Terahertz wireless communications:

A photonics approach

Daniel Mittleman

School of Engineering

Brown University

daniel_mittleman@brown.edu

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.

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

considered in the context 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:

THz wireless test bed at Brown University

Transmitter Receiver

Brown University: Two-year experimental

license from FCC for outdoor tests up to 400 GHz

Carrier Frequency 100 GHz 200 GHz 400 GHz

IF frequency 1 GHz

LO frequency 12.25 GHz

PRBS 27 – 1

Max. Tx output power 24 dBm 20 dBm 10 dBm

Tx/Rx antenna gain 21 dB 21 dB 26 dB

Tx beam directivity (angular full-width)

28 dBi (7.8º)

34 dBi (4º)

42 dBi (1.6º)

Detector responsivity 2400 V/W 6200 V/W 1700 V/W

Detector NEP 3 pW/√Hz 3 pW/√Hz 1.9 pW/√Hz

Tx/Rx polarization vertical

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)

Non-line-of-sight links

total distance: 5.5 m

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

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)

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

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-9

-8

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-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).

Is eavesdropping possible?

If the beam is directional enough,

conventional attacks will fail due

to blockage of the intended receiver.

Alice Bob

Eve

Eve’s shadow

Alice Bob

Eve

A wide-angle broadcast:

A directional beam:

Bob

Directional THz links: eavesdropping

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

Alice

Eve

q

Eavesdropping test bed

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)

Directional THz links: blockage, secrecy capacity

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

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

Bob

Eavesdropping using a tuned beam splitter

Suppose Eve can insert a beam

splitter with an arbitrary split ratio.

Alice

Eve

0.6 0.8 1.00.00

0.25

0.50

0.75

1.00

0.86

Transmission coefficient T

0.82

In this range, the

backscattered

signal changes

significantly.

High

secrecy

capacity

sweet spot for the eavesdropper

A clever

eavesdropper

always wins.

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

Conclusions

• THz communications: will be inevitable

• 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: very inspiring!

Funding: