## 2 Related Works
CFR MAC has the advantage of avoiding merging collisions because it allocates resources by considering both the moving direction and speed of each vehicle. However, when traffic density is high in one direction, vehicles may experience many merging collisions due to finely divided resource allocation units. Figure 4: Frame architecture of Vehicular Ad Hoc Network MAC (VeMAC). Figure 3: Frame architecture of ADHOC Medium Access Control (MAC). Prediction-based TDMA MAC (PTMAC) proposed a method to avoid resource collisions by predicting the position of vehicles using the same resources over a 2-hop communication range [10]. Like ADHOC MAC, PTMAC allocates available time-slots randomly regardless of the vehicle movement direction and speed. Each vehicle stores its occupied time-slot, its own position, its speed, and its moving direction in its FI message and broadcasts it to its own 1-hop neighbor vehicles. As shown in Figure 6, each vehicle can recognize the existence of 2-hop neighbor vehicle(s) using the same time-slot from the FI message and can predict whether there will be resource collision in the near future by estimating the position of vehicles sharing the same time-slot. If a resource collision is predicted, the vehicle abandons the resources occupied by the current frame and selects another unused time-slot in the next frame. In [11], MAC protocols designed or adapted for VANET were studied giving a qualitative comparison with the IEEE 802.11 protocols based on CSMA/CA and inter-frames spaces. The author of [12] proposed MOvement Prediction based Routing (MOPR) for VANETs, which improves the routing process by selecting the most stable route in terms of lifetime considering the movement of vehicles, and the MOPR was applied to position-based routing protocol. The results showed that the MOPR enhanced the performance compared to Greedy Perimeter Stateless Routing (GPSR) and MOvement-based Routing Algorithm (MORA). In [13; 14], the authors suggested the hybrid protocol called Active Signaling-DTMAC (AS-DTMAC), with the purpose of resolving the problem of access collision in the time-slot assignments of the DTMAC protocol. In these works, AS-DTMAC is analyzed based on generating functions and the collision rate of AS-DTMAC is studied. The number of time frames to achieve collision-free resource allocation and the transmission requirements for urgent packets are studied. The simulations showed the enhanced performance of the proposed AS-DTMAC in terms of collision rate and convergence. It is noted that VeMAC and CFR MAC can reduce resource collisions by using slot segmentation techniques. However, in the direction with high traffic density, the slot usage rate is low, resulting in frequent merging collisions. In addition, resource collisions between vehicles traveling in the same direction but at different speeds cannot be avoided. PTMAC suggests a resource collision method  
Figure 5: Architecture of collision-free reservation-based MAC (CFR MAC). Figure 6: Collision prediction of prediction-based Time Division Multiple Access (TDMA) MAC (PTMAC). that is based on the prediction of location, speed, moving direction, and slot information of neighbor vehicles without using a slot partitioning method. However, many vehicles can not accurately predict whether or not there will be resource allocation collision (time-slot collision in TDMA-based VANET) due to sudden change of position arising from rapid acceleration/deceleration and moving direction change. In addition, in some road environments such as intersections with/without traffic lights, circuit intersections, and curved highways, the accuracy of the collision prediction can be greatly reduced if it relies only on the information of the current traveling speed and the simple moving direction (left and right or up and down) of each vehicle.