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In this paper, we study underlay device-to-device (D2D) communication systemsempowered by a reconfigurable intelligent surface (RIS) for cognitive cellularnetworks. Considering Rayleigh fading channels and the general case where thereexist both the direct and RIS-enabled D2D channels, the outage probability (OP)of the D2D communication link is presented in closed-form. Next, for theconsidered RIS-empowered underlaid D2D system, we frame an OP minimizationproblem. We target the joint optimization of the transmit power at the D2Dsource and the RIS placement, under constraints on the transmit power at theD2D source and on the limited interference imposed on the cellular user for twoRIS deployment topologies. Due to the coupled optimization variables, theformulated optimization problem is extremely intractable. We propose anequivalent transformation which we are able to solve analytically. In thetransformed problem, an expression for the average value of thesignal-to-interference-noise ratio (SINR) at the D2D receiver is derived inclosed-form. Our theoretical derivations are corroborated through simulationresults, and various system design insights are deduced. It is indicativelyshowcased that the proposed RIS-empowered underlaid D2D system designoutperforms the benchmark semi-adaptive optimal power and optimal distanceschemes, offering $44\%$ and $20\%$ performance improvement, respectively.

Jointly Optimal RIS Placement and Power Allocation for Underlay D2D Communications: An Outage Probability Minimization Approach