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.

2

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

3

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

4

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.

6

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.

7

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

8

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.

9

Outline:

• Fundamentals of intelligent reflecting surface.

• Applications of IRS in wireless communications.

• Challenges of IRS aided wireless communications.

10

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.

11

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

12

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.

14

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

15

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

16

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.

17

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

18

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.

19

Spatial multiplexing gain

Rank (𝐇i.i.d)= min(𝒏𝒕, 𝒏𝒓) , Well conditioned

Rank (𝐇𝑙𝑜𝑠)= 1

Backhaul Links

IRSs for P2P MIMO in LoS Environments

20

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.

21

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.

22

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

23

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

26