  Reconfigurable intelligent surface (RIS)-assisted communications appear as a
promising candidate for future wireless systems due to its attractive
advantages in terms of implementation cost and end-to-end system performance.
In this paper, two new multiple-input multiple-output (MIMO) system designs
using RISs are presented to enhance the performance and boost the spectral
efficiency of state-of-the-art MIMO communication systems. Vertical Bell Labs
layered space-time (VBLAST) and Alamouti's schemes have been considered in this
study and RIS-based simple transceiver architectures are proposed. For the
VBLAST-based new system, an RIS is used to enhance the performance of the
nulling and canceling-based sub-optimal detection procedure as well as to
noticeably boost the spectral efficiency by conveying extra bits through the
adjustment of the phases of the RIS elements. In addition, RIS elements have
been utilized in order to redesign Alamouti's scheme with a single radio
frequency (RF) signal generator at the transmitter side and to enhance its bit
error rate (BER) performance. Monte Carlo simulations are provided to show the
effectiveness of our system designs and it has been shown that they outperform
the reference schemes in terms of BER performance and spectral efficiency.
