turbocharging ambient backscatter communication aaron parks angli liu shyamnath gollakota joshua r....
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Turbocharging Ambient Backscatter CommunicationAaron ParksAngli LiuShyamnath GollakotaJoshua R. Smith
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Radio Communication Trends
1997 1999 2002 2007 20111
10
100
1000Wi-Fi Bit Rates (Mbps)
Multi-Antenna
• Range (10s of km)• Reliability
Coding
Then
Then
Now
Now
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Our Work
Can we achieve these techniques on battery-free devices?
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If Possible, Benefits New Classes of Devices
RFID
Wearables Localization
Ambient Backscatter
Severe power constraints
Challenge: Expensive Digital Computation
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•Channel estimation•Matrix inversion, etc.
Multiple antennas
•Expensive correlation•Synchronization, etc.
Coding (e.g., CDMA)
Requires power-intensive ADCs
100’s of mW
Battery-free devices have orders of magnitude less power
Our Design Principle
Perform computation in the analog domain
Contributions
• Introduce the first multi-antenna cancellation design for battery-free backscatter devices
• Introduce first analog coding technique for long-range backscatter communication
10 kbps 1 Mbps
2 feet 20 meters
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Ambient Backscatter TX
Ambient Backscatter Communication
No additional powerNo additional spectrum
Modulated reflection
Ambient Backscatter RX
Limited to 10 kbps and 2 feet of range
Contributions
• Introduce the first multi-antenna cancellation design for battery-free backscatter devices 10 kbps 1 Mbps
• Introduce first analog coding technique for long-range ambient backscatter communication 2 feet 20 meters
+ hb2s(t) b(t)hrf2s(t) + hb1s(t) b(t)
Multi-Antennas Without Digital Computation
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Bob(RX)
Alice (TX)
hrf1
hrf2
hb2
hb1 hrf1s(t)s(t)
b(t)
1
2
1 =
2 =
b(t)
s(t)[hrf2 + hb2b(t)]s(t)[hrf1 + hb1b(t)]
Multi-Antennas Without Digital Computation
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Bob(RX)
Alice (TX)
hrf1
hrf2
hb2
hb1s(t) =
=
1
2
hrf2 + hb2
hrf1 + hb1
hrf2
hrf1 ,
b=0 b=11
2=
1
2 Decode b(t) using changes in
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Division in the Analog Domain
• Commercial analog dividers are power hungry!– Build our own.
LogAmplifier
LogAmplifier
Exp.Amplifier ThresholdingSubtractor
= exp(log( ) – log( ))1
21 2
Multi-antenna design without digital computation
Contributions
• Introduce the first multi-antenna cancellation design for battery-free backscatter devices 10 kbps 1 Mbps
• Introduce first analog coding technique for long-range ambient backscatter communication 2 feet 20 meters
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How do we Increase Range?
Pattern for ‘1’ bit
Pattern detected here‘1’
• Add redundancy to data for easier decoding
Cross-correlating is too expensive
Pattern for ‘0’ bit
O(N2)RX
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No shift Gives I component
How do we Increase Range?
• Use periodic code
No synchronization required
Pattern for ‘1’ bit Pattern for ‘0’ bit000000…
Receiver simply correlates with:
½ symbol delay Gives Q component
|I| + |Q| = N
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How do we Increase Range?
• Analog implementation
Simple analog implementation Low Power
Sum Thresholding Circuit
Correlator I
Correlator Q
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Our Hardware Prototype
• Integrated multi-antenna and coding implementation– 422 uW for multi-antenna– 8.9 uW for coding circuit
• Software-defined behavior– 0.3 bps to 1 Mbps
• TV , RFID, and solar harvesting
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1 2 3 4 5 6 71E-04
1E-03
1E-02
1E-01
1E+00
Distance from Scatterer to Receiver (ft)Incident TV power is -20dBm
BER
What Gains Can Multiple Antennas Provide?
10kbps, SIGCOMM ‘13
Multi-antenna, 1 Mbps
100x improvement
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• How do we get orders of magnitude gains by adding an antenna?
• Last year (SIGCOMM ’13)– Average to eliminate big TV signal
• Multi-antenna design– Completely cancel TV signal
What Gains Can Multiple Antennas Provide?
Orders of magnitude increase in rate
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Can our Analog Code Increase the Range?
Transmitter and receiver in line-of-sight
-30 -25 -20 -15 -10 -5 00
20
40
60
80
100
120
TV Signal Strength at Scatterer (dBm)
Rang
e (ft
)
SIGCOMM ‘13
Analog Coding
10-100x improvement across all power levels
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Can our Analog Code Increase the Range?
Transmitter and receiver in non-line-of-sight
-30 -24 -18 -12 -6 0
0
1 1
2 2
3
TV Signal Strength at Scatterer (dBm)
Num
ber o
f Wal
ls
0
1
2
3
4
TX
Conclusions
• Introduce the first multi-antenna and coding designs for battery-free backscatter devices
• Provide orders of magnitude increase in rate and range of ambient backscatter
• Re-design networking primitives with power as a first class citizen- Full-duplex (MOBICOM’14), UWB (?), Random
access (?), TCP/IP (?), …