terahertz wireless communications: a photonics approach · thz wireless test bed at brown...
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Terahertz wireless communications:
A photonics approach
Daniel Mittleman
School of Engineering
Brown University
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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).
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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:
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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
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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)
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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)
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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).
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Coupling two waveguides together
Key component: electrically actuated liquid metal plug
Collaboration with: M. D. Dickey, North Carolina State University
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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)
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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
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Multiplexing: the idea
Terahertz signals are highly directional. Distinct frequencies
can be associated with distinct propagation directions.
nnnn1111nnnn2222
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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)
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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).
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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:
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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
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Eavesdropping test bed
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Directional THz links: measuring scattered light
Metal cylinders: measured and
calculated scattering efficiencies
J. Ma, et al., Nature, 563, 89 (2018)
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Directional THz links: blockage, secrecy capacity
On axis
blockagesecrecy
capacity
Off axis
secrecy
capacity
blockage
J. Ma, et al., Nature, 563, 89 (2018)
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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
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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)
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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: