| Intelligent reflecting surface(IRS)is expected to be widely used in mobile communication in the future.With the assistance of smart reflector,the spectrum efficiency of communication system can be effectively improved by combining active beamforming and passive beamforming.In this paper,the joint beamforming algorithm is designed to optimize the maximum sum spectral efficiency of multi-user downlink multi-input multi-output(MISO)system.Firstly,optimization problem on the maximum downlink multiuser spectrum efficiency for a single-IRS scene is proposed,which is based on the maximum reflection power of IRS.According to the idea of alternating optimization,the manifold optimization method is introduced.The active beamforming at the base station is solved by zero forcing(ZF)and minimum mean square error(MMSE)methods for interference cancellation and interference suppression,respectively.The simulation results show that the two alternative optimization algorithms can improve the spectrum efficiency for different indicators,and have the characteristics of fast convergence.Furthermore,the scenario of single IRS is extended to the scenario of double IRS.In particular,the double-reflection link between two IRSs is introduced in the double-IRS scenario,and the optimization problem of multiuser maximum sum spectrum efficiency is established under the joint constraint of two IRSs reflection coefficients.For single user system,a closed form algorithm assisted by double IRS is proposed.For multi-user system,the alternative optimization method based on manifold optimization is extended to solve the optimization problem in double-IRS scenario.In order to reduce the complexity of the algorithm in the double-IRS scenario,a low complexity iterative algorithm based on the successive convex optimization(SCA)is proposed.The final simulation results show that the proposed algorithm can effectively improve the system performance compared with the benchmark scheme. |
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