Exploration of adaptive beaconing for efficient intervehicle safety communication

Abstract

In the future intervehicle communication will make driving safer, easier, and more comfortable. As a cornerstone of the system, vehicles need to be aware of other vehicles in the vicinity. This cooperative awareness is achieved by beaconing, the exchange of periodic single-hop broadcast messages that include data on the status of a vehicle. While the concept of beaconing has been developed in the first phase of research on VANETs, recent studies have revealed limitations with respect to network performance. Obviously, the frequency of beacon messages directly translates into accuracy of cooperative awareness and thus traffic safety. There is an indisputable trade-off between required bandwidth and achieved accuracy. In this work we analyze this trade-off from different perspectives considering the consequences for safety applications. As a solution to the problem of overloading the channel, we propose to control the offered load by adjusting the beacon frequency dynamically to the current traffic situation while maintaining appropriate accuracy. To find an optimal adaptation, we elaborate on several options that arise when determining the beacon frequency. As a result, we propose situation-adaptive beaconing. It depends on the vehicle's own movement and the movement of surrounding vehicles, macroscopic aspects like the current vehicle density, or microscopic aspects.

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IEEE Network • January/February 2010
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0890-8044/10/$25.00 © 2010 IEEE
ne of the major goals of vehicular ad hoc net-
works (VANETs) is to support traffic safety.
Cooperative awareness applications require fre-
quent and low-delay information exchange
among vehicles, including data such as current position, move-
ment, and acceleration. This is realized by broadcasting so-
called (single-hop) beacon messages. As a result, every vehicle
is aware of other vehicles within a certain range. Beaconing is
also the basic supporting process that enables geographic rout-
ing and message dissemination. However, this also requires a
significant amount of bandwidth. The higher the frequency and
thus the accuracy, the higher the bandwidth consumption.
The first phase of research on VANETs has set the bound-
ary conditions in terms of basic communication protocols and
routing paradigms. Communication in VANETs will be based
on IEEE 802.11p. Message dissemination and routing are
based on geocast principles. Beaconing takes place on a single
communication channel (commonly referred to as the control
channel) that is shared by all nodes.
However, it has also been shown in [1] that the limited
bandwidth of the wireless channel has a severe impact on the
efficiency of the communication. This means that if the bea-
con rate is fixed, channel load may increase too much in sce-
narios with high vehicle density. High channel utilization
increases the information loss as packets are received erro-
neously, which is especially observed at large distances
between sender and receiver.
Simply reducing the beacon rate is not a suitable solu-
tion because it reduces the information quality at the same
time. The error between the real position of a vehicle and
the last known position retrieved from a beacon increases
as the beacon rate is reduced. This results in position inac-
curacies, which may disturb correct operation of active
safety applications, which rely on accurate and up-to-date
information.
Currently, there is no final recommendation for a particular
static beacon rate. No upper boundary in terms of maximum
channel load has been specified. Furthermore, no require-
ments from the different applications have yet been clearly
defined. As a consequence, even minimum and maximum
beacon rate are hard to derive as these two boundaries have
to satisfy all imaginable road traffic situations.
In this article we pave the way for further enhancements of
beaconing algorithms for the second phase of research on
VANETs. After a short assessment of the current state of the
art, we present a detailed analysis of the problem space and
how different beacon rates influence 1) the offered load to the
channel and 2) the resulting average and maximum accuracy of
information
Based on this problem evaluation, we motivate a flexible
approach to control both appropriately. For this purpose, we
propose a situation adaptive beaconing process that adapts
the beacon rate continuously. The design space and different
candidates for such an algorithm are introduced in this article.
O O
Robert K. Schmidt and Tim Leinmüller, DENSO AUTOMOTIVE
Elmar Schoch and Frank Kargl, Ulm University
Günter Schäfer, Technische Universität Ilmenau
Abstract
In the future intervehicle communication will make driving safer, easier, and more
comfortable. As a cornerstone of the system, vehicles need to be aware of other
vehicles in the vicinity. This cooperative awareness is achieved by beaconing, the
exchange of periodic single-hop broadcast messages that include data on the sta-
tus of a vehicle. While the concept of beaconing has been developed in the first
phase of research on VANETs, recent studies have revealed limitations with respect
to network performance. Obviously, the frequency of beacon messages directly
translates into accuracy of cooperative awareness and thus traffic safety. There is
an indisputable trade-off between required bandwidth and achieved accuracy. In
this work we analyze this trade-off from different perspectives considering the con-
sequences for safety applications. As a solution to the problem of overloading the
channel, we propose to control the offered load by adjusting the beacon frequency
dynamically to the current traffic situation while maintaining appropriate accuracy.
To find an optimal adaptation, we elaborate on several options that arise when
determining the beacon frequency. As a result, we propose situation-adaptive bea-
coning. It depends on the vehicle’s own movement and the movement of surround-
ing vehicles, macroscopic aspects like the current vehicle density, or microscopic
aspects.
Exploration of Adaptive Beaconing for
Efficient Intervehicle Safety Communication

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IEEE Network • January/February 2010
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The difficulties with this design are the two optimization
goals which seem to be conflicting at the first glance:
• Low resource usage: Less occupancy of the wireless medium
is achieved by reducing the beacon rate. It is better to
receive less information but still reliably than to have
unpredictable high packet loss.
• High information accuracy: The knowledge of the surround-
ing vehicles must be as accurate and up-to-date as possible,
which can be achieved with a higher beacon rate.
In order to evaluate our concepts, we analyze the influence
of different beacon rates on the channel load and achievable
accuracy. Therefore, we denote the offered load as the number
of beacons sent by all vehicles in relation to the channel
capacity. For determining the accuracy of a particular beacon
rate, we introduce the position accuracy metric where we con-
sider the error between the current physical position and the
last reported position.
Related Work
The adaptation of beaconing may cover different aspects in
general. Widely discussed are parameters such as the transmit
power (e.g., [2]) or beacon rate; the latter is addressed in this
article. As we focus on the impact of adaptation on the accu-
racy of information, in the following we look at related work
in the area of beacon rate adaptation.
In [3] Khorakhun et al. propose that all nodes should adapt
the beacon rate depending on the current channel load, mea-
sured via the channel busy ratio. The goal is to let the offered
load converge to a given maximum allowed channel load. To
achieve a smooth adaptation, an average beacon rate is calcu-
lated and exchanged among the vehicles. Each vehicle then
adapts its beacon frequency to the average.
Another direction of adapting the beacon rate is taken by
Rezaei et al. in [4, 5] where they present a concept to adapt
the beaconing rate depending on differences from position
predictions. They assume that all vehicles run the same posi-
tion prediction algorithm. An extended Kalman filter is local-
ly applied to each vehicle in the neighbor table. It
continuously estimates the current position based on the
received position information history in order to improve the
accuracy of the position during the time between two succes-
sive beacons. The time to send the next beacon is determined
based on the following algorithm. The vehicle knows its own
physical position and is able to estimate its own position the
same way all surrounding vehicles do. Once it determines
that its own physical position has a particular difference from
the remote estimator, it sends the next beacon. This approach
is a suitable concept to inform neighboring vehicles about
movement changes. However, there are several drawbacks
with the presented approach when thinking about active safe-
ty applications. Position prediction is critical because active
safety applications will become active in situations where the
movement of a vehicle changes suddenly, and thus the pre-
diction is inaccurate. Also, it does not account for situations
where higher beacon rates are needed, such as in case of an
imminent collision (but no change of movement). Another
problem is that once an application decision whether to warn
the driver has to be made, the current prediction error is not
known to the receiver. Furthermore, message loss and fre-
quent network topology changes are not treated; for example,
the defined maximum error can be exceeded due to message
loss. A countermeasure would be, for instance, to introduce a
minimum beacon rate.
A step toward situation-adaptive beaconing was made by
Fukui et al. in [6]. Beacons are sent periodically based on a
constant distance a vehicle has to travel. Furthermore, each
vehicle determines the current number of lanes. For multilane
roads, the beacon rate is reduced. Basically, with this approach
situations with potentially high node densities are detected,
and hence the beacon rate is reduced appropriately. A further
adaptation is applied by considering the current packet error
rate. However, the defined goal is only to reduce the offered
load and the number of colliding packets. The effect of
reduced accuracy has been neglected, which may cause prob-
lems, for example, at multilane intersections.
In summary, each approach addresses only one optimiza-
tion goal. The approach of Khorakhun et al. prevents over-
loading of the channel but does not account for maintaining
position accuracy. Rezaei et al. do not consider the node den-
sity nor do they react on the information they receive from
surrounding vehicles. Finally, Fukui et al. neglect the effect of
reduced accuracy.
Adaptive Beaconing in VANETs
As mentioned in the introduction, we consider both goals for
beaconing, to reduce offered load and provide the best possi-
ble information accuracy for safety applications and routing.
Therefore, we propose to adapt the beacon rate according to
the following requirements and influences:
• Offered load limitation: Beaconing may be the application
consuming most of the bandwidth. It must be ensured that
the channel does not get overloaded since it would lead to
significant decrease of performance in terms of high packet
error rates.
• Minimum of available neighbor information: A minimum
beacon rate must be maintained to ensure the discovery of
new neighbors with low delay and keep the neighbor table
up to date.
• Appropriate resolution: A beaconing frequency of 10 Hz is
not necessary in a traffic jam with vehicles standing still,
whereas 1 Hz may be not sufficient on a multilane high-
speed highway with frequent lane changes. Thus, the maxi-
mum time interval and maximum driving distance between
two beacons must be limited. As a traffic situation also
depends on the movement of the surrounding vehicles, the
adaptation of the beacon rate must consider position,
speed, acceleration, direction, and yaw rate of surrounding
vehicles.
These requirements are to be considered for the problem
evaluation as well as the design of adaptive beaconing con-
cepts.
Adaptation Evaluation Criteria
We discuss selected beacon rates based on desired accuracy
and offered load. Our discussion is guided by the following
questions:
• What is an appropriate beacon rate at a particular velocity
for a desired accuracy?
• Which beacon rate is appropriate at a particular node densi-
ty to keep a desired maximum offered load?
For the evaluation of different beacon rates, we introduce
the position accuracy metric as a measure for the accuracy of
a particular beacon rate.
Metric for Position Accuracy
We define the metric to reflect three criteria: the minimum
error, maximum error, and average error of the last received
position information in relation to the current physical posi-
tion of a vehicle. The relevant input parameters for the metric
are the vehicle velocity v, beacon rate fBand transmission
delay tTx. The resulting accuracy metric is as follows:

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IEEE Network • January/February 2010
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• Minimum position error ?E?: denotes the lower error bound-
ary resulting from the transmission delay tTx. The minimum
position error is usually negligible as the transmission delay
tTxis typically around 0.001 s and thus relatively small com-
pared to the lowest beacon interval of 0.1 s.
• Maximum position error ?E?: is the upper boundary that
occurs when the position of a vehicle is looked up (?) right
before receiving the next beacon from this vehicle. This
error is equal to the distance the vehicle travels during a
beacon interval minus a small time value ?.
• Average position error E
that the event of looking up the position is uniformly dis-
tributed between minimum and maximum time difference
to the transmission event of the beacon. Figure 1 sketches
the time dependencies of the metric. The corresponding
equations can easily be derived as
—: expresses the mean error assuming
(1)
In the following evaluation we focus on the discussion of
the maximum and average errors. We do not discuss the mini-
mum error as it solely depends on the transmission delay. As
the parameters have wide ranges (e.g., velocity), and may not
have been clearly defined (e.g., the beacon rate), we define
typical values for the remaining analysis and discussion.
The velocity of a vehicle may vary in a range of 0 to 250
km/h, depending on many factors and conditions. We select
four values for maximum speed, which can be observed in
four different traffic scenarios: residential areas (30 km/h),
metropolitan areas (50 km/h), rural roads (100 km/h), and
highways (200 km/h).
Note that for the sake of simplicity, we assume constant
speed without acceleration or deceleration during two succes-
sive beacons. Also, changes in the direction of a vehicle are
not considered. The possible change of movement would only
provide for a small offset given by maximum acceleration and
yaw rate. For the special case of v = 0, the error would be 0.
Commonly discussed are beacon rates fBbetween 1 and 10
beacons/s. For our discussion we therefore investigate four
typical values: high (10 Hz), medium (5 Hz), low (2 Hz), and
minimum (1 Hz) beacon rate.
Accuracy Evaluation
Figure 2a visualizes the influence of different beacon rates
and vehicle velocities on the average position error calculated
according to Eq. 1 as introduced and explained in the previ-
ous section. Each graph shows a particular error level, ranging
from 1 m up to 10 m average position error, reflecting high
down to low accuracy. We make the following observations.
With high beacon rate of 10 beacons/s, an accuracy of 1 m is
maintained up to a velocity of about 70 km/h. An accuracy of
10 m is achieved with low beacon rate up to a velocity of
about 150 km/h. At high speed (200 km/h), the medium bea-
con rate of 5 beacons/s has to be applied to stay at within 10
m of average error.
The minimum beacon rate provides an accuracy of 5 m
only at low velocities (30 km/h), whereas for a metropolitan
area with 50 km/h, only an accuracy of 10 m is met.
The discussion of the maximum error is similar. As shown
in Eq. 1, the maximum error is twice the average error.
Hence, the graph for 1 m average error is the same for the
maximum error of 2 m. The graph of 5 m average error corre-
sponds to the graph of 10 m maximum error.
Offered Load Evaluation
For the discussion of the offered load, we make the following
assumptions and simplifications. We assume 6 Mb/s as data
rate on the communication channel, in line with research from
Jiang et al. [7]. Furthermore, we consider only the percentage
of the offered load (payload) in relation to the (gross) data
rate where we neglect any (varying) overhead by the medium
access control (MAC) and physical (PHY) layers (e.g., head-
ers, contention window, and interframe spaces).
In Fig. 2b we show two levels of offered load with a
globally applied beacon rate. As packet sizes have not been
fixed or limited yet, we distinguish two packet lengths:
medium-size packets of 200 bytes each and large packets of
1000 bytes each. The main reason for considering large
packets is that beacons might contain overhead from cryp-
tographic signatures and key material. The results for small
packets (50 bytes) are not shown as for the given con-
straints of desired offered load, number of vehicles in com-
munication range, and beacon rate, the channel capacity is
not exceeded.
The two offered loads are chosen according to [8], where
Brakemeier argues that for efficient channel usage the offered
load should be between 40 and 60 percent. According to his
work, an offered load of 60 percent plus PHY/MAC protocol
overhead may fully utilize the channel.
For large packets, the desired offered loads are reached at
300 and 450 vehicles even with the minimum beacon rate.
With 10 beacons/s, only 50 and 90 vehicles are supported at
maximum. We also see only a slight difference for 40 and 60
percent.
The situation relaxes at medium packet size. For example,
300 vehicles in communication range and a beacon rate of 5
beacons/s result in 40 percent offered load. Likewise, with 10
beacons/s 150 vehicles are supported for that load. Still, with
higher densities the offered load can increase to more than 60
percent.
These examples show the limitations of efficient communi-
cation in the particular scenarios. High beacon rates cannot
be supported for all vehicles in cases of high vehicle densities
or larger packets.
Accuracy vs. Offered Load
The previous discussion on accuracy considers the accuracy of
position information of a single vehicle when using a certain
beacon rate. In the discussion of the offered load, we have
considered all vehicles within communication range as they all
contribute with their beacon transmissions. Setting both into
relation does not provide sufficient criteria for beacon rate
adaptation. Basically, one can derive the offered load for a
given average speed (of all vehicles) and a particular beacon
rate. In relation to per-vehicle accuracy, the achieved (aver-
age) accuracy can be estimated.
However, the heterogeneity of velocities should be taken
into account in the discussion of beacon rate adaptation.
Imagine a traffic jam with vehicles driving slowly at the same
velocity. At a constant beacon rate, the position of these vehi-
cles can be determined with the same accuracy. But oncoming
traffic may not be jammed and travel at high speeds. For the
same beacon rate, the accuracy would be much lower. For
E
EE
vt
v f
Tx
B
= + =
2
+
−
−
2
1
( ).
ε
Figure 1. Relation of relevant time parameters that determine the
accuracy.
fB–1
Tx
t
ε
TransmissionTransmission Reception Reception
Next Next
Time

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IEEE Network • January/February 2010
17
these vehicles a much higher beacon rate is required to meet
the same accuracy requirements. This could result in situa-
tions where the overall offered load exceeds the channel
capacity.
With this example we want to highlight that there are many
parameters that have to be considered; especially their sophis-
ticated interrelation has to be taken into account in the design
of adaptive beaconing. Furthermore, the difference of chang-
ing the beacon frequency globally or only locally in individual
vehicles must be investigated further. Summarizing, the bea-
con rate should be adapted based on a vehicle’s current con-
text, which we denote as situation-adaptive beaconing.
Schemes for Situation-Adaptive Beaconing
Situation-adaptive beaconing basically depends on the vehi-
cle’s own status and the road traffic situation in considera-
tion with the currently offered load. Accordingly, we
discuss two categories of schemes for rate adaptation,
depending on the vehicle’s own movement and depending on
surrounding vehicles’ movement. Schemes that act depending
on a vehicle’s own movement are able to adapt the beacon
rate based on the vehicle’s status to maintain a defined
accuracy. These schemes do not use any information from
other vehicles. This, in turn, is the case for the second cate-
gory of adaptation schemes. They either depend on charac-
teristics of the whole traffic situation (macroscopic view) or
adapt the rate with respect to particular situations (micro-
scopic view). Figure 3 gives an overview of the different
schemes. For each of them, we discuss the impact on accu-
racy and offered load.
Adaptation Depending on a Vehicle’s Own
Movement
This adaptation considers solely the current status of the
transmitting vehicle. According to Fig. 3, there are three main
criteria that can be taken into account. The first criterion is
the velocity of a vehicle. As we have already discussed earlier,
the accuracy strongly depends on the current velocity. Thus,
the beacon rate should be higher for higher velocities, by
either linearly increasing the beacon rate fBor changing it in
specified steps. Especially to maintain the maximum offered
load, an additional criterion may be needed. The resulting
offered load is implicitly controlled by the relationship
between average velocity and traffic density. This fundamental
relationship is commonly known from traffic flow theory; for
example, in [9] Kerner states that for increasing traffic densi-
ty, the achievable average speed decreases. As a result, the
offered load remains stable for more vehicles driving at lower
velocities. Nevertheless, additional criteria are required to
maintain the maximum offered load.
For adaptation based on movement changes we basically
share the opinion of Rezeai et al. [4, 5] that situations where a
vehicle changes its movement have a higher potential for dan-
gerous situations. For example, heavy braking, turning, or
changing a lane are events where accidents may occur. Their
approach implicitly considers all movement parameters. We
propose to consider all parameters explicitly. According to
Fig. 3, movement changes can be divided into acceleration
(and deceleration) and yaw rate. Optionally, vehicle-internal
sensors may trigger a higher beacon rate (e.g., in case of
reduced friction).
Figure 2. a) Average position error depending on beacon rate and velocity, b) amount of data produced (offered load) by beaconing
depending on beacon rate and number of vehicles in communication range.
Velocity [km/h]
500
2
0
Beacon rate per vehicle (1/s)
5
4
3
6
7
8
9
10
100200 250150
Number of vehicles in range
1000
2
0
Beacon rate per vehicle (1/s)
5
4
3
6
7
8
9
10
200 400500 350
40% - 1000 byte
60% - 1000 byte
40% - 200 byte
60% - 200 byte
1 meter
2 meters
5 meters
10 meters
Figure 3. Overview on essential schemes for situation-adaptive beaconing.
Depending on own movement Depending on surrounding vehicles’ movement
Movement changeMacroscopic scope Microscopic scope
VelocityAcceleration Yaw rateSpecial
vehicle
Vehicle
density
Close-by
vehicles
Crossing
vehicles
High
relative speed
.......
Situation-adaptive beaconing

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IEEE Network • January/February 2010
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The last criterion covers vehicles with special movement
patterns, such as emergency vehicles. They may have an
increased beacon rate as their position information is assumed
to have higher priority, informing other vehicles to clear the
way, with the additional demand of high accuracy.
Adaptation Depending on Surrounding Vehicles’
Movement
The schemes that are discussed in this section are based on
knowledge of surrounding vehicles. They are essential to con-
sider the situation around a vehicle. The beacon rate may be
adapted by macroscopic aspects like the current vehicle densi-
ty or by microscopic aspects. For example, if there is a vehicle
close by due to a lane change, the beacon rate may be
increased for accuracy reasons.
Macroscopic Traffic Situation — Situations with high vehicle
densities may constitute a problem for reliable communication
as the channel capacity may be exceeded. To proactively
reduce the offered load, the beacon rate should be lowered in
case the vehicle density is high. Reduction of the beacon rate
increases the reliable reception of the reduced offered load.
As discussed earlier, usually the beacon rate is already
reduced when velocity-based adaptation is applied. This
scheme additionally adapts the rate, proactively avoiding
channel overload in case velocity-based is not sufficient. As a
consequence, a reduction of the beacon rate by this scheme
may lead to significant degradation of accuracy. The benefit
would be to maintain the defined maximum offered load (i.e.,
keeping the communication system in a stable state).
Another approach would be to reduce the beacon rate
based on the number of messages received per second instead
of the number of (communicating) vehicles. The problem here
is the potentially uneven distribution of beacon rates at differ-
ent vehicles. Assuming vehicle A increases the rate, vehicle B
would react to this with a reduction of the beacon rate, poten-
tially leading to even further increase of the beacon rate by
vehicle A. Hence, the vehicle-density-based scheme should be
applied as an open-loop approach. The adaptation should not
depend on the influence of the last adaptation.
Microscopic Traffic Situation — Schemes that consider micro-
scopic aspects allow for wider adaptation of the beacon rate
than the schemes described in the previous subsection. The
following schemes address a few vehicles in particular situa-
tions. Hence, the impact on the offered load is small com-
pared to macroscopic schemes where all vehicles react in
parallel. Their influence is in order of the number of all vehi-
cles within range.
Microscopic reactions trigger an increase of the beacon rate
in special situations according to use cases or the require-
ments of currently running applications. An overview on many
use cases can be found in the use case catalog of European
Telecommunications Standards Institute (ETSI) TR 102 638
[10]. Due to the huge number of use cases, we focus on the
explanation of the key ideas for these schemes.
Vehicles that are close to each other should increase the
beacon rate as the probability of collision becomes higher the
closer the vehicle gets. Furthermore, two vehicles potentially
crossing their ways demand a higher beacon rate. This occurs
in situations like lane changes, intersections, a wrong-way
driver, or vehicles at high velocities approaching slow vehicles.
In these situations the beacon rate may already be high as the
autonomous schemes should have detected high velocity or
changes in movement. If so, the beacon rate should be
increased even further if vehicle collisions are likely. The
respective scheme, however, must be constrained not to
exceed the maximum offered load as this may occur in situa-
tions where there are many vehicles close to each other.
Note that this category of situation-specific adaptation
schemes is most sophisticated due to the variety of different
traffic situations. Nonetheless, it appears to also be the class
of mechanisms to cover the variety of application require-
ments in terms of accuracy.
Combination of Schemes
The combination of several schemes from the different cate-
gories into an adaptation framework results in situation-adap-
tive beaconing. The adaptation of the beacon rate has to
depend on the vehicle’s own status in consideration of the sta-
tus of its surrounding vehicles. In the framework each scheme
is provided with a weight defining the total impact on the
increase of the beacon rate. The outcome of all considered
schemes influences the adaptation, that is, how much to
increase the beacon rate additionally to the minimum beacon
rate.
The combination of schemes ensures that vehicles increase
the beacon rate in special (and dangerous) situations (e.g.,
once they detect rapidly approaching vehicles). This supports
not only active safety applications but also the routing scheme,
which relies on up-to-date neighbor tables for an efficient
routing decision.
An example of a highway traffic situation where an emer-
gency vehicle approaches a traffic jam already covers many
schemes described in this article and highlights how to com-
bine them. The emergency vehicle, as a special vehicle with
light bar in use, travels at high velocity. Its beacon rate is set to
a high value by the schemes that depend on its own move-
ment. The vehicles in the traffic jam use the minimum beacon
rate because they are driving slowly. Once the emergency
vehicle approaches the traffic jam at high relative speed being
close by, the beacon rates are adapted by the tail-end vehicles
in the traffic jam while the emergency vehicle keeps its high
beacon rate. After passing the tail-end vehicles, they set the
rate to minimum again. The other vehicles in the traffic jam
increase the beacon rate temporarily as the emergency vehicle
approaches them and return to the minimum rate once they
have been passed (i.e., once there is no longer any imminent
danger).
Conclusions
Exchanging vehicle awareness information via beacon mes-
sages is important for active safety applications as well as for
geographic routing. By its periodicity of transmission, a very
high load may be imposed onto the wireless channel.
In this work we study the effects of adapting the beacon rate
with respect to reduced accuracy and changing offered load.
We identify different accuracy demands in different traffic sit-
uations. Considering both offered load and corresponding
accuracy, we analyze the spectrum of proposed beacon rates.
To account for this, we proposed schemes for adapting the
beacon rate according to the traffic situation. Requirements
for minimum and maximum beacon rate are introduced which
define the boundaries for the adaptation process. Schemes for
this adaptation have been categorized by their scope. We iden-
tify the microscopic traffic-situation-based adaptation as a
promising scheme to fulfill application requirements in special
situations. When vehicles detect a dangerous situation, these
few vehicles temporarily increase their beacon rate. The
advantage of this is only a slight increase in channel load with
the benefit of high accuracy by frequent position and move-
ment updates for these affected vehicles.

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For a final situation-adaptive beaconing scheme, we pro-
pose to combine the outlined schemes in a framework that
allows the aggregation of the various schemes. The aggrega-
tion result increases the beacon rate dynamically, starting
from the minimum required rate. This framework should also
consider weighting of the schemes, where microscopic traffic-
situation-based adaptation has the greatest impact.
The adaptation of the beacon rate has been discussed to
control the offered load. However, there are more techniques
that can be considered. The communication channel load can
be reduced by setting a lower transmit power for vehicles with
temporary high beacon rates. Again, to meet the minimum
requirement for geographic routing, the minimum beacon rate
must be maintained at standard transmit power.
Future research work comprises a lot of challenging aspects
based on beacon rate adaptation. All imaginable road traffic sit-
uations and network loads have to be covered by suitable adap-
tation schemes in order to provide potential life saving
information to drivers. Therefore, research on smart communi-
cation has to be combined with traffic flow aspects. Applications
for safety, in turn, need to define their accuracy requirements
for particular situations. Correspondingly, an additional metric
measuring the potential danger of a situation is needed.
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Biographies
ROBERT KARL SCHMIDT (r.schmidt@denso-auto.de) received his Diploma degree in
computer science in 2008 from the Technical University Ilmenau, Germany.
Afterward, he joined DENSO AUTOMOTIVE Deutschland GmbH where he
focuses his research activities on efficient communication and security in vehicu-
lar ad hoc networks.
TIM LEINMÜLLER (t.leinmueller@denso-auto.de) received his joint degree in electri-
cal engineering from ENST-Paris and the University of Stuttgart in 2003. After-
ward, he worked for DaimlerChrysler AG Group Research and Advanced
Engineering. In 2007 he joined DENSO AUTOMOTIVE Deutschland GmbH,
where his activities focus on research and standardization in the area of car-to-
car communication.
ELMAR SCHOCH (elmar.schoch@uni-ulm.de) received his doctorate degree in com-
puter science from Ulm University in 2009, working on robust and efficient secu-
rity mechanisms for communication in intervehicle networks. Both for Ulm
University and DaimlerChrysler Telematics Research, he was involved in several
European IVC projects, such as SEVECOM, PRECIOSA, and NOW. His current
research interests center around advanced techniques for vehicular networks,
trust, security, and privacy for mobile and ubiquitous systems.
FRANK KARGL (frank.kargl@uni-ulm.de) is an associate professor at the University
of Twente in the Distributed and Embedded Security Group. Until 2009 he was a
senior researcher at Ulm University, leading a research team focusing on various
aspects of VANET communications including information dissemination, applica-
tions, and security and privacy. He has co-authored over 80 peer-reviewed pub-
lications, and is actively involved in research projects like SeVeCom and
PRECIOSA. He also contributes to standardization activities, is a regular member
of program committees, a reviewer for selected journals, and a panelist and
keynote speaker on ITS security and privacy.
GÜNTER SCHÄFER [M] (guenter.schaefer@tu-ilmenau.de) is a full professor of
telecommunications/computer networking at the University of Ilmenau, Germany.
His main subject areas are network security, protection of communication infras-
tructures, as well as communication protocols and architectures. He is a member
of the Association for Computing Machinery (ACM) and the German Gesellschaft
für Informatik (Computer Science Society).