We study this optimization problem in a mix between underlay access strategy, where spectrum is always accessible to the SU subject to an interference constraint at the PU, and overlay access strategy where the secondary service accesses to the spectrum only during the idle periods. Thus, we propose a novel algorithm by taking into account the combination between idle and busy periods of the PUs. The performance of the maximum achievable capacity of the SU is illustrated for different interference power parameters. The simulation results show that the underlay behavior of the mixed strategy is dominant for very low interference and for very high interference, the overlay behavior of the mixed strategy is dominant.