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
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Introduction / Backscatter Sensor NetworksSemi-passive Tag
SDR Reader Design and Backscatter LinksConclusion
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.
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Introduction / Backscatter Sensor NetworksSemi-passive Tag
SDR Reader Design and Backscatter LinksConclusion
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.
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Introduction / Backscatter Sensor NetworksSemi-passive Tag
SDR Reader Design and Backscatter LinksConclusion
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.
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Introduction / Backscatter Sensor NetworksSemi-passive Tag
SDR Reader Design and Backscatter LinksConclusion
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
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Introduction / Backscatter Sensor NetworksSemi-passive Tag
SDR Reader Design and Backscatter LinksConclusion
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
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Introduction / Backscatter Sensor NetworksSemi-passive Tag
SDR Reader Design and Backscatter LinksConclusion
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
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Introduction / Backscatter Sensor NetworksSemi-passive Tag
SDR Reader Design and Backscatter LinksConclusion
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
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Introduction / Backscatter Sensor NetworksSemi-passive Tag
SDR Reader Design and Backscatter LinksConclusion
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
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Introduction / Backscatter Sensor NetworksSemi-passive Tag
SDR Reader Design and Backscatter LinksConclusion
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!
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Introduction / Backscatter Sensor NetworksSemi-passive Tag
SDR Reader Design and Backscatter LinksConclusion
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.
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Introduction / Backscatter Sensor NetworksSemi-passive Tag
SDR Reader Design and Backscatter LinksConclusion
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.
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Introduction / Backscatter Sensor NetworksSemi-passive Tag
SDR Reader Design and Backscatter LinksConclusion
Questions?
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Introduction / Backscatter Sensor NetworksSemi-passive Tag
SDR Reader Design and Backscatter LinksConclusion
Thank You!
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