John Kimionis

Introduction / Backscatter Sensor NetworksSemi-passive Tag

SDR Reader Design and Backscatter LinksConclusion

Design and Implementation of Backscatter Links

with Software Defined Radio for Wireless Sensor

Network Applications

John KimionisThesis Supervisor: Aggelos Bletsas

Telecom Lab, ECE Department, Technical University of Crete

February 17, 2012

John Kimionis Backscatter Links with SDR for WSN Applications 1/14



Basic Idea: Sensing for Precision Agriculture

Micro-climate monitoring for everyplant!

◮ Ultra low-cost.

◮ Ultra low-power.

◮ Ultra high network density.

No existing technology for all above.Existing technologies (e.g. ZigBee) canbe cost-effective(?) but notenergy-efficient.

◮ Radio transceiver needs to beeliminated.




Backscatter: Transmission without Radiation

DigitalLogic

CarrierGenerator

Radiation

Reflection

Reader Tag

Receiverand DSP

Conventional transmitter:◮ Generate carrier, modulate, amplify, radiate.◮ Complex design, high cost and consumption.

Backscatter tag:◮ Reflect carrier induced on its antenna. Switch single transistor

for modulation.◮ Minimal front-end, ultra-low cost and consumption!

⇒ Complexity shift from sensor tags to reader.




Experimental Setup

◮ Signal Generator transmitting carrier at 867MHz, 30dBmoutput power, with a 3dBi omnidirectional antenna.

◮ Spectrum Analyzer and Software Defined Radio tuned at867MHz, receiving bandwidth of 1 MHz.

◮ Custom semi-passive tags.




Semi-passive Tag

◮ Full custom, designed andbuilt in Telecom Lab, ECE,TUC.

◮ Low-power MCU.

◮ RF transistor & antenna.

◮ 3V coin battery.

◮ Capacitive & resistivehumidity sensors, built inTelecom Lab.

◮ Total board cost < $10.

MCUSensorCircuitry

CapacitiveSensor

RF transistor

ScatteringAntenna

+

Battery




Semi-passive Tag RF Part.

◮ Antenna scatteringcontrolled by RF transistor:Short: Negative (−) signal.Open: Positive (+) signal.

◮ System reflection coefficientis Γi =

Zi−Z∗a

Zi+Z∗a, i = 1, 2.

◮ Maximization of |Γ1 − Γ2|required to minimize BER.(Bletsas et al, IEEE MTT 2010)

◮ Measure transistors’ Γi andchoose the best.

BITS

0.2

0.5

1.0

2.0

5.0

+j0.2

−j0.2

+j0.5

−j0.5

+j1.0

−j1.0

+j2.0

−j2.0

+j5.0

−j5.0

0.0 ∞

Γ2

Γ1

B

C

E

50Ω

s11

s22

R

Vdd

Gnd

DUT




Backscatter Link Signal Model

◮ Simple channel model.

◮ Received signal at the reader:ynl(t) = Ae−j2π∆Ft−j∆φ[B + CSDe−jφ0x(t)].

∆F : CFO between thesignal generator and SDR.

◮ Tag’s information modulatedon top of carrier’s DC.

◮ SNR of tag’s signal dependson carrier’s amplitude.

Signal

Generator

Software

Defined

Reader

Tag

A y(t)

Ce−jc

Be−j∆φ

De−jd

Sx(t)

0 1 2 3 4 5 6

x 10−4

0

2

4

6

8

10

12

time

am

plit

ude

DC offset

Tag signal




Software Defined Reader

◮ Ettus Research USRP1 withRFX900 UHF front end.

◮ Software signal processing inMATLAB.

◮ Designed receiver capable ofprocessing On Off Keyingand non-coherent FrequencyShift Keying.

DeinterleavingI/Q SamplesComplex

Magnitude

'Mark' Correlator

&EnvelopeDetector

'Space' Correlator

&EnvelopeDetector

Complex Baseband

Detection&

DecodingBitstream

+-

Packet & Symbol Time Sync

Sampling

PulseMatchedFiltering

FSK OOK




Non-coherent Modulation Schemes

OOK modulation

◮ Bit ‘0’/‘1’: transistor ‘On’/‘Off’

◮ Advantage: Simple amplitudedetection.

◮ Downside: Modulated informationclose to strong carrier, affected byRF clutter.

0 1 2 3 4 5

x 10−4

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

time

ampl

itude

10dBm TX power

4dBm TX power

FSK modulation

◮ Bit ‘0’/‘1’: switch transistor atF0/F1.

◮ Advantage: Narrow bandwidthsubcarrier.

◮ Downside: Low bit-rate but highF0,F1 ⇒ higher sampling rate.

50 100 150 200 250 300 350

99

99.5

100

100.5

101

101.5

102

102.5

103

time

ampl

itude

FSK backscattered signal




Modulation Comparison

◮ Theory, simulation, andexperimental validation.

POOKb = Q

(√

EbN0

)

, PFSK, ncb ≈ 1

2e−

Eb2N0

0 1 2 3 4 5 6 7 8 9 1010

−4

10−3

10−2

10−1

100

Eb/N

0 (dB)

BER

FSK (noncoherent)

OOK (est. threshold)

BER: OOK better than FSK.

◮ But OOK close to carrier.

◮ High noise floor around carrierdue to reflections.

◮ Lower SNR ⇒ shorter range.

◮ Prefer FSK (away from carrier)!

FSK range achieved > 100m!




Multiple Access in Backscatter Networks - FDMA

◮ Frequency division multiplexingwith FSK.

◮ No tag collisions.

◮ Each sensor owns specificsubcarriers.

◮ Odd harmonics should beavoided.

◮ Low bitrate ⇒ narrow spectrum⇒ dense network.




Conclusion

◮ Low-cost, low-power communication.

◮ Environmental sensing through backscatter.

◮ Theory met application.

◮ Papers and conferences came out from this work:◮ A. Bletsas, J. Kimionis, A.G. Dimitriou, and G.N. Karystinos, “Single-Antenna, Coherent Detection of

Collided FM0 RFID Signals,” IEEE Transactions on Communications, to appear in 2012.◮ A. Bletsas, A. Vlachaki, E. Kampianakis, G. Sklivanitis, J. Kimionis, K. Tountas, M. Asteris and P.

Markopoulos,“Building the low-cost digital garden as a telecom lab exercise,” IEEE Pervasive Computing,to appear in 2012.

◮ A. Bletsas, A. Vlachaki, E. Kampianakis, G. Sklivanitis, J. Kimionis, K. Tountas, M. Asteris and P.Markopoulos,“Towards precision agriculture: Building a soil wetness multi-hop WSN from first principles,”invited paper, in ECOSENSE 2011, Belgrade, Serbia, Apr. 2011.

◮ J. Kimionis, A. Bletsas, and J.N. Sahalos, “Demonstration of Backscatter Sensor Network with CommoditySDR Reader,” invited, in European Microwave Week (EuMW) 2011, Manchester, UK, Oct. 2011.

◮ J. Kimionis, A. Bletsas, and J.N. Sahalos, “Design and Implementation of RFID Systems with SoftwareDefined Radio,” to appear in 6th European Conference on Antennas and Propagation (EuCAP), Prague,Czech Republic, March 2012.

◮ J. Kimionis, A. Bletsas, A.G. Dimitriou, and G.N. Karystinos, “Inventory Time Reduction in Gen2 withSingle-Antenna Separation of FM0 RFID signals,” in IEEE International Conference on RFID-Technologyand Applications (RFID-TA) 2011, Sitges, Barcelona, Spain, Sep. 2011.

Distinguished with the Second Best Student Paper Award.




Questions?




Thank You!


John Kimionis

Documents

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software dened

backscatter

backscatter

telecom lab

wsn applications

rf transistor

signal generator