BoundedLatency Alerts in Vehicular Networks

1
Bounded-Latency Alerts in Vehicular Networks

• Rahul Mangharam, Raj Rajkumar Mark Hamilton
• Dept. of Electrical Computer Engineering
• Carnegie Mellon University
• Priyantha Mudalige Fan Bai
• General Motors Research Development , U.S.A

2
Outline

• Requirements for VANET Safety Applications
• Broadcast Storm Problem and Probabilistic
  Solutions
• Location Division Multiple Access Protocol
• Vehicle Network Simulation and On-road testing
• Conclusion

3
Protocol Requirements for VANET
Bounded Broadcast
Scheduled Latency Storm
Flooding
Message Disconnected Adaptive
Persistence Network
Rebroadcast
End-to-End Rapid Topology Connectivity
Changes
Heterogeneous Networks
Alert Zone with delay-sensitive
messages
Warning Zone with persistent messages
4
Driver Reaction Time

• Driver reaction time 1.5-2.5 seconds
• Slower reaction with higher cognitive load
• Driver Perception accounts for 50 of reaction
  distance
• Drivers respond faster to audio signals
• End-to-end Delay budget
• 1.5 sec for 1km
• 2.5 sec for 2km

5
Probabilistic Solutions to the Broadcast Storm
Problem

• Back-off Probability
• Location-based suppression
• Position-based suppression
• Neighbor-based suppression

• Limitations of Adaptive Broadcast Schemes
• Operating point selection is difficult
• Require relative and neighborhood information
• The trade-off between latency and link
  utilization is non-linear
• The bounds on the end-to-end latency are very
  loose
• Multiple flows cause Priority Inversion

How can we tighten the bounds on broadcast
latency?
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Overview of LDMA Operation
Part I V2V LDMA-based Communication
Alert Zone

• Part I Fine-grained Synchronized Active
  Regions
• Region Definition shape with boundary
  coordinates
• Spatial Definition Block and Cell resolution
• Temporal Definition Slot schedule (µs fast time
  scale)
• Activity Lease Hours of operation and validity

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Active Region ProgrammingOut-of-band LDMA
Control Channel
Pilot Tone
Monaural Signal L R
FM Radio Band with RDS
Stereo Signal L - R
Sub-carrier Channel
Sub-carrier Channel
15 17 23 53
57 67
92 kHz

• Use FM/RDS (Radio Data System) with Open Data
  Channel
• A priori scheduling based on historical trends
• Reactive programming with on-road feedback
• At slower time scale (10s sec)
• Regional updates with Active Region Definitions

8
Location Division Multiple Access
Assigned Black time slot
Assigned Red time slot
Embed Location-based slots in Map Database
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Simple LDMA Schedule
Block 10 Block 11 Block12 ..
Block 0 Block 1 Block 2 .
Vehicle crosses Spatial block here
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1-D LDMA Pipelines

• LDMA Spatial Definition
• 300m transmission range
• 100m LDMA cells
• LDMA Temporal Schedule
• A, C, B 200m/10ms ? 20,000m/s
• A, B, C 100m/10ms ? half-speed

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Multiple LDMA Active Regions
Suburban Region A
Urban Region A
Downtown Region
Urban Region B
Suburban Region B
Rural Region A
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Scalable LDMA Spatial and Temporal Representation

• Tree-based Cell activation and Schedule Assignment

13
LDMA 2D Scheduling

• County-wide slot assignment based on 2-D grid
  for dense urban regions
• Use 1D slot assignment for sparse rural roads and
  highways

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Current State of Vehicular Network Design
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GrooveNet Hybrid Simulator Design
Event Queue
GrooveNet Simulator Core
16
GrooveNet Demo
150 vehicles in downtown Boston
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Modular Architecture
Traffic Light Model
Infrastructure Node Model
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Time Synchronization Implementation

• Using GPS/PPS signal on a gumstix embedded
  computer
• Sub-200µs local synchronization accuracy with
  Linux 2.6
• 2ms pair-wise synchronization accuracy

19
LDMA Performance Comparison

• Trade-off between End-to-End Delay and Link
  Utilization
• 1D Chain of vehicles at 25 vehicles/km
• Adaptive Broadcast Schemes
• Neighbor-based best trade-off
• LDMA
• Smallest delay with controllable message receive
  rate

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Conclusion

• LDMA Location Division Multiple Access
• Safety Alerts Bounded end-to-end delay
• Conflict-free communication Multi-hop problem ?
  single hop one
• Flexible scheduling
• Dynamic reconfigurable schedules via FM/RDS
  control channel
• Implemented in GrooveNet VANET Virtualization
  Platform
• Tightly-coupled Time Synchronization
• GPS/1PPS with lt200µs local accuracy
• 2ms pair-wise sync accuracy
• Promising direction for time-critical VANET
  protocols

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GPS Dead Reckoning