  Reconfigurable intelligent surface (RIS) devices have emerged as an effective
way to control the propagation channels for enhancing the end-users'
performance. However, RIS optimization involves configuring the radio frequency
response of a large number of radiating elements, which is challenging in
real-world applications due to high computational complexity. In this paper, a
model-free cross-entropy (CE) algorithm is proposed to optimize the binary RIS
configuration for improving the signal-to-noise ratio (SNR) at the receiver.
One key advantage of the proposed method is that it only requires system
performance indicators, e.g., the received SNR, without the need for channel
models or channel state information. Both simulations and experiments are
conducted to evaluate the performance of the proposed CE algorithm. This study
provides an experimental demonstration of the channel hardening effect in a
multi-antenna RIS-assisted wireless system under rich multipath fading.
