  This work studies the wideband modeling and beamforming design of beyond
diagonal reconfigurable intelligent surface (BD-RIS), which generalizes and
goes beyond conventional RIS with diagonal phase shift matrices to achieve
enhanced channel gain. Specifically, we investigate the response of BD-RIS in
wideband systems by going back to its hardware circuit realizations. We propose
a novel wideband model which has simple expressions while capturing the
response variations of BD-RIS for signals with different frequencies. With this
wideband model, we propose a BD-RIS design algorithm for an orthogonal
frequency division multiplexing system to maximize the average rate over all
subcarriers. Finally, we provide simulation results to evaluate the performance
of the proposed design and show the importance of wideband modeling for BD-RIS.
