This document discusses intelligent reflecting surfaces (IRS), which are metasurfaces that can control the amplitude and phase of reflected wireless signals. An IRS consists of many small elements that each scatter incoming waves. By adjusting the individual element properties, the overall reflected signal can be constructively or destructively combined to focus energy towards or away from users. IRS technology could improve coverage and capacity for wireless networks by optimizing signal propagation through coherent beamforming based on estimated channel state information. Open challenges include developing efficient channel estimation techniques and determining optimal placements of IRS arrays within wireless environments.

1. Fundamentals of
Intelligent Reflecting Surfaces
Emil Björnson
Associate professor
Department of Electrical Engineering (ISY)
Linköping University
Sweden

2. Massive MIMO is a Reality
2
Sprint deployed 1000 sites in 2019
• 64-antenna TDD active arrays
• Compact arrays (2.5 GHz band)
Improves sum rates with 4-20x
What is next? Larger arrays!
1) Metasurfaces
(Intelligent reflecting
surface)
2) Holographic
beamforming

3. What is a Reflecting Surface?
3
Snell’s law:
Scattered wave
strongest for !" = !$

4. Intelligent Reflecting Surface
4
RF transmitter Receiving user
Alternative names:
Software-controlled metasurface
Reconfigurable intelligent surface
…
Controllable beam

5. Two Prospective Use Cases
5
(a) Energy focusing. (b) Energy nulling.
User 1
Destructive reflection
Constructive reflection
Information signal
Information signal
User 2
(a) Energy focusing. (b) Energy nulling.
User 1
User 1
Information signal
IRS controller
IRS controller
IRS controller IRS controller
IRS controller
IRS controller
Transmitter
Transmitter
Destructive reflection
Constructive reflection
(a) Energy focusing. (b) Energy nulling.
User 1
Destructive reflection
Constructive reflection
Information signal
Information signal
User 2
(a) Energy focusing. (b) Energy nulling.
User 1
User 1
Information signal
IRS controller
IRS controller
IRS controller IRS controller
IRS controller
IRS controller
Transmitter
Transmitter
Destructive reflection
Constructive reflection

6. Origin: Reflectarray Antennas
6

7. 7
22 x 24 = 528 element (~27 dB array gain)
Experimental setup
Feed: Horn antenna
Reflection in a wide meaning

8. 8
0 20 40 60 80
-60
-50
-40
-30
-20
-10
0
10
20
“Meta-atom”
Sub-%-sized
Scatters incoming wave with
controllable delay/phase, polarization
Today: Metasurface

9. Communication System Model
9
Channel to atom &:
'(
Channel from atom &:
ℎ(
Received signal:
* = +
(,-
.
'(/012ℎ( ⋅ signal + noise
/012
End-to-end channel
'(/012ℎ(

10. Channel Gain With Line-of-Sight to IRS
10
Distance =>
Channel gain:
?
@ABC
D
/012
End-to-end channel gain:
'(/012ℎ(
E
= '(
E ℎ(
E =
FE
4H=>=I
E
Distance =I
Channel gain:
?
@ABJ
D

11. Optimizing the Phase Shifts: Coherent Beamforming
Channel gain:
+
(,-
.
'(/012ℎ(
E
≤ +
(,-
.
'( |ℎ(|
E
Equality for !( = − arg '( − arg ℎ(
11
Received signal:
* = +
(,-
.
'(/012ℎ( ⋅ signal + noise
Less trivial optimization:
Multiple antennas
Multiple devices
…
Continuing the previous example:
+
(,-
.
'( |ℎ(|
E
= O
F
4H=>=I
E
= OE
FE
4H=>=I
E

12. Simulation Example: Signal Focusing
(a) Energy focusing. (b) Energy nulling.
User 1
Destructive reflection
Constructive reflection
Information signal
Information signal
User 2
(a) Energy focusing. (b) Energy nulling.
User 1
User 1
Information signal
IRS controller
IRS controller
IRS controller IRS controller
IRS controller
IRS controller
Transmitter
Transmitter
Destructive reflection
Constructive reflection
0 200 400 600 800 1000
0
1
2
3
4
5
6
7
8
−75 dB −75 dB
LOS or NLOS
Direct path:
NLOS: −100 dB
Direct path:
LOS: −75 dB

13. How to Learn the Channel?
13
(a) Energy focusing. (b) Energy nulling.
User 1
Destructive reflection
Constructive reflection
Information signal
Information signal
User 2
(a) Energy focusing. (b) Energy nulling.
User 1
User 1
Information signal
IRS controller
IRS controller
IRS controller IRS controller
IRS controller
IRS controller
Transmitter
Transmitter
Destructive reflection
Constructive reflection
Control channel
Use parameteric models?
Estimate position or angle to the user
Send pilot signals via IRS?
Change the IRS simultaneously
to achieve more information
Have a few active elements in the IRS?
Anyway convenient for control channel

14. Where Shall We Place the Metasurface?
14
Source Destination
50 m
5 m
=-
IRS (O = 1024)
0 10 20 30 40 50
3.5
4
4.5
5
5.5
10-8
Preferred choice:
Close to source or destination
What if they are co-located? Source Destination

15. Holographic Beamforming
15
RF generator
Holographic array
(metasurface)
Metasurface with built-in source
No pathloss between RF generator and surface!
Thin form factor – invisible deployment
Ideally a contiguous surface
28 GHz, Small array,
one RF chain

16. Do You Want to Learn More?
16