Research On Transmission Capacity Of Vehicular Ad-hoc Networks

As the basis information platform for Intelligent Transport System (ITS),Vehicular Ad-hoc Networks (VANET) has attracted increasing number of concerns. Different from the traditional wireless networks, in vehicular communication system, the high-speed mobility of vehicles makes the transmission environment change rapidly. Meanwhile, the contradiction between explosive growth of communication requirement and limited local resources has become increasingly prominent. As a key index to reflect the performance of VANET, the research on the network capacity is of great significance to the research and application of VANET.GuPta and Kumar first proposed the transport capacity which is an important network capacity definition. The transmission capacity is studied by the scaling law which can not be expressed accurately. Therefore, based on the theoretical results of transport capacity, many scholars put forward some new definitions of network capacity according to different research objectives.Transmission capacity due to the accurate relationship between network parameters can give more academic attention and recognition. The current research on the transmission capacity of VANET most directly undertakes the system model and the traditional architecture of wireless ad hoc networks. It lacks of reasonable modeling of vehicle behavior, and the mechanism of MAC modeling is not perfect. In this paper, combined with the vehicle behavior model in the field of transportation and the MAC mechanism of the IEEE 802.11p standard, the system model of the actual VANET environment is established,and the specific expression of the VANET transmission capacity is studied.Firstly, we study broadcast transmission capacity in a one dimension vehicular ad hoc network model. We develop our model on the basis of a highway scenario and extend both ends of the highway to infinity, and vehicles are assumed to follow a homogeneous Poisson point process. In 802.11p, the fixed total bandwidth is divided into a large number sub-bands that individual users have access to. In intuition, increasing sub-bands number theoretically allows the network to simultaneously support more parallel and noninterfering communications, however, the same information rate is achieved over less bandwidth, so a higher signal-to-noise ratio for each transmission pair is required. We study this problem and obtain the optimum number of sub-bands.We also derive the broadcast transmission capacity over Nakagami fading channels and the transmission capacity with the secrecy outage constraint.Numerical results show that the transmission capacity under Nakagami fading models and the optimal value of the number of sub-bands exist in different scenarios.Secondly, we describe the CSMA/CA protocol used in IEEE802.11p from the perspective of the geometric relationship amongst simultaneous transmitters.The desired channel and interfering channels are assumed to experience the same amount of path-loss and Rayleigh fading. On the basis of the proposed model, we analyse the attempted transmission probability of each road segment and the maximum intensity of active transmitters, including their theoretical values. Then, we employ the physical model to obtain the outage probability and derive the upper bound of the transmission capacity of a VANET, which is defined as the average spatial density of successful transmissions in the network.Simulation results indicate that the theoretical value offers a good bound on network capacity.Thiredly, we propose a CR-VANET model based on celluar uplink spectrum for large urban road environment, in which the bus Wi-Fi cellular network (primary network) and vecular ad hoc network (secondary network)share the uplink spectrum of celluar network, and vehicles in VANET have opportunity to access the primary network for transmission. Secondly, based on the model we proposed, we propose two ways to access the primary network: (1)Cognitive-Contention protocol based on power threshold, a method to find the spatial spectrum hole by detecting the power of beacon signals, and implement spectrum sensing in the region of spectrum hole; (2) Cognitive-Contention protocol based on priority, a method considering both data type and requirement of transmission distance, which will promote the transmission opportunity of services with high priority. Finnaly, we derive the transmission opportunity and transmission capacity in such model and protocols, and validate the relation between transmission opportunity and various factors and the promotion of transmission capacity.Finally, presents a framework of cellular-based cognitive-radio vehicular ad hoc networks that consist of a cellular network (primary network) and a VANET(secondary network),which share the downlink spectrum of the cellular network.We consider a scalable urban grid scenario in which vehicles opportunistically access the downlink spectrum with a carriersensing multiple-access protocol by detecting the spectrum holes available in the primary network. To restrict vehicle interference to the primary receivers, we set a square preservation region that contains appropriate street blocks around a particular street block in which an active base station is located. The number of street blocks is discussed with the practical assumption that the vehicles know only the locations of the primary transmitters. We then analyze the aggregate interference power from primary and secondary networks, and then derive the lower-bound downlink capacities of base station-to-bus transmission for the primary network and lower-bound channel capacity of vehicle-to-vehicle transmission for the secondary network.The results provide an insight into the effects of network parameters on capacities, including base station density, coverage of base stations, link gains,and length of each road segment.