  Reconfigurable intelligent surface (RIS) is an emerging technique employing
metasurface to reflect the signal from the source node to the destination node
without consuming any energy. Not only the spectral efficiency but also the
energy efficiency can be improved through RIS. Essentially, RIS can be
considered as a passive relay between the source and destination node. On the
other hand, a relay node in a traditional relay network has to be active, which
indicates that it will consume energy when it is relaying the signal or
information between the source and destination nodes. In this paper, we compare
the performances between RIS and active relay for a general multiple-input
multiple-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 RIS
are jointly optimized so as to maximize the end-to-end throughput. Although the
optimization problem is non-convex, it is transformed equivalently to a
weighted mean-square error (MSE) minimization problem and an alternating
optimization problem is proposed, which can ensure the convergence to a
stationary point. In terms of active relay, both half duplex relay (HDR) and
full duplex relay (FDR) are considered. End-to-end throughput is maximized via
an alternating optimization method. Numerical results are presented to
demonstrate the effectiveness of the proposed algorithm. Finally, comparisons
between RIS and relays are investigated from the perspective of system model,
performance, deployment and controlling method.
