Reconfigurable Intelligent Surfaces

I. MOTIVATIONS

Beyond 5G and 6G networks have introduced higher and stricter Key Performance Indicators (KPIs), such as guaranteed data rate of 1 Gbps and guaranteed delay of 1μs. It has also combined with increase in data traffic by 55% every year leading to around 5,016 exabytes (5.3×109 Gbps), as well as data rates increase to 1Tbps till 2030 [1]. Additionally, there has been an increase in the need for heterogeneous services, such as localization and sensing, as well as ability to program and control the wireless network environment. The strict requirements have also introduced a need for smart radio environments which essentially signify the need to jointly optimize the entire system i.e. transmitter, receiver and wireless environment compared to current scenario, wherein you cannot control and optimize the wireless environment. Fig. 1 shows the difference between current radio and environment and future smart radio environments. Hence, the need to control and optimize the wireless environment has led to advent in meta-surfaces such as Reconfigurable Intelligent Surfaces (RIS).

II. RIS INTRODUCTION AND WORKING

RIS are meta surfaces designed to be able to control and manipulate wireless signals and to direct signals in particular direction. Before explaining the workings of RIS, an overview of meta surfaces should be provided.

A. Overview of Meta Surfaces

Meta surfaces are essentially two dimensional surfaces which have the capability to manipulate wireless signals. Currently, there are different types of meta surfaces being researched. Two most prominent ones are Reconfigurable Intelligent Surfaces (RIS) and Large Intelligent Surfaces (LIS). Table I shows the differences between the RIS and LIS. Aside from RIS and LIS, there are some more meta surfaces being researched, such as Digitally Controllable Scatterers and Software Controllable Scatterers.

Fig. 1. Radio Environments v/s Smart Radio Environments [1]

TABLE I
DIFFERENCES BETWEEN RIS AND LIS

Characteristics Reconfigurable Intelligent SurfacesLarge Intelligent Surfaces
Alternate NameIntelligent Reflecting Surfaces (IRS)Next Generation Massive MIMO Surface
Individual ElementsPassive ReflectorsActive Elements such as RF Chains and Power Amplifiers
Wireless ProcessingTunable Phase ResponseAdvanced Signal Processing Capabilities
Power ConsumptionIdeally ZeroNon-Zero

B. RIS Definition, Operating Modes and Design

Reconfigurable Intelligent Surfaces (RIS) are defined as two dimensional meta-surfaces which consist of adaptive and inexpensive elements which comprise of thin composite material sheet that is able to program radio waves for desired response. RIS operate in two different modes,

  • Control and Programming Mode: In this mode, the RIS is programmed by a controller to obtain the desired phase response.
  • Normal Operation Mode: This is normal operating mode in which the RIS acts like a passive device with near zero power consumption.

Hence, the design of RIS consists of a two-dimensional structure of composite material layers comprised of passive elements which is configured using low power tunable electronic circuits, such as PIN Diodes (toggling On/Off states) or Varactor Diodes (altering Bias Voltage) which are controlled using a microcontroller. So, the overall characteristics of RIS are,

  • Nearly Passive
  • Programmable Control for Wireless Signals
  • Good Compatibility with existing Wireless Systems
  • Easy Infrastructure Deployment

Due to the novel nature of meta-surfaces, there are only a few prototypes that have been developed. Fig. 2 shows the prototype developed by MIT RFocus which consists of 3720 antennas on 6 m2 and is controlled by Raspberry Pi controller. It is able to provide 9.5x median Signal Strength and 2x median Channel Capacity. Fig. 3 shows the prototype by NT Docomo. It is a transparent meta-surface with three modes of operation: full penetration, partial reflection and full reflection.

Fig. 2. MIT RFocus Prototype [2]
Fig. 3. NT Docomo Prototype [3]

II. RIS RESEARCH OPPORTUNITIES

Due to the flexible nature of integration and deployment, as well as low power consumption, RIS can potentially be used in multiple different scenarios. Some application scenarios and potential research areas which are currently pursued are mentioned here.

A. RIS Application Scenarios

Here are some of the major application scenarios in which integration and usage of RIS is mentioned. These are,

  • SNR and Capacity Maximization: RIS can be used to increase SNR and Capacity by assisting Line of Sight Signal Path, creating a controlled multipath environment,
  • Channel Estimation: RIS can be used to determine a controlled cascaded channel for a deterministic channel response,
  • Localization: RIS can be used as a controllable multipath for localization,
  • Interference Suppression: RIS can be used to redirect signal in desired direction, thereby reducing the overall interference in the environment,
  • Security Enhancement: RIS can be used to redirect signals away from unauthorized nodes in the wireless environment,
  • Ambient Backscattering: RIS can be used on top on low power sensors in wireless environment and can be programmed to piggyback the sensor reading on the wireless signal incident on RIS thereby leading to low power consumption for the sensor.

Hence, due to the effluent compatibility of RIS, application scenarios for it are quite wide and open for research.

B. RIS Research Areas

As explained in the earlier subsection, there are many open research areas for RIS. The most prominent research areas are,

  • Path Loss and RIS Channel Modelling
  • RIS based Modulation and Channel Encoding
  • RIS based Channel Estimation
  • Stochastic Geometry based Analysis
  • RIS based Resource Allocation
  • Passive Beamforming Optimization
  • RIS based System Energy Minimization
  • RIS assisted Localization
  • Physical Layer Security
  • RIS assisted Non-Orthogonal Multiple Access
  • Internet of Things (IOTs) and Backscattering
  • UAV Communication
  • Wireless Power Transfer
  • Multiple Edge Computing
  • mmWave, TerraHertz and Optical Communications
  • SDN based RIS Design
  • ML based RIS Design

As demonstrated, research with respect to RIS is very open and has a lot of scope.

IV. Conclusion

Briefly, RIS is a two-dimensional nearly passive meta-surface which has a capability to alter wireless signals. Due to the novelty of RIS, there is also a lot of space for research in it.

Anay’s full presentation about the RIS can be found in a video form by clicking on the following link: https://drive.google.com/file/d/1jPw638dfaeixVlxtGiFUAy8UfD_GoSbX/view?usp=sharing

References

[1] Di Renzo, M., Zappone, A., Debbah, M., Alouini, M.S., Yuen, C., de Rosny, J. and Tretyakov, S., 2020. Smart radio environments empowered by reconfigurable intelligent surfaces: How it works, state of research, and the road ahead. IEEE Journal on Selected Areas in Communications, 38(11), pp. 2450-2525.

[2] Arun, V. and Balakrishnan, H., 2020. RFocus: Beamforming using thousands of passive antennas. In 17th {USENIX} Symposium on Networked Systems Design and Implementation ({NSDI} 20) (pp. 1047-1061).

[3] DOCOMO, N., 2020. DOCOMO conducts world’s first successful trial of transparent dynamic metasurface.

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Anay Deshpande

Anay is a PhD student at University of Padova, Italy, where he researches Anticipatory Techniques For Wireless Network Optimisation. He is currently working on predictive drone routing, predictive scheduling for real time systems and integration of RIS in drone assisted communication for green networks.

Comments

  1. Andrea Zanella

    Very nice and useful work, Anay!

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