Turbocharging Ambient Backscatter Communication Aaron Parks Angli - - PowerPoint PPT Presentation

Turbocharging Ambient Backscatter Communication

Aaron Parks Angli Liu Shyamnath Gollakota Joshua R. Smith

1

Radio Communication Trends

1 10 100 1000 1997 2002 2011 Wi-Fi Bit Rates (Mbps)

2

Multi- Antenna

• Range (10s of km)
• Reliability

Coding

Then Then Now Now

Our Work

3

Can we achieve these techniques

• n battery-free devices?

If Possible, Benefits New Classes of Devices

4

RFID Wearables Localization Ambient Backscatter

Severe power constraints

Challenge: Expensive Digital Computation

• 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

Ambient Backscatter TX

Ambient Backscatter Communication

No additional power No additional spectrum

Modulated reflection

8

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

10

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

11

Bob (RX) Alice (TX) hrf1 hrf2 hb2 hb1

s(t)

= =

1 2

hrf2 + hb2 hrf1 + hb1 hrf2 hrf1 , b=0 b=1

1 2

=

1 2

Decode b(t) using changes in

Division in the Analog Domain

12

• Commercial analog dividers are power hungry!

– Build our own.

Log Amplifier Log Amplifier Exp. Amplifier Thresholding Subtractor

= exp(log( ) – log( ))

1 2 1 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

How do we Increase Range?

14

Pattern for ‘1’ bit Pattern detected here ‘1’

• Add redundancy to data for easier decoding

Cross-correlating is too expensive

Pattern for ‘0’ bit

RX

No shift  Gives I component

How do we Increase Range?

• Use periodic code

15

No synchronization required

Pattern for ‘1’ bit Pattern for ‘0’ bit

000000…

Receiver simply correlates with: ½ symbol delay  Gives Q component

|I| + |Q| = N

How do we Increase Range?

• Analog implementation

16

Simple analog implementation  Low Power

Sum Thresholding Circuit Correlator I Correlator Q

Our Hardware Prototype

17

• 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

1.E-04 1.E-03 1.E-02 1.E-01 1.E+00 1 2 3 4 5 6 7

BER

Distance from Scatterer to Receiver (ft) Incident TV power is -20dBm

What Gains Can Multiple Antennas Provide?

18

10kbps, SIGCOMM ‘13 Multi-antenna, 1 Mbps

100x improvement

• 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

19

What Gains Can Multiple Antennas Provide?

Orders of magnitude increase in rate

Can our Analog Code Increase the Range?

20

Transmitter and receiver in line-of-sight

20 40 60 80 100 120

• 30
• 25
• 20
• 15
• 10
• 5

Range (ft)

TV Signal Strength at Scatterer (dBm) SIGCOMM ‘13 Analog Coding

10-100x improvement across all power levels

Can our Analog Code Increase the Range?

21

Transmitter and receiver in non-line-of-sight

1 1 2 2 3

• 30
• 24
• 18
• 12
• 6

Number of Walls TV Signal Strength at Scatterer (dBm)

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 (?), …