| UAV(Unmanned aerial vehicle)can act as flying base stations and construct airborne networks,to provide communication service for UEs(User Equipments)on the ground,which is promising during temporary activities,the earthquake relief and emergency communications.There are two crucial problems in UAV airborne networks:(i)how can UAVs autonomously move to the desired locations to fulfill on-demand coverage for UEs on the ground while maintaining connectivity for UAV airborne networks;(ii)how to make proper decisions to allocate the limited channel resource while reducing the interference among UAVs,in order to increase the aggregate throughput of the airborne networks and reduce packet loss.To this end,aiming at the distributed deployment algorithms and access technology for UAV airborne networks,we employ the on-demand deployment algorithm based on virtual forces and adopt the multi-channel access technology based on directional antennas,considering the actual network environment including the UE density,flying nature of UAVs and limited energy.The main work is summarized as follows.We first propose a deployment algorithm based on virtual forces,aiming at the on-demand coverage for UEs on the ground provided by UAV airborne networks and the demand for service quality of UEs and load balancing of the network.The deployment algorithm uses virtual forces to drag UAVs to realize on-demand coverage while maintaining the stable bi-connected topology by sensing the UEs on the ground,at the meantime it can record not only the final positions of UAVs but also the motion tracks of them.Moreover,on the basis of the deployment algorithm,our access selection algorithm puts different emphasis on the SINR(Signal to Interference and Noise Ratio)and the degree(load number)of each UAV,proposing three access selection methods,which aims at maximizing the load balancing and successful rate of access of the whole network.The access options are handed over to UEs to ensure the quality of service.Then,considering the applicability problem of the distributed deployment algorithm based on virtual forces above,we propose the joint deployment of ancillary UAVs and joint optimization of another centralized deployment algorithm.Firstly,when new UEs join in the deployed UAV airborne networks or the traditional cellular network,coverage blind area may occur.Therefore,we propose the ancillary deployment scheme based on virtual forces,supplemented by the interspersed fight algorithm,for the purpose of adminiculary coverage for blind area on the ground and merge of the original backbone network and ancillary UAVs.Secondly,learning from our deployment algorithm based on virtual forces,we optimize the deployment result of another centralized algorithm,guiding UAVs to adjust the coverage area to fulfill the on-demand coverage with minimum number of UAVs.At last,aiming at increasing the channel capacity of UAV airborne networks and reducing data conflicts,we propose a multi-channel access technology based on directional antennas.Directional antennas can be used for intent transmission with high gain to reduce interference and expand the transmission range.Combining it with frequency division can be applicable for the concurrent transmission in high-density UAV networks.The source UAV notifies the destination UAV by sending circular directional RTS(Request To Send),the destination UAV makes decisions of channels and beams while transforming traditional omnidirectional NAV(Network Allocation Vector)into directional NAV.We divide channel using frequency division and multiplex multiple beams from the spatial perspective.Compared to those MAC protocols using single channel or omni-directional antennas,our access mechanism enables the increase of the shared resource utilization and aggregate throughput of the UAV networks and the reduction of the extend starvation in ad-hoc network. |
