Research On Dynamic Frame Aggregation Schemes For Vehicular Ad Hoc Networks

Vehicular Ad Hoc Network(VANET)is a mobile ad hoc network used in traffic environments to ensure safety of transportation systems and improve the travel experience of the passengers,through Vehicle-to-Vehicle(V2V)and Vehicle-to-Infrastructure(V2I)real-time communication.And it is one of the core technologies of intelligent transportation systems(ITSs).However,due to the high-speed mobility of vehicles and the volatile network topology,VANET will experience intermittent connections and may occasionally be unable to guarantee end-to-end connections.This will bring severe challenges to VANET's Medium Access Control(MAC)layer strategy,especially for real-time traffics.The current MAC layer strategy greatly reduces channel utilization due to huge transmission overheads,such as inter-frame spacing,control frames,physical layer headers,and MAC headers.In order to deal with the above problems,this thesis adopts the frame aggregation scheme in VANET based on the IEEE 802.11 standard.Frame aggregation scheme accumulates multiple subframes with the same target address into one data frame for transmission,which reduces the transmission time of overheads and the back-off time.It also reduces a lot of repetitive overheads and significantly improves channel utilization and throughput.But,the increase of the frame length of the aggregated frame will also increase the access delay.Therefore,how to determine the aggregated frame length and control the access delay will be a great challenge for the aggregation scheme in VANET.Based on the relationship between the throughput and the number of sub-frames under different bit error rates,and being combined with the delay requirement of VANET's traffics,this thesis proposes an adaptive A-MSDU frame length aggregation scheme with the limit of remaining delay(RD).Depending on the features of VANET's traffics,the scheme divides the traffics from the upper layer into four types to guarantees the Quality of Service(Qo S)requirements of the traffics,and it is dynamically adjusting aggregated frame length according to the current channel bit error rates and remaining delay so that the throughput is improved on the premise of satisfying the traffics' delay requirement.The simulation results show that the proposed adaptive A-MSDU frame length aggregation scheme improves throughput and channel utilization,and reduces the amount of overheads and retransmissions.In addition,this thesis conducts an theoretical analysis for the A-MPDU aggregation scheme.According to the results of theoretical analysis and combining the characteristics of A-MPDU aggregation scheme,this thesis proposes a RD-based A-MPDU dynamic frame aggregation scheduling scheme based on the previous classification of traffics in VANETs.In the scheme,the queue buffer is divided into two levels,the first level queue buffer stores the same traffic type data,the secondary level queue buffer can store the data of different types.It is responsible for sorting in ascending order in by remaining delays,and the aggregation unit aggregates the data frames with a small remaining delay each time.At the same time,the secondary level queue buffer maintains a dynamic timer to prevent the invalid of data due to the long waiting time when the data frame is insufficient.The simulation results show that the proposed A-MPDU dynamic frame aggregation scheduling scheme improves the system throughput,decrease the delay and packet loss rate.