WSN and nanotechnology

IN RECENT years, advances in miniaturization, low-power circuit design, simple and reasonably efficient wireless communication equipment, and improved small-scale energy supplies have combined with reduced manufacturing costs to make a new technological vision possible, i.e., wireless sensor networks. These networks combine simple wireless communication infrastructure, minimal computation facilities, and minimally invasive sensors to form a network that can be deeply embedded in our physical environment to create an information world. Typical sensing tasks for such a device could be temperature, light, vibration, sound, and radiation. The desired size would be a few cubic millimeters or even smaller. The target price should be less than US$ 1, including radio front end, microcontroller, power supply, and the actual sensor. All these components are integrated together in a single device to form a “sensor node.” While these networks of sensor nodes share many commonalities with existing ad hoc network concepts, there are also a number of fundamental differences and specific challenges. While designing and deploying a wireless sensor network, we found several major limitations that must be addressed.

BACKGROUND AND APPLICATIONS

wireless sensor networks have benefited from advances in both microelectromechanical systems (MEMS) and networking technologies. Such environments may have many inexpensive wireless nodes, each capable of collecting, storing, and processing environmental information, and communicating with neighboring nodes. A sensor node is made up of four basic components, as shown in Fig. 1, namely 1) a “sensing unit,” 2) a “processing unit,” 3) a “transceiver unit,” and 4) a “power unit.” They may also have additional application-dependent components such as a “location finding system,” “power generator,” and “mobilizer.” Sensing units are usually composed of two subunits, namely 1) sensors and 2) analog-to-digital converters (ADCs). The analog signals produced by the sensors in response to the observed phenomenon are converted to digital signals by ADC and then fed into the processing unit. The processing unit, which is generally associated with a small storage unit, manages the procedures that make the sensor node collaborate with the other nodes to carry out the assigned sensing tasks. A transceiver unit connects the node to the network. One of the most important components of a sensor node is the power unit. Power units may be supported by power scavenging units such as solar cells. There are also other subunits that are application dependent. Most of the sensor network routing techniques and sensing tasks require knowledge of location with high accuracy. Thus, it is common that a sensor node has a location finding system. A mobilizer may sometimes be needed to move sensor nodes when it is required to carry out their assigned tasks. All of these subunits may need to fit into a matchbox-sized module or even smaller. In the past, sensors are connected by wire lines. Today, this environment is combined with the novel ad hoc networking and wireless technologies to facilitate intersensor communication, which greatly improves the flexibility of installing and configuring a sensor network. Sensor nodes coordinate among themselves to produce high-quality information about the physical environment. A base station (the sink) may be a fixed node or a mobile node capable of connecting the sensor network to an existing communications infrastructure or to the Internet where a user can have access to the reported data. Networking unattended sensor nodes may have profound effects on the efficiency of many military and civil applications such as target field imaging, intrusion detection, weather monitoring, security, tactical surveillance, and distributed computing; on detecting ambient conditions such as temperature, movement, sound, and light; or the presence of certain objects, inventory control, and disaster management. Deployment of a sensor network in these applications can be in random fashion (e.g., dropped from an airplane) or can be planted manually (e.g., fire alarm sensors in a facility). For example, in a disaster management application, a large number of sensors can be dropped from a helicopter. These networked sensors can assist rescue operations by locating survivors, identifying risky areas, and making the rescue team more aware of the overall situation in the disaster area, such as tsunami, earthquake, etc. Sensor nodes are ensely deployed in close proximity or embedded within the medium to be observed. Therefore, they usually work unattended in remote geographic areas. They may be working in the interior of large machinery, at the bottom of an ocean continuously, in a biologically or chemically contaminated field, in a battlefield beyond enemy lines, and in a home or large building. Driven by all these exciting and demanding applications of wireless sensor network, several critical requirements have been addressed actively from the network prospective for supporting viable deployments, as follows:

• long longevity;
• noninvasive form factor;
• optimal sensing coverage and connectivity;
• high sensing resolution.