SOTIS: A Self-Organizing Traffic Information System based on Car-2 ...

<strong>SOTIS</strong>: A <strong>Self</strong>-<strong>Organizing</strong> <strong>Traffic</strong> <strong>Information</strong>
<strong>System</strong> <strong>based</strong> on Car-2-Car Communications
Hermann Rohling ∗ and Holger Busche ∗∗
Department of Telecommunications
Hamburg University of Technology, Germany
Emails: ∗ rohling@tu-harburg.de, ∗∗ busche@et2.tu-harburg.de
Abstract—Inter-Vehicle Communication (IVC) is an important
research and future application topic which experiences increasing
attention from all major car manufacturers. This paper
describes a <strong>Self</strong>-<strong>Organizing</strong> <strong>Traffic</strong> <strong>Information</strong> <strong>System</strong> (<strong>SOTIS</strong>)
which is purely <strong>based</strong> on IVC and which does not need any
expensive infrastructure. A <strong>SOTIS</strong> car is equipped with a satellite
navigation receiver, a digital map and a communication unit. The
new traffic information system <strong>based</strong> on <strong>SOTIS</strong> technology will
give the driver many benefits even in situations where only a very
small percentage of cars are equipped with the <strong>SOTIS</strong> technology.
Index Terms—inter-vehicle communication, car-to-car communication,
traffic information system
I. INTRODUCTION
Conventional <strong>Traffic</strong> <strong>Information</strong> <strong>System</strong>s (TIS) are mainly
organized in a centralized signal processing structure as illustrated
in Figure 1.
<strong>Traffic</strong> <strong>Information</strong>
Center
Cars with radio receiver or cellular transceiver.
Cars with <strong>SOTIS</strong> system.
Radio broadcast station or
cellular base station.
Fig. 1: Conventional centralized traffic information system.
In this case a large number of traffic monitoring sensor
devices, which are directly mounted at highway bridges,
measure and collect floating car data (FCD), like traffic density
and average velocity. These data are used to characterize the
current traffic situation. All these data are transferred to a
central <strong>Traffic</strong> <strong>Information</strong> Center (TIC), where the current
road situation analysis is carried out. Finally, the result of this
situation analysis is packed into data packets and will be transmitted
to the driver via FM radio or mobile communication
networks.
The service of such a centralized traffic information system
has some advantages but also several technical disadvantages:
• A large number of sensors is needed to get a full coverage
of all streets. <strong>Traffic</strong> information service is available only
on these streets where sensors have been mounted and
where FCD have been measured. <strong>Traffic</strong> information is
therefore not available in all cities for the time being.
• The recorded FCD is transmitted to the TIC for a situation
and traffic analysis. This procedure causes a relatively
long delay (typically 20-50 minutes) before the result of
the situation analysis is broadcasted to the driver.
• The centralized TIS is therefore not suited for timecritical
messages, e.g. some emergency notifications. In
this case a direct car-to-car communication technology is
needed.
• Since a central unit covers a relatively large area, only
major traffic events are reported and announced to the
driver.
• The centralized TIS is expensive. If the traffic information
is transmitted via mobile phone for example, service
charges have to be paid.
For all these reasons, an alternative and completely different
TIS is proposed in this paper. This new system is <strong>based</strong> on
the <strong>SOTIS</strong> technology and is organized in a pure decentralized
and self-organized way. In this case the large number of traffic
Radio broadcast station or
sensors and the centralized TIC are not needed at all. Instead
cellular base station.
a direct car-to-car communication technology is established
<strong>Traffic</strong> <strong>Information</strong>
which isCenter Center the basis for a car-to-car traffic information exchange.
In this case, vehicles inform each other about the local traffic
situation inside the radio range. The considered situation is
illustrated in Figure 2. There is absolutely no traffic sensor
infrastructure needed in the <strong>SOTIS</strong> technology case, each car
is measuring the FCD, like average velocity, itself.
Cars with radio receiver or cellular transceiver.
Cars with <strong>SOTIS</strong> system.
Fig. 2: Decentralized and self-organizing traffic information
system.
<strong>SOTIS</strong> technology applies a completely decentralized and
self-organized procedure. Based on a digital map which is
divided into several road segments, the current traffic situation
is not reported to a central TIC but will be alternatively
analyzed on board of each car.

II. <strong>SOTIS</strong> SYSTEM
The general idea of the <strong>SOTIS</strong> system is to collect all
available FCD, especially the current average speed inside
all adjacent road segments, analyze the traffic situation autonomously
and distribute this knowledge by standardized data
packets into the car-to-car communication network [1].
Therefore, each individual vehicle collects specifically for
each road segment all available traffic and especially speed
information from its own speed measurement and from data
packets which have been received from cars in the local
environment. After the traffic situation analysis each car is
going to transmit for example the average speed information
for all individual road segments of the local environment into
the IVC network by broadcast techniques.
<strong>SOTIS</strong> offers very accurate and detailed traffic information
(e.g. traffic jam length, expected time delay) inside the local
environment with an extremely low time delay even in traffic
situations where only a very few percent of cars are equipped
with the <strong>SOTIS</strong> technology. The radio range of each transmitter
is approximately 1 km. But it will be shown in this
paper that the resulting traffic information range inside the
IVC network is approximately 50 km and more.
Two relevant <strong>SOTIS</strong> applications are:
1) Safety <strong>System</strong>: In case of an accident an emergency data
packet will be transmitted automatically. This information is
either presented to the driver or will be used directly to activate
the brake system or throttle control automatically.
2) Comfort and Route Guidance <strong>System</strong>: This type of SO-
TIS application improves passenger comfort, traffic efficiency
and/or optimizes the traffic route. Examples for this category
are: traffic information system <strong>based</strong> on FCD, weather information,
location of gas stations and restaurants including price
information, interactive communication such as Internet access
or music downloads.
It should be taken into account that in any application case
only a very small number of cars will be equipped with the
<strong>SOTIS</strong> technology at the beginning. But nevertheless even in
this low market penetration situation the driver should benefit
from the discussed application.
The objective in this section is to describe a traffic information
system which has a certain system performance and
benefit for each driver in real traffic applications even if the
market penetration of <strong>SOTIS</strong> equipped cars is still quite low.
Furthermore, if critical changes in the traffic situation will
occur (e.g. an accident), an emergency packet is instantly
transmitted and broadcasted to all vehicles inside the radio
transmission range. In this special case, a small data packet
including the precise position of the emergency is transmitted
with high priority. The comfort application of <strong>SOTIS</strong> is
structured in three different parts.
A. Data Aggregation
It is assumed in this paper that each car has a digital map
where each road is divided into several road segments of
approximately 500 m length. Each car measures per road segment
the current speed and collects all FCD traffic information
<strong>based</strong> on the received data packets from other cars in the local
environment.
If for a road segment s the current velocity Vs has been
measured by the individual car or has been reported by a data
packet of an adjacent car, the average velocity information
for this specific road segment will be updated in a recursive
procedure.
In addition, a road segment specific time stamp indicates the
last update time. Relevant data will be further processed and
expired data are deleted. Thus, a single velocity information
�Vs and a time stamp ts per road segment s will be recorded
for each road segment.
B. Data and Situation Analysis
Each vehicle receives data packets with road segments
information from all cars in local radio range. Figure 3
illustrates the data packet structure. The data packet contains
the average velocity and a time stamp inside each road segment
for a fixed number of road segments. Therefore, the data packet
length and the data packet structure is identical for all nodes.
Each data packet covers all road segment information in the
local environment of 50 km radius for example. From this
information the current average velocity and other road traffic
features are calculated and recorded.
Packet
Header 1 1
Vˆ , t Vˆ 2, t Vˆ L
2 3, t3
Fig. 3: <strong>SOTIS</strong> data packet.
Vˆ N, tN
The measured and received traffic information will be
analyzed inside each individual car and all traffic information
are updated continuously, see equation (1). The time stamp
indicates whether the traffic data are still relevant or have
already expired.
�Vs,new = (1 − α) · �Vs,prev. + α ·Vs
(1)
The updated new average velocity is indicated by �Vs,new
and �Vs,prev. is the previous calculated average velocity for road
segment s. The parameter α is between [0,1].
C. Data Dissemination
The segment specific average velocity information �Vs and
the time stamp are transmitted by local broadcast. Figure 4
shows an example where each vehicle considers the road
segment specific average velocity information Vs for each road
segment s. The information range of the <strong>SOTIS</strong> system is
much larger than the radio range. This fact is illustrated in
the following figure.
Vehicle A and vehicle C are driving on the same road
and in the same direction. Vehicle B drives in the opposite
direction and is currently inside the radio range of vehicle C.
For that reason it will receive all road segment information
from vehicle C. Later vehicle B is inside the radio range with
vehicle A which will be informed by vehicle B about all
road segments traffic conditions which have been originally

t=t 1
t=t 2
90
91
95 50
A
Data packet: … 90 20 10 5
91 95 92 93 98 96 99
Distance > Tx-range
95 50 100 99
Data packet: … 90 20 10 5
90
20 10 5
FCD value, available for car A FCD value, not available for car A
Fig. 4: <strong>SOTIS</strong> data distribution in cases of low market penetration.
sensed by vehicle C. In this case vehicle A has all average
velocity information for the next road segments in his driving
direction available which increases the traffic information
range dramatically.
III. <strong>SOTIS</strong> SYSTEM HARDWARE STRUCTURE
The <strong>SOTIS</strong> functional structure components inside each
vehicle is depicted in Figure 6. A digital map with the detailed
road segment information is needed. A satellite receiver is
integrated into the <strong>SOTIS</strong> system which gives the current
position and road segment location. Finally a radio system
for a car-to-car communication functionality is necessary.
Each car and individual node is going to update the traffic
information for each road segment <strong>based</strong> on the measured
velocity or reported traffic information from all other adjacent
cars [3] and [2]. Each data packet contains the traffic information
for all road segments in the local environment of the
vehicle. If the time stamp shows an expired time the related
traffic information will not be used in the update process.
It is very important to notice that the traffic situation
analysis is processed continuously inside each car <strong>based</strong> on the
currently available traffic information stored in the individual
knowledge base. In a conventional and centralized TIS the
situation analysis will be processed in the central unit but in
<strong>SOTIS</strong>
Knowledge
Base
Memory of of available
traffic information
Road ID
CAN Bus Interface
Map
Vehicle
Speed
B
<strong>SOTIS</strong> Core
FCD processing,
data packet generation
UDP
Broadcast,
Port X
Air Interface
(WLAN)
User Interface
Visualization,
User Interaction
C
Satellite
Nav.
Geo.
Position
Fig. 6: <strong>SOTIS</strong> system implemented in each vehicle.
B
TABLE I: Parameters used in the traffic simulation.
Road segment length 500 m
Number of lanes 2 per direction
Deceleration prob. 0.1
Constitution of traffic 15 % slow, 85 %regular vehicles
Desired velocity 108 (slow), 142 km/h (regular)
Number of vehicles 1600, 1100, 850 (cars/h/lane)
Average Velocity 110 km/h
Radio range 1000 m
Update rate 2 s
<strong>SOTIS</strong>, the situation analysis is done in each car and in a
decentralized way.
The analysis results are included into the next broadcasted
data packet. The broadcast update rate can be additionally
adapted to the local conditions to avoid overload situations
in the radio channel [7], [6].
IV. ROAD TRAFFIC SIMULATION
In a first step of the simulation the road traffic and movement
of individual cars will be described by a mathematical
model. In this respect the typical movement pattern in an
inter-vehicle network is completely different in comparison
with general wireless network simulations. Therefore, the ns-
2 simulator has been extended by a movement model which is
<strong>based</strong> on a traffic simulation tool using a cellular automaton
approach [4] and [5].
The considered street scenario which has been assumed in
this paper simulates a regular highway situation with two lanes
per direction.
Table I lists the road and system parameters used for
traffic simulation. Arrival times are assumed to be Poisson
distributed. Initial time gaps between two adjacent vehicles
are therefore chosen from an exponential distribution.
V. SIMULATION RESULTS
Figure 5a to Figure 5c illustrate the simulation results. For
three different traffic models with low, medium and high traffic
density the resulting delay time, which is called information
delay, between a far away event and the considered node
is illustrated. If a traffic jam happens at 50 km distance,
the <strong>SOTIS</strong> network needs 6 minutes to report this event to
the individual node in a high density traffic situation and a
penetration rate of 5 %.
The resulting information delay is relatively low in high
traffic density situations. But in case of a low traffic density,
the information delays for a market penetration of 5 % and
10 % increase significantly up to 20 or 10 minutes. In low
density road situations the probability to find a communication
partner inside the radio range which can forward the data
packet and traffic information is decreased.
The performance results in these road scenarios demonstrate
the potential of the <strong>SOTIS</strong> technology. The system is able to
provide traffic information for the local area with reasonable
delay even if a low <strong>SOTIS</strong> market penetration of 5 % is
assumed.

Delay [minutes]
30
25
20
15
10
5
2%
5%
10%
0
0 10 20 30 40 50
Distance ahead [km]
(a) Low traffic density (850 cars/h/lane)
Delay [minutes]
30
25
20
15
10
5
2%
5%
10%
0
0 10 20 30 40 50
Distance ahead [km]
(b) Medium traffic density (1100 cars/h/lane)
Delay [minutes]
30
25
20
15
10
5
2%
5%
10%
0
0 10 20 30 40 50
Distance ahead [km]
(c) High traffic density (1600 cars/h/lane)
Fig. 5: Performance of the basic <strong>SOTIS</strong> system for a penetration of 2 %, 5 % and 10 % when the traffic density is varied.
Until now, the considered basic <strong>SOTIS</strong> implementation
was assumed to be static: Broadcast messages were generated
in time intervals with constant length and the radio
range/transmission power of the vehicle was assumed to be
fixed not least because of low market penetrations, where the
impact of overload conditions were neglected.
With a very low percentage of <strong>SOTIS</strong> equipped vehicles
inside the road traffic, the information about a traffic event in
a road segment 50 km away from the current node location
is still very useful traffic information if it is received with a
delay of less than 10 minutes. Conventional traffic information
systems have a delay time between 20 and 50 minutes.
VI. CONCLUSION
IVC technology can significantly increase driver and passenger
safety and comfort. A critical requirement for the
market introduction of a <strong>SOTIS</strong> like system is the market
penetration. The good news is that the traffic information
system offers significant benefits and relevant information even
in a penetration rate of 5 % and outperforms conventional
traffic information systems <strong>based</strong> on a centralized structure.
The paper describes a <strong>Self</strong>-<strong>Organizing</strong> <strong>Traffic</strong> <strong>Information</strong>
<strong>System</strong> (<strong>SOTIS</strong>) for the distribution of data packets with
detailed, relevant and up-to-date travel and traffic information
for the local environment of a vehicle. The information range
can be much larger than 50 km.
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