Down the Block and Around the Corner The Impact of Radio Propagation on Inter-vehicle Wireless Communication

1. Down the Block and Around the CornerThe Impact of Radio Propagation onInter-vehicle Wireless Communication John Otto, Fabián Bustamante & Randall Berry EECS, Northwestern University

2. Distributed systems on wheels • Size- and power-unlimited mobile network platform • Infrastructure-less • Mobility facilitates rapid information dissemination • Many promising applications • Traditional Internet access • Environmental sensing • Traffic advisory and driver safety • Challenging environment • Rapidly changing topology • Network density depends on vehicular density Down the Block & Around the Corner

3. VANETs and the need for simulation • Live experimentation • Viable when a few nodes are enough • OK for a proof of concept • Not an option with 100’s of vehicles • Simulation-based experimentation and its risks • No agreed-upon platform • Vehicular mobility • Traces and models • Signal propagation • Trading scalability and realism Down the Block & Around the Corner

4. A building between us – urban networking • Performance of the network stack’s physical layer defines the boundaries of a system’s ability • … and your environment determines the performance of the physical layer • How does this impact our applications’ performance? • Signal propagation varies widely between open field and urban settings Down the Block & Around the Corner

5. Measurement studies and where we fit in • Challenging assumptions • Kotz et al. (2004) • Opportunistic connectivity • Ott & Kutscher (2004) • Wu et al. (2005) (multi-hop V2V) • Bychkovsky et al. (2006) • Hadaller et al. (2007) • Varied environments • Singh et al. (2002) • DSRC 5.9 GHz band • Taliwal et al. (2004) • Cheng et al. (2007) • We focus on • Vehicle-to-vehicle (V2V) • Varied environments • Line-of-sight (LOS) versus non-LOS communication Down the Block & Around the Corner

6. Modeling the physical layer • Deterministic models • Free space and two-ray ground • Ideal LOS (and ground reflection) signal strengths • Do not account for variations in environment • Empirical models • Based on measurements taken in an environment • Ray Tracing1 • Requires detailed knowledge of the environment • Incurs significant computational cost • Does not scale • Probabilistic empirical model • Two parameters used to describe the environment • Typically a good compromise between realism, scalability 1McKown & Hamilton. “Ray tracing as a design tool for radio networks.” 1991. Down the Block & Around the Corner

7. Log-normal path loss with shadowing • Parameters • Path Loss Exponent (β) : environment decay rate • Shadowing (σdB): variation due to obstacles • Can complex environments be modeled using just two parameters? Free Space path loss Environment path loss Random variations (obstacles) Down the Block & Around the Corner

8. Goals and methodology • Characterize signal propagation in urban settings • Pick representative environments • Measure signal propagation in • line of sight (LOS) and • non-LOS (Around the Corner – ATC) settings • Pick a signal propagation model, a good simulator, and a simple application • Free-space, probabilistic shadowing … • ns, GloMoSim, JIST/SWANS … • Evaluate application-level impact of environment • This work appeared in Proc. of ICDCS, 2009 Down the Block & Around the Corner

9. Roadmap • Overview of radio propagation models • Experimental characterization of radio propagation in an urban setting (Chicago) • Measurement platform • Measured environments • Data analysis • Understanding the impact of signal propagation parameters on application performance • Conclusion Down the Block & Around the Corner

10. Soekris net4801 running Linux Garmin GPS 18 USB 7 dBi omni-directional antenna Measurement platform • Set of equipped vehicles with • Soekris net4801-60 machines, 256 MB memory, 1GB flash storage • Garmin GPS 18 USB for positioning • Ubiquiti Networks 2.4 GHz 802.11b/g • 7 dBi 2.4 GHz omni-directional antenna • Software • Linux (2.6.19 kernel) • iperf (CBR UDP stream) • tcpdump Down the Block & Around the Corner

11. Understanding the environment • Measurement in representative environments & times Open field – Provides a baseline; no buildings or any other obstacles Suburban – Residential area with trees, cars and houses set back from the road with space between them Urban – Large and tall buildings, very close to the street, few gaps between buildings, etc • Run experiments: • Daytime (high traffic) • At night (low traffic) Down the Block & Around the Corner

12. Simple environment – Open field No traffic Path loss exponent stabilizes at 3.10 Path loss exponent Line-of-Sight (LOS) Communication Same road Distance (meters) Down the Block & Around the Corner

13. Simple environment – Open field No traffic Median path loss exponent = 3.29 Path loss exponent Distance (meters) Around the Corner (ATC) Communication Perpendicular roads Distance from intersection (meters) Down the Block & Around the Corner

14. Simple environment – Open field • Same road • Perpendicular roads Down the Block & Around the Corner

15. More complex settings – Line of sight • Suburban • Open field Down the Block & Around the Corner

16. … Around the corner (ATC) • Suburban • Open Field Down the Block & Around the Corner

17. … Line of sight and Around the corner • Same road • Perpendicular roads • At 50 meters apart, LOS and ATC β = 3.2 • At 80 meters apart, LOS β = 3.1… but ATC β > 4 ! Down the Block & Around the Corner

18. Urban ATC • Urban 50 meters apart, in LOS non-LOS communication, higher path loss exponent due to diffraction, reflection > 100 meters apart, no communication possible Down the Block & Around the Corner

19. Suburban vs. Urban – Around the corner • Suburban • Urban Can be 20 meters from intersection before observing PLE increase Immediate increase in PLE after leaving intersection • Distance of obstructions from the road: • Suburban: wide front lawns • Urban: narrow sidewalks Down the Block & Around the Corner

20. Line-of-sight experimental summary • Obstacles increase signal variability (shadowing parameter) • e.g. from σ = 3.23 in an open field to 9.15 in an urban setting • Vehicular traffic degrades signal strength • Overall, path-loss exponent is not significantly impacted • e.g. from 3.10 in an open field to 3.17 in an urban setting • Transmit range reduced by 14% • Open field: 1070 m • Urban: 915 m • (predicted with model) Down the Block & Around the Corner

21. Around-the-corner experimental summary • Path loss exponent varies significantly • e.g. 3.29 in an open field to 4.05 in an urban setting • Transmit range reduced by 70% • Open field: 715 m • Urban: 208 m • (predicted with model) • Non-LOS communication is possible • Reflection, diffraction • Gaps between buildings • Distance of obstacles from road is a significant factor Down the Block & Around the Corner

22. Experimental impact • Challenge assumption: one set of parameters is sufficient • Experiments contradict this assumption • For complex environments (suburban, urban) • LOS vs. non-LOS (ATC) is a key factor in communication • So, we actually need at least two sets of parameters: • LOS and non-LOS (ATC) • What is the impact at the application layer? • Use simulations to evaluate application performance under • Environments • Parameter settings (e.g. LOS, ATC) Down the Block & Around the Corner

23. The impact of signal propagation parameters • Pick a signal propagation model, a good simulator, and a simple application • Signal propagation model • Log-normal path loss with shadowing • Sample application – Epidemic-based data dissemination • e.g. Communicating road (traffic) conditions • Push-based protocol, based on Vahdat & Becker (2000) • Beacon • Exchange digest • Send messages • Application performance metric: Delivery latency • e.g. Lower latency gives fresher data and better detouring ability Down the Block & Around the Corner

24. The impact of signal propagation parameters • For simple environments • LOS vs. ATC does not affect performance • However… for complex environments • LOS performance much higher than ATC • Combining data sets does not give average performance • We evaluate LOS&ATC • Switch between LOS and ATC parameters: same / different street • Gives expected intermediate performance • Compromise between scalability and realism Down the Block & Around the Corner

25. Simulation configuration • For simulation – JiST/SWANS++ • http://www.aqualab.cs.northwestern.edu/projects/swans++/ • For vehicular mobility – STRAW • Using real cities’ road maps • Lights, signals, speed limits • IDM car-following • MOBIL lane-changing • http://sourceforge.net/projects/straw/ • Parameters • Map: downtown Chicago (approximate Manhattan grid), 1.76 km2 • Radio settings: match experiment configuration • 26 dBm transmit power, 7 dBi antenna gain, 2 Mbps fixed data rate • 150 vehicles • 2 hour duration Down the Block & Around the Corner

26. Application performance in LOS vs. ATC In an open field, the locations of the communicating vehicles (in line-of-sight or not) have no performance impact Open field setting with traffic LOS ATC Down the Block & Around the Corner

27. Application performance in downtown area In urban settings, around-the-corner parameters mean smaller transmit range, hence lower performance Urban setting LOS ATC Down the Block & Around the Corner

28. Urban – Combining datasets Averaging parameters – by combining datasets – doesn’t yield averaged performance Urban setting LOS Combined ATC Intermediate PLE, but increased shadowing Down the Block & Around the Corner

29. LOS & ATC – A compromise • Using two parameter sets and relative vehicle position,select LOS or ATC parameters based on node position Urban setting LOS LOS&ATC ATC Down the Block & Around the Corner

30. Application-level impact summary • Simple environments (open field) • One set of parameters is sufficient • No difference in performance between LOS and ATC parameters • Complex environments (suburban, urban) • Using one set of parameters (LOS or ATC) is not sufficient • Combining LOS and ATC gives worse than expected performance • LOS&ATC approach gives the expected intermediate performance • Possible extensions to LOS&ATC • Tolerance for distance from the intersection • Simulating heterogeneous environments on the same map • Utilizing LOS/ATC information at the protocol or application layers • … Down the Block & Around the Corner

31. Conclusion • LOS is a major factor of signal propagation characteristics in complex environments • Accounting for LOS versus non-LOS has a significant impact on application-level performance • LOS&ATC is a computationally scalable and more realistic approach for modeling complex environments • Part of C3R, a project on urban environmental monitoring through vehicular networks, working towards • Ensuring sustainable urban growth • Participatory sensing with a mobile platform • Applications including traffic advisory, air quality and noise monitoring Down the Block & Around the Corner

32. Simple environment – Open field • Same road • Perpendicular roads With traffic, Increased β (3.31) and σ Down the Block & Around the Corner

33. More complex settings – Urban LOS • Urban • Open field Similar to suburban: larger variations in path loss exponent Down the Block & Around the Corner