# Data Rate Selection Strategies for Periodic Transmission of Safety Messages in VANET
###### Abstract  
Vehicular ad hoc networks (VANETs) facilitate communication among vehicles and possess designated infrastructure nodes to improve road safety and traffic flow. As the number of vehicles increases, the limited bandwidth of the wireless channel used for vehicle-to-vehicle (V2V) communication can become congested, leading to packets being dropped or delayed. VANET congestion control techniques attempt to address this by adjusting different transmission parameters, including the data rate, message rate, and transmission power. In this paper, we propose a decentralized congestion control algorithm where each factor adjusts the data rate (bitrate) used to transmit its wireless packet congestion based on the current load on the channel. The channel load is estimated independently by each vehicle using the measured channel busy ratio (CBR). The simulation results demonstrate that the proposed approach outperforms existing data rate-based algorithms, in terms of both packet reception and overall channel load. c pacs: 12.37.-a, 12.37.+a, 12.37.+a, 12.37.+b, 12.37.+b, 12.37.  
aware of, such as the vehicle's current position, speed, acceleration, and heading. Vehicles send BSMs through the channels allocated in the DSRC/WAVE system [12], and they are processed using the on-board units (OBUs) that are placed inside each vehicle. In the United States, the FCC has allocated 75 MHz of spectrum in the 5.9 GHz band for dedicated short-range communication (DSRC) [13]. This spectrum is divided into seven 10 MHz channels with associated guard bands, from which channel 172 is assigned for the exchange of safety messages [14]. Many VANET simulations [13; 15; 16] and some standardization activities [17] use a BSM transmission data rate of 6 Mbps, although some papers have considered other data rates; for example, a data rate of 3 Mbps was used in Refs. [18; 19] because of its low signal to interference and noise ratio (SINR) requirement. The typical BSM transmission rate for vehicles is 10 Hz, resulting in one BSM being sent every 0.1 s; however, an increase in vehicle density can cause a heavy channel load. Channel congestion occurs when the load is so high that the nodes (vehicles) have to compete with each other in order to access the channel [20]. A channel load of 40% or higher results in a rapid increase in the number of packet collisions and delays [21], so it is important to implement appropriate congestion control methods in order to avoid the channel being congested and ensure that messages are delivered properly. Many VANET congestion control algorithms attempt to reduce the channel load by lowering the BSM transmission rate, transmission power, or a combination of both, but this can come at the cost of significantly lowering the awareness of the surrounding vehicles. Some recent papers have examined the effects of adjusting the BSM transmission data rate (i.e., bitrate) on congestion. For the remainder of the paper, we will use the terms data rate and bitrate interchangeably. Packets sent with lower bitrates take longer to send but have a lower SINR threshold for proper frame reception [22]. This means that BSMs transmitted with a lower bitrate have a lower chance of becoming corrupted due to noise interference and are more likely to be received at longer distances. Conversely, packets that are sent with a higher bitrate are sent faster, reducing channel congestion, but have a greater chance of not meeting the required SINR threshold. Therefore, when a vehicle sends a BSM, it should use a bitrate that can balance the desired characteristics of having low channel congestion and also having a high chance of the beacon being successfully received by nearby vehicles. In this paper, we propose a new congestion control algorithm that selects a suitable bitrate value for each BSM sent, determined by the current channel busy ratio (CBR).