Transcript Dedicated Short-Range Communications Department of Electrical Engineering Daniel T. Fokum Apr. 2008

University of Kansas | School of Engineering
Dedicated Short-Range Communications
Department of Electrical Engineering
and Computer Science
Daniel T. Fokum Apr. 2008
University of Kansas | School of Engineering
Abstract
In the next decade it is expected that vehicles
would become part of the Intelligent
Transportation System. The MAC and physical
layers of this system would be supported by
IEEE 802.11p Wireless Access in Vehicular
Environments (WAVE) standard. In what
follows we give an introduction to IEEE
802.11p, showing its PHY and MAC layers as
well as research issues connected to each.
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University of Kansas | School of Engineering
Outline
Motivation
•Issues with Vehicle Communications
Overview
•Terminology
Physical Layer
MAC Layer
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Terminology
OBE(U) = On-board equipment (unit)
RSE(U) = Road side equipment (unit)
VII = Vehicle Infrastructure Integration
ITS = Intelligent Transportation Services
VANET = Vehicular Ad Hoc Network
WAVE = Wireless Access in Vehicular
Environments
AC = Access Category
CW = Contention Window
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Role of DSRC
From Intelligent Transportation System, High Level Architecture Description, [16]
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University of Kansas | School of Engineering
Motivation
“Relatively short-range, high-bandwidth, [and]
low latency communications technology” for
traffic safety.
FCC has allocated 75 MHz of bandwidth around
5.9 GHz for VII.
VII takes two forms:
•vehicle-to-vehicle (V2V)
•vehicle-to-roadside communications (V2R) [1]
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Motivation
Supporting vehicular wireless communications
capabilities within a 1000 m range at highway
speeds [3]
•Standardization efforts include IEEE 802.11p
IEEE 802.11p also known as Wireless Access in Vehicular
Environment (WAVE)
•Relies on location and timing information from GPS
Vehicles will be equipped with OBE to collect
sensor information and relay to neighboring
vehicles [1].
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University of Kansas | School of Engineering
Applications
Applications include:
•Coordinated traffic control
•Electronic toll collection
•Hazard warnings,
•Road-level weather advisories
•Different types of safety warnings [1].
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University of Kansas | School of Engineering
Issues with Vehicle Communications
Privacy issues
•Should not divulge identity of vehicle reporting incident
Reliability
•Vehicles are in range for limited period
Timely reporting
Note: Energy conservation not issue
•OBE has access to power from car [2]
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University of Kansas | School of Engineering
Physical Layer
Variant of IEEE 802.11a PHY
In North America standard provides seven
channels in the 5.9 GHz licensed band [4]
•Each channel designated for different applications [3]
•Channels are 10 MHz wide, with 5 MHz margin at lower
end of band [4]
•Central channel is control channel [4]
•Other channels are service channels [4]
•Has six (6) service channels and one control channel
Two service channels designated for special safety critical
applications [18]
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University of Kansas | School of Engineering
Physical Layer
Variant of IEEE 802.11a PHY [4]:
•Uses 64 subcarrier OFDM, 52 subcarriers used for actual
transmission;
48 data subcarriers and 4 pilot subcarriers
Pilot signals used to get frequency offset and compute phase noise
•Training symbols in each packet preamble
Used for signal detection, coarse frequency offset estimation, time
synchronization and channel estimation
•Guard time associated with each OFDM symbol to
combat ISI.
•Data bits are coded and interleaved to combat fading.
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University of Kansas | School of Engineering
Physical Layer
Variant of IEEE 802.11a PHY [4]:
•Each vehicle broadcasts status 10 times per second.
•Lower priority communication is carried out on service
channels after negotiation on control channel.
•Two adjacent service channels may be used together as
a single 20 MHz channel
•Frequency bandwidth is 10 MHz to increase tolerance to
multipath propagation effects
Results in reduced Doppler effects
Reduces ISI caused by multipath propagation
Data rate for IEEE 802.11p is half that of IEEE 802.11a
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Physical Layer
•Channels available for IEEE 802.11p [8]
•Negotiation for service channels is done on control
channel
From S. Eichler, “Performance Evaluation of the IEEE 802.11p WAVE Communication Standard” [8]
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University of Kansas | School of Engineering
MAC Layer
Uses prioritized channel access developed for
IEEE 802.11e [4]:
•No frame exchange prior to actual data transmission
Reduces communication overhead
•Basic Service Set (BSS) is initiated by provide station
transmitting service announcement frame regularly
No restrictions on transmission intervals
No authentication or frame exchange needed to join BSS
•Each station contains four queues representing four
different types of traffic
•Each queue contends independently for medium access
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University of Kansas | School of Engineering
MAC Layer
Uses prioritized channel access developed for
IEEE 802.11e [4]:
•Each station maps eight user priorities (UP) into four
access categories (AC)
Each AC is modeled as a separate queue contending
independently for medium [17]
Each AC has different MAC layer parameters [17]
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University of Kansas | School of Engineering
MAC Layer
Uses prioritized channel access developed for IEEE
802.11e [4]:
•EDCA parameters for IEEE 802.11p [8]
Used for access to control channel
aCWmin = 15
aCWmax = 1023
AC
CWmin
CWmax
AIFSN
tw
0
aCWmin
aCWmax
9
264 μs
1
(aCWmin + 1)/2 -1
aCWmin
6
152 μs
2
(aCWmin + 1)/4 -1
(aCWmin + 1)/2 -1
3
72 μs
3
(aCWmin + 1)/4 -1
(aCWmin + 1)/2 -1
2
56 μs
From S. Eichler, “Performance Evaluation of the IEEE 802.11p WAVE Communication Standard” [8]
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MAC Layer
How to communicate:
•Stations use Enhanced Distributed Contention Access
(EDCA) scheme.
•AIFS[AC] = AIFSN[AC]*aSlotTime + SIFS
•If frame arrives in an empty AC queue and medium has
been idle for more than AIFS[AC] + aSlotTime [17]
Packet is transmitted immediately
•If frame arrives when medium is busy ([6] and [17])
Wait until medium idle
Defer for AIFS[AC] + aSlotTime
Pick random CW size and countdown to zero [6]
Additional period is given by CW size for this traffic category
Transmit
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University of Kansas | School of Engineering
MAC Layer
How to communicate:
•If a transmission fails, the station uses the binary
exponential back-off (BEB) scheme [8]:
BEB equation: CW = 2*(CW+1) – 1
BEB continues until:
CW = CWmax or
maximum number of retries is achieved
•Station cannot gain access to SCH and CCH for more
than 100 ms [8]
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MAC Layer
Some IFS Relationships Fig. 9-3 in [6]
From IEEE Std. 802.11-2007, Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY)
Specifications, [6]
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University of Kansas | School of Engineering
Implementation Issues
How is routing done?
•Traditional MANET routing protocols cannot be used in
VANET
MANET protocols have an explicit route-establishment phase [3]
•Cannot use traditional routing techniques since message
recipients are unknown beforehand [3].
Department of Electrical Engineering
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Daniel T. Fokum Apr. 2008
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University of Kansas | School of Engineering
Implementation Issues
How is routing done?
•Direction-aware broadcast forwarding [3]
Vehicle forwards emergency situation message to all cars behind it
•Naïve broadcast [3]
Vehicle immediately broadcasts message on emergency situation
•Intelligent broadcast with Implicit Acknowledgement [3]
Vehicle broadcasts emergency situation message to its neighbors
If vehicle eventually receives the same message, it ceases
broadcast
Simulations show that scheme shows good performance.
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Daniel T. Fokum Apr. 2008
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University of Kansas | School of Engineering
Implementation Issues
Improving reliability (from [7])
•Lower layers of DSRC are variant of IEEE 802.11a
Manages medium poorly for broadcasts.
•Failed broadcasts are not retransmitted
•Contention window size is not adjusted for failed
broadcasts
•Suggest using an adaptive scheme
If reception rate exceeds threshold contention window is reduced.
Department of Electrical Engineering
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Daniel T. Fokum Apr. 2008
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University of Kansas | School of Engineering
Implementation Issues
Providing security [19]
•Need to provide:
Anonymity
Can be provided by using:
Anonymous certificates
Random MACs
Changing IP addresses when the OBU moves to new RSU
Authentication
Ensure that fake messages cannot be inserted into the system
Prevent eavesdropping
Prevent competitors from eavesdropping on commercial vehicle operations
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University of Kansas | School of Engineering
Implementation Issues
Deployment timeline (from [1])
•Proof of concept testing in 2007
•Decision on deployment by vehicle manufacturers and
Department of Transportation by late 2008.
•Potential introduction in vehicles in 201x
IEEE 802.11 completion by 12/31/08 [15]
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University of Kansas | School of Engineering
References
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
J. McNew et al., “Safe in Traffic,” GPS World, vol. 17, no. 10, pp. 41-48, Oct. 2006.
M. Conti and S. Giordano, “Multihop Ad Hoc Networking: The Reality,” IEEE Communications
Magazine, vol. 45, no. 4, pp. 88-95, April 2007.
S. Biswas et al., “Vehicle-to-vehicle Wireless Communication Protocols for Enhancing Highway Traffic
Safety,” IEEE Communications Magazine, vol. 44, no. 1, pp. 74-82, Jan. 2006.
L. Stibor et al., “Neighborhood Evaluation of Vehicular Ad-hoc Network Using IEEE 802.11p,” in Proc.
13th European Wireless Conf., Paris, France, 2007
S. K. Shanmugam and H. Leung, “A Novel M-ary Chaotic Spread Spectrum Communication Scheme
for DSRC System in ITS,” in Proc. 60th IEEE Vehicular Technology Conference, Fall 2004, Los
Angeles, CA, USA, vol. 2, pp. 803-807.
IEEE Std. 802.11-2007, Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY)
Specifications, IEEE, 2007.
N. Balon and J. Guo, “Increasing broadcast reliability in vehicular ad hoc networks,” in Proc. 3rd Int’l
Workshop Vehicular Ad Hoc Networks, 2006, Los Angeles, CA, USA, pp. 104-105.
S. Eichler, “Performance Evaluation of the IEEE 802.11p WAVE Communication Standard,” in Proc.
IEEE 66th Vehicular Technology Conference, (VTC-2007 Fall), Baltimore, MD, USA, pp. 2199-2203.
D. Jiang et al. “Design of 5.9 GHz DSRC-based Vehicular Safety Communication,” IEEE Wireless
Communications, [see also IEEE Personal Communications], vol. 13, no. 5, pp. 36-43, Oct. 2006.
M. Torrent-Moreno, D. Jiang, and H. Hartenstein, “Broadcast reception rates and effects of priority
access in 802.11-based vehicular ad-hoc networks,” in Proc. 1st ACM Int’l Workshop on Vehicular Ad
Hoc Networks, 2004, Philadelphia, PA, USA, pp. 10-18.
Department of Electrical Engineering
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Daniel T. Fokum Apr. 2008
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University of Kansas | School of Engineering
References
11.
12.
13.
14.
15.
16.
17.
18.
19.
Q. Xu et al., “Layer-2 protocol design for vehicle safety communications in dedicated short range
communications spectrum,” in Proc. 7th Int’l IEEE Conf. Intelligent Transportation Systems, 2004, pp.
1092-1097.
F. Yu and S. Biswas, “Self-Configuring TDMA Protocols for Enhancing Vehicle Safety With DSRC
Based Vehicle-to-Vehicle Communications,” IEEE Journal on Selected Areas in Communications, vol.
25, no. 8, pp. 1526-1537, Oct. 2007.
J. Zhu and S. Roy, “MAC for Dedicated Short Range Communications in Intelligent Transport System,”
IEEE Communications Magazine, vol. 41, no. 12, pp. 60-67, Dec. 2003.
M. D. Dikaiakos et al., “Location-Aware Services over Vehicular Ad-Hoc Networks using Car-to-Car
Communication,” IEEE Journal on Selected Areas in Communications, vol. 25, no. 8, pp. 1590-1602,
Oct. 2007.
IEEE 802.11 Official Timelines, Mar. 2008,
http://grouper.ieee.org/groups/802/11/Reports/802.11_Timelines.htm
Intelligent Transportation System, High Level Architecture Description, Feb. 2008
http://www.its.dot.gov/arch/arch_longdesc.htm
Q. Ni, L. Romdhani, and T. Turletti, “A Survey of QoS Enhancements for IEEE 802.11 Wireless LAN,”
Journal of Wireless Communications and Mobile Computing, vol. 4, no. 5, pp. 547-566, Aug. 2004.
M. Weigle, “Standards: WAVE/ DSRC/ 802.11p,” class notes CS 795/895, Old Dominion University,
Spring 2008.
W. Whyte, “Safe at Any Speed: Dedicated Short Range Communications (DSRC) and On-road Safety
and Security,” presented at RSA Conference 2005.
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University of Kansas | School of Engineering
Backup Slides
Department of Electrical Engineering
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Daniel T. Fokum Apr. 2008
University of Kansas | School of Engineering
Physical Layer
Research Issues:
•Using a chaotic spread spectrum modulation scheme [5]
Baseband symbols split into in-phase and quadrature phase
components and each is modulated with chaotic parameter
modulation.
Proposed system achieves the same performance as a
conventional M-ary QAM system with a relatively low complexity
receiver.
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Daniel T. Fokum Apr. 2008
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University of Kansas | School of Engineering
MAC Layer
Research issues (from [8])
•Recall WAVE has control channel and six service
channels
Each station would use both control channel and service channel
for no more than 100 ms.
•Contention mechanism in WAVE uses specific
parameters
•Simulation results show that number of received
messages for all AC decreases linearly due to more
collisions on channel.
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Daniel T. Fokum Apr. 2008
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University of Kansas | School of Engineering
MAC Layer
Research issues (from [8])
•Suggests using mechanism to reduce number of high
priority messages
Will result in slightly shorter message queues
•Suggest using different EDCA parameters to minimize
effects of high collision probability
Department of Electrical Engineering
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Daniel T. Fokum Apr. 2008
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University of Kansas | School of Engineering
MAC Layer
Research issues (from [9])
•Congestion control mechanism necessary in DSRC.
Vehicles could regulate message generation rates and
transmission powers according to context.
•Propose using Piggybacked Acknowledgement protocol
for performance feedback.
•Propose ECHO protocol to proactively forward other
nodes’ messages
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University of Kansas | School of Engineering
MAC Layer
Research issues (from [10])
•Assume VANETs will operate in saturated state
Need to determine network parameters to reduce probability of
collision
Propose priority access scheme
•Simulation results show that decreasing AIFS and CW
size results in higher packet reception probability
AIFS has larger effect on probability
Department of Electrical Engineering
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Daniel T. Fokum Apr. 2008
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University of Kansas | School of Engineering
MAC Layer
Research issues (from [11])
•Develop MAC protocol that can meet latency and
reliability requirements for safety messages, while making
economical use of the control channel.
•Propose new MAC protocols that have lower probability
of reception failure and occupy the channel less than
IEEE 802.11
Department of Electrical Engineering
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Daniel T. Fokum Apr. 2008
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University of Kansas | School of Engineering
MAC Layer
Research issues (from [12])
•Introduces Vehicular Self-Organizing MAC (VeSOMAC)
TDMA protocol which copes with vehicular topology changes
•Simulations show that VeSOMAC has smaller packet
latency than IEEE 802.11
Results in fewer vehicles colliding in a VANET
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University of Kansas | School of Engineering
MAC Layer
Research issues (from [13])
•Presents state of art on IEEE 802.11, and how that
applies to VANETs.
•State that most current research on multi-hop networks
assumes slowly-changing topology.
Not necessarily case for VANETs.
MAC design for DSRC complicated by shortened connection time
and frequent topology changes
Must support higher data rates due to shorter connection time.
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Daniel T. Fokum Apr. 2008
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University of Kansas | School of Engineering
Application Layer
Research issues (from [14])
•Introduces Vehicular Information Transfer Protocol (VITP)
VITP is stateless and analogous to HTTP
•VITP architecture consists of
VITP peers
Location encoding scheme and
Additional protocol features
•VITP performance depends on return condition for VITP
requests
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