Intelligent Reflecting Surface (IRS) consists of passive elements that can reflect signals with adjustable phase shifts to maximize SNR through passive beamforming. IRS reflects signals in a controlled manner for improved communication without requiring transmission power. Key challenges include passive beamforming under power constraints and joint active/passive beamforming in multipath environments. Research directions include developing high-quality reflecting surfaces, RF propagation control, and optimizing IRS-enhanced wireless networks.

1. Intelligent Reflecting Surface (IRS)
Domain: 5G Beyond Wireless Communication Research
Dr. Varun Kumar
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2. Outlines
1 Introduction to Intelligent Reflecting Surface (IRS)
2 Existing Key Technology for Improvising the SNR
3 Need of IRS
4 Research Direction in IRS
5 References
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3. Intelligent Reflecting Surface (IRS):
Intelligent Reflecting Surface (IRS)
⇒ Intelligent reflecting surface (IRS) is a cost-effective solution for
achieving high spectrum and energy efficiency.
⇒ It consists of massive low-cost passive elements that are able to
reflect the signals with adjustable phase shifts.
⇒ It minimize the transmit power at the access point (AP).
⇒ SNR is maximized by passive beamforming through the IRS.
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4. Fundamentals of reflection phenomenon
⇒ When a electro-magnetic wave strike to the surface then three things
may occur at a same time
1. Refraction
2. Reflection
3. Scattering
Electrical characteristics of surface
⇒ Skin depth:The skin depth is a measure of the penetration of a plane
electromagnetic wave into a material. It is denoted by δ.
δ =
1
α
⇒ α = ω
µ
2
1 +
σ2
ω2 2
− 1
1/2
≈
ωµσ
2
=
ωµ0µr σ
2
(1)
α is called as attenuation factor.
⇒ Higher the skin depth, greater be the chance of absorption/refraction
from the respective surface.
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5. Continued–
δ =
2
ωµ0µr σ
(2)
From (2), it is clear that skin depth decreases (more chance of
reflection) by
ω → Frequency of incident wave
σ → Conductivity of the surface
µr → Relative permeability
⇒ When the nature of surface is metallic then reflected wave has high
amount of energy.
⇒ For non-intelligent surface, reflection phenomena dominates, when
surface has high conductivity.
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6. Continued–
⇒ On the other side, if the nature of surface is non-metallic/di-electric
then that surface absorbs huge amount of incident energy.
⇒ If surface is not smooth then multiple reflected and refracted wave
components can be observed from that surface. → Scattering
⇒ From figure given below, reflection, refraction and scattering can be
observed through the ray tracing operation.
Note: If surface is non-intelligent and passive in nature then the angle of
incidence is equal to the angle of reflection.
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7. What is intelligent surface?
⇒ The inherent quality of IRS should be as follows
1 The nature of surface should be metallic.
2 Surface should be smooth enough, so that uni-direction of reflected
wave could be achieved.
3 Direction change ability of the reflected wave.
⇒ In communication system, the strength of reflected signal decides the
SNR quality in multi path environment.
⇒ The signal strength becomes prominent in case small IoT devices or a
mobile user.
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8. Continued–
⇒ IRS should have the beam steering capability, which has been shown
in above figure.
⇒ It never transmit the power from reflecting surface to the desired user
equipment.
⇒ Phase shifter is an important ingredient of IRS aided communication
set-up.
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9. Fundamental advantage and challenges of IRS
Advantage
⇒ It forwards the RF signals using passive reflection beamforming.
⇒ The power consumption of IRS is much lower than that of the AF
relay.
⇒ There is nearly no additional thermal noise added during reflecting.
Challenges
1 The reflection element suffers a stringent instantaneous power
constraint, which makes the passive beamforming more challenging.
2 Joint beamforming (active and passive) in multi-path environment.
3 Computational complexity
4 Passive reflector has discrete set of phase shifter that may or may not
track the user equipment.
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10. Similar key technology for improvising the SNR/data rate
⇒ Massive MIMO
Large arrays of antennas are utilized to improve spectral and energy
efficiencies.
One of the important breakthrough for 5G wireless communication.
⇒ mm-Wave communication
It helps in active beamforming operation.
Very high carrier frequency of operation helps for integrating large
number of antenna elements within a fixed physical space. → Base for
massive MIMO network formation
Spatial multiplexing and beamforming enhances the data rate as well
end user received signal strength.
⇒ Back-scatter communication
It enable the low-power communications as required in the Internet of
things (IoTs) and green communications.
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11. Research direction
1 In the field of material science for making good quality reflecting
surface.
2 To develop a software based RF propagation control mechanism.
3 How to integrate the reconfigurable meta-surfaces into wireless
networks?
4 What are the ultimate performance limits of wireless networks in the
presence of reconfigurable meta-surfaces?
5 How to attain such performance limits in practice?
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12. References
M. Di Renzo, M. Debbah, D.-T. Phan-Huy, A. Zappone, M.-S. Alouini, C. Yuen,
V. Sciancalepore, G. C. Alexandropoulos, J. Hoydis, H. Gacanin et al., “Smart
radio environments empowered by reconfigurable ai meta-surfaces: An idea whose
time has come,” EURASIP Journal on Wireless Communications and Networking,
vol. 2019, no. 1, pp. 1–20, 2019.
H. Guo, Y.-C. Liang, J. Chen, and E. G. Larsson, “Weighted sum-rate optimization
for intelligent reflecting surface enhanced wireless networks,” arXiv preprint
arXiv:1905.07920, 2019.
Q. Wu and R. Zhang, “Intelligent reflecting surface enhanced wireless network via
joint active and passive beamforming,” IEEE Transactions on Wireless
Communications, vol. 18, no. 11, pp. 5394–5409, 2019.
J.-P. Niu and G. Y. Li, “An overview on backscatter communications,” 2019.
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