# Data Rate Selection Strategies for Periodic Transmission of Safety Messages in VANET
## 2 Background Review
### Power- and Message Rate-Based Approaches  
Transmission power affects the distance over which a message can be successfully received by another vehicle. A higher transmission power increases the range across which a message can be received, so vehicles that are farther away can receive it. Controlling the transmission power based on the level of channel congestion involves reducing the power when the channel is congested; this results in vehicles only receiving messages from vehicles that are nearby. Although this limits the vehicle's awareness, it also serves to reduce congestion, and nearby vehicles are the most important vehicles to receive messages from because they have the most immediate impact on a vehicle. In Ref. [33], the vehicles adjusted their transmission power according to their speed, which resulted in a reduction in the beacon error rate (BER) and CBR. In Ref. [34], vehicles used different levels of transmission power depending on the local vehicle density. Vehicles adapted to high densities by using a low power level, while medium and high densities resulted in the use of a medium and high power level, respectively. In Ref. [35], the authors proposed the distributed fair transmit power adjustment for VANET (D-FPAV) algorithm, which adjusted the transmission power based on the vehicle density and the traffic in the application layer. In Ref. [36], the authors proposed the CLF-BTPC algorithm, where all vehicles started with the same initial value for the transmission power, and the algorithm then calculated a forecasted value for the congestion and an assigned power based on this value. All vehicles in the network would be assigned the new power value; if the forecasted value was greater than a predefined threshold value, the assigned power would be less than the current power, and it would be greater if the forecasted value was less than the threshold. In Ref. [37], the authors set out to increase the awareness quality. A random transmission power level was selected for each packet transmission, and each vehicle controlled its power selection by using a complementary cumulative distribution function (CCDF) due to its strong correlation with awareness quality. While the default transmission rate for BSMs is 10 Hz, message rate-based approaches change the frequency at which a vehicle sends out BSMs as an adaptive response to its surroundings and the congestion of the channel. As the channel congestion increases, the message rate is reduced accordingly, with the goal of reducing the channel load. The main limitation of these approaches is that reducing the message rate of a vehicle also restricts the frequency at which it updates other vehicles on its status, so the vehicles will be less aware of their surroundings. This reduces the safety of the network, since current information and frequent updates from the vehicles are vital to most safety applications. Some well-known congestion control algorithms using message rate control are discussed below. In Ref. [16], the authors proposed a new scheme called linear message rate control (LIMERIC), where vehicles can sense the current channel load and adapt to changes by adjusting their message rates to meet a predefined required CBR value. In Ref. [38], the authors proposed a method that extended the LIMERIC algorithm by using a predefined target value to control the total channel load. In Ref. [39], the authors used a function of both the channel load (the LIMERIC component) and vehicle dynamics (the suspected tracking error (STE) component) to calculate the transmission rate. The LIMERIC component calculates an ideal message rate based on the channel load using the LIMERIC algorithm, while the STE component estimates the time when the CBR is expected to reach a given threshold and ensures that the next packet is sent no later than the estimated time. In Ref.