| With the fast development and wide spread of mobile communication technology, researches on the mobile ad hoc networks have become a hot topic which is aimed at acentric networks. Since the medium access control (MAC) protocol operates above the physical layer in the protocol stack of mobile ad hoc networks, it determines how the nodes receive and send messages. So, the MAC protocol plays an important role in the performance and efficiency of the whole network. Due to the mobile ad hoc network is independent of other facilities, it will bring much interferences and collisions in case that some nodes compete for the utilization of the wireless channel at the same time. Therefore, adopting what kind of medium access protocol for the networks becomes a necessary and urgent issue, since it is used to avoid or decrease the collisions and then greatly improve the efficiency and capability of the networks.The allocation and utilization of the channel resource in mobile ad hoc networks is not only indicated in the temporal reuse, but also the spatial reuse. That means the network allows multiple nodes to communicate with others at the same time, under the condition of only one frequency. It is one of the advantages of the mobile ad hoc networks. Whether the spatial reuse of the wireless channel is reasonable determines the performance of the networks when it runs. An excellent medium access protocol must ensure the reliable communications and enhance the degree of the spatial reuse as much as possible. This paper first briefly summarizes the characteristics, key performance of the media access protocol and the general situation. Then it particularly analyzes the principles of the IEEE802.11 DCF standards and the previous researches on the transmitting rate and carrier sense mechanism used in the mobile ad hoc networks. Base on these, the author builds the spatial reuse and interference models, exactly describes the relationships among the spatial reuse, overhead of MAC protocol and the network throughput.This paper proposes a kind of dynamic spatial backoff algorithms which is based on power and rate adaptation. It makes use of the control packets to estimate the distance between the sending and receiving nodes, then forecasts the signal-to-noise level at the receiving node for the purpose of adjusting the transmit power and channel rate self-adaptively, Under this scheme, the network will gradually work toward to the optimal state, which is not only increasing the degree of wireless channel spatial reuse, but also ensuring the certain transmitting speed. In addition, a kind of algorithm to solve the power consumption and delay in MAC is proposed, which combines the genetic algorithms principles and the model of mobile ad hoc networks. This algorithm adopts the variable-length chromosomes to encode the problem and applies genetic algorithms to optimize the route. At last, the analysis and simulating results can prove the dynamic spatial backoff mechanism based on PRA algorithms can greatly decrease the probability of collisions and increase the network throughput when compared with 802.11 DCF protocol and DSB algorithms. Moreover, the improving scheme based on genetic algorithms can effectively decrease the system consumption. |
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