Reconfigurable Intelligent Surfaces (RIS) in 6G: Advantages and Disadvantages

Introduction

Reconfigurable Intelligent Surfaces (RIS) are a potentially revolutionary technology, poised to redefine wireless communication in 6G networks. By dynamically controlling electromagnetic waves, RIS enhances signal coverage, improves energy efficiency and ensures smarter connectivity. This article explores the key advantages and disadvantages of RIS in 6G wireless networks, offering insights into its potential and constraints in future communication systems.

What are Reconfigurable Intelligent Surfaces (RIS)?

A Reconfigurable Intelligent Surface (RIS), also known as an intelligent reflecting surface or a software-controlled metasurface, is a surface that uses passive reflecting elements controlled by software. The principle of operation is based on a variation of Snell’s law. RIS receives plane waves as input and outputs scattered waves whose phase shifts are controlled to achieve the desired reflection.

The metasurface consists of ‘N’ number of meta-atoms with sub-λ size. These elements scatter incoming waves with controllable delay/phase and polarization, effectively changing the wireless propagation channel environment by carefully tuning the phase shifts of a large number of low-cost passive reflecting elements.

Let’s understand how RIS works:

• EM (Electromagnetic) waves transmitted from a Base Station (BS) impinge on the RIS, inducing a current within the RIS.
• The RIS reflects these signals towards the users.
• During reflection, the RIS changes its response by controlling phase and amplitude. Phase shifts are controlled by PIN diodes used within the RIS.

RIS is an ultra-thin meta-surface composed of three layers:

• Outer layer: A dielectric substrate with RIS elements that directly interact with incident signals. These elements utilize PIN diodes to control the phase response of the scattered wave by switching ON and OFF as needed. They support frequencies from sub-6 GHz to THz range.
• Middle layer: A copper plate that prevents signal energy leakage.
• Inner layer: A control circuit board, known as the RIS controller.

Metasurfaces are used in various applications, aiding systems such as: cell edge systems, mmWave systems (e.g., 6G wireless systems), IoT (Internet of Things), Indoor WiFi systems, UAV sensor-based systems, Physical layer secure systems, SWIPT systems, etc.

Active RIS vs. Passive RIS

Reconfigurable Intelligent Surfaces (RIS) play a pivotal role in optimizing wireless communications by controlling the propagation of electromagnetic waves. These surfaces can be broadly categorized into Active RIS and Passive RIS, each offering distinct functionalities and use cases.

Active RIS

• Definition: Active RIS includes integrated amplifiers or active components that can amplify the reflected or refracted signals.
• Key Features:
  • Signal Amplification: Active RIS not only reflects but also amplifies the incoming signals, compensating for path losses.
  • Power Supply: Requires external power sources to drive the active components and maintain signal amplification.
  • Complex Design: Incorporates complex circuitry and components such as amplifiers, which increases the system’s cost and power consumption.

Passive RIS

• Definition: Passive RIS relies solely on reflective elements to redirect the electromagnetic waves without any signal amplification.
• Key Features:
  • Signal Reflection Only: Passive RIS uses low cost, energy efficient reflective surfaces to redirect signals.
  • No External Power Supply: Does not require a power source, as it does not amplify signals.
  • Simpler Design: Consists of basic metasurfaces, making it lightweight, cost effective, and easy to deploy.

Advantages of Reconfigurable Intelligent Surfaces (RIS)

Here are some of the benefits of using Reconfigurable Intelligent Surfaces (RIS):

1. Wireless subscribers or mobile phones blocked from direct Base Station (or eNB) service can be reached via RIS, which adjusts the channel using its ‘N’ elements.
2. Metasurfaces outperform SISO (Single Input Single Output) systems with the same distance coverage.
3. It offers better beamforming gain compared to massive MIMO. The received power increases by a factor of ‘N’ in massive MIMO, while it increases by ‘N²’ in RIS.
4. It enhances spectrum efficiency by providing extra spatial diversity gain.
5. It extends network or Base Station coverage by serving cell edge users.
6. It improves energy efficiency as RIS doesn’t require energy-hungry hardware. It offers low energy consumption, better than relay, massive MIMO, and backscatter technologies.
7. It uses low-cost passive reflecting elements. It does not require ADC/DAC and amplifier devices as used in classical relay. Hence Reconfigurable Intelligent Surfaces are available at an affordable cost.
8. It is hard to design and build conventional arrays for operation above 100 GHz. RIS can address this issue.
9. It is easy to implement, flexible to deploy, and extend.
10. RIS integrates a large number of antenna elements in a compact space.

Disadvantages of Reconfigurable Intelligent Surfaces (RIS)

Here are some drawbacks of Reconfigurable Intelligent Surfaces:

1. It does not outperform a relay. To achieve performance similar to a relay, a metasurface with a higher number of elements (~200) is required.
2. RIS has a worse SNR (Signal to Noise Ratio) than massive MIMO.
3. RIS elements do not support digital processing capabilities, as they are designed based on the concept of analog beamforming.

Conclusion

Reconfigurable Intelligent Surfaces (RIS) are poised to revolutionize 6G wireless communication by enhancing coverage, reducing energy consumption, and enabling smarter network control. However, challenges such as high implementation costs, complex deployment, and potential security concerns must be addressed for RIS to achieve its full potential. As 6G networks continue to evolve, RIS remains a promising innovation, offering unparalleled possibilities for a connected and efficient future.