Optimal Data Rate Selection for Vehicle Safety Communications

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Optimal Data Rate Selection for Vehicle Safety
Communications

• ACM Workshop VANET 2008
• Daniel Jiang, Qi Chen, Luca Delgrossi
• Mercedes-Benz Research and Development North
  America, Inc.
• Sep 15th 2008

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Contents
1. Overview of Data Rate in 802.11p
2. Theory and Methodology
3. Simulation Results and Summary
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Data Rate Selection In VANET Communications
54Mbps
Data Rate choicesFor IEEE 802.11p(Mbps) 27 24 18
12 9 6 4.5 3
The best selection of data rate depends on the
scenario
Choose
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6
1
100m
200m
300m
Data rate vs. operating distance for MAC 802.11
a/b/g

• Is there a data rata most suitable for DSRC?
• If so, how do we find out the most suitable one?

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Influential Parameters of a DSRC Scenario
Influential Parameters of a DSRC Scenario
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Data Rate Supported in IEEE 802.11p

• Impact of the Data Rate Selection
• Higher rate leads to shorter MAC Frame
  transmission duration
• Higher rate makes MAC frame reception more
  difficult and prone to errors

IEEE802.11p OFDM PHY Parameters
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Contents
1. Overview of Data Rate in 802.11p
2. Theory and Methodology
3. Simulation Results and Summary
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Methodology of Finding the Most Suitable Data Rate
3-step procedure to figure out the most suitable
data rate Step 1 (Make different parameter
combinations) Create scenarios with different
combinations of transmission parameters Step 2
(Compare combinations ) Compare the performance
of different combinations Step 3 (Choose best
combination) Select the combination with highest
performance
Challenges How to select representative values
of the 6 influential parameters. How to make
sure scenarios with different parameters can be
compared fairly? Whats the criterion of
highest performance?

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Concept of Communication Density
Data Rate
Packet Size
Vehicle density
Packet Size
Vehicle density
Number of Lanes
Transmission power
Number of Lanes
Transmission power
Message frequency
Message frequency
Communication Density
4 into 1
CD Transmission Power Message Frequency
Nr. Lanes Vehicle Density

• A system can be now defined by a triplet
• ltCD value, Packet Size, Data Rategt

• A system is defined by 6 parameters
• ltVehicle Density, Number of Lanes, Message
  Frequency, CD value, Packet Size, Data Rategt

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Understanding of Communication Density

• Channel load at a location is determined by the
  number of transmission sensible there and their
  durations.
• Number of sensible transmissions is related to
  the number of nodes around the location and their
  transmission power and frequency.
• (CD is a simple metric value, please see
  Communication Density paper in Movenet2007)
• The duration of a sensible transmission is
  determined by the MAC frame size and the data
  rate being used.

T2
T1
Channel Busy
Channel Clear
t
t1
t2
CSMA Channel Status of Node A
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Use CD to Measure the Channel Load

• Channel load is determined by a triplet ltCD, data
  rate, message sizegtCD Transmission Power
  Message Frequency Nr. Lanes Vehicle Density
• Power, Message Frequency and Vehicle Density are
  inter-changable
• CD is additive. Two groups of nodes with CD1 and
  CD2 are mixed together, their total CD CD1 CD2

How to measure?
Attach a reference group,
If the reference group keeps the same
performance, the tested group shall produce the
same level of load to the reference group
Both groups yield the same Channel Load as long
as their CD values are the same
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Transmission Performance Under a Channel Load
Level

• Performance of a transmission is determined by a
  triplet ltCD, size, data rategt, together with the
  power being used.
• Performance degrades under high channel load

Range 500m
Range 100m
Range 300m
Tested Group
Reference Group
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Answers to the Challenges

• How to select representative values of the 6
  influential parameters?
• (using Communication Density as a metric to
  combine 4 parameters)
• How to make sure scenarios with different
  parameters can be compared fairly?
• (use an attached reference group to test if the
  tested group generates the same level of channel
  load, even though its parameters have been
  selected in different combinations)
• Whats the criterion of highest performance?
• (the best transmission performance in the tested
  group, in terms of successful reception
  probability)

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Contents
1. Overview of Data Rate in 802.11p
2. Theory and Methodology
3. Simulation Results and Summary
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Simulation Settings
Simulator Environment
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Simulation Settings (Cont.)

• 9 channel load levels
• System CD level (3 options) 200, 400, 800
• Packet Size (3 options) 100B, 200B, 500B

6Mbps
100B, 200B, 500B
200, 400, 800
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Defining Parameter Combinations

• In each channel load level, parameters are varied
  in combinations of ltrange, message frequency,
  lanes, vehicle density,data rate, message sizegt
• Data rate (5 options) 3, 4.5, 6, 9, 12 Mbps
• Transmission Range (3 options)100m, 300m, 500m

For each channel load level with ltCD,size,6Mbpsgt
System Load is divided into 3 groups
Making 15 different combination of the parameters
of the study group
Varying with15 combinations
System Load
Using reference group to test the study group
maintains the same channel load contribution
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Transmission Performance in a Given Channel Load
Performance of Study Group
Performance of Reference Groups
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Simulation Result
Study Group Range 100m
Study Group Range 300m
Study Group Range 500m
Best combination with 4.5Mbps
Best combination with 4.5Mbps
Best combination with 6Mbps
Performance of reference group1 matches to each
other
Performance of reference group 2matches to each
other
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Simulation Result (Cont.)
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Summary

• 6Mbps is the most optimum data rate for DSRC
  communications across a general spectrum overall
  channel load and transmission powers. This result
  removes one dimension for future research.
• Triplet lttotal CD values, message size, data
  rategt is an simple but effective value to
  indicate channel load level.
• Proposed new methodology COMBINE, COMPARE,
  CHOOSE allows the further optimization study of
  communications parameters.

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Questions and Discussions