  This paper presents a promising design concept for reconfigurable intelligent
surfaces (RISs), named plug-in RIS, wherein the RIS is plugged into an
appropriate position in the environment, adjusted once according to the
location of both base station and blocked region, and operates with fixed beams
to enhance the system performance. The plug-in RIS is a novel system design,
streamlining RIS-assisted millimeter-wave (mmWave) communication without
requiring decoupling two parts of the end-to-end channel, traditional control
signal transmission, and online RIS configuration. In plug-in RIS-aided
transmission, the transmitter efficiently activates specific regions of the
divided large RIS by employing hybrid beamforming techniques, each with
predetermined phase adjustments tailored to reflect signals to desired user
locations. This user-centric approach enhances connectivity and overall user
experience by dynamically illuminating the targeted user based on location. By
introducing plug-in RIS's theoretical framework, design principles, and
performance evaluation, we demonstrate its potential to revolutionize mmWave
communications for the limited channel state information (CSI) scenarios.
Simulation results illustrate that plug-in RIS provides power/cost-efficient
solutions to overcome blockage in the mmWave communication system and a
striking convergence in average bit error rate and achievable rate performance
with traditional full CSI-enabled RIS solutions.
