The Research On Collision Avoidance Strategy For LR-WPAN/IEEE802.15.4

As the low rate wireless personal area networks(LR-WPAN) communication protocol, IEEE802.15.4 standard make the wireless connection between low-cost devices a reality. It has the characteristics of low power consumption, low rate, less complexity, strong networking capabilities and extra-large capacity, and it defines the PHY layer and the MAC layer, based on which the ZigBee Alliance has published the ZigBee protocol by standardizing the NWK layer, security layer and the framework for application layer. Given its unique characteristics, IEEE802.15.4 becomes the most suitable standard for wireless sensor network, while it has been widely used in the fields of electronic manufacture, home and building automation, industrial control, medical equipment and so on, enjoying a broad application prospects.This paper firstly summarizes the IEEE802.15.4 standard for LR-WPAN, introduces the stratified relationship between IEEE802.15.4 and ZigBee in OSI system, and the development of their own as well. For a better understanding of the basic knowledge and research background in the domain of IEEE802.15.4, the protocol structure and technology characteristics and other related knowledge are described in this paper. The relevant definition of this protocol in PHY layer and MAC layer are also illustrated in detail ,more efforts are put to discuss the function realization of every layer and the superframe structure. After analyzing the research results of IEEE802.15.4 protocol and hidden node problem at home and abroad, the paper delivers its main purpose: to improve the existing channel access mechanism on MAC layer of IEEE802.15.4, to propose an collision avoidance strategy which has small additional cost, low energy comsumption and high flexibility, and mainly to solve the hidden node problem in IEEE802.15.4 network, meanwhile to reduce occerence of contention collision, improving the whole performance of LR-WPAN further.Energy saving is the most prominent characteristics as well as the demand of LR-WPAN. But Unfortunately The present theoretical studies have confirmed that the existing IEEE802.15.4 protocol has not fully show the good performance of this technology, so how to reduce power consumption as much as possible becomes a key issues. Decreasing the occurrence of collision to reduce reduplicative transimmisons is one of the effective ways to slove this problem. In general, there are two kinds of collision in LR-WPAN: one is the contention collision; the other is the hidden node collision. IEEE802.15.4 MAC layer includes modified CSMA / CA algorithm to solve contention collision. But in terms of smaller energy consumption, BE parameters can only change in a small range defined by protocol. Thus, devices are easier to select the same number of Backoff Period, increasing the the possibility of collision.Hidden node collision is a common problem in all WPAN which adopts CSMA mechanism in their MAC layer, Although transmission region of the coordinator is able to cover all nodes,nodes are not guaranteed to hear the signals from all the other nodes. More importantly, considering the simplicity of the protocol and characteristics of low energy consumption, IEEE802.15.4 does not contain any mechanism like RTS / CTS to avoid the hidden node problem. Moreover, most of current researches related to energy consume of IEEE 802.15.4 MAC layer, have assumed that the hidden node problem has not been existed. but previous studies have found that the probability of occurrence of hidden node collision in a network where nodes randomly distribute is up to 41%. Occurrence of hidden node collision can make a very negative impact on the throughput and communication delay of the entire network, and also can cause a large number of unnecessary energy consumption.Collision problems on IEEE802.15.4 MAC layer is the core of this paper. We make a introduction about channel accessing mechanism of MAC layer, then analyse the CSMA/CA algorithm in two different operation mode---beacon-enable network and nonbeacon-enabled network in detail. Afer that, we describe the principles and differences of contention collision and hidden node collision existing on MAC layer, clarify the negative impact on network performance brought by collision ulteriorly and the reason why RTS/CTS is not suitable for IEEE802.15.4 standard is explained detailedly. In order to solve the collision problem of IEEE802.15.4 effectively, a hidden node collision avoidance strategy(HN-CAS) based on both region and superframe divison is proposed: Dividing the region of network coverge to make sure nodes in same sub-region within the carrier sense range of each other, which can destroy the hidden relationship of nodes from the perspective of space. Meanwhile adjusting superframe structure slightly by dividing contention access period so as to allot slots to grouped nodes in different sub-regions , which can make transmission of hidden nodes be staggered from the perspective of time. Then the grouped nodes in different sub-regions are able to communicate within corresponding R-slots in superframe while nodes in same sub-region still use CSMA/CA algorithm for accessing channel with competition. The simplicity of IEEE 802.15.4 protocol has taken sufficiency into account, so this strategy will not bring too much modification and overhead.At same time,The method of software simulation has also been used for studying collision problems on IEEE802.15.4 MAC layer in this paper. A brief introduction about NS2 network simulation software platform is provided, we also describe the principle of IEEE802.15.4 protocol realization in NS, and then discuss the simulation environment configurations and parameters choice in detail. After that we run several simulations respectively about contention collision, hidden node collision and proposed HN-CAS in IEEE802.15.4 network. By caculating packet delivery ratio,end to end delay, network throughput and energy consume, we make a comprhensive assessment of the performance of IEEE802.15.4 network in various environments. Simulation results clearly show the negative impact on network performance brought by collision, and validate the strategy proposed in this paper not only slove the IEEE802.15.4 hidden node problem but also inhibit the occurrence of contentence collision to some extent.Finally, a conclusion of this thesis is given and also the expectation to next step.