Dedicated Short-Range Communications

1
Dedicated Short-Range Communications
2
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.

3
Outline

• Motivation
• Issues with Vehicle Communications
• Overview
• Terminology
• Physical Layer
• MAC Layer

4
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

5
Role of DSRC
From Intelligent Transportation System, High
Level Architecture Description, 16
6
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

7
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.

8
Applications

• Applications include
• Coordinated traffic control
• Electronic toll collection
• Hazard warnings,
• Road-level weather advisories
• Different types of safety warnings 1.

9
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

10
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

11
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.

12
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

13
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
14
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

15
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

16
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
17
MAC Layer

• How to communicate
• Stations use Enhanced Distributed Contention
  Access (EDCA) scheme.
• AIFSAC AIFSNACaSlotTime SIFS
• If frame arrives in an empty AC queue and medium
  has been idle for more than AIFSAC aSlotTime
  17
• Packet is transmitted immediately
• If frame arrives when medium is busy (6 and
  17)
• Wait until medium idle
• Defer for AIFSAC aSlotTime
• Pick random CW size and countdown to zero 6
• Additional period is given by CW size for this
  traffic category
• Transmit

18
MAC Layer

• How to communicate
• If a transmission fails, the station uses the
  binary exponential back-off (BEB) scheme 8
• BEB equation CW 2(CW1) 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

19
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
20
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.

21
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.

22
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.

23
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

24
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

25
References

1. J. McNew et al., Safe in Traffic, GPS World,
   vol. 17, no. 10, pp. 41-48, Oct. 2006.
2. M. Conti and S. Giordano, Multihop Ad Hoc
   Networking The Reality, IEEE Communications
   Magazine, vol. 45, no. 4, pp. 88-95, April 2007.
3. 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.
4. L. Stibor et al., Neighborhood Evaluation of
   Vehicular Ad-hoc Network Using IEEE 802.11p, in
   Proc. 13th European Wireless Conf., Paris,
   France, 2007
5. 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.
6. IEEE Std. 802.11-2007, Part 11 Wireless LAN
   Medium Access Control (MAC) and Physical Layer
   (PHY) Specifications, IEEE, 2007.
7. N. Balon and J. Guo, Increasing broadcast
   reliability in vehicular ad hoc networks, in
   Proc. 3rd Intl Workshop Vehicular Ad Hoc
   Networks, 2006, Los Angeles, CA, USA, pp.
   104-105.
8. 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.
9. 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.
10. 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 Intl Workshop on
    Vehicular Ad Hoc Networks, 2004, Philadelphia,
    PA, USA, pp. 10-18.

26
References

1. Q. Xu et al., Layer-2 protocol design for
   vehicle safety communications in dedicated short
   range communications spectrum, in Proc. 7th
   Intl IEEE Conf. Intelligent Transportation
   Systems, 2004, pp. 1092-1097.
2. 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.
3. 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.
4. 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.
5. IEEE 802.11 Official Timelines, Mar. 2008,
   http//grouper.ieee.org/groups/802/11/Reports/802.
   11_Timelines.htm
6. Intelligent Transportation System, High Level
   Architecture Description, Feb. 2008http//www.its
   .dot.gov/arch/arch_longdesc.htm
7. 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.
8. M. Weigle, Standards WAVE/ DSRC/ 802.11p,
   class notes CS 795/895, Old Dominion University,
   Spring 2008.
9. W. Whyte, Safe at Any Speed Dedicated Short
   Range Communications (DSRC) and On-road Safety
   and Security, presented at RSA Conference 2005.

27

• Backup Slides

28
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.

29
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.

30
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

31
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

32
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

33
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

34
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

35
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.

36
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