Research On Multichannel Communications Of Wireless Sensor Networks

As wireless sensor networks are widely used in the field of industrial monitoring, medical care, efficient agriculture and intelligent buildings, the requirements to network communication performance are increasing. Multichannel communication technology can effectively improve the reliability and throughput of networks, and solves the communication competition due to the numerous nodes and the network breakdown as a result of the channel be interfered. But the application of multichannel communication technology also has higher requirements to the design of networks communication protocol, such as how to effectively use the available multiple channels, how to extend the available channel resources, these are problems needed to be solved. Although multichannel communication can improve the ability of network communication, it also brings some new problems, the broadcast is difficult because the nodes live in different channels, as this is easy to realize in single channel. In addition, with the expansion of the wireless sensor networks applications, networks security attracted more attention, time synchronization is the base of multichannel communication, it is also the condition for wireless sensor networks to realize some applications related to time. So a security time synchronization protocol is the foundation of multichannel communication.In this thesis, some problems related to multichannel communication were studied, such as multichannel MAC protocols, multichannel spectrum sensing, multichannel broadcast and security time synchronization. The main work of this thesis is as follows:First, the background of multichannel communication is analyzed. Wireless sensor networks are multi-hop self-organizing networks, which are formed by a large number of cheap micro sensor nodes in a region, connected with wireless communication to monitor. Compared with the traditional network, wireless sensor networks are self-organizing, multi-hop communication and data-centric, the quantity of nodes is numerous, and the communication competition among nodes is serious. So traditional wireless network communication protocols are not suitable for wireless sensor networks. On the other hand, restricted with cost and size of wireless sensor networks nodes, their computing power, storage capacity and communication capability are all weak, so they can not run complex programs, and their reliability of wireless communication are very low. Another we should consider is the node runs with battery, and its energy is restricted, and the wireless communication consumes most energy in a node. The above characteristics of wireless sensor networks, are challenges to the reliability, complexity and energy efficiency of communication technology.Multichannel communication uses multiple channels to transmit information in wireless sensor networks, avoiding serious communication competition caused by many nodes gathered in single channel. Nodes can simultaneously communicate with multiple channels, avoiding interfered with each other, and this improved the network throughput. The usage of multiple channels avoids the breakdown of networks caused by the single channel being interfered. Multichannel communication has many advantages, but also increases the complexity of the communication protocol. How to assign multiple channels to many nodes, how to make the nodes in different channels to communicate with each other, and how to reduce the control overhead of communication protocols, all these problems need to be solved. In addition, the problems about broadcasting in multiple channels, avoiding interfered channel, opportunity accessing the authorized channel, all need to be studied in multichannel communication technology.In this thesis, many traditional multichannel MAC protocols have been studied. Many multiple rendezvous multichannel MAC protocols, such as McMAC, are distributed controlled, have low control overhead and do not depend on control channel. In multiple rendezvous multichannel MAC protocols, the nodes switch in different channels according to their default sequence, do not need management node to assign the channels, this improves the reliability and scalability of protocols. The default sequence is generated from a pseudo-random sequence, each node can get the default sequence of other node with the local computing, and this method reduces the exchange of control information. The operation of multiple rendezvous multichannel MAC protocols do not rely on control channel, this avoids the congestion caused by the communication competition, and avoids the breakdown of the network caused by the control channel being interfered. But the traditional multiple rendezvous multichannel MAC protocols still have some problems, such as the disappear node and unfair communication competition. Based on the original protocols, the time slot mechanism is used in this thesis. In one period, the node wakes up in one slot to receive the information and sleeps in other slot. The awake slot is selected according to a pseudo-random sequence. The usage of pseudo-random time slot not only reduces the energy consumption with the sleep mechanism, but also resolves the problems about the disappear node and unfair communication competition. Simulation results show that this protocol has good network throughput and energy efficiency.There is an overlapping problem between the channels of wireless sensor networks and wireless local area networks, they will interfered each other when they located in the same area. Because wireless sensor networks are resource-constrained, which are weak in competition. How to avoid using interfered channel in multichannel communication, this is a practical problem need to be resolved. As the frequency resource is scarce, it is important for multichannel communication to find new method to use more channels. Cognitive radio sensor networks present a new approach to develop multichannel communication, and the key problem is how to do spectrum sensing in sensor networks. Effective spectrum sensing is not only the foundation of cognitive radio sensor networks, but also the requirement of avoiding interfered channel, but the spectrum sensing come from the cognitive radio, does not adapt the wireless sensor networks because of its high complexity. In this thesis, we proposed a simple and reliable cooperative spectrum sensing algorithm, based on finite-dimensional random matrix theory. In this algorithm, the detection threshold is calculated from the accurate and simple expression of DCN in random matrix. Simulation results show that, compared to the asymptotic method, the algorithm needs fewer measurements and has more reliable detection performance. This algorithm proposed a practicable method to use spectrum sensing in wireless sensor networks.Multichannel can improve the communication capability of wireless sensor networks, but also caused some problems that do not exist in single channel. In the single channel, using wireless broadcast advantage of the wireless radio, the node can easily transmit message to its neighbours with broadcast. Many applications of wireless sensor networks and routing protocols are depend on the broadcast. But in multichannel, the neighbour nodes locate in different channels, they can not receive the broadcast packet sent by a node in a fixed channel, and the sleep mechanism increases the difficulty of broadcasting, because it makes the nodes waking at different time. So how to broadcast in multichannel is a key problem of multichannel communication in wireless sensor networks. In this thesis, a broadcast protocol based on distributed broadcast tree is proposed. In this protocol, the node which initiates the broadcast will establish an index broadcast tree with training broadcast packets. When it begins to broadcast, the broadcast packet will be forwarded according to the tree. Broadcast tree ensures that all nodes in the network can receive the broadcast packets, and without redundant broadcast packets. Simulation results show that the broadcast tree can be established quickly, and the protocol has good performance in broadcast coverage and broadcast delay.As the expansion of wireless sensor networks applications, its security attracted much attention. Time synchronization protocol is related to many applications of wireless sensor networks, and it is also the foundation of many network protocols. The dividing of time slot and the estimating of channel switching time all need time synchronization. But most traditional time synchronization protocols focus on the accuracy of time synchronization, and do not consider the security, so there are some security vulnerabilities in traditional time synchronization protocols, and this will affect their accuracy. In this thesis, we analyzed the security of the traditional time synchronization protocols, and proposed a fault-tolerant time synchronization protocol. This protocol has intrusion detection function, it can detect the attacks against time synchronization from the malicious nodes. Using random weighted average method, the node can calibrate its local time with fusing many synchronization information from different nodes, this can resist smaller slow attacks with smaller deviation, and reduce the inherent error of transmit time in synchronization packets. Simulation results show that this time synchronization protocol has good performance in intrusion detection and fault tolerance.In this thesis, we analyzed different aspects of multichannel communication in wireless sensor networks. We mainly studied the design of multichannel communication MAC protocols, multichannel spectrum sensing, broadcast in multichannel, and security time synchronization protocols, we gave our own solutions to these problems. These solutions will promote the application of multichannel communication in wireless sensor networks, it is important to the development of wireless sensor networks.