Research Of Delay Tolerant Routing In Cognitive Radio Vehicular Ad Hoc Networks

With the rapid development of the vehicle industry,the limited spectrum resources of vehicular ad hoc networks cannot meet the increasing demand for vehicular communications.Cognitive radio(CR)technologies enable in-vehicle communication devices to dynamically access the spectrum,and it can alleviate the spectrum shortage problem and improve the spectrum utilization efficiency.It produces a new communication paradigm called cognitive radio vehicle ad hoc networks(CR-VANETs),which brings new challenges and becomes a research hotspot in China and abroad.Routing problems for vehicular ad hoc networks or cognitive radio ad hoc networks have been well studied extensively.However,the existing works have the following restrictions,making them impossible to be directly applied in CR-VANETs.(1)The rapid movement of nodes,network sparsity,environmental interference,and rapid changes of the CR spectrum make the existing solutions impossible to ensure a stable connection path,and difficult to achieve the expected performance.(2)Without fully considering the spectrum resource scheduling problem,the spectrum utilization efficiency of existing solutions is low.It is still an unsolved problem that how to maximize the utilization of the spectrum resource while reducing the spectrum band consumption caused by more routing hops.(3)Because existing solutions are designed for real-time applications,most of them have high transmission overhead and low packet delivery ratio.For non-real-time applications,it is a challenging problem that how to minimize the transmission overhead while maximizing the packet delivery ratio.(4)Existing route selection methods only consider the spectrum and link characteristics,and ignore the social behavior of networks.Because it is difficult to precisely predict the high-speed of vehicles and the dynamic changes of available channels,the existing routing schemes which are based on the spectrum information and link condition,may lead to sub-optimal or unavailable routing decisions.The delay-tolerant routing is more suitable for the network with random node movements and frequently disconnected links,and it adopts the "storage-carry-forward" strategy to solve the network partitioning problem.To this end,this thesis investigates delay-tolerant routing problems in CR-VANETs,concentrating on the routing selection,spectrum resource scheduling,message scheduling,and social-aware message replication and forwarding issues.Specifically,the main research contents and contributions of this thesis are described as follows.(1)We propose a delay-tolerant routing scheme based on store-and-forward in CR-VANETs to maximize the packet delivery ratio.We first established a CR channel availability probability model and a V2 V contact duration model to adapt to the dynamic characteristics of CR spectrum and vehicle high-speed mobility.Secondly,a store-and-forward routing algorithm is proposed.The algorithm consists of a forwarding strategy and a packet replication strategy.In the forwarding strategy,we first select available relay nodes based on the CR spectrum characteristics,and then select a joint forwarding set of relay nodes based on the effective V2 V contact duration.In the packet replication strategy,candidates are used to store and greedily forward packet replications to as many relay nodes as possible to increase the packet delivery ratio.At the same time,a hybrid adaptive strategy is adopted,which combines the Drop-oldest and Drop-most mechanisms to reduce the overhead.Simulation results show that our proposal presents on average gains of 17.4% in the delivery ratio when compared with other routing schemes.(2)We propose a delay-tolerant routing scheme based on joint scheduling in CR-VANETs to optimize routing hops while scheduling limited CR spectrum resources.First,we built a network model in CR-VANETs to predict vehicle movement and define the effective contact durations between vehicles.Secondly,we establish a CR spectrum model based on V2 V communication,and obtained the CR spectrum availability probability.Through the parameters obtained by modeling,we define the link consumption of the V2 V communication,use it as a weight factor to weigh the link quality,and use this weight factor to perform CR spectrum resource scheduling.We analyze the optimization goal and transform it into the problem of minimizing route hops under the constraint of limited CR spectrum resources,and prove that the problem is NP-hard.Finally,in view of this joint routing scheduling problem,we propose a hybrid heuristic algorithm that combines the fast convergence of particle swarm optimization and the population diversity of genetic algorithms to schedule routing hops while scheduling limited CR spectrum resources.Simulation results show that our proposal presents on average gains of 27.2% in the hop count when compared with other routing schemes.(3)We propose a delay-tolerant routing and message scheduling scheme in CR-VANETs to maximize the packet delivery ratio and reduce the transmission overhead.First,we build a joint forwarding set and design a forwarding strategy.In this set,we select relay candidates by considering CR channel availability and V2 V contact duration.In addition,we evaluate the priority of relay candidates to provide a reliable forwarding strategy,and take this priority as one of the reference elements of buffer management.We also give the message complexity of our routing scheme.Secondly,we propose an optimized binary tree replication algorithm,which can quickly propagate messages and terminate the replication process.We give the calculation process of the upper bound of message replications.Finally,considering node priority,message tag and message remaining lifetime,we propose a buffer management strategy to make the tradeoff between the packet delivery ratio and the transmission overhead.Simulation results show that our proposal presents on average gains of 25.1% and 10.9% in terms of delivery ratio and overhead ratio,respectively,when compared with four CR-VANET routing schemes.(4)We propose a social-aware delay-tolerant routing scheme in CR-VANETs to maximize the packet delivery ratio and reduce the transmission overhead.First,we use social centrality to evaluate the PU’s active probability,and then obtain the accurate CR channel availability probability for SUs.Secondly,we propose a SU-based community partitioning algorithm based on encounters and related to geographic locations.It dynamically divides SUs into intra-community groups and inter-community groups.Then,we apply different strategies to different community communication processes to increase the packet delivery ratio and reduce the overhead.For the intra-community communication process,we use a single copy strategy and the contact duration as the forwarding rank.Moreover,for the inter-community communication process,we propose an optimized binary tree replication algorithm that can quickly replicate packets and terminate replication,and utilize the bridge coefficient to evaluate the forwarding rank.Simulation results show that our proposal presents on average gains of 9% and 17.5% in terms of delivery ratio and overhead ratio respectively,when compared with four CR-VANET routing schemes.