Beaconing Approaches in Vehicular Ad Hoc Networks: A Survey <s> Beacon Transmission Power Control <s> Abstract This paper summarizes the principal results of a series of statistical studies in the last seven years on the intensity distributions due to rapid fading. The method of derivation and the principal characteristics of the m-distribution, originally found in our h.f. experiments and described by the author, are outlined. Its applicability to both ionospheric and tropospheric modes of propagation is fairly well confirmed by some observations. Its theoretical background is also discussed in detail. A theoretical interpretation of the log-normal distribution is given on the basis of this formula. An extremely simplified method is presented for estimating the improvement available from various systems of diversity reception. The mutual dependences between the m-formula and other basic distributions are fully discussed. Some generalized forms of the basic distributions are also investigated in relation to the m-formula. Two methods of approximating a given function with the m-distribution are shown. The joint distribution of two variables, each of which follows the m-distribution, is derived in two different ways. Based on this, some useful associated distributions are also discussed. <s> BIB001 </s> Beaconing Approaches in Vehicular Ad Hoc Networks: A Survey <s> Beacon Transmission Power Control <s> We present a new protocol for power control in ad hoc networks. We describe the issues in conceptualizing the power control problem, and provide an architecturally simple as well as theoretically well founded solution. The solution is shown to simultaneously satisfy the three objectives of maximizing the traffic carrying capacity of the entire network, extending battery life through providing low power routes, and reducing the contention at the MAC layer. Further, the protocol has the plug and play feature that it can be employed in conjunction with any routing protocol that pro-actively maintains a routing table. The protocol, called COMPOW, has been implemented in the Linux kernel and we describe the software architecture and implementation details. <s> BIB002 </s> Beaconing Approaches in Vehicular Ad Hoc Networks: A Survey <s> Beacon Transmission Power Control <s> Vehicular Ad Hoc Networks (VANET) have several characteristics that distinguish them from other ad hoc networks. Among those is the rapid change in topology due to traffic jams, which also disturbs the homogenous distribution of vehicles on the road. For this reason, a dynamic transmission range is more effective in maintaining connectivity while minimizing the adverse effects of a high transmission power.We provide a relationship that allows vehicles to estimate the local density and distinguish between two phases of traffic, free-flow and congested traffic. The density estimate is used to develop an algorithm that sets a vehicle transmission range dynamically according to local traffic conditions.Simulations of various road configurations show that the algorithm is successful in maintaining connectivity in highly dynamic networks. <s> BIB003 </s> Beaconing Approaches in Vehicular Ad Hoc Networks: A Survey <s> Beacon Transmission Power Control <s> The discrepancy between real-world radio channel behavior and its standard modeling in simulations (unit disk graph) is a major reason for protocols to perform differently - often worse - than predicted when deployed in a real-world setup. As researchers having to deal with real ad hoc networks are aware of, assuming a fixed border for a node's communication range might not only lead to inaccurate results but also to a wrong judgment on the comparison between different protocols. We have set up a simulation study to investigate the effects of realistic channel characteristics on packet forwarding strategies for vehicular ad hoc networks. The contributions of this paper are threefold: i) we provide a performance evaluation of various routing/forwarding strategies under the realistic non-deterministic Nakagami radio propagation model and compare the results with the ones obtained using the standard two-ray-ground model. Validated German highway movement patterns are used to model node mobility. ii) We demonstrate that realistic channel conditions present an opportunity and not only a drawback for some forwarding strategies. More specifically, we show that for contention-based forwarding (CBF) techniques, realistic channel characteristics provide a positive impact in terms of an increased average hop distance. iii) We provide an analytical derivation of the expected hop distance for CBF that provides a basis to optimally adjust CBF parameters <s> BIB004 </s> Beaconing Approaches in Vehicular Ad Hoc Networks: A Survey <s> Beacon Transmission Power Control <s> The main goal of inter-vehicle communication technologies is to provide each vehicle with the required information about its surrounding in order to assist the driver avoiding potential dangers. The required information level, or awareness, can be achieved by the exchange of periodic status messages (beacons) among neighboring vehicles together with the quick dissemination of information about potential hazards. In previous work, we proposed an algorithm (D-FPAV) to control the beaconing load on the medium by adjusting the transmission power in a fair and distributed fashion. In this paper, we adapt a promising position-based message forwarding strategy in order to disseminate time-critical safety information. Moreover, we evaluate its performance when making use of D-FPAV, i.e., limiting the beaconing load on the medium, in a realistic highway environment with probabilistic radio propagation characteristics <s> BIB005 </s> Beaconing Approaches in Vehicular Ad Hoc Networks: A Survey <s> Beacon Transmission Power Control <s> We consider the problem of adjusting transmission power for vehicle-to-vehicle broadcast safety communication in vehicular ad hoc networks. Given a target communication range designated by a vehicle safety application, the power control algorithm is designed to select a transmission power no greater than necessary for the targeted range. The power control algorithm results in higher communication reliability since collisions are minimized for safety communications. Our main idea is to add a power tuning feedback beacon during each safety message exchange. Our simulation results show that the power control algorithm can reduce the transmission power significantly to achieve higher packet reception rate than the case without any power control. This algorithm is mainly applied to communications-based vehicle safety applications but could be useful for other applications as well. <s> BIB006 </s> Beaconing Approaches in Vehicular Ad Hoc Networks: A Survey <s> Beacon Transmission Power Control <s> Direct radio-based vehicle-to-vehicle communication can help prevent accidents by providing accurate and up-to-date local status and hazard information to the driver. In this paper, we assume that two types of messages are used for traffic safety-related communication: 1) Periodic messages (ldquobeaconsrdquo) that are sent by all vehicles to inform their neighbors about their current status (i.e., position) and 2) event-driven messages that are sent whenever a hazard has been detected. In IEEE 802.11 distributed-coordination-function-based vehicular networks, interferences and packet collisions can lead to the failure of the reception of safety-critical information, in particular when the beaconing load leads to an almost-saturated channel, as it could easily happen in many critical vehicular traffic conditions. In this paper, we demonstrate the importance of transmit power control to avoid saturated channel conditions and ensure the best use of the channel for safety-related purposes. We propose a distributed transmit power control method based on a strict fairness criterion, i.e., distributed fair power adjustment for vehicular environments (D-FPAV), to control the load of periodic messages on the channel. The benefits are twofold: 1) The bandwidth is made available for higher priority data like dissemination of warnings, and 2) beacons from different vehicles are treated with ldquoequal rights,rdquo and therefore, the best possible reception under the available bandwidth constraints is ensured. We formally prove the fairness of the proposed approach. Then, we make use of the ns-2 simulator that was significantly enhanced by realistic highway mobility patterns, improved radio propagation, receiver models, and the IEEE 802.11p specifications to show the beneficial impact of D-FPAV for safety-related communications. We finally put forward a method, i.e., emergency message dissemination for vehicular environments (EMDV), for fast and effective multihop information dissemination of event-driven messages and show that EMDV benefits of the beaconing load control provided by D-FPAV with respect to both probability of reception and latency. <s> BIB007 </s> Beaconing Approaches in Vehicular Ad Hoc Networks: A Survey <s> Beacon Transmission Power Control <s> Position-based routing has proven to be well suited for highly dynamic environment such as Vehicular Ad Hoc Networks (VANET) due to its simplicity. Greedy Perimeter Stateless Routing (GPSR) and Greedy Perimeter Coordinator Routing (GPCR) both use greedy algorithms to forward packets by selecting relays with the best progress towards the destination or use a recovery mode in case such solutions fail. These protocols could forward packets efficiently given that the underlying network is fully connected. However, the dynamic nature of vehicular network, such as vehicle density, traffic pattern, and radio obstacles could create unconnected networks partitions. To this end, we propose GeoDTN+Nav, a hybrid geographic routing solution enhancing the standard greedy and recovery modes exploiting the vehicular mobility and on-board vehicular navigation systems to efficiently deliver packets even in partitioned networks. GeoDTN+Nav outperforms standard geographic routing protocols such as GPSR and GPCR because it is able to estimate network partitions and then improves partitions reachability by using a store-carry-forward procedure when necessary. We propose a virtual navigation interface (VNI) to provide generalized route information to optimize such forwarding procedure. We finally evaluate the benefit of our approach first analytically and then with simulations. By using delay tolerant forwarding in sparse networks, GeoDTN+Nav greatly increases the packet delivery ratio of geographic routing protocols and provides comparable routing delay to benchmark DTN algorithms. <s> BIB008 </s> Beaconing Approaches in Vehicular Ad Hoc Networks: A Survey <s> Beacon Transmission Power Control <s> Problem statement: Vehicular Ad hoc Networks (VANET) is one of the most challenging research area in the field of Mobile Ad Hoc Networks. Approach: In this research we proposed a dynamic power adjustment protocol that will be used for sending the periodical safety message. (Beacon ) based on the analysis of the channel status dependi ng on the channel congestion and the power used for transmission. Results: The Beacon Power Control (BPC) protocol first sensed and examined the percentage of the channel congestion, the result ob tained was used to adjust the transmission power fo r the safety message to reach the optimal power. Conclusion/Recommendations: This will lead to decrease the congestion in the channel and achieve good channel performance and beacon dissemination. <s> BIB009
As mentioned in previous section, periodic and even-driven messages are required to activate safety applications. Periodic beacon messages are necessary to enable awareness of nearby vehicles by exchanging state information such as position, speed, direction and other vehicle's information. This beaconing message handshaking captures important information of vehicles in the vicinity and thereby each vehicle will be able to detect emergency situations in the surroundings. The beaconing load on the wireless channel should precisely be monitored to guarantee bandwidth requirement of active safety applications. To this purpose, the authors in BIB007 conducted performance analysis of active safety related applications in order to identify mechanisms to improve VANET performance through reducing packet level interference. For this purpose, they proposed a distributed fair transmit power adjustment algorithm for vehicular ad Hoc networks called D-FPAV. It maintains the beaconing load so that the active safety applications meet a strict deadline of the event driven message. In addition, they developed a fast beacon message dissemination towards destination using receiver based relay node self election. D-FPAV limits the channel overhead on the wireless medium due to periodic beacon message transmission and provides the entire channel capacity in case of the occurrence of any emergency situations. The priority of message transmission is achieved through EDCA of the IEEE 802.11p. In other words, event driven messages have highest priority and maximum transmit power as compared to the periodic beacon message. For the fast dissemination of beacon safety messages, the authors in BIB005 BIB004 proved desirable and satisfactory performance of beaconless geographical forwarding. According to this forwarding mechanism, the emergency message is transmitted to all neighbour nodes and they independently elect themselves as a next forwarder. The proposed mechanism favours farthest node to be a next beacon message forwarder. The performance evaluations were done using a popular Network Simulator (ns-2) [30] . Although ns-2 is a widely used network simulator among researchers, there are some defects in terms of accuracy of lower layers. To tackle this issue, the authors added a cumulative noise module to the physical layer of the IEEE 802.11p. A node accumulates the power level of all interfering packets that generated from neighbour nodes with the background noise.Then, the node decides whether the beacon message is successfully received. The probabilistic Nakagami BIB001 propagation model was used since it can estimate the fading phenomena of wireless channel in vehicular scenario with buildings and trees. In the performance evaluation, the D-FPAV On is compared with the D-FPAV Off. It is worth mentioning that with D-FPAV Off all beacon messages are sent with maximum Carrier Sense range (CS= 1000 m) whereas in D-FPAV On the beacon transmission power is computed based on the D-FPAV algorithm. Results show that D-FPAV On always performs better than when it is compared to D-FPAV Off in terms of probability of successful packet reception and channel access time. The aims of transmission range adaptation in mobile wireless networks are to increase throughput, network and link lifetime BIB006 BIB002 . These conventional objectives of power control can not be applicable in vehicular networks since network topology and communication between vehicles have (a) Tx is larger than R. (b) R is larger than Tx. unique characteristics when they are compared with traditional mobile networks. In addition, vehicleto-vehicle communication is emerged to improve traffic safety on the road. Thus, power control in vehicular networks should fulfill reliability and delay sensitive active safety applications. This interesting application has motivated some researchers to develop suitable power control mechanism. For instance, the authors in BIB006 proposed a transmission power control algorithm of safety messages that are exchanged between nearby vehicles. The algorithm is based on the fact that each vehicle periodically broadcast its status information and the required information for power control is piggybacked in the safety message header. The piggybacked information is the target range and feedback beacon. The target range is defined as the specified range up to which the transmitted safety message is effective. As depicted in Fig.4 , vehicles are traveling along a highway with three lanes. Vehicle effective communication range and target range are denoted as Tx and R respectively. The source node first broadcasts a safety message to nearby vehicles. When the neighbour vehicle received the message, the relative distance is computed between the message sender and receiver. The source node realizes that the Tx range is larger than R range based on the feedback from the neighbour nodes that are outside the R range. In this case, the transmission power will be reduced ( Fig.4a ). If this case is not happen, the transmission power of the safety message is increased by the source (Fig.4b ). The scheme proposed by the authors in BIB006 is simulated on the OPNET network simulator with modifying 802.11a to model 802.11p MAC layer. The log-normal radio propagation model is used to model slow fading in which the wireless channel is not susceptible to fast variation during communication between vehicles. Simulation results show that the proposed scheme performs better as compared to a scheme with no power control on terms of probability of message loss. However, the performance of proposed power control scheme is not optimal since the response packet from neighbour receivers might be lost due to channel collisions and leads to higher transmit power by the source node. Following the trend of adjusting the transmission power of packet carrier vehicles, the authors in BIB008 proposed an adaptive power control mechanism based traffic and propagation modeling. Lognormal shadowing is used to model the power propagation in highway vehicular scenario and real highway data has been used to model vehicular traffic distribution from late night to the daytime rush hours. Performance evaluation results of the proposed mechanism is compared with the no-power control algorithms in three traffic situations: early morning, moderate morning traffic and evening rush hour. Results show that the expected transmission power reduces from the early morning to the evening rush hour. As witnessed by the authors in  and  , a dynamic adaptation of transmission range is important in heterogeneous traffic distribution like vehicular networks. This improves the network performance in terms of packet collisions, interference and network overhead. To tackle the connectivity issues in such bipolar traffic environment, the authors in BIB003 proposed a mechanism to adapt vehicles transmission range based on local vehicles traffic density estimation. The traffic density estimation is based on the movement of vehicles i.e., when a vehicle travels with high speed, the local density is low and vice versa. Simulation results shown in this paper reveal that the proposed mechanism maintains the link life time of the wireless channels between high speed vehicles. In another attempt of dynamic power transmission, Samara et al. in BIB009 proposed a scheme based on the numerical analysis to control the transmission power of the periodic beacon message. The basic idea of the proposed scheme is that the a vehicle counts the number of beacons lost or successfully received in 1 second from the neighbouring vehicles. As depicted in Fig.5 , if the beacon generation rate is 10 ms, vehicle S must receive 10 beacon messages from each neighbour vehicle (A,B,C,D) every 1 second. More precisely, the proposed scheme calculates the scheduled transmission power as in Equ.1: where TP is the new transmission power, MinP is minimum power of neighbour vehicles (this parameter is included into beacon message), PD is the power difference between maximum and minimum received power of neighbour vehicles and PS is the percentage of success of beacon message reception. When a vehicle (such as S in Fig.5 ) receives a beacon message extracts minimum and maximum received power from them. Moreover, PD as well as PS can be determined based on the number of beacon messages that are received successfully.