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Experimental Study of Concurrent Transmission in Wireless Sensor Networks
Dongjin Son, Bhaskar Krishnamachari (USC/EE),
and John Heidemann (USC/ISI)
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Motivation• Prior work
– Understanding wireless propagation essentials
• Zhao, Ganesan, Aguayo, Cerpa, Woo, Lal, Zuniga, Son, etc.
– Only few consider concurrent packet transmission
• Whitehouse, Jamieson, Kochut
• Concurrent transmission is endemic in dense networks– Applications
• Event detection and target tracking • Code distribution and flooding for route discovery
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Research goals
Understanding concurrent packet transmissions !
– Systematic experimental study
– Single and multiple interferers
– Develop a better interference model
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Main findings
• Single Interferer effects– Capture effect is significant– SINR threshold varies due to hardware– SINR threshold does not vary with location– SINR threshold varies with measured RSS– Groups of radios show ~6 dB gray region– New SINR threshold (simulation) model
• Multiple interferer effects– Measured interference is not additive– Measured interference shows high variance– SINR threshold increases with more interferers
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Main findings
• Single Interferer effects– Capture effect is significant– SINR threshold varies due to hardware– SINR threshold does not vary with location– SINR threshold varies with measured RSS– Groups of radios show ~6 dB gray region– New SINR threshold (simulation) model
• Multiple interferer effects– Measured interference is not additive– Measured interference shows high variance– SINR threshold increases with more interferers
November 11, 2003 6
Part I: Single interferer
• Main research questions– Does concurrent transmission imply a
collision ?
– Can we identify a constant SINR threshold
(SINRӨ) for capture?
• Experiments– Two concurrent senders
• varying transmitter hardware and power
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Methodology
Sender1(SRC1)
Sender2(SRC2)
Receiver
Synchronizer(Sync)
Time
Sync
PC104
Mica2
Synchronizes the clocks of both senders
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Methodology
Receiver
Synchronizer(Sync)
Time
Sync SRC1
Measure the RSS of Sender1 (S1)
Measure an ambient Noise (N)
Sender1(SRC1)
Sender2(SRC2)
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Methodology
Receiver
Synchronizer(Sync)
Time
Sync SRC1 Sync SRC2
Measure the RSS of Sender2 (S2)
Measure the ambient Noise (N)
Sender1(SRC1)
Sender2(SRC2)
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Methodology
Receiver
Synchronizer(Sync)
Time
Sync
Sender1(SRC1)
Sender2(SRC2)
SRC1 Sync SRC2
Test the delivery of the sender’s packet under the CTX
Sync SRC1
SRC2
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Methodology
Receiver
Synchronizer(Sync)
Time
Sync
Sender1(SRC1)
Sender2(SRC2)
SRC1 Sync SRC2
Test the delivery of the sender’s packet under the CTX
Sync SRC1
SRC2
• Stronger packet ► Signal• Weaker packet ► Interference
Repeat this epoch and measure PRR
vary Tx power, hardware, location
epoch
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Power and PRR based regions
Gray10~90% PRR
Black< 10% PRR
White> 90% PRR
• Black-Gray-White due to power change• Prior work (Zhao, woo etc) use a distance based definition
• SINR threshold (SINRθ)
– SINR (Signal-to-interference-plus-noise) value which ensures reliable packet reception
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Capture effect
[Finding] Capture effect is significant & SINRθ is not constant
• Concurrent packet transmission does not always means packet collision (capture effect: recently studied by Whitehouse et al.)
• Systematically study capture effects and quantify the SINRθ value
White
White
Black
Gray
Gray
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Modeling SINR to PRR relationship
▪ ß0 changes the shape
(ß0 is set to 2.6 based on the empirical data)
▪ ß1 changes the location
f: frame size of the packet in bytesl: preamble size in bytes- Model based on the link layer model by Zuniga and Krishnamachari
ß0=2ß0=3
ß0=1
-1 0 1 2ß1
β0,β1
β0,β1
β0,β1
β0,β1
β0,β1
β0,β1
• Regression model for simple description of experimental data
)2(8)exp5.01( 10 lfSINRPRR
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Transmitter hardware effect
• How much SINR threshold change does transmitter hardware can make ? – Does hardware variation dominate other effects?
• E.g., compared to the location effect
• Experiments– Hold location constant– Swap one of the transmitter hardware
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Does transmitter hardware affect SINRӨ ?• Vary transmitter hardware (SRC1-SRC2, SRC1-SRC3)
while keeping the same receiver
[Finding] SINRӨ changes with different transmitter hardware
SRC1(with SRC2)
SRC1(with SRC3) SRC3
(with SRC1)
SRC2(with SRC1)
-1.7 dB+1 dB 5.3 dB3.4 dB
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Signal strength effect
• Is SINR threshold constant at different
signal (or interference) strength level? – I.e., Can we always identify a constant SINR
threshold for the same hardware pair ?
• Experiments– Hold location and use the same transmitter pair– Vary transmission power of both transmitters
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Does signal strength level affect SINRӨ?
• Same transmitter hardware, but vary both sender and interferer’s transmission power levels.
[Finding] SINRӨ changes at different signal strength levels
-74 -72 -70 -68 -66 -64 -62 -60 -58 -561
2
3
4
5
6
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Received signal strength (dBm)
SIN
R th
resh
old
(dB
)
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Implications of findings
• Protocols based on constant SINR threshold assumption will fail– Power control protocol and capture-aware protocol
should consider variable SINRθ
– New interference model is necessary
Signal strength
(4.6 dB)
Hardware Signal strength+ Hardware(d
B)
November 11, 2003 20
Part II: Multiple interferers
• Main research questions– Textbook says “Interference is additive”, – How about the reality with low-power RF
transceiver ?
• Experiments– Empirically test the additive signal strength
assumption • Varying the number of interferers and Tx power
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Methodology
Sender
Interferer1
(IFR1)
Interferern
(IFRn)
Receiver
Synchronizer(Sync)
Time
Sync Sender
Measure the RSS of Sender (S)
Measure an ambient Noise (N)
PC104
Mica2
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Methodology
Sender
Interferer1
(IFR1)
Interferern
(IFRn)
Receiver
Synchronizer(Sync)
Time
Sync Sender Sync IFR1
Measure the RSS of Interferer1 (I1)
Measure an ambient Noise (N)
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Methodology
Sender
Interferer1
(IFR1)
Interferern
(IFRn)
Receiver
Synchronizer(Sync)
Time
Sync Sender Sync IFR1 Sync IFRn
Measure the RSS of Interferern (In)
Measure the ambient Noise (N)
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Methodology
Sender
Interferer1
(IFR1)
Interferern
(IFRn)
Receiver
Synchronizer(Sync)
Time
Sync Sender Sync IFR1 Sync IFRn Sync IFR1
IFRn
Measure the Joint Interference
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Methodology
Sender
Interferer1
(IFR1)
Interferern
(IFRn)
Receiver
Synchronizer(Sync)
Time
Sync Sender Sync IFR1 Sync IFRn Sync IFR1
IFRn
Test the delivery of the sender’s packet
Sync Sender
IFR1
IFRn
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Joint interference (JRIS) estimatorsTime
RIS RIS RIS
IFR1
IFR2
IFR3 IFR3
IFR2
IFR1
IFR1 IFR2 IFR3
Summation of independent interference measurement
Average of the actual joint interference measurements
Jointly Measured
Independently Measured
JRIS(e)
JRIS(m)
Textbook prediction!
Direct measurement!
expected
measured
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Does joint interference show additivity?
Comparison between JRIS(e) and JRIS(m) when two interferers (IFR1 and IFR2) have equivalent RISs at the receiver
[Finding] Measured interference is not additive• JRIS(e) is higher than JRIS(m) • Additive behavior is different at different signal strength levels
Individual RIS of IFR1 and IFR2 (dBm)
RIS
(d
Bm
)
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Joint Interference and SINRθ
SINR threshold measurements with different number of interferers
2 Interferer
3 Interferer
4 Interferer
JRIS(e)JRIS(m)
-73 dBm
-68.8 dBm -64.1 dBm
-73 dBm
-68.8 dBm -64.1 dBm
1 Interferer
[Finding] SINR threshold increases with more interferers
• SINR threshold changes with different number of interferers which changes the joint received interference strength
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Potential of capture-aware MAC
• Compare the number of CTXable (Concurrently Transmittable) links
• Methodology• Trace-based Simulation
• Uses real measured RSS• Without Tx power control• Assume red link Tx, who
can CTX together?
• Observation– More available links for the
capture-aware medium access
CTXable links with RTS/CTS based MAC
RTS/CTS based
CTXable links with capture-aware MAC
Capture-aware
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Generalized for all links in the testbed
Capture-aware
Capture-unaware
• The number of CTXable links comparison between traditional and capture-aware MAC
[Finding] Capture-aware MAC shows about 3 times more CTXable links on average
31USC ANRG: http://ceng.usc.edu/~anrg I-LENSE: http://www.isi.edu/ilense
Conclusion• Experimental results show
– the significance of capture effects as Tx power varies
– some of the theoretical assumption does not hold for the measurements
(1) SINR threshold varies (not constant)
(2) Multiple interference worse than addition (not additive)
– better understanding of single and multiple interference on
packet delivery
• Experimental results imply– need better SINR threshold simulation models
– more efficient use of wireless channel is possible with better understanding of concurrent packet transmission
E.g.,) Capture-aware medium access protocol