In this paper, we consider the reconfigurable intelligent surface (RIS)aided MIMO systems to realize a high quality link between the base station (BS) and the users via a RIS. To achieve this goal, the active beamforming at the BS and the passive beamforming at the RIS should be jointly optimized. However, most of the existing joint optimization schemes that maximize the sum-rate have high computational complexity due to the complex derivative calculations and matrix inversion. To this end, a biologically inspired particle swarm optimization (PSO) algorithm is exploited to solve the non-convex sum-rate optimization problem with low-complexity. Simulation results show that the proposed scheme provides a better trade-off between performance and complexity than existing solutions for RIS-aided MIMO systems.

Cell-free (CF) network is a favorable technique against inter-cell interferences to improve the network capacity. However, to further improve the network capacity, a large number of base stations (BSs) are required to be deployed with high cost and power consumption. To tackle this problem, an energyefficient technique called reconfigurable intelligent surface (RIS)-aided CF network has been recently proposed. By replacing some of the required power-hungry BSs with low-power RISs, the energy efficiency of CF network can be enhanced with guaranteed performance. To achieve this goal, in this paper, we first formulate a joint active and passive beamforming problem to maximize the energy efficiency of RIS-aided CF networks. Then, we propose an alternate optimization algorithm to solve this problem. Specifically, we decompose the original energy efficiency maximization problem into multiple subproblems and solve them alternatively. Particularly, we adopt the zero-forcing (ZF) beamforming scheme to optimize the active beamforming at BSs, while the sequential programming (SP) method is adopted to realize the passive beamforming at RISs. Moreover, a realistic energy-consumption model for wideband RIS-aided CF networks is provided, and the effectiveness of the proposed scheme is evaluated by simulations. Finally, simulation results verify that, the proposed scheme is able to achieve a higher energy efficiency than the existing benchmark solution.INDEX TERMS Cell-free network, reconfigurable intelligent surface (RIS), beamforming, wideband, energy efficiency.

The reconfigurable intelligent surface (RIS) is an emerging technology, which will hopefully bring a new revolution in wireless communications. The RIS technology can be deployed in an indoor/outdoor environment to dynamically manipulate the propagation environment. The RIS consists of a large number of independently controllable passive elements, and these elements are involved in realizing high passive beamforming gain. Different from the conventional active phased antenna array, there is no dedicated radio-frequency (RF) chain installed at the RIS to perform complex signal processing operations. Therefore, it does not incur additional noise while retransmitting the incident wave, which is substantially a unique feature from the conventional wireless communication systems.Taking advantage of its working principle, RIS has been deployed in various practical scenarios. In this tutorial, at first we will review the latest advances in RIS, including the application scenarios such as the system and channel model, the information theoretic analysis, the physical realization and design, key signal processing techniques such as precoding and channel estimation, and prototyping. Finally, we discuss interesting future research problems for the RIS-aided communications.

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