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Intelligent Reflecting Surfaces: Fundamentals and Applications
Transcript of Intelligent Reflecting Surfaces: Fundamentals and Applications
Intelligent Reflecting Surfaces: Fundamentals and Applications
byEmad IbrahimPhD Student
Signal Processing GroupLTU
Outline:
• Fundamentals of intelligent reflecting surface (IRS).
• Applications of IRSs in wireless communications.
• Challenges of IRS aided wireless communications.
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Reflecting element
Intelligent Reflecting Surface
• 2D surface.
• Composed of a large number of sub-wavelength reflectingelements (small antennas such as micro-strip patches).
• Each reflecting element is connected to a tunable chip tochange its load impedance such as PIN diode or varactor .
• The on/off state of the PIN diodes results in different loadimpedances and generate a phase-shift difference of π.
PIN diode On
PIN diode Off
Equivalent CircuitReflecting Element
Phase shift =0
Phase shift = π
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Intelligent Reflecting Surface
• Also if we control bias voltage of the varactors, this results in continuously tunable loadimpedance and induces in continuous phase shift.
• In addition, variable resistor can be attached to change the amplitude of reflectioncoefficient.
• So, we control the reflection coefficient (amplitude and phase) of each reflecting elementindividually.
Load impedance varies from S.C to O.C Phase shift varies from 0 to 2π
IRS Controller
Copper board Control Circuit board
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Specular or Anomalous Reflection
• The angle of reflection can be controlled, by accurately tuning the phase shifts of all reflectingelements in a way that, the scattered waves are added constructively at certain direction.
-π
π
θ𝑖θ𝑟
Incident wave Reflected waveIncident wave
Reflected wave
θ𝑟
Metal PlateSpecular Reflection
Constant Surface ImpedanceSnell’s Law (θ𝑖 = θ𝑟)
IRSAnomalous Reflection
Alternate Surface Impedance Control phase shift per element
Generalized Snell’s Law (θ𝑖 ≠ θ𝑟) 5
Other names:
• IRS and its reflecting elements are widely named as meta-surface and meta-atom,respectively.
• Meta is a Greek word meaning “beyond” to represent the beyond normal function of the IRS.
• In addition IRS has multiple other names:
• Large intelligent surface.
• Large intelligent meta-surface.
• Reconfigurable meta-surface.
• Reconfigurable intelligent surface.
• Software-defined surface.
• Software-defined meta-surface.
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Active or Passive IRS
RF generator
User
IRS
UserRF transmitter
IRS
• Connected with RF chains, PA, and DSPCapabilities.
• Next step beyond massive MIMO.
• Holographic MIMO.
• No RF chains, PA, and DSP Capabilities.
• Minimal Power to control the tunable chips .
• Nearly Passive.
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In analogy with Relay
• IRS resembles a full-duplex multi-antenna AF relay.
• However, IRS is nearly a passive element (ultra-low power consumption).
• No self-interference.
• No additional thermal noise added to the forwarded signal.
UserRF transmitter
IRS
. . .
. . .
Relay
RF transmitterUser
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Smart Radio Environment
• The wireless environment is fixed by nature.
• Current methods to design wireless systems usually rely on the optimization of transmittersand receivers to compensate the wireless environment effects.
• IRSs are capable of shaping the radio waves before that observed by the receivers tocustomize the wireless environment.
• IRS turns the wireless environment into an optimization variable, which, jointly with thetransmitters and receivers, can be programmed.
• SREs: The environment is generated by nature but is programmable by design.
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Outline:
• Fundamentals of intelligent reflecting surface.
• Applications of IRS in wireless communications.
• Challenges of IRS aided wireless communications.
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Coverage Enhancement
• IRS enhances coverage by creating virtual links for users with blocked direct link to the BS.
• This is useful for the coverage extension especially in mmWave communications that arehighly affected by blockage.
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IRS assisted cell edge users
• Cell-edge users suffer from both high signal attenuation from its serving cell and severe co-channel interference from neighbor cells.
• An IRS can be deployed at the cell edge to improve the desired signal power and suppress theinterference.
BS 1 BS 2
Cell edge user
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IRS assisted NOMA
• Conventional OMA is to serve a single user per orthogonal channel.
• NOMA is to serve multiple users per orthogonal channel and utilize successive interferencecancellation at the receiver.
• Combining NOMA with MIMO means to serve multiple users on the same spatial direction.
• However, NOMA is not always preferable.
• If users’ channel vectors are orthogonal to each other, OMA is more preferable than NOMA.
• While NOMA is preferable, when the directions of the users’ channel vectors are aligned.13
IRS assisted NOMA
• By employing IRSs in the wireless environment the directions of users’ channel vectors canbe effectively controlled.
• In particular, we can align one user’s channel with the other and makes NOMA possible.
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IRS and Index Modulation
• In index modulation, data bits are encoded in both information symbol and antenna index
• Index modulation increases the spectral efficiency while using low-complexity transmitterwith a single RF chain.
• Bits per channel use=log2 M + log2 NT
QPSK
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IRS and Index Modulation
• By dividing the IRS into groups of elements, data bits can be encoded in both informationsymbol and a group of unit cells index.
• Bits per channel use=log2 M + log2 N .
Index 1 Index 2 ……… Index N
RxTx
IRS
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IRS assisted D2D communications:
• IRS for enabling massive device-to-device (D2D) communications.
• The IRS acts as a signal reflection hub to support simultaneous low-power transmissions andinterference mitigation.
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IRS for Wireless Power Transfer (WPT):
• Wireless power transfer targets the transmission of electrical energy without using wires as aphysical link.
• The main idea is that the receiver extracts power from the EM field and supplies it to anelectrical load.
• IRS can be used for WPT to multiple devices in an Internet-of things (IoT) network.
Information transmission
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Channel rank enhancement:
• The propagation channel plays an important role in the capacity of point-to-point MIMOcommunications.
• In rich scattering environments, the channel matrix has full rank. Thus, spatial multiplexing ofindependent data streams is possible .
• In LoS environments, P2P MIMO channel matrix turns out to be rank deficient.
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Spatial multiplexing gain
Rank (𝐇i.i.d)= min(𝒏𝒕, 𝒏𝒓) , Well conditioned
Rank (𝐇𝑙𝑜𝑠)= 1
Backhaul Links
IRSs for P2P MIMO in LoS Environments
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𝐇o
TX RX
IRS1
IRS2
IRS L
𝒕1
𝒕𝐿
𝒓1
𝒓𝐿
..
..
.
.... .. .
.
.
.
.
.
.
.
• By introducing distributed IRSs to theLoS environment
• Artificial multi-path propagation canbe synthesized.
• This makes spatial multiplexingpossible and directly enhance theachievable rate.
Outline:
• Fundamentals of intelligent reflecting surface.
• Applications of IRS in wireless communications.
• Challenges of IRS aided wireless communications.
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Channel Estimation in IRS aided wireless network systems
• The performance of IRS-assisted wireless communications is guaranteed by the accuratetuning of the phase shifts of all IRS elements.
• This require accurate channel estimation.
• There are three channel links to be estimated:
a) BS to user link. b) User to IRS link . c) IRS and BS link.
• IRS is a nearly passive device with limited capability of transmitting, receiving, and processingsignals.
• So, we can not rely on the IRS to estimate the channel links by processing the pilots from theusers and the BS.
• Also, IRS can not transmit pilot signals to facilitate the channel estimation at the users and theBS.
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Channel Estimation in IRS assisted wireless network systems
• So channel estimation is commonly done at the BS on two stages.
• First stage: by turning the IRS off, CSI between BS and user is estimated using uplink pilots.
• Second stage: by iteratively turn on single reflecting element on the IRS, while keeping therest reflecting element off, the cascaded channel from user to BS through IRS can beestimated.
• The size of a IRS is usually very large. This results a huge overhead for the channel estimationprocess.
• Finally, the BS has to feedback the computed phase shifts to the IRS through a dedicatedChannel (wired or wireless).
BSUser
ℎo
𝒉1𝒉2
Controller
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Path Loss of IRS assisted wireless system
• Although, IRS controls the phases of scattered waves to create a constructive interference atthe intended user.
• However, the reflected signal on the IRS faces an unfavorable path loss model.
• It is a two-hops channel:
• 1st hop channel : path loss α 𝑟12.
• 2nd hop channel : path loss α 𝑟22.
• So given all scattered waves are phase aligned then, SNR α𝑁
𝑟1𝑟2
2
24BS User
𝑟1𝑟2
N reflecting elements
Q&A25
Thank You
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