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Reliable Time-CriticalWireless Communication in Vehicular NetworksElisabeth Uhlemann
PhD student course in:
Communications for Cyber-Physical SystemsMälardalen University
2014 11 20
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The Connected World of Cooperative ITS
Source: CVIS, 2008
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What is Cooperative ITS?
● Intelligent Transport System (ITS) is a system whereyou blend transport with information and communication technologies (e.g., GPS, WiFi, 3G, etc.)
● E.g., intelligent bus stop signs tell you when the next bus arrives, variable speed signs
● The end user is only a consumer of information
● Cooperative ITS (C-ITS) is a subgroup of ITS, wherethe end user also provides information and does not only consume information
● E.g., vehicles transmitting position messages wirelessly toneighboring vehicles and the ego vehicle can use receivedinformation to predict an upcoming hazardous event
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Benefits with C-ITS
Tranarpsbron, E6, Östra Ljungby, Sweden
January 15, 2013 March 31, 2013
Interstate 17, Virginia, US
Around 100 vehicles involved in both accidents.
4Slide courtesy of: Katrin Sjöberg, Volvo
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Intelligent Transport Systems
Intelligent transport systems (ITS) complemented by wireless vehicular communications currently receive a great deal of attention worldwide:● Safety: reduce traffic accidents and human
injuries by introducing traffic safety applications based on vehicular communications.
● Efficiency: reduce congestion, travel-time and pollution through traffic efficiency applications.
● Value-added services: other types of services may be implemented and offered in order to facilitate system introduction and provide sustainable business and operation models.
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Traffic Safety Applications
Extend the driver’s safety margin for detection and prevention of potential accidents:
● Speed reduction and braking● Wrong way driving warning● Intersection support● Reduce crash severity● Hidden object support● Stop light assist● Overtaking vehicle warning● Traffic condition warning● Roadwork warning● Collision risk warning● Blind spot warning ● Emergency vehicle warning● Motorcycle approaching indication● Lane change assistant ● Cooperative glare reduction
Source: Car2Car-CC, 2008
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RearDetection
Blind Spot
SideCrash
LaneSupport
CollisionMitigation
Extended RearDetection &Blind Spot
CooperativeWarning
Infrastructure BasedWarning
Road Side Equipment (local or remote)
Extended Safe Distance and Speed
Extend the driver’s “Safety
Margin” to detect and prevent potential accidents
Dynamic inter-vehicle and vehicle-to-roadside network extends range of on-board vehicle systems
Lane ChangeAssistant
Source: Safespot Project, 20087
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Local Dynamic Map
● In-Vehicle Platform Output Interface
map fromprovider
landmarks for referencing
treeidpos…
com nodes,fusion result
temporaryregional info !
accidentidpos…
fogidposa,b…
congestionidposlengthdir…
egoposvel…
rsuidpostype…
vehidposveltype…
Source: Safespot Project, 20088
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Wireless Access Technologies
Variable Message Sign
Terrestrial BroadcastRDS, DAB
UMTSWiMAX
Info-Broadcaster
GSM-GPRS
Sat-Comm
COOPERS CVIS SafeSpot SISTER
BroadcastTransmitter
Vehicle-to-Vehicle (M5, IR, MM)
Hot-Spot(Wireless LAN)
GPS, Galileo
Beacon
•CALM-M5
•CALM-IR
•CEN-DSRC
Source: the Coopers, CVIS, Safespot and SISTER Projects, 2008
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Standardisation – CALM
CommunicationsAccess for
LandMobiles
● Provide user transparent continuous communications
● A set of more than 20 standards
● Formally under ISO TC204 (ITS) WG16 (Communications)
● Base standards available, rest complete 2010
● First implementation within the CVIS project
Source: ISO, 2009
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ProprietaryProtocol
B
Proprietaryor
DedicatedRadio
Interface
ProprietaryProtocol
C
Proprietaryor
DedicatedRadio
Interface
ProprietaryProtocol
A
Proprietaryor
DedicatedRadio
Interface
App. 1 App. 2 App. 3
ALL ITS
APPLICATIONS
TCP UDP
Stream &
Realtime
Protocols
ISO
DSRC
L7
HTTP/
SMTP
Protocols
MAC
802.11p
WAVE
Init
Hnd-
ovr
Secur
MAC
2G/3G
GSM
Init
Hnd-
ovr
Secur
MAC
IR
Init
Hnd-
ovr
Secur
L2/UDP
…
IPv6 layer
Service
QoS
Interface
Selection
Handover
Interface
QoS
Source: CVIS Project, 2008
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Ad Hoc or Cellular
Communications?
V2VR2V
V2V
V2R
Peer-to-peer communication Communicating via an access point
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Vehicular Ad Hoc Networks
● A Vehicular Ad hoc NETwork (VANET) has some benefits for traffic safety applications as compared to e.g., UMTS or LTE: ● it eliminates the problem of guaranteeing coverage ● it reduces the average communication delay ● no billing between VANET nodes (participation
without account)
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Ad hoc no central entity.
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Standards are a necessity!
● Standards outline how the communication should be performed, i.e., the same language must be spoken by the ones communicating
● Example of standards is WiFi, 3G/LTE, IP
● Protocols are agreements on how the wireless communication should be performed at different abstraction levels
● Standardization on Cooperative ITS has in Europe mainly been done within ETSI Technical Committee on ITS (ETSI TC ITS)
ETSI = European Telecommunications Standards Institute14
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Local area network – IEEE LAN
IEEE = Institute of Electrical and Electronics Engineering
IEEE LAN family 802
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IEEE 802.11 WLAN
WLAN = Wireless Local Area Network802.11
802.11a 802.11b 802.11g
802.11a/b/g PHY amendments
802.11e QoS
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DSRC in US – IEEE 802.11p
● 75 MHz at 5.850-5.925 GHz Intelligent Transportation Systems Radio Service (ITS-RS)
● IEEE 802.11 CSMA● IEEE 802.11e Quality of Service ● IEEE 802.11a Orthogonal Frequency Divison Multiplexing
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IEEE 802.11p
● Ratified July 2010
● PHY and MAC amendment
● No support for access points
● Peer-to-peer mode (ad hoc)
● IEEE 802.11a OFDM physical layer
● 3, 4.5, 6, 9, 12, 18, 24 and 27 Mbps
● 5.850-5.925 GHz Intelligent Transportation Systems Radio
Service (ITS-RS)
● 10 MHz channels
● 1 control channel and 6 service channels (WAVE 1609.4)
● European standard (ETSI) – ITS G5
● Worldwide standard (ISO) – CALM M5
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WAVE
WAVE = IEEE 802.11p, 1609.0, 1609.1, 1609.2, 1609.3, 1609.4 and 1609.5
WAVE = Wireless Access in Vehicular Environment
802.11pPhysical
802.11p 1609.4Medium access
LLC 802.2Logical link
IPv6 Network
WSMP
TCP/ UDP
Transport
1609.31609.2Security
WSMP – WAVE Short Message protocol
HTTP etc
Application 1609.1
1609.5 WAVECommunications Managerstarted during spring 2008.
1609.0 WAVEArchitecture
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Frequency bands for ITS
● North America● 10 MHz channels at a carrier frequency of 5.850-5.925
GHz (in total a 75 MHz wide band)● Intelligent Transportation Systems Radio Service (ITS-
RS)● WLAN look-a-like
● Europe● 5.795-5.805 GHz Road Transport and Traffic
Telematics (RTTT)● RFID look-a-like
● 5.875-5.905 30 MHz ● Japan
● 715-725 MHz available in 2012 ● WLAN look-a-like
● 5.770-5.850 GHz● RFID look-a-like with TCP/IP support
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Source: National Telecommunications and Information Administration, http://www.ntia.doc.gov/osmhome/allochrt.html
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● After less than 5 days and 31 rounds of bids, it was decided which operators got licenses in the 800 MHz band: six blocks of frequencies was auctioned out, but only 3 operators won:
● Hi3G Access, 2 x 10 MHz : Price 431 000 000 SEK
● Net4Mobility, 2 x 10 MHz : Price 469 000 000 SEK● have promised coverage improvements for another 300 000 000 SEK
● behind Net4Mobility we find Tele2 and Telenor
● TeliaSonera, 2 x 10 MHz : Price 854 000 000 SEK
● The reason why the different frequency blocks had different prices is that some blocks require adjustments of the mobile technology. Telia Sonera’s block does not require this.
22
Frequency Auction in Sweden
for 4G at 800 MHz band in 2011
Source: NyTeknik
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-70-60-50-40-30-20-10
0102030
5450 5500 5550 5600 5650 5700 5750 5800 5850 5900 5950
dB
m/M
Hz
ITS Spectrum in Europe
CE
N D
SR
C
Common Control Channel, Broadcast data, 10MHz@5900 (ch180) (Used by all)
Vehicle-Roadside data, 10MHz@5890 (ch178) (Safespot/C2C-CC, not used initially)
Service Channel: GeoRoute multihopping, 10MHz@5880 (ch176) (Safespot/C2C-CC)
Aux Channel for roadside initiated data, 20MHz @5480+ (ch96) (CVIS/COOPERS)
Blue line represent European spectrum mask for BRAN (conditional use for ITS)Red line represents European spectrum mask for ITS 5.9
V2
V (IS
M b
an
d)
V2
R –
re
str
icte
d e
mis
sio
n
SC
H: V
2R
-r
es
tric
ted
S
CH
: Ge
oR
ou
te M
ultih
op
CC
H
Au
x c
ha
nn
el –
R2
V
ITSBroadband Radio Access (BRAN)
Source: COMeSafety, 2008
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Characteristics of VANETs
●Dezentralized network topology● No central controller like an access point or a base station
● Peer-to-peer communication
●Can contain roadside units (RSU)
●Share a common communication channel
●Broadcast● Traditional Automatic Repeat reQuest (ARQ) cannot be used
●The number of participating nodes in a VANET cannot be restricted
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Reliable, Real-Time Communication
● The wireless communications protocols available today enables either reliable communications with low error rate or time-critical communications with real-time constraints – but not integrated high levels of both:● Voice has real-time requirements
but is relatively error tolerant. ● E-mailing requires reliable
communications but is delay tolerant. ● Control traffic on a fieldbus has
requirements on both reliability and real-time but the fieldbus is wired.
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Examples of Communication
Requirements
bit error probability: < 0.1 %delay: < 100 ms
data rate: 10-15 kbit/s
bit error probability: < 10-9
delay: < 1 daydata rate: irrelevant
bit error probability: < 10-5
delay: < secondsdata rate: < 80 Mbit/s
Cell phones:
E-mailing:
Mobile broadband:
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Retransmissions
42
What?
42
Ok
• When humans cannot hear we ask the speaker to repeat
• Computers can do the same.
• The reliability increases (the packet error rate reduces)
• The delay increases
• The deadline limits the number of attempts
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Identified Problems
● Mobile nodes that moves quickly
● Retransmissions may not be possible
● Broadcasted messages with several recipients
● If 75% got the message should we retransmit or not?
● The lower delay, the longer the time for the driver to react
● We cannot suddenly loose coverage
● Ad hoc network
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Scalability
V2VR2V
V2V
V2R
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Medium Access Control
● It two vehicles transmit their position messages at the same time it is difficult for anyone to hear anything…
I’m here
I’m thereOh no!
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Predictable MAC for Voice
● GSM (2G) uses TDMA ● Predictable
● Fair
● Not scalable
● Centralized (hardware can be added to improve scalability)
● UMTS (3G) uses CDMA● Predictable
● Can be unfair
● Scalable (graceful degradation)
● Centralized (hardware can be added to improve scalability)
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● Currently the only standard supporting VANET is IEEE 802.11p using CSMA
● Listen-before-talk protocol:When the node wants to send a packet it starts by listening to the channel to determine if it is free. If free, the node sends directly, otherwise the node has to perform a backoff.
● Suitable for lightly loaded networks with bursty (event-driven) data traffic
● Pros:● Supports arbitrary packet sizes
● Low complexity, e.g., no need for synchronization
● Cons: ● Unpredictable
● Unfair (scales badly)
802.11pPHY
802.11p 1609.4MAC
LLC 802.2LLC
IPv6 Net.
WSMPTCP/ UDP
Trans.
HTTP etc
App.1609.1
Carrier Sense Multiple Access withCollision Avoidance (CSMA/CA)
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Challenges for the MAC Layer
● Predictable access guaranteeing timely channel access delay in decentralized VANET
● The number of nodes participating in a VANET cannot be restricted – fairness and good scalability is needed.
● Interference depends on data collisions which in turn depends on the distance to the nearest concurrent transmitter.
● If a vehicle is not successful in transmitting its periodic update during a time interval, it becomes “invisible” to the surrounding vehicles.
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Identified Problems
● Unpredictable channel access delay● Periodic messages need to be sent within its time
period
● The random backoff may cause a delay longer than the time period
● Causes packet drops at sending node
● Collisions ● The random backoff time chosen are discrete and
thus nodes may choose the same● For example in 802.11e highest priority have only 4
values {0 µs, 13 µs, 26 µs, 39 µs}
● Two concurrently transmitting nodes may be located very close together
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CSMA/CA in 802.11p
Sensing range=1000 mPacket size = 500 byteReport rate = 10 HzApprox. 230 nodes
Packet drops/Deadline miss ratio
84%
23%
52% Channel access delay
Slide courtesy of: Katrin Sjöberg, Volvo
Overloaded network!
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● Cooperative Awareness Messages:
● Hard deadlines but Medium reliability
● Hazard Warning Messages:
● Soft deadlines but High reliability
● A medium access method that is:
● Decentralized (works in ad hoc)
● Self-organizing (works even when nodes move)
● Fair (lets everybody talk at even intervals)
● Predictable (does not cause unexpected delays)
● allows every other cooperative awareness message through and enables repetition of hazard warnings
Desired Solution
36
Picture courtesy of Volvo
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Self-Organizing Time Division
Multiple Access (STDMA)
● Already in commercial use: ● Automatic Identification System (AIS)
● VDL mode 4
● Specially designed for periodic messages, LDM is similar to AIS● In overloaded situation “collisions” are scheduled to minimize
interference
● Pros:● Predictable: always channel access regardless of the number of nodes in
the network – no packet drops!● Scalable: The network can be overloaded up to 700% without collapsing –
fair access is provided
● Cons: ● Packet sizes are fixed ● Higher complexity, e.g, needs synchronization through a GNSS
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Self-Organizing TDMA (Håkan Lans)
NI=nominal increment, SI=selection interval, NSS/NS=nominal slot, NTS=transmission slot
SI SI
NI
….. …..
SI
NI
….. …..
SI
NI
SI
NI
NSNSS
NS NSS NS
Node 1
Node n
.
.
.
.
.
NTSNTS
NTSNTS
NTS
…..NI
…..
…..NI
…..
Choose a free slot within SI. If all slots in SI are occupied we pinch a slot from the node situated furthest away from us.
38Slide courtesy of: Katrin Sjöberg, Volvo
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Channel access delay
300 byte packets, 6 Mbps, 10 Hz,
Channel access delay is not a problem in STDMA!
39Slide courtesy of: Katrin Sjöberg, Volvo
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Interference Distance
Probability that two nodes initiatea transmission at the same time.
2 Hz, 800 byte, 6 Mbps
40Slide courtesy of: Katrin Sjöberg, Volvo
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TxTx
Distance betweenconcurrent transmissions
● In CSMA concurrent transmission takes place randomly in space
● Higher interference level locally for the nodes situated closest to the transmitters
● Receivers cannot decode the packets correctly due to the high interferencein the system
Tx Tx
• In STDMA the position information sent is used for schedulingconcurrent transmission to be as far apart as possible in space
• The nodes closest to the transmitters are ”protected”, and can hopefullydecode the packet correctly
STDMACSMA
41Slide courtesy of: Katrin Sjöberg, Volvo
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Research Challenges in
Vehicular Ad Hoc Networks
● Todays' wireless access technologies are not optimized for VANETs. To function properly, the communication needs to be predictable, fair, scalable and reliable.
● Challenges:● Ad hoc network with inherent interference and scalability
problems● Concurrent requirements on real-time and reliability● Traditional performance metrics not directly applicable
● Since communication can never be made 100% robust –the application has to be! However, reliability can be traded off for autonomy: ”Inform – Warn – Advice – Guide – Steer”.
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Typical Communication Requirements
● Traffic safety applications relying on cooperative driving systems have exceptionally challenging requirements:
● Low-delay, ● reliable, ● time-critical,● scalable, ● wireless communication
is to be provided in a harsh, dynamic environment.
Source: CVIS http://www.cvisproject.org/
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Message Types for VANETs
● Hazard Warning Messages:
● Event-driven warning messages are broadcasted only in the event of a hazard.
● Requires very high reliability – each warning must be delivered.
● A lower delay gives the driver more time to react.
● Cooperative Awareness Messages:
● Time-triggered position messages are broadcasted periodically.
● Since the messages are repeated periodically and do not signal imminent hazard the requirements reliability are moderate.
● However, to avoid invisible, mute vehicles, not too many consecutive messages can be lost.
● The maximum delay should be related to the message period.
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Real-Time Communications
● A real-time system interacts with a physical system.
● This means that it is not only the result itself that is important but also when in time it is presented.
● An application with real-time requirements therefore has a deadline to meet.
ttdl
Usefulness of
presented value
ttdl
Usefulness of
presented value
Hard deadline Soft deadline
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● Cooperative Awareness Messages:
● Time-triggered position messages are broadcasted periodically.
● Hazard Warning Messages:
● Event-driven warning messages are broadcasted only in the event of a hazard.
ttdl
Usefulness of
presented value
ttdl
Usefulness of
presented value
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● Cooperative Awareness Messages:
● Time-triggered position messages are broadcasted periodically.
Hard deadlines Medium reliability
● Hazard Warning Messages:
● Event-driven warning messages are broadcasted only in the event of a hazard.
ttdl
Usefulness of
presented value
ttdl
Usefulness of
presented value
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● Cooperative Awareness Messages:
● Time-triggered position messages are broadcasted periodically.
Hard deadlines Medium reliability
● Hazard Warning Messages:
● Event-driven warning messages are broadcasted only in the event of a hazard.
Soft deadlines High reliability
ttdl
Usefulness of
presented value
ttdl
Usefulness of
presented value
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Qualitative Requirements
● So, how low-delay, reliable, and scalable, does the deadline dependent communication need to be?
● Well, that depends…
Source: CVIS http://www.cvisproject.org/
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Problem #1: What is to be communicated?
●How long are the messages?
●How often is something sent?
●What QoS requirements do the messages have?
●What happens if a message is lost?
●How many consecutive messages can be lost?
Source: SAFESPOT, http://www.safespot-eu.org/
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Different Realizations
● Hazard Warnings: Information is communicated only in the event of a hazard. Event-driven messaging with requirements on very low delay and very high reliability (the hazard is imminent and the data is critical). However, the system is useful also at moderate penetration rates since it can be implemented as an enhancement of existing sensors.
● Cooperative Awareness: Requires that all vehicles are equipped with a communication system and periodic time-triggered messages are broadcasted. Since the messages are repeated periodically and do not signal imminent hazard the requirements on delay and reliability are moderate. However, in order to avoid a system with invisible, mute vehicles, a very high penetration is likely needed
● Cooperative Autonomous Driving: Not only is this system required to warn or predict and advice – but also to act. Typically both periodic time-triggered and event-driven messages are needed, requiring some sort of service differentiation between the two types of messages, and very likely close to 100% penetration will be necessary.
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Observation or Evaluation?
● The control loop that is using the communicated data consists of four steps: observation, evaluation, decisionand action.
● The communicated messages could either be raw data (observation) or some level of refined data (evaluation) or even the actual driver intentions.
● Transmitting processed data is likely to require less bandwidth but higher penetration rate such that decisions are made based on the same information.
● When penetration increases, more and more processed data could be transmitted to compensate for the increase in required collective bandwidth.
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Problem #2: How is the information
used?
● What can the local dynamic map be used for? Warnings? Predictions? Advice?
● How trustworthy is the information?
● How do we make sure that the information indeed helps rather than confuses the driver?
AIDE truck platform based on Volvo FH12Source: AIDE, http://www.aide-eu.org/ Source: CVIS http://www.cvisproject.org/
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Reliability versus Autonomy
● An issue that greatly influences the requirements of the wireless communication is the selected level of autonomy.
● The more reliable the communication system is, the higher autonomy can be given to the application:
● ”Inform – Warn – Advice – Guide –Steer”
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Problem #3: Do Cooperative Driving
Systems Offer Improvements?
● How do we know that safety is improved?
● How do we verify that these distributed systems function as intended?
● If something goes wrong – who's fault is it?
● Do people want this?
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Validation and Verification
● Information theory has provided a scientific foundation for the development of both the internet and cellular phones. But: ● performance limits of decentralized wireless networks are
presently not known, ● a central concept for information theory is capacity,
characterized by the maximum reliable throughput between two mobile terminals
● In real-time communication systems, traditional performance measure throughput is of less importance and the performance measure “deadline miss ratio” is instead used.
● However, even this performance measure needs to be redefined in due to the broadcast nature for both event-driven and time triggered realizations.
● Successful message reception should instead be defined as a function of the number of vehicles in communication range and interest range.
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Platooning
• First step towards autonomous driving
• Platoon of vehicles (e.g. trucks) on a highway
• Vehicles follow a leading vehicle with minimum distance
++ reduced fuel consumption
+ increased safety and efficiency
– tighter control needed
• Exchange of status data and hazard warnings between
platoon vehicles
• Relatively stable topology
Fast and reliable inter-vehicle communication
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Platooning
58
https://www.youtube.com/watch?v=tasa3D1vVTc
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Challenges of Cooperative Driving
• Ideally
– Always up-to-date status information about every
surrounding vehicle
– Event warning dissemination with minimum delay
• Challenge
– Packet loss over wireless medium
– Packet drops due to limited, shared bandwidth
No channel access
Lost or erroneous packet
DENM
59Slide courtesy of: Annette Böhm, Halmstad University
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Performance Measure: CAM
• CAM
• Goal: Every vehicle is informed about the up-to-
date status of its surrounding platoon members at
any given time
• Performance measure: CAM Age
•What is the worst-case inter-arrival time of
CAMs from a specific neighbor?
•Measured from the point in time when a vehicle
receives a CAM from a specific neighbor until
the point in time when the next CAM from that
neighbor is successfully received.
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Performance Measure: DENM
• DENM
• Goal: Every vehicle in the platoon is warned of
unexpected situations with minimal delay
• Performance measure: DENM Dissemination Delay
• How fast can a hazard warning be spread throughout
the entire platoon?
•Measured from the time when one vehicle in the
platoon detects an event (e.g. the platoon leader
detecting the need to break immediately) until the
point in time when the entire platoon received a
warning about the event
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MAC Comparison
a) Decentralized– Standard-compliant using 802.11p MAC and send rates
– CAM and DENM coexist
– Dedicated ITS Control Channel
b) Centralized– MAC method, message types and send rates not acc. to
standard
– CAM and DENM separated
– Dedicated Service Channel
CCH
SCH
62Slide courtesy of: Annette Böhm, Halmstad University
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Proposed Centralized MAC
What is the suitable SF size and CBP/CFP ratio to support
guaranteed channel access for CAMs and reasonable
dissemination delays for DENMs?
• 2 steps
1.Determine CBP size based on knowledge of typical
DENM Dissemination Delay for the platoon length at
hand (e.g. from simulation)
2.Determine CFP size by applying real-time
schedulability analysis
63Slide courtesy of: Annette Böhm, Halmstad University
Superframe
Collision-Free PhaseCFP
Contention-Based PhaseCBP
CFP/CBP ratio adapted to current CAM traffic
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DENM Dissemination Strategy
• How to stop dissemination of DENMs in a platoon?
• No acknowledgements of successful packet reception, i.e.
no way for a sending node to know who has and has not
received the packet
• Dissemination strategies
• Repeat 1 time
• Repeat 5 times
• Repeat until DENM is received from vehicle situated
further back in the platoon
64
DENM disseminationCAM Exchange
Platoonleader
Control vehicle for centralized channel access control
Slide courtesy of: Annette Böhm, Halmstad University
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Delay in Communications
● Channel access delay
● Time from message creation until access is granted to access the channel
● Must be upper bounded
● Depends on the MAC method
● Overall delay
● The time from message creation until the message is successfully received by the destination(s)
● Can never, ever, be upper bounded
● Depends on the channel access delay and packet error rate
● Can be improved by diversity (redundancy)
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The Wireless Channel
● The resulting error rates are generally higher
● The noise level varies with time, space and frequency
● The pathloss is considerable (and frequency dependent)
● Shadowing and multipath fading affect the error rates
● The medium has inherent broadcast nature
● The link capacity is limited by interference from other users (intentional or non-intentional)
Transmitter
ReceiverLOS
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Propagation Mechanisms
Slide 67
● Reflection on smooth surfaces
● Transmission through objects
● Scattering on rough surfaces● Diffraction around sharp
edges
● Line-of-sight paths are not necessarily needed for communication
● Increasing frequency gives● more “optical” propagation● smaller antennas● higher path loss
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If there is a LOS path, the fading h(t) is Rician distributed.
If there is no LOS the fading h(t) is Raylegih distributed
Unknowns:
● The number of paths, N
● The pathloss, alpha, for each path
● The delay, tau, for each path
● If the receiver is mobile each path also has a Doppler shift, phi
Statistical Multipath Model
2 ( )
0
( ) Re ( )c i Di
Nj f t
i i
i
r t s t e
Transmitter
Receiver
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Multipath Fading
● The path loss depends on the distance between transmitter and receiver.
● The signal is scattered and different parts of the signal travel different distances to reach the receiver.
● At the receiver, the signals interfere with each other, constructively or destructively.
● The result is a fading channel.
0 200 400 600 800 1000 1200 1400 1600 1800 2000-35
-30
-25
-20
-15
-10
-5
0
5
10
time
att
enuation
Transmitter
Receiver
( ) ( ) ( ) ( )r t h t s t n t
69
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● If both the receiver and the transmitter moves, everything becomes dependent of time:
● The paths, but also the number of paths changes with time, N(t)
● The pathloss component, alpha, varies with time for each path
● The delay, tau, varies over time for each path
● The Doppler shift, phi, varies with time
( )
2 ( ( )) ( )
0
( ) Re ( ) ( ( ))c i Di
N tj f t t t
i i
i
r t t s t t e
Multipath Fading for Mobile Communications
70
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020
4060
80100
0
5
10
15
20-40
-30
-20
-10
0
10
timefrequency
att
enuation
spacefrequency
@time = 1a
tte
nu
ati
on
[d
B]
Channel Characterization
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Slide 72
Source: Albin Dunand, Jean-Marc Conrat, France Telecom Research & Development,
COST2100 TD(08)406, Carrier freq. 2.2 GHz, bandwidth 62.5 MHz, downtown Mulhouse, France
0 360°
Diffraction over roof top
Transmission/scattering
through trees
ReflectionStreet waveguide
(guided reflections) Scattering
72
Channel Characterization
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Slide 73
Fading
● Constructive and destructive interference of multiple propagation paths gives radically different received signal strength as we move around
● With many propagations paths and no strong a line-of-sight component, the received power changes dramatically by moving about half a wavelength
● The wavelength is83 10
5 cm for 6 GHzc
c c
cf
f f
• At 15 m/s (54 km/h), a wavelength at 6 GHz corresponds to 3ms
• It can be challenging to maintain reliable links in fading
• Remedies: error correcting codes, multiple antennas, wideband transmission…
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Time-Frequency Response
Slide 74
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Time and Frequency Selective Fading
020
4060
80100
0
5
10
15
20-40
-30
-20
-10
0
10
timefrequencyatt
enuation
• The fading will vary with time and frequency.
• Time diversity is one way to conquer fading. ARQ is an example of this.
• OFDM and DMT is a way to conquer frequency selective fading.
• Both examples may use the same error correcting code, but the redundancy is spread over time and/or frequencies.
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Diversity for Increased Rate, Range or Reliability
● If retransmissions are used (time diversity) the range or alternatively the reliability is increased, but so is the jitter.
● Frequency hopping (frequency and time diversity) increases reliability in environments where frequency selective fading occurs, but also increases complexity (cost)
● Multiple-input-multiple-output (MIMO) systems (spatial diversity) gives a higher reliability, higher rate/throughput or longer range – but also increases complexity and has some form factor requirements (i.e., the antennas need to be located at least half a wavelength apart: 6.25 cm for 2.4 GHz and 3 cm for 5 GHz).
● Laptops can have antennas 6.25 cm apart – but most phones cannot.
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● Would you hear better if you had four ears?● What if both your ears were located on the same side of
the head?
● MIMO gives: ● Increased throughput● Greater range
● Cell size versus transfer rate
77
MIMO – Multiple Multiple Output
1 feet = 0.3 m225 feet = 70 m
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Retransmissions for Increased Reliability
42
What?
42
Ok
78
● When humans cannot hear, we ask the speaker to repeat
● Computers can do the same.
● The reliability increases (the packet error rate reduces)
● The delay increases
● A deadline limits the number of attempts
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Retransmissions
42What?
He said 42 Ok
• Retransmit at another time
• Retransmit on another frequency
• Retransmit at another place, relaying
020
4060
80100
0
5
10
15
20-40
-30
-20
-10
0
10
timefrequency
att
enuation
79
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80
Co-Existing Event-Driven and Time-Triggered
. . . . . . . . . . . . . . .
Last slot of previous hyperframe
First slot in current hyperframe
Beacon, vehicle 1 Beacon, vehicle 2 Beacon, vehicle N
Last slot in
current hyperframe
First slot of
new hyperframe
available slots for event -driven messages
Slide courtesy of: Le Nam Hoang, Halmstad University
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Relaying in Platooning
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One time-slot: only one option 1 -> 12, Pe = 0.2337, after 0.546 ms
Two time-slots: 1 -> 4 -> 12 or 1 -> 9 -> 12; Pe = 0.0265, after 1.092 ms
1 -> 1 -> 12; Pe = 0.0546
Three time-slots: 1 -> 4 -> 9 -> 12; Pe = 2.1 10-4, after 1.638 ms1 -> 1 -> 1 -> 12; Pe = 0.0128
Four time-slots: 1 -> 4 -> 6 -> 9 -> 12; Pe = 2.18 10-7, after 2.184ms
1 -> 1 -> 1 -> 1 -> 12; Pe = 0.003
Slide courtesy of: Le Nam Hoang, Halmstad University
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Minimizing Dissemination Delay in Platooning
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Slide courtesy of: Le Nam Hoang, Halmstad University
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Research Challenges
● To function properly, the communication needs to be predictable, fair, scalable and reliable.
● Challenges:● Decentralized ad hoc networks ● Inherent interference and scalability problems● Concurrent requirements on real-time and reliability● Traditional performance metrics not directly applicable
● Delays in Communications
● Channel access delay● Depends on the MAC method
● To schedule this needs to be upper bounded
● Overall delay● Can never, ever, be upper bounded
● Depends on the channel access delay and packet error rate
● Only probabilistic guarantees can be given
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Key References
● Journals:● Larson, J.; Liang, K.-Y.; Johansson, K.H., "A Distributed Framework for Coordinated Heavy-Duty
Vehicle Platooning," IEEE Transactions on Intelligent Transportation Systems, no.99, pp.1-11, 2014.● Bansal, G.; Kenney, J.B., "Controlling Congestion in Safety-Message Transmissions: A Philosophy for
Vehicular DSRC Systems," IEEE Vehicular Technology Magazine, vol.8, no.4, pp.20-26, Dec. 2013.● A. Böhm, M. Jonsson and E. Uhlemann, “'Performance comparison of a platooning application using the
IEEE 802.11p MAC on the control channel and a centralized MAC on a service channel,” in Proc. IEEE International Conference on Wireless and Mobile Computing, Networking and Communications, Lyon, France, October 2013, pp. 545-552.
● Kenney, J.B., "Dedicated Short-Range Communications (DSRC) Standards in the United States," Proceedings of the IEEE , vol.99, no.7, pp.1162-1182, July 2011
● Mecklenbrauker, C.F.; Molisch, A.F.; Karedal, J.; Tufvesson, F.; Paier, A.; Bernado, L.; Zemen, T.; Klemp, O.; Czink, N., "Vehicular Channel Characterization and Its Implications for Wireless System Design and Performance," Proceedings of the IEEE , vol.99, no.7, pp.1189-1212, July 2011
● Alexander, P.; Haley, D.; Grant, A., "Cooperative Intelligent Transport Systems: 5.9-GHz Field Trials," Proceedings of the IEEE , vol.99, no.7, pp.1213-1235, July 2011
● Sepulcre, M.; Mittag, J.; Santi, P.; Hartenstein, H.; Gozalvez, J., "Congestion and Awareness Control in Cooperative Vehicular Systems," Proceedings of the IEEE , vol.99, no.7, pp.1260-1279, July 2011
● Kaul, S.; Gruteser, M.; Rai, V.; Kenney, J., "Minimizing age of information in vehicular networks," 2011 8th Annual IEEE Communications Society Conference on Sensor, Mesh and Ad Hoc Communications and Networks (SECON), 27-30 June 2011, pp.350-358.
● Hong, Kezhu; Kenney, J.B.; Rai, V.; Laberteaux, K.P., "Evaluation of Multi-Channel Schemes for Vehicular Safety Communications," 2010 IEEE 71st Vehicular Technology Conference (VTC 2010-Spring), 16-19 May 2010, pp.1-5.
● Bilstrup, E. Uhlemann, E. G. Ström and U. Bilstrup, “On the Ability of the 802.11p MAC Method and STDMA to Support Real-Time Vehicle-to-Vehicle Communications,” EURASIP Journal on Wireless Communications and Networking, vol. 2009, Article ID 902414, 13 pages, 2009.
● Theses:● Michele Rondinone, “Connectivity-based Routing and Dissemination Protocols for Vehicular Networks,”
PhD Thesis, Universidad Miguel Hernandez de Elche, Spain, November 2013.● Annette Böhm, “Delay-Sensitive Wireless Communication for Cooperative Driving Applications,” PhD
Thesis, Halmstad University, Sweden, June 2013.● Katrin Sjöberg, “Medium Access Control for Vehicular Ad Hoc Networks,” PhD Thesis, Chalmers
University of Technology, Sweden, April 2013.
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Acknowledgements
● This presentation would not have been possible without lots of input from:
● Katrin Sjöberg, Volvo Group Trucks Technology
● Annette Böhm, Halmstad University
● Le Nam Hoang, Halmstad University
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