What are Reconfigurable Intelligent Surfaces?

A Reconfigurable Intelligent Surface (RIS) is a programmable surface structure that can be used to control the reflection of electromagnetic (EM) waves by changing the electric and magnetic properties of the surface. These surfaces can be strategically placed in the radio channel between a transmitter and receiver to control the way the signal reflects off a surface in its propagation path. Reconfigurable Intelligent Surfaces can be used to steer signals to the receiver resulting in better reception or link quality.

In conventional wireless systems, the radio channel is seen as an uncontrollable entity that usually distorts transmitted signals. The transmitted signals usually interact with a wide range of surfaces in their propagation path and reflect off them in random directions resulting in random constructive and destructive interference. RIS provides a way to control the surfaces found in radio channels by directing them in a specific direction to improve the reliability and energy efficiency of wireless systems. This can increase the range of a wireless system and direct signals to hard-to-reach places.

There are two main approaches can be used to implement an RIS to control the characteristics of signals (e.g., reflection, refraction, absorption, focusing and polarization), namely, conventional antenna arrays and metasurfaces.

Metasurfaces: Metasurfaces are electrically thin and dense two-dimensional arrays of structural elements possessing desired properties granted by their constitutive elements. Elements are called meta-cells, unit-cells or meta-atoms and their size is much smaller than the signal’s wavelength. In addition to the control of EM propagation, metasurfaces can be used to realize complex operations such as data modulation & mathematical operations and they have been demonstrated to also be capable of storing EM pulses for short time periods. 

Antenna Arrays: The other simpler implementation of RIS includes electrically tunable reflecting antenna arrays that can be used to dynamically adjust their radiation patterns. Hence, within the context of RIS, antenna arrays can be employed as reflecting structures. Specifically, a large number of low-cost and passive antenna elements can be employed as RIS, which operate as reflectors with no need for RF chains as in the conventional multiple-input multiple-output (MIMO) systems using multiple transmit and multiple receive antennas.

Envisioned scenarios for RIS applications vary from cases where a single RIS is placed on a wall to direct signals coming from a predetermined direction, e.g., base station, to the environment where almost all surfaces (walls, furniture, clothes, etc.) are covered with a metasurface based RIS. Mostly RIS structures will be used to deflect the signals in the desired direction. 

How do Reconfigurable intelligent surface work?

Reconfigurable intelligent surfaces would first absorb a signal that is directed towards it, which then would be processed by a microcontroller and then reproduced again and transmitted in a specific direction using antenna arrays and metasurfaces. Thus they can be used to increase availability of cellular or GNSS signals in areas where they are not naturally accessible. Deflection of signals can also be used to shorten the path taken by the signals to reach the receiver so as to maximize efficiency of the network system.

As frequency bands used for 5G and 6G go into the mmWave and THz region, the need for Reconfigurable intelligent surfaces (RIS) will increase. This is because signals in these frequency bands have can not propagate very far or penetrate walls like the sub-6 GHz signal used by most cellular technologies today (3G/4G/ sub-6GHz 5G). Such high-frequency signals would need to be directed artificially using RISs to reach inside buildings and other structures. The signals may also need to be modulated according to a specific algorithm mid-propagation which can also be done by using RISs.