# A Swarming Approach to Optimize the One-Hop Delay in Smart Driving Inter-Platoon Communications
###### Abstract  
Multi-platooning is an important management strategy for autonomous driving technology. The backbone vehicles in a multi-platoon adopt the IEEE 802.11 distributed coordination function (DCF) mechanism to transmit vehicles' kinematics information through inter-platoon communications, and then forward the information to the member vehicles through intra-platoon communications. In this case, each vehicle in a multi-platoon can acquire the kinematics information of other vehicles. The parameters of DCF, the hidden terminal problem and the number of neighbors may incur a long and unbalanced one-hop delay of inter-platoon communications, which would further prolong end-to-end delay of inter-platoon communications. In this case, some vehicles within a multi-platoon cannot acquire the emergency changes of other vehicles' kinematics within a limited time duration and take prompt action accordingly to keep a multi-platoon formation. Unlike other related works, this paper proposes a swarming approach to optimize the one-hop delay of inter-platoon communications in a multi-platoon scenario. Specifically, the minimum contention window size of each backbone vehicle is adjusted to enable the one-hop delay of each backbone vehicle to get close to the minimum average one-hop delay. The simulation results indicate that, the one-hop delay of the proposed approach is reduced by 12% as compared to the DCF mechanism with the IEEE standard contention window size. Moreover, the end-to-end delay, one-hop throughput, end-to-end throughput and transmission probability have been significantly improved. multi-platoon; inter-platoon communications; swarming; one-hop delay +
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0000-0001-6978-2288-2888-2888-2888-2888-2888-2888-2888-2888-2888-2888-2888-2888-2888-2888-2888-2888-2888-2888-2888-2888-28888-2888-2888-2888-28888-2888-28888-288  
thus saving thousands of lives [3]; traffic jams can be mitigated, thus reducing the fuel consumption [3]; user experience can be enhanced by liberating people from a long time's driving, enabling them to drink coffee, deal with their business and so on [4]. Platooning is an important management strategy for autonomous vehicles, which can reduce traffic congestion, conserve energy consumption, improve traffic safety and facilitate the management of autonomous vehicles [5]. With platooning strategy, autonomous vehicles can maintain a small constant distance between them on the common lane with a constant speed [6]. In this case, autonomous vehicles follow one after another in a queue and organize themselves as a set called a platoon. Each platoon consists of a leader vehicle, a tail vehicle, and some member vehicles. Specifically, the leader vehicle is the first vehicle, which controls the speed and driving direction of the platoon. The tail vehicle is the last vehicle within the platoon. The rest are the member vehicles. Note that both the leader vehicle and tail vehicle refer to backbone vehicles, while the member vehicles are called non-backbone vehicles. To keep a stable platoon formation, autonomous vehicles periodically transmit a packet with their kinematics information such as velocity and acceleration through vehicle-to-vehicle (V2V) communications [7]. Once the kinestate of a vehicle changes according to the surrounding environment, the vehicle, i.e., source vehicle, would transmit a packet with the updated kinematics information to other vehicles, i.e., destination vehicles. The destination vehicles can achieve the updated kinematics information in time and take prompt action to keep a stable platoon formation. When there are many vehicles in a platoon, the distance between the source vehicle and some destination vehicles may be long. The packet may be transmitted to some destination vehicles through multi-hop communications. The end-to-end delay would be long due to the plenty of collisions and hidden terminal problems caused by the simultaneous transmissions of many vehicles during the multi-hop communications.