Ambient Backscatter : Wireless Communication

Out of Thin Air-Liu et. al.

By: MANJOT SINGH(2011CS10228)

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.

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

Communication Through Ambient Backscatter

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.

Key InsightDifference between the transfer rates.

Low-power analog operations using components like capacitors and comparators.

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

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

DESIGN

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.

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.

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

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

Decoding on Ultra Low Power Device Cannot use Analog-to-Digital Convertors ( power constraint ) .

Two stages: Average envelop stage

Compute threshold stage

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.

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.

Prototype

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

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.