Reconfigurable Intelligent Surface-Assisted Secondary Communication System Coexisting With Multiple Primary Networks

Abstract

In a secondary communication system (SCS) coexisting with multiple primary networks, the coupling interference between the SCS and primary networks would deteriorate the spectral efficiency (SE) of the SCS and impede the spectrum access of massive secondary devices. In this paper, a well-designed reconfigurable intelligent surface (RIS) is employed to control wireless channels, which can achieve a high SE of the SCS and relieve the scarcity of the spectrum resource. Specifically, the SE enhancement problem is decomposed into two subproblems which are a second-order cone programming and a fractional programming of the convex quadratic form (CQFP), respectively, to optimize alternatively the beamforming vector at the secondary access point and the reflecting coefficients at the RIS. The CQFP subproblem is solved by the domain and envelope shrinking (DES) algorithm, providing the best SE performance. Besides, a low-complexity method of gradient-based linearization with domain (GLD) is proposed for obtaining a sub-optimal reflecting coefficients for fast deployment. Moreover, a nearest point searching with penalty method is also developed to realize the discretization of the phase shifts at the RIS in practice. The simulation results validate the effectiveness of the DES algorithm and show that the proposed GLD algorithm improves the SE performance significantly.