ambient backscatter ppt
TRANSCRIPT
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Ambient Backscatter : Wireless Communication
Out of Thin Air-Liu et. al.
By: MANJOT SINGH(2011CS10228)
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Introduction Small computing devices are increasingly embedded in objects and environments.
Key issues is powering the devices as their size decrease.
The new system of wireless communication allows devices to interact without any power source.
System harvests all the existing signals in air and converts them into power for communication.
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Different From RFID-style backscatter Use existing RF signals
Does not require a special-purpose power infrastructure
Reduced installation and maintenance cost
Small environmental footprint.
Device to device communication
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Communication Through Ambient Backscatter
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Challenges in Design Backscattered signals are weak.
Traditional backscatter uses constant signal.
Ambient signals with information already encoded in them.
No power hungry components like oscillators ,ADCs etc. for battery-free receiver.
No centralized controller.
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Key InsightDifference between the transfer rates.
Low-power analog operations using components like capacitors and comparators.
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TV Transmissions
Towers transmit up to 1MV effective radiated power (ERP)
Serve location :More than 100 miles in flat terrain
Up to 45 miles in denser terrain
Broadcast uninterrupted, continuous signals
Fast changing amplitude varying signals
Ability of TV receivers to account for multipath distortion
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Legality Illegal to broadcast random signal on TV spectrum
Backscattering devices are unregulated and not tested by FCC ( Federal Communications Commission )
No active emission
Only modulating the reflecting signals
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DESIGN
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Block diagram of An Ambient Backscattering Device
The transmitter, receiver, and the harvester are all connected to a single antenna and use the same RF signals. The transmitter and receiver communicate by backscattering the ambient signals. The harvester collects energy from the ambient signals and uses it to provide the small amount of power required for communication and to operate the sensors and the digital logic unit.
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Transmitter A wave encountering media boundary with different impedance/densities is reflected back
Backscattering achieved by changing the impedance of antenna
Switch modulates the impedance Input is one : Reflecting state
Input is zero: little reflection
RF switches are deigned for only specific frequency range.
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Receiver TV signal encode at a bandwidth of 6MHz.
Transmit at a larger time scale than 6MHz.
Averaging the received signals across multiple samples
Two average power levels used to decode information
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Receiver (continued.)
0 100 200 300 400 500 600 700 800 900 1000 0
0.1
0.2
0.3
0.4
0.5
0.6
Time Sample #
0 100 200 300 400 500 600 700 800 900 1000 0.19
0.21
0.23
0.25
0.27
Time Sample #
(a) Original TV plus backscattersignal
(b) Signal after averaging
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Decoding on Ultra Low Power Device Cannot use Analog-to-Digital Convertors ( power constraint ) .
Two stages: Average envelop stage
Compute threshold stage
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Bit Encoding and Detecting Transmission Backscattered signal could interfere constructively or destructively with ambient signal.
‘1’ bit : an increase or decrease in received power.
FM0 encoding is used Symbol transition at beginning of every bit to represent ‘1’.
No such transition in ‘0’ bit.
Continuous correlation is power consuming .
Comparator has built in threshold to detect bit transition.
Correlation process activated when power difference crosses threshold.
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Carrier SensingEach device can perform carrier sense .
By both energy detection and preamble correlation.
𝐷 = 1 −(#𝑜𝑛𝑒𝑠 −#𝑧𝑒𝑟𝑜𝑠)
(#𝑜𝑛𝑒𝑠 + #𝑧𝑒𝑟𝑜𝑠)
Used to detect backscattering transmitter.
Bits output by comparator are mostly ‘0’ or ‘1’ in absence.
In presence, the average number of ‘0’ and ‘1’ is same.
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Prototype
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Further OptimizationsMultiple bit-rates Current prototypes operate at specific bit rate ( 100bps, 1 kbps, or 10 kbps)
Design rate adaption algorithms
Collision AvoidanceReduce the number of collisions by designing collision avoidance mechanisms
Hidden TerminalsRTS-CTS (Request to Send / Clear to Send ) to address the problem
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Future Applications
Smart sensors could be built and placed permanently inside nearly any structure, then set to communicate with each other. For example, sensors placed in a bridge could monitor the health of the concrete and steel, then send an alert if one of the sensors picks up a hairline crack. The technology can also be used for communication – text messages and emails, for example – in wearable devices, without requiring battery consumption.
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