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This paper investigates reconfigurable intelligent surface (RIS)-assistedsecure multiuser communication systems subject to hardware impairments (HIs).We jointly optimize the beamforming vectors at the base station (BS) and thephase shifts of the reflecting elements at the RIS so as to maximize theweighted minimum secrecy rate (WMSR), subject to both transmission powerconstraints at the BS and unit-modulus constraints at the RIS. To address theformulated optimization problem, we first decouple it into two tractablesubproblems and then use the block coordinate descent (BCD) method toalternately optimize the subproblems. Two different methods are proposed tosolve the two obtained subproblems. The first method transforms each subprobleminto a second order cone programming (SOCP) problem, which can be directlysolved using CVX. The second method leverages the Minorization- Maximization(MM) algorithm. Specifically, we first derive a concave approximation function,which is a lower bound of the original objective function, and then the twosubproblems are transformed into two simple surrogate problems with closedformsolutions. Simulation results verify the performance gains of the proposedrobust transmission method over existing nonrobust designs. In addition, the MMalgorithm is shown to have much lower complexity than the SOCP-based algorithm.

Robust Transmission Design for RIS-assisted Secure Multiuser Communication Systems in the Presence of Hardware Impairments