Wireless Ad Hoc and Sensor Networks

Background on Networking 45active tags have a battery and up to 1 MB of data storage. RFIDdevices operate in a low-frequency range of 100 kHz to 1.5 MHzor a high-frequency range of 900 MHz to 2.4 GHz, which has anoperating range up to 30 m. RFID tags are very inexpensive, andare used in manufacturing and sales inventory control, containershipping control, and for homeland security applications. RFIDtags are installed on water meters in some cities, allowing ametering vehicle to simply drive by and remotely read the currentreadings. They are also being used in automobiles for automatictoll collection. RFID networks can be viewed as mobile WSNs andtherefore the DPC scheme developed for cellular and ad hocnetworks is shown to be extended to a dense RFID network forthe improvement of read rates and coverage in Chapter 10.1.5.5 Self-Organization and LocalizationAd hoc networks of nodes may be developed using, for example, aircraftsor ships. Self-organization of ad hoc networks includes both communicationsself-organization and positioning self-organization. In the former,the nodes must wake up, detect each other, and form a communicationnetwork. Technologies for this are by now standard, by and large developedwithin a mobile phone industry. Distributed surveillance sensornetworks require information about the relative positions of the nodes fordistributed signal processing, as well as absolute positioning informationfor reporting data related to detected targets. Chapter 8 presents a selforganizationscheme for WSNs.Relative layout positioning (localization): Relative positioning or localizationrequires internode communications and a TDMA messageheader frame that includes both communications and localizationfields. There are various means for a node to measure distance toits neighbors, mostly based on RF time-of-flight information. Inair, the propagation speed is known, so time differences can beconverted to distances. Given the relative distances betweennodes, we want to organize the web into a grid specified in termsof relative positions.Absolute geographical positioning: A network is said to be relativelycalibrated if the relative positions of all nodes are known. Now, itis necessary to determine the absolute geographic position of thenetwork. For the net to be known as a (fully) calibrated flat twodimensional(2-D) net, at least three nodes in the net must determinetheir absolute positions. There are many ways for a node todetermine its absolute position, including GPS and techniquesbased on stored maps, landmarks, or beacons (Bulusu et al. 2002).