Energy-Efficient Medium Access Control for Wireless Sensor Networks Realised Through Aerial Platforms

A wireless sensor network is a collection of small electronic devices, which have the ability to measure and record environmental conditions such as temperature, light, humidity, and noise. They talk to each other by radio, so that information can be transferred through the network to a larger device capable of reporting it back to a central office. Current devices are similar in size to a ten pence piece and operate with a small battery, similar to that found in a wristwatch. The importance of wireless sensor networks lies in their ability to continuously monitor physical conditions in remote and inaccessible locations without human involvement. A huge range of applications is foreseen for sensor networks, many of which will improve quality of life. Some important examples include prediction of natural disasters; environmental monitoring of pollutants; and medical applications. A large number of devices can be dropped out of a plane around a volcano, to monitor the gases emitted and report any unusual conditions. Early prediction of an eruption will allow appropriate action to be taken to prevent loss of life. They can similarly be used to detect the presence of chemical pollutants in rivers and the sea, such as oil spillages, enabling rapid clean up before harming wildlife. Use of sensor nodes to monitor the movement of an elderly person around their home permits falls to be detected and reported back to the family or carer. One of the main limitations of these small devices is battery life, since they will often be required to monitor, detect and report on events for a long period of time without external intervention and without access to an alternative power source. Radio communication consumes a significant amount of energy, as is often apparent with mobile telephones which need to be regularly charged despite having much larger batteries. Most communication systems have large devices to manage the network. A common example is the mobile phone base station, which is responsible for setting up and terminating telephone calls, and ensuring that appropriate radio channels are used so that multiple calls can take place at the same time. In a wireless sensor network, the individual devices often have to control their own access to the network, which consumes a lot of energy and significantly reduces their lifetime. Each device wishing to send some information has to talk to its neighbours to arrange appropriate frequency channels and times at which it can transfer data to another device. This ensures that only one device transmits on the same frequency channel at the same time, otherwise the information would be lost. Multiple transmissions are often required by different devices to relay the information to the receiver, since it is desirable for each device to transmit over a short distance. The aim of this research project is to develop more energy-efficient means by which information can be transferred through a sensor network by radio. The potential of using an aerial platform (either a plane, balloon, or airship at high altitude) to remove some of the communications burden from the energy-constrained devices is to be explored in two distinct ways. The first concept is to use an aerial platform to coordinate transmissions from the different devices by telling them when and how to transmit their information. The aerial platform performs the network management functions that would normally be carried out by a terrestrial base station, and removes the energy burden from the individual devices. The second concept is to have the aerial platform as the direct recipient of information, since this reduces communication activities within the network to a minimum and reduces the energy burden associated with relaying information through the network. Such a novel approach is vital to bring about the necessary improvements in energy-efficiency for successful long-term operation in the presence of energy-constrained devices.