Backscatter Bundle Matteo Panzacchi 1 Backscattered signals - - - PowerPoint PPT Presentation

Backscatter Bundle

Matteo Panzacchi

14-05-2014 1 Backscattered signals - Matteo Panzacchi

What is backscatter?

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Scanning devices

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Battery free wireless communication

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Outline

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 Full Duplex Backscatter:

 Transforming our smartphone into Star Trek’s Tricoder

Backscattered signals - Matteo Panzacchi

Outline

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 Full Duplex Backscatter:

 Transforming our smartphone into Star Trek’s Tricoder

 Ambient Backscatter:

 RF battery free communication

Backscattered signals - Matteo Panzacchi

Full Duplex Backscatter

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 Transforming our smartphone into a futuristic

Tricoder

Backscattered signals - Matteo Panzacchi

Limited sampling rate

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Self-interference

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Limited dynamic range

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Model of backscatter

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 : complex attenuation for the kth reflection  : added phase shift for kth

reflection at mth antenna, relative to the first receiver

 : carrier frequency  : wavelength  : distance between two consecutive antennas

Backscattered signals - Matteo Panzacchi

Estimation of the parameters

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 Estimation of the linear channel  Estimation of the parameters of the constituent

backscatter by solving the following optimization problem minimize where

Backscattered signals - Matteo Panzacchi

Signal cancellation

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Formal algorithm

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First evaluations

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 Checking the possibility of progressive cancellation

by using an emulated backscatter setup

 Checking the accuracy of the parameters estimation

algorithm via MatLab simulations

Backscattered signals - Matteo Panzacchi

Progressive cancellation results

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Parameter estimation accuracy

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Conclusions

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 Basic building blocks for transforming a smartphone

into a scanning device

 Changes in the hardware are needed  First evaluation of the solutions for the main

challenges gave good results

 More in depth evaluations are needed

Ambient Backscatter

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 Idea: enable communication among devices by

using only ambient RF signals as only source of power

RF signals

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 It includes: TV, radio and cellular transmissions  TV signals in particular have these characteristics:

 Carry up to 1 MW power of Effective Radiated Power  Serve locations 100 mi away from the source (flat terrain),

45 mi (denser terrain)

 Excellent coverage  Broadcast signals 24/7  Amplitude changes at a very fast rate  Synchronization symbols to compute multipath channel

characteristics

Backscattered signals - Matteo Panzacchi

Designing an Ambient Backscatter

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 Three main challenges:

 Mechanism to extract the backscattered information

carried by the RF signals

 Low power infrastructure  Channel arbitration and bit error detection

 It differs from traditional backscatter technologies

because:

 They relies on power hungry components

Ambient Backscatter design

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Extracting the backscattered signal

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Formal extracting technique

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 Averaging the instantaneous power in the N receiver

samples:

 B is either ‘0’ or ‘1’, w[n] is uncorrelated with x[n]:  We have two power levels |1 + 𝛽|2𝑄 and P

Receiver circuit

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 RC circuit for the averaging stage (it acts a low-pass

filter)

 Comparator which has a threshold (

𝑊

0+𝑊 1

2

) as input of the – pin and it detects the two levels of power

Physical and Link layer structure

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 Three main challenges:

 Mechanism to extract the backscattered information

carried by the RF signals

 Low power infrastructure  Channel arbitration and bit error detection

 No presence of a centralized controller

 A new packet format  Link layers techniques

Packet format

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 10…10: sequence of ‘1’ and ‘0’ used to awake the

logical unit

 Preamble: used to detect the packet  Type: which can be data/ACK

Link Layer

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 The detection of bit errors is done using CRC  No centralized authority to arbitrate the channel

 Devices perform carrier sense by overhearing the channel  In absence of a transmitter you have a constant bit, so:  In presence of a transmission D is close to 1  In absence of a transmission D is close to 0

 RTS-CTS can be used to avoid Hidden Terminal

problem

BER at different locations (Far vs Near)

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BER at different locations (Indoor vs Outdoor)

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Interference with TV

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Real world application into a Grocery Store

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Conclusions

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 They implemented a prototype of a power free

communication using TV signals

 The results were impressive  It is a first step into a direction of battery free

communication

References

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 Dinesh Bharadia, Kiran Raj Joshi, Sachin Katti: “Full

Duplex Backscatter”

 Vincent Liu, Aaron Parks, Vamsi Talla, Shyamnath

Gollakota, David Wetherall, Joshoua R. Smith: “Ambient Backscatter: Wireless Communication Out

• f Thin Air”

Q&A

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