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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 (?), …