Power Conservation Techniques in Wireless Sensor Networks

Introduction

Due to the recent advancements in the wireless networks, and the manufacturing of small, costless and energy-efficient devices, WSNs have been developing in a rapid speed. WSNs consist of a large number of energy-limited sensors that are used to monitor some area. Since WSNs are comprised of a large number of collaborating sensors (usually in the order of thousands), the cost of these sensors needs to be minimized. These sensors usually operate in areas where it is difficult for humans to work in. For example, WSNs are used to monitor a battle field, monitor the temperature fluctuations in the south and north poles, and work for extended periods in high temperature deserts. One of the future applications for WSNs is to be deployed in space or on the surface of planets.

Due to the size and cost constraints, the sensors are powered with small batteries. These batteries do not hold much energy in them. Also, since the sensors operate in usually inaccessible territories, it is not possible to change the sensors’ batteries. Therefore, power conservation is important to reduce energy consumption in the WSN environment. The main goal for energy conservation is to enable sensors work for the longest possible time (i.e. network life time) while maintaining the quality of service (QoS). The QoS in the WSN environment refers to the network ability to cover 100% of the area that’s being monitored.

Figure 1 shows a typical WSN. Usually WSNs have several hundred sensors and a Base Station. The function of the sensors is to collect data and send it to the Base Station. Another function for the sensors is to serve as a bridge between other sensors and the Base Station. The reason for the need of relays is because the communication range for a sensor is very limited due to energy constraints. In addition, the required energy to transmit data grows with the increase of the transmission range. In a WSN, all of the sensors’ data will be sent to a Base Station. The Base Station collects the information, organizes them, deletes any redundant information, and put the information in a readable format for users to read. Usually the Base Station has higher processing capability than the scattered sensors. It also has larger size and more stored energy, and may have a power link that provides with constant power supply.

Figure 1.

A typical WSN

Figure 2 illustrates how the sensors collaborate to deliver their data to the Base Station. In the figure, sensor A wants to deliver its data to the Base Station, it uses sensors B, C, D as relays.

Figure 2.

Sensors with their transmission ranges

There are a number of techniques that can be used to reduce the energy used in the data collection and access operation. Some of these techniques deal with the sensors hardware (Zhong, 2007). They try to reduce the operational voltage for the sensors. Moreover, they looked into how to schedule the packets in a way that consumes the least amount of energy (Zhong, 2007). Another group of methods to reduce energy consumption is to employ sleep/awake intervals. They allow sensors not be active 100% of the time. There are a number of definitions for “sleeping” in the context of WSNs. It could mean that the only inactive part of a sensor is the radio since it is the most energy consuming part of sensor hardware. The radio is either the transmitter or the receiver (AKA transceiver). Figure 3 shows a simple sensor construction with the radio components. In another possible sleeping mode, the sensor hardware except the preprocessor is completely shutdown. The function of the preprocessor is to awake the sensor when it is time for the sensor to be active.

Figure 3.

Sensor’s internal design