The design of an efficient routing protocol and energy supply for wireless sensor networks

© 2012 Xiaobo LiuIn recent years, there has been a rapid development of wireless sensor network (WSN) technology due to an increasing need for automatic monitoring and control in many areas. The key benefits of using WSN technology for monitoring and control include low cost, low power consumption and self-organizing operation of the sensor network. However, there are still several challenges remaining in practical deployments. Major concerns are improving WSN efficiency in terms of data routing and energy consumption. In this thesis, we address three problems, namely efficient converge-cast routing in tree-based WSNs, battery lifetime extension for outdoor WSN applications and the measurement of soil moisture over large areas using a WSN. Converge-cast data collection is a typical pattern in WSN communications. Given a large number of sensor nodes formed in a tree topology, the objective is to achieve low latency for source-destination data packet delivery, while keeping the network overhead at low levels. In this thesis, we propose a converge-cast routing protocol for tree-based WSNs. In particular, a new frame structure is proposed, which is compliant with the IEEE 802.15.4 standard and gives considerable flexibility in scheduling communication between sensor nodes. Importantly, the proposed beacon flooding scheme requires less overhead for network synchronization, compared with traditional hierarchical scheduling schemes. An experimental WSN test bed is used to evaluate the performance of our proposed routing protocol. The results show that our protocol achieves comparative performance to an existing hierarchical routing protocol, but with lower complexity. We then focus on the issue of extending the battery lifetime for unattended WSN nodes. We propose a systematic design approach for this purpose by utilizing solar energy harvesting and a simple circuit for battery recharging based on the Harvest-Store-Use architecture. A 4-step design procedure is used for selecting the appropriate elements in our charge circuit subject to the requirements of load power consumption. Our design approach accounts for variations in environmental conditions. Indefinite operation of a sensor node can be achieved without the use of any adaptive schemes that require frequent adjustment of the sensor node duty cycle setup. Field experiments are conducted to evaluate the performance of our designed power supply subsystem using a WSN test bed. Finally, we investigate the feasibility of monitoring soil moisture levels in a field using low-cost imaging sensors. The underlying principle is that plant water stress can be loosely translated to a non-optimum soil moisture level, which also affects a plant photosynthesis indicator called the Normalized Difference Vegetation Index (NDVI). Based on this principle, the decision for irrigation scheduling can be made based on the NDVI parameter instead of soil moisture measurements. This enables the use of image-based sensing, which avoids the deployment of a large number of soil moisture sensors across a field, thus significantly reducing the cost. Based on a WSN platform, a series of experimental results are collected to evaluate the feasibility of this approach using low-cost imaging sensors