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This paper investigates the passive beamforming and information transfer(PBIT) technique for the multiuser multiple-input multiple-output (MIMO)systems with the aid of a reconfigurable intelligent surface (RIS), where theRIS enhances the primary communication via passive beamforming and at the sametime delivers additional information by the spatial modulation (which adjuststhe on-off states of the reflecting elements). For the passive beamformingdesign, we propose to maximize the sum channel capacity of the RIS-aidedmultiuser MIMO channel and formulate the problem as a two-step stochasticprogram. A sample average approximation (SAA) based iterative algorithm isdeveloped for the efficient passive beamforming design of the consideredscheme. To strike a balance between complexity and performance, we then proposea simplified beamforming algorithm by approximating the stochastic program as adeterministic alternating optimization problem. For the receiver design, thesignal detection at the receiver is a bilinear estimation problem since the RISinformation is multiplicatively modulated onto the reflected signals of thereflecting elements. To solve this bilinear estimation problem, we develop aturbo message passing (TMP) algorithm in which the factor graph associated withthe problem is divided into two modules: one for the estimation of the usersignals and the other for the estimation of the RIS's on-off states. The twomodules are executed iteratively to yield a near-optimal low-complexitysolution. Furthermore, we extend the design of the multiuser MIMO PBIT schemefrom single-RIS to multi-RIS, by leveraging the similarity between thesingle-RIS and multi-RIS system models. Extensive simulation results areprovided to demonstrate the advantages of our passive beamforming and receiverdesigns.

Passive Beamforming and Information Transfer Design for Reconfigurable Intelligent Surfaces Aided Multiuser MIMO Systems