# A Swarming Approach to Optimize the One-Hop Delay in Smart Driving Inter-Platoon Communications
###### Abstract
In this case, the packet delivery would suffer from the long end-to-end delay. Therefore, it is difficult to manage vehicles in a platoon especially when there are many vehicles [7]. To solve this problem, vehicles are organized into a multi-platoon instead of a single platoon [8]. A multi-platoon consists of a few platoons, as shown in Figure 1. In each platoon, the backbone vehicles are equipped with two transceivers [7]. One transceiver is used for inter-platoon communications, i.e., backbone vehicles communicate with other backbone vehicles. Another transceiver is used for intra-platoon communications, i.e., backbone vehicles communicate with the member vehicles in the same platoon. For a multi-platoon, the backbone vehicles in each platoon periodically transmit packets with their kinematics information to other backbone vehicles through inter-platoon communications, and then forward these packets to the member vehicles of the platoon through intra-platoon communications to keep a platoon formation. For inter-platoon communications, backbone vehicles adopt the IEEE 802.11 distributed coordination function (DCF) mechanism to access a control channel (CCH) and communicate with each other through multi-hop communications [7]. There are several major factors that may affect the one-hop delay of inter-platoon communications:  
1. The minimum contention window size defined in the IEEE 802.11 DCF mechanism. In fact, the IEEE 802.11 DCF mechanism uses the carrier sense multiple access with collision avoidance (CSMA/CA) mechanism to access a channel. It adopts a binary exponential back-off rule to  
Figure 1: A multi-platoon scenario. reduce collisions. According to the IEEE 802.11 standard [9], the minimum contention window size is 64, which is too large. In this case, a vehicle would wait a lot of time during a back-off procedure for transmitting a packet. Owing to the long one-hop delay, the standard IEEE 802.11 DCF mechanism may not be suitable for inter-platoon communications. 2. The hidden terminal problem. For inter-platoon communications, if two backbone vehicles are not in the communication range of each other, packets transferred between them may arrive at a third backbone vehicle simultaneously, thus incurring a collision at the third backbone vehicle. This is the hidden terminal problem. One of the two backbone vehicles is the hidden terminal of another backbone vehicle. The hidden terminal problem would cause the two backbone vehicles retransmit the packet, resulting in the increased one-hop delay. In a multi-platoon, as the number of hidden terminals of each backbone vehicle is different, the one-hop delay of these backbone vehicles would be unbalanced. 3. The number of neighbors. The number of neighbors may be different for different backbone vehicles. The number of neighbors is 1 for the first and last backbone vehicle and 2 for the remaining backbone vehicles. When a backbone vehicle is transmitting a packet, if more than one neighbor is transmitting other packets at the same time, there would be a collision. In this case, the backbone vehicle has to retransmit the packet, and thus prolong the one-hop delay. For different backbone vehicles, if the number of neighbors is different, the collision probability would also be different and thus incurs an unbalanced one-hop delay. As mentioned above, the parameters of DCF, the hidden terminal problem and the number of neighbors may affect the one-hop delay of inter-platoon communications and incur a long and unbalanced one-hop delay. It would affect the end-to-end delay from each source backbone vehicle to its destination backbone vehicles and make the destination backbone vehicles fail to receive vehicles' kinematics information timely through inter-platoon communications. After a destination backbone vehicle receives vehicles' kinematics information, it would forward the kinematics information to the member vehicles in the platoon through intra-platoon communications.