July 2019

Daniel Mittleman, Brown UniversitySlide 1

Project: IEEE P802.15 Working Group for Wireless Personal Area Networks (WPANs)

Submission Title: Terahertz Wireless Communications: A Photonics Approach

Date Submitted: 15 July 2019

Source: Daniel Mittleman Company: Brown University, School of Engineering

Address: 184 Hope St., Box D, Providence RI 02912 USA

Voice: +1-401-863-9056, FAX: N/A, E-Mail: daniel_mittleman@brown.edu

Re: N/A

Abstract: To accommodate the rapid increase in global wireless traffic, which will reach to 49 exabytes per month by 2021, wireless

networks operating beyond 95 GHz will very likely be required. The operation and characteristics of such networks are quite different from

those of conventional wireless systems, or even of 5G systems which will employ millimeter-wave links at lower frequencies. The distinctions

arise from the much shorter wavelength, which implies both a significantly higher directionality and a very different propagation and

diffraction characteristic. This offers both challenges and opportunities for future networks operating at these frequencies. In this presentation,

we discuss several new measurements to characterize aspects of these high-frequency channels, including non-line-of-sight links. A first study

of the implications of high directionality on eavesdropping and physical-layer security will also be described.

Purpose: Information on terahertz technology

Notice: This document has been prepared to assist the IEEE P802.15. It is offered as a basis for

discussion and is not binding on the contributing individual(s) or organization(s). The material in this

document is subject to change in form and content after further study. The contributor(s) reserve(s) the right

to add, amend or withdraw material contained herein.

Release: The contributor acknowledges and accepts that this contribution becomes the property of IEEE

and may be made publicly available by P802.15.

doc: IEEE 802.15-19-0256-00-0thz

Terahertz wireless communications:

A photonics approach

Daniel Mittleman

School of Engineering

Brown University

daniel_mittleman@brown.edu

Collaborators

Kimberly Reichel

Nicholas Karl

Nico Lozada-Smith

Chia-Yi Yeh

Yasaman Ghasempour

Edward Knightly

Rice University

Zahed Hossein

Josep M. Jornet

University at Buffalo

Sarah Bretin

Guillaume Ducournau

University of Lille

Ishan Joshipura

Michael Dickey

North Carolina

State University

Masaya Nagai

Osaka University

Jianjun Ma

Rajind Mendis

Rabi Shrestha

Brown University

Martin Koch

University of Marburg

Terahertz wireless pico-cells: a vision

Cartoon credit:

Martin Koch, 2000

THz systems: an ongoing merger

of electronics and photonics

Lots of progress in recent

years in some key metrics…

…but there are many

important metrics.

A recent review:

K. Sengupta, T. Nagatsuma, & D. Mittleman, Nature Electronics, 1, 622 (2018).

THz links are highly directional

Signals which propagate as

beams, not broadcasts, can

often conveniently be

envisioned through the lens

of optics.

-30

-20

-10

0

10

20

30

0

5

10

15

20

SNR (dB)

Angle

(deg)

Alice Bob

Directivity 28 dBi 34 dBi 42 dBi

Frequency 100 GHz 200 GHz 400 GHz

Angular width 7.8º 4.0º 1.6º

Brown University test bed:

Reflections off a wall

Model accounts for

• absorption

• surface roughness

absorption only

absorption plus

surface roughness

J. Ma, et al., APL Photonics, 3, 051601 (2018)

A link with no direct line-of-sight path

total distance:

5.5 m

Specular non-line-of-sight links are surprisingly robust in indoor environments.

THz

beam

J. Ma, et al., APL Photonics, 3, 051601 (2018)

E

Input pulse

Output: TEM mode

-0.2

0.0

0.2

-0.4

0.0

0.4

0 50 100 150

Time (ps)

Ele

ctr

ic F

ield

(a

rb. u

nits)

Metal parallel-plate waveguides:

a platform for terahertz devices

plate spacing

= 0.5 mm

metal plates

L = 25 mm

No low-

frequency

cutoff

0.0 0.2 0.4 0.6 0.8 1.00.0

0.2

0.4

0.6

0.8

1.0

Fie

ld A

mplit

ude

Frequency (THz)

R. Mendis and D. Mittleman, Opt. Express, 17, 14839 (2009).

Coupling two waveguides together

Key component: electrically actuated liquid metal plug

Collaboration with: M. D. Dickey, North Carolina State University

Electrically actuated filter at 120 GHz

Resonant coupling

between two adjacent

waveguides.

Frequency determined

by geometry of coupling

region – a classic

problem in optics!

K. Reichel, et al. Nature Commun. 9, 4202 (2018)

Frequency (THz)

Simulation

Experiment

Multi-channel add-drop filter for

frequency multiplexing

K. Reichel, et al.

Nature Commun.

9, 4202 (2018)

A stack of

waveguides!

To overcome issues of 2D optics (out-of-plane diffraction):

M. Nagai, et al., Opt. Lett., 39, 146 (2014).

Waveguide stack: an artificial dielectric medium

A lens

antenna for

4 GHz

(ca. 1940)

…and for 170 GHz:

35

mm

R. Mendis, et al., Sci. Rep. 6, 23023 (2016)

Artificial dielectric: a passive isolator

THz

Beam

E

‖E

(f cutoff)

Narrow-band passive

isolation of ~50 dB

R. Mendis, et al., Sci. Rep.,

7, 5909 (2017).

Leaky wave devices: a candidate for multiplexing

ffff

A guided wave device with an opening so that some

energy can “leak” out…

rectangular

waveguide

parallel-plate

waveguide

For this to work, the guided mode must be a fast wave,

with vphase > c0 TE waveguide mode

Multiplexing: the idea

Terahertz signals are highly directional. Distinct frequencies

can be associated with distinct propagation directions.

nnnn1111nnnn2222

0.1 0.2 0.3 0.40.0

0.1

0.2

0.3

0.4

Ele

ctr

ic f

ield

am

plit

ud

e (

arb

. u

nits)

Frequency (THz)

Tx2

Tx1

Tx1: ffff = 43.6°

Tx1: ffff = 39.5°

Tx1: ffff = 33.1°

Multiplexing two independent THz signals

N. Karl, et al., Nature Photonics, 7, 717 (2015)

Multiplexing two independent THz signals

-40 -35 -30 -25 -20 -15 -10

-11

-10

-9

-8

-7

-6

-5

-4

-3

8

106

log

(BE

R)

Power at 312 GHz (dBm)

2.5 Gb/sec2.5 6

10

demux power penalty

input to

demux output

from

demuxinput demuxed

30 35 40 45 50 55 60-9

-8

-7

-6

-5

-4

-3

-2

-1

330 GHz

channel

log

(bit e

rror

rate

)

Angle

280 GHz

channel

Up to 50 Gbit/sec!

J. Ma, G. Ducournau, et al., Nature Commun. 8, 729 (2017).

Wireless links: eavesdropping is a concern

Question: How does eavesdropping change if the link is directional?

Eavesdropping looks different

The conventional wisdom:

if the beam is directional enough,

then Eve will fail due to blockage of

the intended receiver.

Alice Bob

Eve

Eve’s shadow

Alice Bob

Eve

A wide-angle broadcast:

(e.g., 120° sector)

A directional beam:

Directional THz links: an alternative strategy for Eve

Is eavesdropping possible

using scattering from a

passive metal object?

• Large enough to scatter

sufficient radiation to be

detected

• Small enough to avoid

casting a shadow on Bob

BobAlice

Eve

q

Eavesdropping test bed

Two figures of merit:

• blockage: the fraction of Bob’s

signal that is blocked by the object.

A value of zero means:

SNRBob unchanged by Eve

• secrecy capacity: how good is

Eve’s SNR compared to Bob’s?

A value of zero means:

SNREve = SNRBob

Flat and cylindrical metallic

scattering objects:

Directional THz links: measuring scattered light

Metal cylinders: measured and

calculated scattering efficiencies

J. Ma, et al., Nature, 563, 89 (2018)

Directional THz links: blockage, secrecy capacity

On axis

blockagesecrecy

capacity

Off axis

secrecy

capacity

blockage

J. Ma, et al., Nature, 563, 89 (2018)

20 40 60 800.0

0.2

0.4

0.6

Fra

ctio

na

l ch

an

ge

in

ba

ckscatte

r

Pipe diameter (mm)

200 GHz

off axis100 GHz

on axis

100 GHz

off axis

A counter-measure

Suppose Alice has a transceiver (not just a transmitter),

and can monitor the back-scatter from the channel.

For these four configurations, the

back-scatter changes significantly:

eavesdropping fails!

Directional THz links: blockage, secrecy capacity

Alice Bob

Eve

metal plate

Metal plates: even more

effective (although Eve

has less freedom)0 2 4 6 8 10

0.0

0.2

0.4

0.6

0.8

1.0

100 GHz

200 GHz

400 GHz

Blo

ckage, norm

aliz

ed s

ecre

cy c

apacity

Plate size (cm)

secrecy

capacity

blockage

J. Ma, et al., Nature, 563, 89 (2018)

A clever eavesdropper always wins.

(although less easily at higher frequencies)

Conclusions

• THz communications: many challenges remain

• THz links: this is not merely ‘microwaves with a few

extra zeros.’ Things are fundamentally different.

• Channel characteristics: there is still a lot of

‘conventional wisdom’ that needs correcting.

• Borrowing ideas from optics is very inspiring!

Funding: