Experimental%20Study%20of%20Concurrent%20Transmission%20in%20Wireless%20Sensor%20Networks

1
Experimental Study of Concurrent Transmission in
Wireless Sensor Networks

• Dongjin Son, Bhaskar Krishnamachari (USC/EE),
• and John Heidemann (USC/ISI)

2
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

3
Research goals

• Understanding concurrent packet transmissions
  !
• Systematic experimental study
• Single and multiple interferers
• Develop a better interference model

4
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

5
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

6
Part I Single interferer

• Main research questions
• Does concurrent transmission imply a collision ?
• Can we identify a constant SINR threshold
• (SINR?) for capture?
• Experiments
• Two concurrent senders
• varying transmitter hardware and power

7
Methodology
Mica2
PC104
Sender1 (SRC1)
Receiver
Sender2 (SRC2)
Synchronizer (Sync)
Time
Sync
Synchronizes the clocks of both senders
8
Methodology
Sender1 (SRC1)
Receiver
Sender2 (SRC2)
Synchronizer (Sync)
Time
Sync
SRC1
Measure an ambient Noise (N)
Measure the RSS of Sender1 (S1)
9
Methodology
Sender1 (SRC1)
Receiver
Sender2 (SRC2)
Synchronizer (Sync)
Time
Sync
SRC1
Sync
SRC2
Measure the ambient Noise (N)
Measure the RSS of Sender2 (S2)
10
Methodology
Sender1 (SRC1)
Receiver
Sender2 (SRC2)
Synchronizer (Sync)
Time
Sync
SRC1
Sync
SRC2
Sync
SRC1
SRC2
Test the delivery of the senders packet
under the CTX
11
Methodology
vary Tx power, hardware, location
Sender1 (SRC1)
Receiver

• Stronger packet ? Signal
• Weaker packet ? Interference

Sender2 (SRC2)
Synchronizer (Sync)
Time
Sync
SRC1
Sync
SRC2
Sync
SRC1
SRC2
Test the delivery of the senders packet
under the CTX
epoch
Repeat this epoch and measure PRR
12
Power and PRR based regions
White gt 90 PRR
Black lt 10 PRR
Gray 1090 PRR

• Black-Gray-White due to power change
• Prior work (Zhao, woo etc) use a distance based
  definition
• SINR threshold (SINR?)
• SINR (Signal-to-interference-plus-noise) value
  which ensures reliable packet reception

13
Capture effect
White
Gray
Black
Gray
White

• Finding Capture effect is significant SINR?
  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 SINR? value

14
Modeling SINR to PRR relationship

• Regression model for simple description of
  experimental data

f frame size of the packet in bytes l preamble
size in bytes - Model based on the link layer
model by Zuniga and Krishnamachari
? ß0 changes the shape (ß0 is set to 2.6
based on the empirical data) ? ß1 changes
the location
ß01
ß02
ß03
-1 0 1 2
ß0,ß1
ß1
ß0,ß1
ß0,ß1
ß0,ß1
ß0,ß1
ß0,ß1
15
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

16
Does transmitter hardware affect SINR? ?

• Vary transmitter hardware (SRC1-SRC2, SRC1-SRC3)
• while keeping the same receiver

SRC1 (with SRC2)
SRC2 (with SRC1)
SRC1 (with SRC3)
SRC3 (with SRC1)
-1.7 dB
1 dB
5.3 dB
3.4 dB
Finding SINR? changes with different
transmitter hardware
17
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

18
Does signal strength level affect SINR??

• Same transmitter hardware, but vary both sender
  and interferers transmission power levels.

Finding SINR? changes at different signal
strength levels
19
Implications of findings
(4.6 dB)
Signal strength Hardware
Hardware
(dB)
Signal strength

• Protocols based on constant SINR threshold
  assumption will fail
• Power control protocol and capture-aware protocol
  
• should consider variable SINR?
• New interference model is necessary

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

21
Methodology
Mica2
PC104
Sender
Receiver
Interferer1 (IFR1)
Interferern (IFRn)
Synchronizer (Sync)
Time
Sync
Sender
Measure an ambient Noise (N)
Measure the RSS of Sender (S)
22
Methodology
Sender
Receiver
Interferer1 (IFR1)
Interferern (IFRn)
Synchronizer (Sync)
Time
Sync
Sender
Sync
IFR1
Measure an ambient Noise (N)
Measure the RSS of Interferer1 (I1)
23
Methodology
Sender
Receiver
Interferer1 (IFR1)
Interferern (IFRn)
Synchronizer (Sync)
Time
Sync
Sender
Sync
IFR1
Sync
IFRn
Measure the ambient Noise (N)
Measure the RSS of Interferern (In)
24
Methodology
Sender
Receiver
Interferer1 (IFR1)
Interferern (IFRn)
Synchronizer (Sync)
Time
Sync
Sender
Sync
IFR1
Sync
IFRn
Sync
IFR1
IFRn
Measure the Joint Interference
25
Methodology
Sender
Receiver
Interferer1 (IFR1)
Interferern (IFRn)
Synchronizer (Sync)
Time
Sync
Sender
Sync
IFR1
Sync
IFRn
Sync
IFR1
Sync
Sender
IFR1
IFRn
IFRn
Test the delivery of the senders packet
26
Joint interference (JRIS) estimators
Time
IFR1
IFR1
Jointly Measured
Independently Measured
IFR2
IFR2
IFR3
IFR3
RIS
RIS
RIS
IFR1
IFR2
IFR3
measured
JRIS(m)
Average of the actual joint interference
measurements
expected
JRIS(e)
Summation of independent interference measurement
Direct measurement!
Textbook prediction!
27
Does joint interference show additivity?
RIS (dBm)
Individual RIS of IFR1 and IFR2 (dBm)
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

28
Joint Interference and SINR?
4 Interferer
3 Interferer
2 Interferer
-64.1 dBm
-68.8 dBm
-64.1 dBm
-68.8 dBm
1 Interferer
-73 dBm
-73 dBm
JRIS(e)
JRIS(m)
SINR threshold measurements with different number
of interferers

• Finding SINR threshold increases with more
  interferers
• SINR threshold changes with different number of
  interferers which changes the joint received
  interference strength

29
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

RTS/CTS based

CTXable links with RTS/CTS based MAC

Capture-aware
CTXable links with capture-aware MAC
30
Generalized for all links in the testbed

• The number of CTXable links comparison between
  traditional and capture-aware MAC

Capture-aware
Capture-unaware
Finding Capture-aware MAC shows about 3 times
more CTXable links on average
31
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

USC ANRG http//ceng.usc.edu/anrg I-LENSE
http//www.isi.edu/ilense