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Caraoke

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An E-Toll Transponder Network for
Smart Cities
Omid Abari
Deepak Vasisht, Dina Katabi, Anantha Chandrakasan
Electronic Toll Transponders
80% of the cars in US have them
Some states are making it mandatory
Opportunities
7
6
5
4
1
2
3
Opportunities
Adjust
traffic-light timing
Smart
Detect
red-light runner street-parking
One infrastructure for many smart services
Challenge: No MAC to Prevent Collision
Wireless query
One car responds
Physical Isolation
Wireless query
Interference
All cars respond
What can we do?
at the same time
I don't care 'Cause I've never been so high
Follow me to the dark
Let me take you past our satellites
Caraoke
• A system that delivers smart services using
existing e-toll transponders despite interference
• Software-Hardware solution:
– Count cars
– Localize cars
– Decode transponders
• Built and evaluated on our campus streets with
existing e-toll transponders
Count cars: How to count despite
interference?
Localize cars
Decode transponders
Counting Despite Interference
Freq-Domain
Time-Domain
  −  +   − 
Data
Spike
Amplitude
·  2 
Carrier
Frequency
Counting Despite Interference
Freq-Domain
Time-Domain
  −  +   − 
Data
Spike
Amplitude
·  2 
Carrier
Frequency
Amplitude
Toll transponders have difference in their
carrier frequency
Caraoke can count transponders despite
interference
Frequency
Count cars
Localize cars: How to localize despite
interference?
Decode transponders
Localizing Transponders
Localization requires knowing the channel
How to extract channels in the presence of
interference?
Localizing Transponder
Time-Domain
·  2 
ℎ1 ·
Freq-Domain
ℎ1 ·   −  +ℎ1   − 
Spike
ℎ1 ℎ3
ℎ2
ℎ1
ℎ2
ℎ1
ℎ3
Frequency
Use the channel to find the Angle-of-Arrival
Localizing Transponder
z
y
x
x
Localizing Transponder
Cars are always on the road plane
z
y
x
Intersect spatial angle
with road plane
Hyperbola
Intersects hyperbolas across multiple
readers to find the exact location of the car
Count cars
Localize cars
Decode transponders: How to decode
despite interference?
Imag
Solution: Coherent Combining
Real
Imag
Solution: Coherent Combining
Real
Imag
Solution: Coherent Combining
Real
+
+
=
Caraoke uses channel information to combine
responses coherently and decode the ID
Custom Hardware Design
• Self-Sustaining
– Harvest Solar Energy
Custom Hardware Design
• Self-Sustaining
– Harvest Solar Energy
• Low-Cost
– Replace TX chain with simple PLL
Power: 9mW; Cost: $40
Experimental Results
Evaluation
• MIT campus- four streets
• Caraoke readers were placed on 12.5-feet poles
• Standard E-ZPass transponders on the cars
Accuracy of Counting Transponders
Accuracy (%)
100
98
96
94
92
90
10
20
30
40
Number of Transponders
50
Higher accuracy and significantly lower cost
than camera-based systems [R.L., JEI’13; J.M. Urbana‘08 ]
Accuracy of Localizing Transponders
1
CDF
0.8
0.6
0.4
0.2
0
0
2
4
6
8
Localization Error (in degree)
Caraoke enables enough accuracy to detect
occupied versus available parking spots
Detected Speed(mph)
Speed Detection Accuracy
60
50
40
30
20
10
0
10
20
30
40
50
Actual Speed(mph)
Caraoke detects the speed to within 8%
Identification Time (mS)
Required Time for Decoding
60
50
40
30
20
10
0
1
2
3
4
5
6
7
8
9
10
Number of Transponders
Caraoke enables real-time smart city services
Related Work
• Wireless Interference
– They require changing transmitter or leverage
intrinsic asynchrony in protocols (S. G.
Sigcomm’12, J. W. Sigcomm’08, etc.)
• Smart Parking
– They require sensors on parking spots or Wi-Fi
devices in cars (Street Line, S. M. MobiSys’10,
S. N. Mobicom’13, etc.)
• Automated Payment
– They require a gate and physical isolation
(McDonald’s drive-through)
Conclusion
• A system for delivering smart services using
existing e-toll transponders
• Can count, localize and decode transponders
in the presence of interference
• We built it into a small PCB
An E-Toll Transponder Network for
Smart Cities
Omid Abari
Deepak Vasisht, Dina Katabi, Anantha Chandrakasan
Author
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