How RIS technology can plug coverage holes in 5G and 6G

In the era of 5G millimetre band and 6G THz band wireless networks, the coverage range of traditional base stations undergoes a reduction. To address this challenge, RIS technology is deployed. RIS is useful for:

1. Coverage extension. They extend coverage for users in challenging cell-edge, especially with blockage.

2. RIS provides dynamic control of reflected beam direction with PA (Power Amplifier)-less operations.

3. Achieving better network energy efficiency

4. Improving the network capacity

In RIS implementation, large panels are installed on the sides of buildings and they are used to reach the locations of the network which cannot be covered otherwise.

This is an example where there is blockage due to the wall and by using a simple cartoon we are plugging in the coverage holes by going round the wall.

Success of this technology depends on cost, size, weight, and power consumption of the implementation. The main point here is that the RIS consists of all passive elements that can be controlled with very low power by adjusting their characteristics.

What does RIS consist of?

RIS consists of large panels with the individual elements. These individual elements can be controlled for the phase of the reflected beam or in some of the more advanced implementations for the polarisation and other aspects of the RF signal. RIS comes in sizes ranging from 100 square centimetres up to about 5 square metres and cost will be less than cellular antennas. RIS products are passive and can be powered with a simple battery and a small solar panel.

RIS contains many thousands of metamaterial elements. Metamaterials have properties that exceed that of the base materials. They exhibit anomalous reflection or refraction that can be exploited to increase 5G/6G penetration. When an electromagnetic wave strikes a metasurface, they alter the direction of the incident wave and reconfigure the signal in real time in response to the changes in the wireless channel. The efficiency of a wireless channel varies depending on the buildings, hills, trees, fog, rain, snow, and vehicles it has to negotiate. RIS consist of the materials that tweak the wireless environment to facilitate the smooth, error less signal propagation in the high bandwidth applications. Metasurfaces have the ability to vary the magnitude and phase of the signal using a software approach. Programmable metasurfaces can enhance the wireless channel environment for optimal reception and also transmission.

RIS is a two dimensional surface of engineered materials whose properties are reconfigurable. Antenna arrays are part of the transmitter or receiver whereas RIS is part of the wireless environment. RIS technology controls the propagation environment between the transmitter and receiver. The salient feature of RIS is the ability to adapt to the wireless signals propagation in the channel. The wireless environment is controllable (the channel entities can be optimised with the transmitter and receiver) using the channel state information. RIS adaptively control the radio environment by periodically repeating the following operations:

1. Estimate the radio channel characteristics, required for determining RIS operation

2. Control the scattering intensity and phase distribution on RIS, based on the above estimated information to obtain the desired propagation channel.

Metamaterial/ Metasurface technologies

Metamaterial/metasurface technologies form the foundation of RIS. Metamaterial features a periodic arrangement of structures each sufficiently smaller than the wavelength of electromagnetic waves. This periodic structure can, in effect, behave as material having a negative refractive index, so it can be used to obtain characteristics that can’t be achieved by ordinary material. At the frequency bands of mm band/THz band, the electrical resistance of metals can be ignored. Metamaterial is a three dimensional periodic structure whereas metasurface is a two dimensional periodic structure. Controlling the reflection phase distribution on the metasurface enables the propagation of reflected waves to be controlled.

RIS adaptively control the radio environment for achieving non-line of sight coverage in mm band/ THz band

Research areas in this field

In the mm band of 5G and THz band of 6G, the wavelength of the signal is very small and the signal can easily be reflected, absorbed or scattered. That is the reason why RIS is regarded as vital to 5G/6G and research is happening in this field. Extending in-building cell coverage by affixing transparent RIS films to windows and thus allowing 5G/6G signals to be refracted and become strong as they pass through RIS film into a building- is being experimented.

RIS nodes available today are not that smart and they carry out instructions from a base station. Research is in progress to develop autonomous RIS nodes that will make their own decisions and remove the need for control by a base station thus removing the need for signalling, synchronisation along with power consumption. The idea of integrating very low power AI chips in RIS nodes is being explored.

The following are the further topics on RIS on which research is going on:

1. Materials and meta-surface designs with wide bandwidth and high- speed configuration

2. Cascade channel modelling (Can we access each one of the individual components of the channel? Or we have to look at cascade channel, system design and link adaptation based on that)

3. Can RIS support MIMO (Multi Input Multi Output)?

4. Network optimisation with RIS entities

Lot of papers are coming out of academia on this technology.

Way forward

RIS technology is an exciting one and it is showing up. The International Institute of Information Technology, Bangalore (IIT-B) is involved in the development of this technology. It is actively engaged in the development and design of a novel prototype for RIS aided communications.

RIS technology also enables the creation of Smart Radio Environments which can be tailored to specific use cases such as the Industrial Internet of things. By combining RIS technology with other futuristic technologies, significant enhancements in data rates, coverage and energy efficiency can be achieved.

Mm wave communications and massive MIMO have achieved success in 5G. Mm wave increases the data rate per user and massive MIMO increases the number of users. Hope RIS technology will also be successful to cover the uncovered.

(The author is retired Advisor, Department of Telecommunications (DOT), Government of India)