Modeling And Analyzing Directional Carrier Sense Access Protocol For Wireless Ad Hoc Networks

Wireless ad hoc network does not relay on fixed infrastructures.It has advantages in flexible networking,high reliability and survivability.The futher use of directional antennas can not only expand the transmission distance of nodes,but also increase the network throughput,and reduce the access delay of packets effectively.How to develop a mathematical model to analyze per link saturation throughput and access delay in directional ad hoc networks,so as to provide a theoretical basis for the optimization of directional carrier sense access protocol,has been the focus of research in this field.The research work of this paper is focused on the above issues,and the main contributions are summarized as follows:(1)The limitations of the application of Markov chains in the modeling of directional random access protocol are analyzed.In omni-directional wireless ad hoc networks,researchers have proposed several markov chain modeling methods for randomchannel access protocols.These methods model the backoff and retransmission mechanism of protocols,and get the theoretical expressions of saturation throughput and access delay through non-null one step state transition probability of each un-fixed slot and normalizing condition for probabilities.However,in directional wireless ad hoc networks,‘deafness problems' and ‘directional hidden terminals' generated by random access will affect the throughput and access delay of the neighborhood flows.In response to this effect,we analyze the intrinsic reasons why the traditional Markov chainsbased on un-fixed slot are not applicable to the modeling of directional randomaccess protocolby simulation experiments.(2)A four dimensional markov chain model based on fixed slot is proposed for directional carrier sense access protocols.The model uses a fixed slot as a time unit for state transter,and its non-null one step state transfer process can be divided into three parts: backoff process,transmission process and suspension process.The collision probability calculation and the suspended state analysis of the node'sbackoff counter are two difficult problems of modeling.Considering the effects of ‘deafness problems' and ‘directional hidden terminals' on the collision probability of MAC frames,the collision zones of nodesin the directional wireless ad hoc network are divided into two parts: instantaneous collision zone and persistent collision zone.The effects of these two kinds of collision on network performance are considered.When the channel becomes busy,and the node's backoff counters remain suspended,we use the loop iteration method of continuous and discrete Markov chain models to calculate thesuspension probability and suspension time expectation of nodes.(3)Based on the proposed four dimensional markov chain model,we further analyze the saturation throughput and the MAC access delay of the directional wireless ad hoc network.The transmission probability,collision probability and saturation throughput of each link in the directional network are obtained through the model solution.The calculation of the access delay can be divided into two situations: packet transmission success(including retransmission)and packet transmission failure(packets are discarded when the number of retransmission reaches the upper limit).Then the mathematical expectation of access delay of each link is obtained.The effects of the directional beam width and the contending window size on the performance of per link saturated throughput and MAC access delay are analyzed in detail.Finally,the validity of the model is proved by comparing the numerical results of the model to the simulation results obtained in the network simulation environment.