Beaconing Approaches in Vehicular Ad Hoc Networks: A Survey <s> Beacon Rate Control <s> The inter-vehicle communications (IVC) is one of the most important technologies to realize advanced intelligent transport systems (ITS). Conventional IVC is vehicle-to-vehicle communication through line of sight communication by millimeter wave in a narrow area. Here, we deal with more progressive IVC in a wide area. In this paper, we propose Dynamic Integrated Transmission Control (DITRAC) for IVC. DITRAC optimizes transmission power and transmission interval of vehicles according to the fluctuation of the communications circumstance of the ad-hoc vehicular communications network. In order to estimate the fluctuation of the neighboring communications, we present an effective and simple method, the road-weight with receiving history (WH) method. The simulations are conducted to show the validity of DITRAC for IVC. <s> BIB001 </s> Beaconing Approaches in Vehicular Ad Hoc Networks: A Survey <s> Beacon Rate Control <s> A mobile ad hoc network (MANET) is a collection of wireless mobile nodes forming a temporary network without using any existing infrastructure. Since not many MANETs are currently deployed, research in this area is mostly simulation based. Random waypoint is the commonly used mobility model in these simulations. Random waypoint is a simple model that may be applicable to some scenarios. However, we believe that it is not sufficient to capture some important mobility characteristics of scenarios in which MANETs may be deployed. Our framework aims to evaluate the impact of different mobility models on the performance of MANET routing protocols. We propose various protocol independent metrics to capture interesting mobility characteristics, including spatial and temporal dependence and geographic restrictions. In addition, a rich set of parameterized mobility models is introduced including random waypoint, group mobility, freeway and Manhattan models. Based on these models several 'test-suite' scenarios are chosen carefully to span the metric space. We demonstrate the utility of our test-suite by evaluating various MANET routing protocols, including DSR, AODV and DSDV. Our results show that the protocol performance may vary drastically across mobility models and performance rankings of protocols may vary with the mobility models used. This effect can be explained by the interaction of the mobility characteristics with the connectivity graph properties. Finally, we attempt to decompose the routing protocols into mechanistic "building blocks" to gain a deeper insight into the performance variations across protocols in the face of mobility. <s> BIB002 </s> Beaconing Approaches in Vehicular Ad Hoc Networks: A Survey <s> Beacon Rate Control <s> We present an adaptive communication scheme for cooperative active safety system (CASS). CASS uses information communicated from neighboring vehicles via wireless communication in order to actively evaluate driving situations and provide warnings or other forms of assistance to drivers. In CASS, we assume that vehicles are equipped with a GPS receiver, a dedicated short range communications (DSRC) transceiver, and in-vehicle sensors. The information exchanges between vehicles include position, speed, heading, and other vehicle kinematic and dynamic information, and the information is broadcast to all neighbors within a certain communication range. The literature surmises CASS may need a vehicle to broadcast information as often as every 100 msec which may lead to channel congestion resulting in message loss rates above 20%. Here we present a new communication design scheme, supported by simulations, which indicates that CASS could be enabled by broadcasting, on average, as little as once every 500 msec. <s> BIB003 </s> Beaconing Approaches in Vehicular Ad Hoc Networks: A Survey <s> Beacon Rate Control <s> This paper presents an adaptive communication scheme for Cooperative Active Safety System (CASS). CASS uses information communicated from neighboring vehicles via wireless communication in order to actively evaluate driving situations and provide warnings or other forms of assistance to drivers. In CASS, vehicles are equipped with a GPS receiver, a Dedicated Short Range Communications (DSRC) transceiver, and in-vehicle sensors. The information exchanges between vehicles include position, speed, heading, and other vehicle kinematic and dynamic information, and the information is broadcast to all neighbors within a certain communication range. The authors review the literature, which surmises that in order to be successful, CASS may need a vehicle to broadcast information as often as every 100 msec which may lead to channel congestion resulting in message loss rates above 20%. They present a new communication design scheme, supported by simulations, which indicates that CASS could be enabled by broadcasting, on average, as little as once every 500 msec. In the proposed scheme, each vehicle compares its self state estimates of the state outputs of their remote estimator. Then, based on a threshold crossing policy defined for longitudinal position error and lateral position error violation, the vehicle decides to broadcast or not. <s> BIB004 </s> Beaconing Approaches in Vehicular Ad Hoc Networks: A Survey <s> Beacon Rate Control <s> Optimizing vehicular communication strategies is important for an efficient usage of the available wireless bandwidth and also critical for the success of VANETs. In this paper we address the fundamental and practical question whether the load on the wireless channel can be reduced if periodic beacon messages are transmitted over multiple hops with reduced transmit power instead of being transmitted over one hop with high transmit power. In particular, we look at the possible bandwidth savings that can be achieved by piggybacking forwarded messages into the own next beacon transmission. For that matter, we first propose an analytical model to compute a lower bound for the resulting channel load when single- or multi-hop dissemination of beacons is performed. In this model we assume optimal channel conditions and perfect relaying and piggybacking decisions to show that a reduction of the load by multi-hop is possible and closely related to piggybacking. Further, we show that the possible savings depend on the ratio between the size of the header and the payload of a beacon and that a reduction of the load is theoretically possible if the header is larger than the payload - what would be the case in VANETs if security overheads are considered part of the header. We then perform a simulative comparison of single- and multi-hop beaconing to evaluate the impact of effects such as packet collisions and channel fading. We show that the possible savings of multi-hop beaconing are difficult to exploit under non-perfect channel conditions and suboptimal relaying decisions. <s> BIB005 </s> Beaconing Approaches in Vehicular Ad Hoc Networks: A Survey <s> Beacon Rate Control <s> Vehicular Ad Hoc Networks (VANETs) have been widely studied, and the deployment of such networks is likely to happen soon as the required technology and commercial opportunities are already available. Most of applications proposed for these networks require a localization mechanism with reasonable accuracy. In addition to applications, most protocols rely on the availability of a system that determines vehicles' positions. As many vehicles are (or are in the process of being) equipped with GPS, the accurate localization of vehicles in VANETs is achievable. ::: ::: However, one issue that has been ignored is the localization of neighboring vehicles. In VANETs, nodes usually move at high speeds; the information about the position of a neighbour vehicle is therefore fast outdated. The naive approach for handling this issue is to increase the frequency by which periodic messages containing a node's position are exchanged. This solution might lead an unfeasible overhead significantly dropping the bandwidth available for the exchange of services' data. ::: ::: In this paper, we study this problem further, and propose a solution where vehicles predict the position of a neighbor for the near future. Through extensive experiments, we show that a prediction model of low complexity was able to considerably increase the accuracy of neighbor localization. Our mechanism has achieved an accuracy of 50 centimeters with a frequency of exchange of beacons 75% smaller than the naive approach. <s> BIB006 </s> Beaconing Approaches in Vehicular Ad Hoc Networks: A Survey <s> Beacon Rate Control <s> 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. <s> BIB007 </s> Beaconing Approaches in Vehicular Ad Hoc Networks: A Survey <s> Beacon Rate Control <s> We present Adaptive Traffic Beacon (ATB), a fully-decentralized car-to-X protocol built around the central ideas of delay sensitivity and congestion awareness. From previous research findings, we see that centralized solutions, VANETs, and broadcasting based approaches each show benefits and drawbacks depending on traffic density, penetration, network utilization, and other parameters; intelligent transportation systems are therefore tuned for specific settings. In order to overcome this limitation, we developed a broadcast-based solution that has been designed to carefully use only the remaining capacity of the wireless channel. Thus, it will not influence other applications using the same radio network. ATB is adaptive in two dimensions: First, the beacon interval is adapted dynamically according to the channel quality and the importance of messages in the local knowledge base, and, secondly, the protocol can dynamically make use of available infrastructure elements. Simulation experiments demonstrate that ATB performs well in a broad range of settings. It maintains a non-congested wireless channel to prevent collisions during the data exchange. <s> BIB008 </s> Beaconing Approaches in Vehicular Ad Hoc Networks: A Survey <s> Beacon Rate Control <s> Traffic information systems are one of the key non-safety application areas of vehicular ad hoc networks. As such, TIS are much less delay sensitive than safety applications, which have recently attracted a lot of attention in VANET research. We propose a new message dissemination protocol, Adaptive Traffic Beacon (ATB), which is fully distributed and uses adaptive beaconing based on two key metrics: message utility and channel quality. It is shown that adaptive beaconing leads to much broader dissemination of messages (in terms of penetration rate) than flooding-based approaches, albeit at a slower rate. Adaptive beaconing thus seems to be much more suitable for TIS than flooding-based protocols. <s> BIB009 </s> Beaconing Approaches in Vehicular Ad Hoc Networks: A Survey <s> Beacon Rate Control <s> The recently standardised IEEE 802.11p Wireless Access in Vehicular Environment (WAVE) protocol is envisioned to be used for Vehicle-to-Vehicle and Vehicle-to-Infrastructure communications. We provide an analysis of 802.11p's performance when used for the exchange of small status messages known as beacons. These beacons enable vehicles to establish a Cooperative Awareness from which many applications can draw their inputs. Applications have been proposed to increase safety, efficiency and comfort of future road traffic. This paper investigates the impact of adapting settings for the Contention Window (CW) based on the traffic density in order to increase the efficiency of the 802.11p broadcasts w.r.t. reception probability and delay with a focus on real-time vehicle control. In contrast to the literature available on adapting CW settings in unicast, we find that the initial CW value of 15 proposed in the IEEE 802.11p standard performs significantly well under almost all studied circumstances. <s> BIB010 </s> Beaconing Approaches in Vehicular Ad Hoc Networks: A Survey <s> Beacon Rate Control <s> Vehicular Ad-Hoc Networks (VANETs) are special forms of Mobile Ad-Hoc Networks (MANETs). Due to vehicles moving with high speed, network topology frequently changes. Communicating and exchanging information between vehicles could be done by beaconing. Nevertheless, most of previously-proposed protocols for VANETs are designed to use constant beaconing rate. This results in an increase of beacon overhead and a degradation of protocols' performances in dense area. Therefore, in this paper, we study parameters that affect adaptive beaconing, and then propose two methods to apply to adaptive beaconing. One is based on a statistical technique and the other is based on the machine learning technique. Simulation results indicate that our proposed methods can reduce beacon overhead while maintaining the reliability and speed of data dissemination of the protocol. <s> BIB011
As we have aforementioned, high beacon rate leads to network congestion while low beacon rate causes inaccurate shared position information. To tackle this problem, the authors in BIB004 and BIB003 considered adaptation of beacon rate in vehicular environment. The proposed beacon rate adaptation is based on differences in predicted positions. In their prediction scheme, all vehicles are embedded with modified Kalman estimators to provide continuous estimations of existing positions. This position estimate can be obtained via the last beacon message, enhancing positional accuracy between two sequential beacons. Moreover, the prediction scheme requires that the next beacon message is triggered based on a vehicle's current position and an estimated position. Once the vehicle determines a change in its physical position, it triggers the next beacon message. In this way, vehicles independently estimate the duration of the next beacon message. However, rapid topology changes of vehicles and mobility traffic characteristics were not considered. Fukui et al. in BIB001 proposed a beacon adaptation scheme which considers the distance travelled by vehicles. Moreover, vehicles independently determine the number of lanes the current road has, and the higher the number of lanes, the lower the beacon rate. In addition, another beacon adaptation technique is based on packet loss rate. But, changing beacon rates based on multi lanes is unfair because multi lanes do not directly imply higher traffic density. Further, the accuracy of information has not been considered. Authors in BIB007 first studied the adaptation of beacon rate in order to find the compromise between information accuracy and bandwidth consumption. After the analysis of the parameters, which affect to the beacon rate, they proposed a scheme to adapt beacon rate according to the VANET traffic behaviour. In their study, however, intelligently combined traffic parameters like direction, density and status of a vehicle have been neglected. Moreover, their study is based on theoretical analysis. In our previous research  , we proposed an intelligent Adaptive (situation-aware) Beaconing Rate (ABR) approach based on fuzzy logic to control the frequency of beaconing by taking traffic characteristics into consideration. The proposed ABR considers the percentage of vehicles travelling in the same direction, and status of vehicles as inputs of the fuzzy decision making system, in order to tune the beaconing rate according to the vehicular traffic characteristics. For comparison purpose with fixed beaconing rate approach, the ABR approach is implemented in JIST/SWANs. The simulation results show that the proposed ABR approach is able to improve channel load due to beaconing, improve cooperative awareness between vehicles and reduce average packet delay in lossy/lossless urban vehicular scenarios. The authors in BIB005 modelled and simulated multi-hop and single hop beaconing in vehicular networks. The aim of the study is to address the fundamental question whether the beaconing load on the wireless channel can be reduced if periodic beacon messages are transmitted over multiple hops with reduced transmitted power instead of being transmitted over single hop with high transmit power. In their study, they noted that the multi-hop beaconing is necessary in situations where non-line-ofsight area obstruct a radio range by single hop beaconing. In BIB008 , Sommer et al. proposed Adaptive Traffic Beacon (ATB) approach to achieve congestion aware and low latency wireless channel. The ATB approach is a broadcast-based approach that precisely designed to use the residual capacity of wireless channel. In particular, the proposed solution is adaptive in the sense that the beacon generation rate is changed dynamically with respect to the channel quality and importance of the message as well as their solution can dynamically use infrastructure networks in the vicinity. Simulation results show that the ATB maintains a non-congested wireless channel to prevent packet collisions during the data exchange. In another attempt, the authors in BIB009 extended their ATB in order to compare it with flooding-based approaches. Simulation experiments demonstrate that the proposed ATB achieves broader coverage than flooding-based approaches, but at the expenses of transmission rate. The above approaches BIB008 BIB009 did not consider the effect of MAC layer settings on beacon reception probability and delay. The authors in BIB010 investigated the effect of MAC layer Contention Window (CW) on the efficiency of IEEE 802.11p beacon broadcasts. More precisely, their analysis improves the performance of the 802.11p broadcasts in terms of reception probability and delay with a focus on real-time vehicle control. Because vehicles are travelling in a very high speed, network topology is continuously changing along the time. In this harsh environment, vehicles could communicate and synchronize through exchanging of beacon messages. To promote situation-aware beacon communication between vehicles, adaptive beaconing is necessary in order to reduce network overhead. In BIB011 , Thaina et al. proposed two methods to adapt beacon rate with frequent topology changing of vehicular networks. The first method is linear regression analysis which can be represented as follows: where a and b are regression coefficients, which are computed in equ.3 and equ.4: where x and y are the average value of x and y respectively. As a result, each vehicle counts number of neighbour nodes and queued messages to estimate network traffic and density. The value of V is calculated as the summation of these two parameters. After this the value of X is obtained. Then, it is substituted in equ.2. From equ.2, we can compute the value of Y which is the next beacon interval. Their second method for estimating next beacon interval is k-nearest neighbour which is used to calculate the approximate value of target function. In this method, each vehicle creates a table which contains the parameters of node's environment condition (network density and traffic) and beacon interval. As mentioned early, vehicles usually moves at very high speed. The positional information exchanged between nearby vehicles promptly becomes outdated. The simple method to tackle this issue is to increase the transmission rate of beacon message transmission. But, this method might generate high traffic overhead in the network. To tackle this issue, the authors in BIB006 proposed an improved neighbour localization scheme that predicts the location of neighbours in the near future. Besides the node's current location, neighbour nodes use additional information stored in the beacon such as node's previous location, speed and direction. With these positional information, a vehicle uses predicted position of a neighbour vehicle instead of the position that exists in the received beacon. Each vehicle tracks the distance between actual and predicted position of the neighbour vehicle in order to define a threshold δ . New beacon will be scheduled for transmission if the difference between predicted and actual position is greater than δ . Fig.6 shows the information flow and beacon rate scheduling inside a node. Next beacon message is scheduled in two cases: due to large error threshold between actual and predicted vehicle position, and periodically with constant rate. In BIB006 , the authors conducted an extensive simulation study of the proposed localization scheme in realistic highway scenario (Freeway) BIB002 . The Freeway simulation model is modified to incorporate vehicle overtaking in multi-lane vehicular scenario. The proposed solution, jointly with the ones presented in the state of the art, are evaluated based upon the parameters of mean error and number of transmitted beacons. Simulation results show that proposed localization algorithm dramatically increases the accuracy of localization of neighbour nodes and number of beacons sent as compared to the naive approach of beaconing. However, the proposed scheme is evaluated in unrealistic vehicular environment since radio obstacles are not considered in the simulation scenarios. In addition, only beacon transmission in single hop are considered in the devised localization scheme.