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

Introduction : Reconfigurable Intelligent Surfaces (RIS) are transformative technologies 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) ?

The surface which uses passive reflecting elements which are controlled by software is known as RIS or reconfigurable Intelligent Surface or intelligent reflecting surface. It is also known as software controlled metasurface. The principle of working is as per variation to snell's law. The input to RIS is plane waves where as output is scattered waves whose phase shifts are controlled to meet desired reflection.

The metasurface is made of 'N' number of meta atoms with sub-λ size. These elements scatter incoming waves with controllable delay/phase and polarization. It changes wireless propagation channel environment by carefully tuning phase shifts of a large number of low cost passive reflecting elements.

Let us understand working of RIS.
• The EM (Electromagnetic) waves transmitted from BS are impinged on RIS which produces induction current in the RIS.
• RIS reflects these signals toward the users.
• During reflection, RIS changes response by controlling phase and amplitude. Phase shifts are controlled by PIN diodes used in RIS.

RIS is ultra thin meta-surface which is composed of three layers viz. Outer layer, Middle layer and Inner layer.
Outer layer: It is a dielectric substrate with RIS elements. These elements directly interact with incident signals. RIS elements use several PIN diodes which helps in controlling the phase response of scattered wave by turning themselves ON and OFF as required. They support frequency from sub 6 GHz to THz range.
Middle layer: It is a copper plate which prevents signal energy leakage.
Inner layer: It consists of control circuit board. It is known as RIS controller.

Metasurfaces are used in various applications. It aids in various systems which include 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)

Following are some of the benefits of Reconfigurable Intelligent Surfaces (RIS) :
1. Wireless subscribers or mobile phones which are behind any blockage are not served by Base Station or eNB directly. RIS with 'N' elements serves these blocked subscribers by varying the channel.
2. Metasurfaces outperform SISO (Single Input Single Output) system having same distance coverage.
3. It offers better beamforming gain compare to massive MIMO. The received power is increased by factor 'N' in massive MIMO where as it is increased by factor 'N²' in RIS.
4. It enhances spectrum efficiency by providing extra spatial diversity gain.
5. It extends network coverage or BS (Base Station) coverage by serving cell edge users or subscribers.
6. It improves energy efficiency as RIS does not require energy hungry hardware. It offers low energy consumption which is 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 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 large number of antenna elements in compact space.

Disadvantages of Reconfigurable Intelligent Surfaces (RIS)

Following are some of the drawbacks of Reconfigurable Intelligent Surfaces :
1. It does not outperform relay. To make performance similar to relay, requires metasurface with higher number of elements (~200).
2. RIS has worst SNR (Signal to Noise Ratio) than massive MIMO.
3. RIS elements do not support digital processing capability as it is designed based on concept of analog beamforming.

Conclusion

Reconfigurable Intelligent Surfaces (RIS) are set 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.

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