Research On Adaptive Backoff Algorithm In MAC Protocol For Ad Hoc Network

Ad Hoc network is a multihop wireless network that does not rely on any fixed infrastructure, with the characteristics of fast and flexible networking, strong self-healing capacity, and good extensibility, etc, therefore Ad Hoc network has raised widespread concern. The design of the Media access control (MAC) protocol is the one of the key technologies in Ad Hoc network, which determines that how do multiple users share limited wireless resources reasonably and efficiently. Since Ad Hoc network using a distributed control method that causes a conflict when users simultaneously send packets in the network, then what kind of backoff algorithm to adopt for reducing the probability of collision, improving network throughput and guaranteeing access fairness among users, which become a difficulty of MAC protocol design.In this thesis, based on in-depth analysis of various types of MAC protocols and backoff algorithms, in view of shortcomings of the backoff algorithm used in the distributed coordination function (DCF) of IEEE802.11MAC protocol, an improved backoff algorithm—adaptive logarithmic backoff (ALB) algorithm is proposed. The main achievements of the thesis are as follows:(1) Analyzing the characteristics of Ad Hoc network and the difficulties of its MAC protocol design, and researching on the classification of MAC protocols for Ad Hoc Network, which focuses on analysis of the realization principle of IEEE802.11DCF mechanism.(2) Analyzing the disadvantages in access fairness, throughput, etc, existed the binary exponential backoff (BEB) in DCF mechanism, and researching on the classification of existing backoff algorithms, which focuses on analysis of the realization principle, advantages and disadvantages of the logarithmic backoff (LB) algorithm.(3) In order to overcome the shortcomings of BEB algorithm and LB algorithm, an improved backoff algorithm—adaptive logarithmic backoff (ALB) algorithm is proposed. In ALB algorithm, when the transmission success or failure, node will use an adaptive adjustment factor to adjust the contention window size, since the adaptive adjustment factor is a function on the number of competing nodes, and may dynamically vary with network channel contention situation, so the contention window size can change with Ad Hoc network adaptively, which reduces the probability of conflict among nodes, and improves the fairness of the access channel. Simulation results show that at saturated network status, the ALB algorithm has better performance in aspects of throughput, delay, packet loss rate and fairness, than BEB, LB backoff algorithm, namely the ALB algorithm improves the overall performance of Ad Hoc network.