3. Wireless Access for Vehicular Environments
Rationale
What was the motivation behind a vehicle specific WLAN? What
prevented the existing IEEE 802.11-family from being adopted as is?
4. IEEE 802.11 in C2C
Requirements to be used for C2C
Changes in baseline 802.11 standards are required to:
support longer ranges of operation (up to ~1000 meters),
the high speed of the vehicles (up ~500 km/h relative velocities),
the extreme multipath environment (many reflections with long
delays (up to ~5 μs)),
the need for multiple overlapping
ad-hoc networks to operate with
extremely high quality of service,
and
the nature of the automotive
applications (e.g. reliable
broadcast) to be supported.
5. IEEE 802.11 in C2C
VANET communication entities – not only cars
Communication between:
roadside units and mobile radio units (Vehicle-2-Infrastructure),
mobile units (Vehicle-2-Vehicle), or
portable units and mobile units (Vehicle-2-Pedestrian)
Infrastructure:
Roadside Units (RSUs)
Gantries (e.g. tolling gantries)
Poles, traffic lights, etc.
Mobile/Portable equipment:
On-board Unit (OBU)
Based on IEEE 802.11p
DSRC platform
11. IEEE 802.11p
Frequency band
U.S. FCC allocated 75 MHz band in 1999 for ITS
Shared Public Safety/Private
Short Rng
Control Medium Rng
Service
Service
Power Limit
Dedicated Public Safety
InterHigh
Availability sections
44.8 dBm
Power Limit
40 dBm
33 dBm
Power Lim it
23 dBm
Uplink
Based on B. Cash (2008): North American 5.9 GHz DSRC Operational Concept / Band Plan
5.925
5.920
5.915
5.910
5.905
5.900
Public
Public Safety
Safety/
Intersections
Private
Ch 182 Ch 184
5.895
Public
Control Safety/
Channel Private
Ch 178 Ch 180
5.890
5.880
Public
Safety/
Private
Ch 176
5.875
5.870
Public
Safety/
Private
Ch 174
5.865
5.860
5.855
5.850
5.845
5.835
5.830
5.825
Public
Safety
V eh
eh-V
Ch 172
5.885
Downlink
12. IEEE 802.11p
Multi-channel
Control Channel (CCH):
Broadcast communication
Dedicated to short, high-priority, data and management frames:
Safety-critical communication with low latencies
Initialization of two-way communication on SCH
Service Channel (SCH):
Two-way communication between RSU and OBU or between
OBUs
For specific applications, e.g. tolling, internet access
Different kinds of applications can be executed in parallel on
different service channels
Requires the setup of a WAVE Basic Service Set
(WBSS – “Ad-hoc group”) prior to usage of the SCH
14. IEEE 802.11p
Operation modes
Operation modes
Without W VE Basic
A
Service Set (WBSS)
Safety-critical, low latency
messages and control messages
Mainly broadcast
Only on CCH
With W VE Basic
A
Service Set (WBSS)
Two-way transactions (e.g.
tolling, internet access)
Required to use a SCH
Requires initiation on CCH
In contrast to the Independent
Basic Service Set (IBSS), WBSS
does not require authentication
and association procedures
15. IEEE 802.11p
PHY
OFDM-based modulation similar to
IEEE 802.11a
Halved channel bandwidth of IEEE
802.11a:
10 MHz channels
half data rate: 3-27 Mbps
doubled symbol duration: 8.0 μs
156.25 kHz
10 MHz
16. IEEE 802.11p
PHY: Comparison to IEEE 802.11a
IEEE 802.11a
IEEE 802.11p
Data rate
6, 9, 12, 18, 24,
36, 48, 54 Mbps
3, 4.5, 6, 9, 12,
18, 24, 27 Mbps
Modulation
BPSK OFDM
QPSK OFDM
16-QAM OFDM
64-QAM OFDM
BPSK OFDM
QPSK OFDM
16-QAM OFDM
64-QAM OFDM
Error Correction Coding Convolutional
Coding with K=7
Convolutional
Coding with K=7
Coding Rate
1/2, 2/3, 3/4
1/2, 2/3, 3/4
# of subcarriers
52 net
52 net
OFDM Symbol Duration 4.0 μs
8.0 μs
Guard Period
0.8 μs
1.6 μs
Occupied bandwidth
20 MHz
10 MHz
Frequency
5 GHz ISM band
5.850-5.925 GHz
Longer guard period
Less Inter-symbol Interference
Better resistance against multipath error
Re-order of sub-carriers
Better multipath mitigation
Dedicated frequency band
Less Co-Channel Interference
17. IEEE 802.11p
MAC
Based on Distributed Control Function (DCF) with CSMA/CA
MAC-level acknowledgements for unicast communication,
but no acknowledgements for broadcast communication
unreliable broadcast communication
RTS/CTS is only used on SCH
Because of higher range, slot time and SIFS should be longer
Addressing:
IEEE
IEEE
RSUs have a fixed 48-bit MAC address
802.11a
802.11p
OBUs generate a random MAC address
Slot time 9 μs
13 μs
upon start-up of the device
SIFS time 16 μs
32 μs
If a MAC address collision occurs the
CW min
15
15
OBU automatically changes its MAC
CW max
address
1023
1023
Prioritization based on IEEE 802.11e EDCA
SIFS – Short Inter-Frame Space
(Enhanced Distributed Channel Access),
defined in IEEE 1609.4
18. IEEE 1609.4
Extension for multi-channel coordination
IEEE 1609.4 is a functional extension to IEEE 802.11e MAC to enable
multi-channel coordination
Functions:
Channel routing
Data buffers (queues)
Prioritization
Channel coordination
20. IEEE 1609.4
Channel Coordination
Each Universal Time Coordinated (UTC) second is split
into 10 Sync Intervals
Every Sync Interval is composed of alternating:
CCH Intervals: Every node monitors the CCH and
SCH Intervals: Nodes can monitor one of the SCHs
All WAVE devices have to monitor the CCH during the CCH Interval
During the SCH Interval nodes may switch to a SCH (RX or TX)
At the start of each UTC second the first Sync Interval begins
Synchronization is performed via GPS time base
21. IEEE 1609.3
Networking Services
IP-based communication:
IPv6-based with optional:
Mobile IPv6 (MIPv6) and
Network Mobility (NEMO)
enhancements
UDP or TCP on transport layer
Transmission on SCH only
Non-IP-based communication:
Based on
WAVE Short Message Protocol
(WSMP)
Transmission on CCH or SCH
No.
of
layer
4
TCP/UDP
3
IPv6
Data Plane
WSMP
2b
802.2 LLC
2a
WAVE MAC
1b
WAVE PLCP
1a
WAVE PMD
SCH
CCH/SCH
22. IEEE 1609.3
WAVE Short Message Protocol (WSMP)
Networking protocol specifically designed for V2X communications
WAVE Short Message (WSM)
structure:
WSMP can use CCH and SCH
During the SCH Interval low priority messages can be transmitted on
CCH for stations that do not switch to a SCH, high priority frames and
WAVE Announcement frames shall be transmitted during the CCH
Interval
In order to access a SCH, the nodes have to be member of the WBSS
WBSS roles:
Provider: Initiates a WBSS by sending a WAVE Announcement
User: Joins a WBSS based on the receipt of the WAVE
Announcement
23. SAE J2735
Message Dispatcher
Implementation specific
common
Implementation specific
Based on: Robinson et al. (2006): Efficient Coordination and
Transmission of Data for Cooperative Vehicular Safety Applications
24. SAE J2735
Basic message set definition
SAE J2735: Dedicated Short Range Communication (DSRC) Message
Set Dictionary
ASN.1 representation of message structures
Hierarchical definition of messages and substructures
Basic message set is not so basic any more, i.e. comprehensive:
16 different message frames, which use
54 different data frames, which are parametrized through
162 different data elements
