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Transcript of Http://aqualab.cs.northwestern.edu John Otto, Fabián Bustamante & Randall Berry EECS, Northwestern...
http://aqualab.cs.northwestern.edu
John Otto, Fabián Bustamante & Randall BerryEECS, Northwestern University
Down the Block and Around the Corner
The Impact of Radio Propagation onInter-vehicle Wireless Communication
Otto, Bustamante & Berry 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
Otto, Bustamante & Berry 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
Otto, Bustamante & Berry 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
Otto, Bustamante & Berry 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
Otto, Bustamante & Berry 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
Otto, Bustamante & Berry 7
),0(log104
log1020
100
10 dBNd
ddPL
Parameters– Path Loss Exponent (β) : environment decay rate– Shadowing (σdB): variation due to obstacles
Can complex environments be modeled using just two parameters?
Log-normal path loss with shadowing
Down the Block & Around the Corner
Free Space path loss
Environment path loss
Random variations
(obstacles)
),0(log104
log1020
100
10 dBNd
ddPL
Otto, Bustamante & Berry 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
Otto, Bustamante & Berry 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
Otto, Bustamante & Berry 10
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
GarminGPS 18 USB
Soekris net4801 running Linux
7 dBi omni-directionalantenna
Down the Block & Around the Corner
Otto, Bustamante & Berry 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
Down the Block & Around the Corner
Run experiments:
• Daytime (high traffic)
• At night (low traffic)
Otto, Bustamante & Berry 12
Simple environment – Open field
Down the Block & Around the Corner
No traffic
Path loss exponent stabilizes at 3.10
Line-of-Sight (LOS)
Communication
Same roadP
ath
loss
exp
onen
t
Distance (meters)
β / σ Open Field Suburban Urban
LOS 3.10 / 3.23
ATC
Otto, Bustamante & Berry 13
Simple environment – Open field
Down the Block & Around the Corner
No traffic
Median path loss exponent = 3.29
Around the Corner(ATC) Communication
Perpendicular roads
Pat
h lo
ss e
xpon
ent
Distance from intersection (meters)
Dis
tanc
e (
met
ers)
β / σ Open Field Suburban Urban
LOS 3.10 / 3.23
ATC 3.29 / 3.35
Otto, Bustamante & Berry 14
Simple environment – Open field
Same road Perpendicular roads
Down the Block & Around the Corner
β / σ Open Field Suburban Urban
LOS 3.10 / 3.23
ATC 3.29 / 3.35
Otto, Bustamante & Berry 15
More complex settings – Line of sight
Suburban Open field
Down the Block & Around the Corner
β / σ Open Field Suburban Urban
LOS 3.10 / 3.23 3.14 / 7.28
ATC 3.29 / 3.35
Otto, Bustamante & Berry 16
Suburban
… Around the corner (ATC)
Open Field
Down the Block & Around the Corner
β / σ Open Field Suburban Urban
LOS 3.10 / 3.23 3.14 / 7.28
ATC 3.29 / 3.35 3.87 / 8.44
Otto, Bustamante & Berry 17
… Line of sight and Around the corner
Same road Perpendicular roads
Down the Block & Around the Corner
β / σ Open Field Suburban Urban
LOS 3.10 / 3.23 3.14 / 7.28
ATC 3.29 / 3.35 3.87 / 8.44
• At 50 meters apart, LOS and ATC β = 3.2
• At 80 meters apart, LOS β = 3.1… but ATC β > 4 !
Otto, Bustamante & Berry 18
Urban ATC
Urban
Down the Block & Around the Corner
β / σ Open Field Suburban Urban
LOS 3.10 / 3.23 3.14 / 7.28 3.17 / 9.15
ATC 3.29 / 3.35 3.87 / 8.44 4.05 / 10.74
non-LOS communication, higher path loss exponent due to diffraction, reflection
50 meters apart, in LOS
> 100 meters apart, no communication possible
Otto, Bustamante & Berry 19
Suburban vs. Urban – Around the corner
Suburban Urban
Down the Block & Around the Corner
Can be 20 meters from intersection before
observing PLE increaseDistance of obstructions from the road:• Suburban: wide front lawns• Urban: narrow sidewalks
Immediate increase in PLE after leaving intersection
Otto, Bustamante & Berry 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
Otto, Bustamante & Berry 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
Otto, Bustamante & Berry 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
Otto, Bustamante & Berry 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)
1. Beacon
2. Exchange digest
3. Send messages
Application performance metric: Delivery latency– e.g. Lower latency gives fresher data and better detouring ability
Down the Block & Around the Corner
Otto, Bustamante & Berry 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
Otto, Bustamante & Berry 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
Otto, Bustamante & Berry 26
Application performance in LOS vs. ATC
Down the Block & Around the Corner
LOS
ATC
In an open field, the locations of the communicating vehicles (in line-of-sight or not) have no performance impact
Open fieldsetting
with traffic
Otto, Bustamante & Berry 27
Application performance in downtown area
Down the Block & Around the Corner
LOS
ATC
In urban settings, around-the-corner parameters mean smaller transmit range, hence lower performance
Urban setting
β / σ Urban
LOS 3.17 / 9.15
ATC 4.05 / 10.74
Otto, Bustamante & Berry 28
Urban – Combining datasets
Down the Block & Around the Corner
LOS
ATC
Combined
Averaging parameters – by combining datasets – doesn’t yield averaged performance
Urban setting
β / σ Urban
LOS 3.17 / 9.15
ATC 4.05 / 10.74
Combined 3.43 / 11.95
Intermediate PLE, but increased shadowing
Otto, Bustamante & Berry 29
LOS & ATC – A compromise
Down the Block & Around the Corner
Urban setting LOS
ATC
LOS&ATC
Using two parameter sets and relative vehicle position,select LOS or ATC parameters based on node position
Otto, Bustamante & Berry 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
Otto, Bustamante & Berry 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
Otto, Bustamante & Berry 32
Simple environment – Open field
Same road Perpendicular roads
Down the Block & Around the Corner
With traffic,Increased β (3.31) and σ
β / σ Open Field Suburban Urban
LOS 3.10 / 3.23
ATC 3.29 / 3.35
Otto, Bustamante & Berry 33
More complex settings – Urban LOS
Urban Open field
Down the Block & Around the Corner
β / σ Open Field Suburban Urban
LOS 3.10 / 3.23 3.14 / 7.28 3.17 / 9.15
ATC 3.29 / 3.35 3.87 / 8.44
Similar to suburban: larger variations in path loss exponent