  Reconfigurable intelligent surface (RIS)-based transmission technology offers
a promising solution to enhance wireless communication performance
cost-effectively through properly adjusting the parameters of a large number of
passive reflecting elements. This letter proposes a cosine similarity
theorem-based low-complexity algorithm for adapting the phase shifts of an RIS
that assists a multiple-input multiple-output (MIMO) transmission system. A
semi-analytical probabilistic approach is developed to derive the theoretical
average bit error probability (ABEP) of the system. Furthermore, the validity
of the theoretical analysis is supported through extensive computer
simulations.
