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Reconfigurable intelligent surface (RIS) is an emerging technique employingmetasurface to reflect the signal from the source node to the destination nodewithout consuming any energy. Not only the spectral efficiency but also theenergy efficiency can be improved through RIS. Essentially, RIS can beconsidered as a passive relay between the source and destination node. On theother hand, a relay node in a traditional relay network has to be active, whichindicates that it will consume energy when it is relaying the signal orinformation between the source and destination nodes. In this paper, we comparethe performances between RIS and active relay for a general multiple-inputmultiple-output (MIMO) system. To make the comparison fair and comprehensive,both the performances of RIS and active relay are optimized with best-effort.In terms of the RIS, transmit beamforming and reflecting coefficient at the RISare jointly optimized so as to maximize the end-to-end throughput. Although theoptimization problem is non-convex, it is transformed equivalently to aweighted mean-square error (MSE) minimization problem and an alternatingoptimization problem is proposed, which can ensure the convergence to astationary point. In terms of active relay, both half duplex relay (HDR) andfull duplex relay (FDR) are considered. End-to-end throughput is maximized viaan alternating optimization method. Numerical results are presented todemonstrate the effectiveness of the proposed algorithm. Finally, comparisonsbetween RIS and relays are investigated from the perspective of system model,performance, deployment and controlling method.

Performance Comparison between Reconfigurable Intelligent Surface and Relays: Theoretical Methods and a Perspective from Operator