Cognitive radio (CR) systems offer an efficient solution to improve spectral and energy efficiency, addressing the growing demand for wireless communication. Intelligent reflecting surfaces (IRS) enhance network performance by dynamically configuring the propagation environment using passive elements. This paper explores IRS integration into CR networks to optimize resource allocation and maximize spectral efficiency for secondary users (SUs) in a downlink scenario from a secondary base station (BS) while ensuring interference to the primary network remains below acceptable limits. This work aims to enhance the sum-rate of SUs by addressing a joint optimization problem that encompasses the allocation of IRS elements to SUs, adjustment of IRS reflection parameters, and the design of beamforming vectors at the secondary BS, modeled as a non-convex mixed-integer program. The problem is decomposed using a strategy based on variable decoupling and constraint relaxation, resulting in two subproblems that are solved in an iterative manner: initially, the secondary BS beamforming vector is obtained via fractional programming (FP), followed by a simulated annealing-based optimization for configuring the phase-shifts and determining IRS-user associations. Simulation results show the proposed method outperforms random allocation and phase-shift scenarios. Performance analysis in practical settings reveals up to a 61 % enhancement in sum-rate in proportion to zero-forcing beamforming utilizing randomly assigned IRS phase-shifts and user association.
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