1. 1 Experimental Study of Concurrent Transmission in Wireless Sensor Networks Dongjin Son, Bhaskar Krishnamachari (USC/EE), and John Heidemann (USC/ISI)

2. 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. 3 Research goals Understanding concurrent packet transmissions ! –Systematic experimental study –Single and multiple interferers –Develop a better interference model

4. 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. 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. 6 Part I: Single interferer Main research questions –Does concurrent transmission imply a collision ? –Can we identify a constant SINR threshold (SINR Ө ) for capture? Experiments –Two concurrent senders varying transmitter hardware and power

7. 7 Methodology Sender1 (SRC1) Sender2 (SRC2) Receiver Synchronizer (Sync) Time Sync PC104 Mica2 Synchronizes the clocks of both senders

8. 8 Methodology Receiver Synchronizer (Sync) Time SyncSRC1 Measure the RSS of Sender1 (S 1 ) Measure an ambient Noise (N) Sender1 (SRC1) Sender2 (SRC2)

9. 9 Methodology Receiver Synchronizer (Sync) Time SyncSRC1SyncSRC2 Measure the RSS of Sender2 (S 2 ) Measure the ambient Noise (N) Sender1 (SRC1) Sender2 (SRC2)

10. 10 Methodology Receiver Synchronizer (Sync) Time Sync Sender1 (SRC 1 ) Sender2 (SRC2) SRC1SyncSRC2 Test the delivery of the sender’s packet under the CTX SyncSRC1 SRC2

11. 11 Methodology Receiver Synchronizer (Sync) Time Sync Sender1 (SRC 1 ) Sender2 (SRC2) SRC1SyncSRC2 Test the delivery of the sender’s packet under the CTX SyncSRC1 SRC2 Stronger packet ► Signal Weaker packet ► Interference Repeat this epoch and measure PRR vary Tx power, hardware, location epoch

12. 12 Power and PRR based regions Gray 10~90% 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 (SINR θ ) –SINR (Signal-to-interference-plus-noise) value which ensures reliable packet reception

13. 13 Capture effect [Finding] Capture effect is significant & SINR θ 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 θ value White Black Gray

14. 14 Modeling SINR to PRR relationship ▪ ß 0 changes the shape ( ß 0 is set to 2.6 based on the empirical data) ▪ ß 1 changes the location f: frame size of the packet in bytes l: preamble size in bytes - Model based on the link layer model by Zuniga and Krishnamachari ß 0 =2 ß 0 =3 ß 0 =1 -1 0 1 2 ß1ß1 β0,β1β0,β1 β0,β1β0,β1 β0,β1β0,β1 β0,β1β0,β1 β0,β1β0,β1 β0,β1β0,β1 Regression model for simple description of experimental data

15. 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. 16 Does transmitter hardware affect SINR Ө ? Vary transmitter hardware (SRC1-SRC2, SRC1-SRC3) while keeping the same receiver [Finding] SINR Ө 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

17. 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. 18 Does signal strength level affect SINR Ө ? Same transmitter hardware, but vary both sender and interferer’s transmission power levels. [Finding] SINR Ө changes at different signal strength levels

19. 19 Implications of findings Protocols based on constant SINR threshold assumption will fail –Power control protocol and capture-aware protocol should consider variable SINR θ –New interference model is necessary Signal strength (4.6 dB) Hardware Signal strength + Hardware (dB)

20. 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. 21 Methodology Sender Interferer 1 (IFR 1 ) Interferer n (IFR n ) Receiver Synchronizer (Sync) Time SyncSender Measure the RSS of Sender (S) Measure an ambient Noise (N) PC104 Mica2

22. 22 Methodology Sender Interferer 1 (IFR 1 ) Interferer n (IFR n ) Receiver Synchronizer (Sync) Time SyncSenderSyncIFR 1 Measure the RSS of Interferer 1 (I 1 ) Measure an ambient Noise (N)

23. 23 Methodology Sender Interferer 1 (IFR 1 ) Interferer n (IFR n ) Receiver Synchronizer (Sync) Time SyncSenderSyncIFR 1 SyncIFR n Measure the RSS of Interferer n (I n ) Measure the ambient Noise (N)

24. 24 Methodology Sender Interferer 1 (IFR 1 ) Interferer n (IFR n ) Receiver Synchronizer (Sync) Time SyncSenderSyncIFR 1 SyncIFR n SyncIFR 1 IFR n Measure the Joint Interference

25. 25 Methodology Sender Interferer 1 (IFR 1 ) Interferer n (IFR n ) Receiver Synchronizer (Sync) Time SyncSenderSyncIFR 1 SyncIFR n SyncIFR 1 IFR n Test the delivery of the sender’s packet SyncSender IFR 1 IFR n

26. 26 Joint interference (JRIS) estimators Time RIS IFR 1 IFR 2 IFR 3 IFR 2 IFR 1 IFR 2 IFR 3 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

27. 27 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 (dBm)

28. 28 Joint Interference and SINR θ 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

29. 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 CTXable links with RTS/CTS based MAC RTS/CTS based CTXable links with capture-aware MAC Capture-aware

30. 30 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

31. 31 USC ANRG: http://ceng.usc.edu/~anrg I-LENSE: http://www.isi.edu/ilensehttp://ceng.usc.edu/~anrghttp://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