Performance Analysis And Improvement On MAC Protocol In Wireless Sensor Networks

The paper studies the principle and performance of Medium Access Control (MAC) protocols for wireless sensor networks, and two enhanced mechanisms are proposed in this dissertation.Firstly, applications, system structure, development, the features and key technologies of wireless sensor networks are introduced.Secondly, this paper introduces the feature of Zigbee and the PHY and MAC principles of IEEE 802.15.4. Then,there is detailed introduction about the channel accessing mechanism of MAC layer and the slotted CSMA/CA mechanism under star network topology.In 802.15.4, before each new transmission attempt, a node generates a random slotted backoff interval, and the number of the backoff slots is uniformly chosen in the range [0, W0]. The minimum backoff period, W0, is set equal to a value 2BE-1, where BE is the backoff exponential. The backoff counter value decrements at every slot without sensing the channel. Once the backoff counter reaches zero, the node performs twice clear channel assessment (CCA). The probability of the first CCA and the second CCA are deduced with mathematical formula. Then the analytic formulas of network throughput and power efficiency are obtained. And according it find out the throughput and power efficiency about the transmission probability and the minimum backoff period. Finally, the performance of IEEE 802.15.4 with different parameters (the length of packet, backoff exponential) is simulated by using the network simulator OPNET.Simulation results can match with the theoretical values very well, which indicates that the analytical model is right. Then a method to estimate the number of competing nodes in a network is studied, which is easily to be established but also accurate enough. From the above analysis, two optimization mechanisms - BeMAC and PeMAC are proposed. In IEEE 802.15.4, the minimum backoff period is fixed, but in the optimization mechanisms, it can be adjusted according to the number of nodes. From the simulation results, we can see that the performance of BeMAC is better than the legacy IEEE 802.15.4, and PeMAC is better than the legacy IEEE 802.15.4.