IRS: Intelligent reflecting surface in 6G

The upcoming 6G of wireless communication aims to deliver extremely fast networks, minimal delays, seamless connectivity everywhere, and smart integration of devices. A key technology to make this possible is Intelligent Reflecting Surfaces (IRS). IRS uses an innovative method to control how wireless signals travel, improving network performance, saving energy, and making systems more flexible.

What is IRS?

Intelligent Reflecting Surfaces (IRS) are advanced technologies designed to enhance wireless communication by controlling how electromagnetic waves propagate through the environment. IRS consists of a large surface, often embedded with many small, programmable reflecting elements called meta-atoms. These elements can adjust the phase, amplitude, and direction of signals, creating an optimized communication pathway between the transmitter (e.g., base station) and receiver (e.g., user device).

Unlike traditional wireless systems that rely heavily on active devices like relays or antennas, IRS passively reflects and reshapes wireless signals without requiring additional power for amplification or transmission

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As shown in above diagram we can outline the points –

• The image demonstrates an Intelligent Reflecting Surface (IRS)-enabled wireless communication ecosystem.
• IRS panels are installed on buildings to enhance signal reflection, coverage, and communication efficiency.
• A UAV (Unmanned Aerial Vehicle) equipped with IRS supports extended wireless communication, especially in areas with limited direct base station coverage.
• Physical layer security is achieved by directing signals toward intended users, minimizing risks of eavesdropping.
• Vehicles on the road, including emergency vehicles, rely on vehicular communication supported by IRS to maintain uninterrupted connectivity.
• Multi-Access Edge Computing (MEC) services are shown, where IRS assists in providing faster data processing for nearby users by connecting them to edge servers.
• Blocked users behind obstacles like trees experience signal challenges, which IRS overcomes by reflecting signals around the obstacles.
• UAVs play a role in extending network connectivity to areas that are out of reach of base stations.
• The diagram highlights a seamless communication ecosystem for mobile users, UAVs, smart vehicles, and edge computing, all enhanced by IRS technology.

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Why IRS?

As wireless communication evolves, challenges such as signal interference, high energy consumption, and inefficient use of the spectrum have become significant hurdles. Intelligent Reflecting Surfaces (IRS) address these issues by introducing a novel way to enhance network performance through real-time manipulation of electromagnetic waves.

Addressing Key Challenges

• Overcoming Propagation Limitations: Wireless signals often face obstacles like walls, furniture, and atmospheric conditions, leading to signal weakening (attenuation) and multipath fading. IRS helps by intelligently redirecting signals to bypass these barriers.
• Improving Energy Efficiency: Traditional solutions like active relays and base stations consume considerable power, making them less scalable for large networks. IRS offers a passive alternative, reconfiguring the environment without additional power-hungry equipment.
• Simplifying Beamforming: Current technologies for signal directionality are costly and complex. IRS simplifies this process by dynamically reflecting signals, reducing setup and operational challenges.

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Advantages of IRS in 6G

• Enhanced Throughput: IRS boosts data rates by adding multiple reconfigurable reflection points that optimize signal paths.
• Ubiquitous Coverage: By intelligently reflecting signals, IRS extends coverage to remote and hard-to-reach areas, addressing connectivity gaps in rural and urban environments.
• Improved Spectrum Efficiency: IRS maximizes the use of available spectrum by controlling the spatial paths of signals, ensuring efficient communication for multiple users.

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The architecture of IRS

The hardware of an Intelligent Reflecting Surface (IRS) is built around a concept called a "metasurface." This is essentially a thin, flat layer made from a special material that can be controlled electronically.

The metasurface is made up of a grid-like structure with many small components called meta-atoms. These components are very thin, much smaller than the wavelength of the signals they work with.

By carefully designing the shape, size, alignment, and arrangement of these tiny components, the way they interact with signals—such as reflecting or altering the signal's strength and phase—can be adjusted to meet specific requirements. This allows the IRS to control how signals behave for better communication performance.

• The figure represents the architecture of an Intelligent Reflecting Surface (IRS).
• The metasurface at the top consists of multiple tunable elements that can adjust the phase and amplitude of incoming electromagnetic waves, enabling precise signal control.
• Below the metasurface lies the dielectric substrate, which provides structural support and ensures efficient wave propagation through the surface.
• The copper backplane is situated beneath the dielectric substrate and prevents signal leakage by reflecting waves back toward the metasurface.
• A control network connects the metasurface to the IRS controller, facilitating real-time adjustments of the tunable elements.
• The controller chip acts as the interface between the metasurface and the IRS controller, executing commands to dynamically adapt the IRS properties.
• The IRS controller, shown as a separate module, manages the overall operation, making decisions based on network requirements and external inputs.
• This layered design allows the IRS to dynamically manipulate wireless signals, enhancing communication efficiency by reflecting, refracting, or redirecting signals as needed.

Working Principle of IRS

The working principle of an Intelligent Reflecting Surface (IRS) combines the concept of metasurfaces and a modified version of Snell's law to control and manipulate electromagnetic (EM) waves. Here’s how it works:

• Interaction with Plane Waves: Electromagnetic waves, such as those transmitted by a base station, hit the IRS in the form of plane waves. The metasurface of the IRS interacts with these waves, inducing currents on its surface.
• Controlled Scattering: The IRS reflects these waves as scattered waves. Unlike traditional surfaces that reflect waves naturally, the IRS actively adjusts the properties of the reflected signals, such as their phase and amplitude, to achieve the desired direction and intensity of reflection.
• Phase Shift Control: By using electronically tunable components, such as PIN diodes embedded in the metasurface, the IRS can dynamically control the phase shifts of the reflected waves. This control allows the IRS to direct signals precisely toward the intended users or devices.
• Manipulation of Signal Properties: The IRS also adjusts the amplitude of the reflected waves, ensuring that the signal strength is optimized for the target destination while reducing interference or signal loss.
• Reflection Toward Users: After processing the waves, the IRS redirects them toward specific users or devices. This capability is particularly useful for bypassing obstacles, extending coverage, and enhancing signal quality.
• Enhanced Signal Efficiency: By tailoring the behavior of the reflected waves, the IRS improves the efficiency of wireless communication systems, ensuring stronger connections, reduced interference, and better overall performance.

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In summary, the IRS acts like a programmable mirror, receiving input signals as plane waves, inducing current through its metasurface, and reflecting these waves in a controlled manner using adjustable phase shifts and amplitude modulation. This process, grounded in a variation of Snell's law, enables precise control of signal direction and strength, enhancing communication systems' effectiveness.

Here are enhanced examples showcasing various wave manipulations performed by an Intelligent Reflecting Surface (IRS):

1. Original Signal: The baseline wave represents the initial electromagnetic signal received by the IRS, with no modifications.
2. Phase-Shifted Signal (π/4\pi/4π/4): This demonstrates a moderate phase shift, aligning signals to improve synchronization or mitigate destructive interference.
3. Phase-Shifted Signal (π/2\pi/2π/2): A stronger phase shift shown here enables advanced control over the timing of signals for focused beamforming.
4. Amplitude Increase (x2): Doubling the amplitude enhances the signal strength, improving communication quality for devices in poor reception areas.
5. Amplitude Decrease (x0.5): Reducing amplitude helps in minimizing interference or conserving energy for specific communication scenarios.
6. Reflected Signal: This inverted wave represents how the IRS can reflect signals in the opposite direction, bypassing obstacles or focusing the signal toward a specific receiver.

These graphs illustrate the IRS's ability to dynamically manipulate waves in real time, optimizing wireless networks for efficiency and reliability.

Advantages of IRS

IRS technology not only improves energy efficiency and signal quality but also provides versatile solutions for various communication challenges in 6G, making it indispensable for next-generation networks.

Energy Efficiency

• IRS operates as a nearly passive system, consuming minimal energy for its operation.
• It significantly reduces network power consumption compared to traditional solutions like relays and active MIMO systems.

Noise Mitigation

• IRS does not introduce additional noise because it avoids active signal processing components like amplifiers or converters.

Deployment Flexibility

• IRS can be easily mounted on walls, ceilings, building facades, or other surfaces.
• It is suitable for deployment in various environments, including urban, rural, and indoor settings.

Enhanced Coverage

• IRS redirects signals to areas with weak connectivity, serving edge users and eliminating coverage holes.

IRS in Emerging 6G Use Cases

IRS demonstrates immense potential in supporting 6G networks across challenging and innovative use cases, including non-terrestrial networks, high-frequency communication, and modern smart environments, ensuring reliable, efficient, and adaptive wireless connectivity.

Non-Terrestrial Networks (NTNs)

IRS plays a crucial role in bridging communication gaps in satellite and UAV networks by optimizing signal paths and enhancing connectivity.

• Redirecting Satellite Signals

Example: IRS panels installed on remote ground stations can redirect satellite signals to extend coverage to rural or mountainous regions, ensuring reliable communication in hard-to-reach areas.

• Enhancing UAV-Based Wireless Backhaul

Example: UAVs equipped with IRS can reflect signals between a central base station and users in disaster-affected areas, establishing quick and efficient temporary communication links.

Terahertz and Optical Communication

IRS addresses the challenges of terahertz (THz) and optical frequencies, where signals suffer from significant path losses and blockages due to their high sensitivity.

• Compensating for Path Losses

Example: In a high frequency 6G network, IRS can focus THz waves around obstacles, like buildings, to maintain strong and reliable connectivity in dense urban environments.

§? Overcoming Blockages

Example: For indoor optical wireless communication (e.g., Li-Fi), IRS can reflect and redirect light signals around furniture or walls, ensuring uninterrupted data transmission in smart offices.

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Smart Environments

IRS integrates seamlessly into smart systems, enhancing connectivity and adaptability in modern environments.

• Smart Homes

Example: IRS panels on walls can dynamically reflect Wi-Fi signals to optimize coverage for all devices in a house, including those in signal-dead zones like basements.

• Smart Cities

Example: In urban areas, IRS can be deployed on streetlights to reflect 5G signals, ensuring uninterrupted connectivity for autonomous vehicles and IoT sensors.

• Industry 4.0

Example: In smart factories, IRS can enhance communication between robotic systems and central controllers by redirecting signals around heavy machinery that often blocks conventional wireless paths.

Challenges in IRS Deployment

• Channel Estimation and Complexity-Accurately estimating channels between base stations, IRS, and users is challenging due to IRS's passive nature.
• Mobility and Dynamic Environments-Dynamic factors such as user mobility, moving vehicles, and environmental changes affect IRS performance.
• Scalability and Deployment-Large-scale deployment faces infrastructure limitations (e.g., power supply, placement challenges) and high costs for widespread metasurface installations.

References

• Nadeem, Q.-U.-A., Kammoun, A., Chaaban, A., Debbah, M., & Alouini, M.-S. (2019). Intelligent Reflecting Surface Assisted Wireless Communication: Modeling and Channel Estimation. Retrieved April 16, 2021, from arXiv.
• Zhao, J., & Liu, Y. (2019). A Survey of Intelligent Reflecting Surfaces (IRSs): Towards 6G Wireless Communication Networks. Retrieved April 16, 2021, from ResearchGate.
• Tang, W., et al. (2021). Wireless Communications with Reconfigurable Intelligent Surface: Path Loss Modeling and Experimental Measurement. IEEE Transactions on Wireless Communications, 20(1), 421–439. doi: 10.1109/TWC.2020.3024887.
• Wu, Q., & Zhang, R. (2020). Towards Smart and Reconfigurable Environment: Intelligent Reflecting Surface Aided Wireless Network. IEEE Communications Magazine, 58(1), 106–112. doi: 10.1109/MCOM.001.1900107.
• Basar, E., Di Renzo, M., De Rosny, J., Debbah, M., Alouini, M. S., & Zhang, R. (2019). Wireless Communications through Reconfigurable Intelligent Surfaces. IEEE Access, 7, 116753–116773. doi: 10.1109/ACCESS.2019.2935192.
• Gong, S., et al. (2020). Toward Smart Wireless Communications via Intelligent Reflecting Surfaces: A Contemporary Survey. IEEE Communications Surveys & Tutorials, 22(4), 2283–2314. doi: 10.1109/COMST.2020.3004197.
• Elmossallamy, M. A., Zhang, H., Song, L., Seddik, K. G., Han, Z., & Li, G. Y. (2020). Reconfigurable Intelligent Surfaces for Wireless Communications: Principles, Challenges, and Opportunities. IEEE Transactions on Cognitive Communications and Networking, 6(3), 990–1002. doi: 10.1109/TCCN.2020.2992604.
• Tang, W., et al. (2018). Wireless Communications with Programmable Metasurface: Transceiver Design and Experimental Results. Retrieved April 16, 2021, from arXiv.
• Abdelhady, A. M., Salem, A. K. S., Amin, O., Shihada, B., & Alouini, M.-S. (2020). Visible Light Communications via Intelligent Reflecting Surfaces: Metasurfaces vs Mirror Arrays. IEEE Open Journal of Communications Society, 2, 1–20. doi: 10.1109/ojcoms.2020.3041930.
• Chen, Z., Han, C., Ning, B., Tian, Z., & Li, S. (2021). Intelligent Reflecting Surfaces Assisted Terahertz Communications toward 6G. Retrieved April 16, 2021, from arXiv.

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