Wideband, Polarization-Agile Reconfigurable Intelligent Surfaces

With the ever-increasing demand for high data-rate wireless communications, higher frequency mm-wave bands are becoming more and more lucrative, mainly for the current 5G and the upcoming 6G communication architectures. In an urban environment that comprises buildings, vehicles, trees, etc., lower frequency signals from a transmitting node can get bounced off easily, and enable sufficient signal strength at a non-line-of-sight (NLOS) receiving node by virtue of their longer wavelengths. However, at the mm-wave frequency bands, the corresponding signals will get absorbed and dissipated by the same obstacles due to their very small wavelengths, which can cause a loss of signal at the receiver. As a remedy, the concept of Reconfigurable Intelligent Surfaces (RISs) has been gaining popularity lately in the field of wireless communication. These RISs would allow dynamic control of the communication channel by efficiently reflecting/refracting the incoming mm-wave signals while enabling features like gain enhancement, polarization agility, beam steering, and the creation of multiple beams for catering to multiple receiver nodes. Although a relatively new concept in the fields of wireless communication and signal processing, such surfaces have been explored in electromagnetics for a long time, and are commonly known as Reflectarrays (RA) or Transmitarrays (TA), depending on whether the incident beam is being reflected or refracted off of the surface. These surfaces typically comprise numerous periodically arranged resonant-sized or subwavelength conducting structures - known as unit cells - on a flat/conformal surface. The construction of the unit cells determines the operating bandwidth and polarizations supported by the RIS. In order to dynamically control the gain, steering direction, and polarization of the reflected/refracted beam (i.e., to attain reconfigurability), the phases at each of these unit cells need to be controlled, which is typically accomplished by using tunable elements like P-I-N or Varactor diodes, or sometimes RF-MEMS, liquid-crystal or Graphene-based materials that are particularly useful for mm-wave frequencies. These elements can be controlled simply by using an integrated circuit (IC) chip that can regulate the corresponding bias voltages. Lastly, some selective intelligence can be incorporated into the operation of these surfaces by making the output beam either track a moving receiver or by improving the signal-to-noise ratio (SNR) at a receiving node in real-time by dynamically controlling the unit cell properties.