| The Internet of Things is called the third wave of the world information industry after the computer and Internet. As one of the core technologies of the Internet of Things, Wireless Sensor Networks (WNSs) receive more and more researchers’attention. Wireless Sensor Network is a network that consists of many sensors, which can contact each other, process and transmit information through wireless communication. It can be used to monitor, sense and collect various environment and monitored object information that within the region of network distribution. The collected information can be processed and transmitted to the needed user through WSNs. However, due to the size and cost limitations of sensors, the sensors of WSNs are often powered by battery. The battery is capacity limited and it often can not be replaced in many applications, therefore, how to save energy and prolong the network lifetime is a key issue in the design of WSNs.To this end, many energy efficient new technologies have been proposed to save energy and prolong the lifetime of WSNs. Especially the wireless communication energy consumption is the dominant of the whole energy consumption of a sensor node. Therefore, in recent years, a new research trend in the energy saving design of WSNs is the introduction of advanced physical layer transmission technology, such as virtual MIMO (Multiple Input and Multiple Output) technology. In addition, due to the addition of mobility in WSNs, mobile WSNs can save energy through short-range wireless communication. The exploitation of mobility for energy saving design became a new research field of WSNs.The agriculture WSN used for monitoring covers a wide range of area. The information collected by it is simple and allows a certain degree of delay. For these characteristics of agriculture WSN, this paper intensively investigates from the energy saving perspective the virtual MIMO based energy saving technologies and mobility based energy saving technologies for WSNs. The thesis elaborates a new collaborative energy saving transmission scheme design and a mobile Sink based energy saving algorithm design for data collection. Firstly, starting from the sensor node and network characteristics, this paper discusses the technical backgrounds of virtual MIMO based WSNs and mobile WSNs (mWSN). At the same time, the related research papers for mWSN were classified and summarized in this paper. Then, energy efficient virtual V-BLAST transmission scheme, STBC based virtual MIMO transmission scheme and mobile Sink based energy saving algorithm for data collection were investigated. The main contributions of this paper are as follows:Firstly, the data collected by a large volume of sensor nodes that distributed randomly in agriculture and forestry field have much redundancy. In addition, the Sink used for data collection often situated far from the monitoring area and size unconstrained. For these characteristics, a cluster-based virtual MIMO transmission architecture is proposed for energy-constrained wireless sensor networks. In the proposed architecture, instead of using cluster members as cooperative nodes, multiple cluster heads cooperate to form virtual antenna array so that MIMO transmission can be implemented. In the research, V-BLAST (vertical Bell Laboratories layered space-time) based MIMO technology was integrated into this architecture and a V-BLAST cluster heads cooperative transmission (VCHCT) scheme was proposed. Through the BER analysis of Decorrelating Decision Feedback Detector of V-BLAST, a total energy consumption model for VCHCT was developed. Simulation results show that when compared with the traditional LEACH (Low-energy Adaptive Clustering Hierarchy) protocol, VCHCT can save more energy and prolong the network lifetime.Secondly, based on the cluster heads cooperative transmission architecture, this research combines the STBC based virtual MIMO technology and the cluster heads cooperative transmission architecture. A STBC based virtual MIMO transmission scheme for clustered WSNs was proposed to reduce energy consumption and prolong the network lifetime. This scheme is called SCHCT (STBC-based Cluster Heads Cooperative Transmission). This research analyses the BER performance of STBC based MIMO system and develops a total energy consumption model for SCHCT based on the single bit energy consumption model. When compared with LEACH scheme, numerical and simulation results together show that the proposed scheme can prolong the sensor network lifetime greatly when the distance to sink is above a threshold, especially in situations where the sink is far from the sensor area.Thirdly, when compared with LEACH, VCHCT scheme and SCHCT scheme are more energy efficient than LEACH. But when consider at the same time the different energy saving characteristics of these two schemes and the additional communication consumption for cluster heads involved in the SCHCT scheme, it is not clear which scheme is more energy efficient. Under the same BER performance and the same node transmission rate conditions, detailed comparison between these two schemes and the original LEACH is performed to investigate the performance of these two schemes. Simulation results not only verify the theoretical analysis but also show that the two schemes have their specific application scenarios. When the sink is far from the sensor area, SCHCT scheme is much more energy efficient than LEACH and VCHCT scheme even if it consumes additional inter-cluster communication energy. When the distance to sink is near the sensor area, VCHCT is the best choice.Fourthly, large delay is one of the key issues of mobile WSNs, some related work have achieved some success in reducing data delay of mobile WSNs. But for the large scale WSNs, these methods still can’t satisfy the needs of data delay. In addition, exiting research work considered the path of mobile Sink and the routing from sensor nodes to rendezvous separately, it does not consider the relation between rendezvous selection, routing and path scheduling. For this problem, combined with the characteristics of large scale and delay tolerated, a data collection scheme was proposed based on mobile Sink and rendezvous. Based on this scheme, a heuristic algorithm that jointly considered rendezvous selection, routing and mobile Sink path scheduling was proposed. Under the assurance of condition for data latency requirement, this algorithm can reduce the data transmission from sensor nodes to rendezvous and achieves the purpose to save energy and prolong the network lifetime. |
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