Smart Radio Environments Empowered by Reconfigurable Intelligent\n Surfaces: How it Works, State of Research, and Road Ahead

What is a reconfigurable intelligent surface? What is a smart radio\nenvironment? What is a metasurface? How do metasurfaces work and how to model\nthem? How to reconcile the mathematical theories of communication and\nelectromagnetism? What are the most suitable uses and applications of\nreconfigurable intelligent surfaces in wireless networks? What are the most\npromising smart radio environments for wireless applications? What is the\ncurrent state of research? What are the most important and challenging research\nissues to tackle?\n These are a few of the many questions that we investigate in this short opus,\nwhich has the threefold objective of introducing the emerging research field of\nsmart radio environments empowered by reconfigurable intelligent surfaces,\nputting forth the need of reconciling and reuniting C. E. Shannon's\nmathematical theory of communication with G. Green's and J. C. Maxwell's\nmathematical theories of electromagnetism, and reporting pragmatic guidelines\nand recipes for employing appropriate physics-based models of metasurfaces in\nwireless communications.\n