Wireless Ad Hoc and Sensor Networks

12 Wireless Ad Hoc and Sensor Networks1.3.1 Cellular Wireless NetworksMobile radio systems were designed with the main objective of providinga large coverage area to users by using a single, high-powered transmitter(Rappaport 1999). This approach achieved good coverage, but it wasimpossible to increase the number of users in the system beyond a certainlimit. Attempts to increase the number of users in the network resultedin greater interference due to simultaneous transmissions.The cellular networking concept was a major breakthrough in solving theproblem of spectral congestion and user capacity. The cellular conceptreplaced the existing high-power transmitters with many low-power transmitters(base stations serving smaller cells) to combat interference and toincrease capacity within a limited spectrum. Each of the low-power transmittersserved only a small service area, defined as a cell. Each cell wasprovided with different groups of channel frequencies so that all the availablebandwidth is assigned to a relatively small number of neighboringcells. Neighboring base stations or cells are assigned different groups offrequencies so that the interference between base stations is minimized.Success of the Internet has initiated the search for technologies that canprovide high-speed data access even to mobile users. The third-generation(3G) in cellular systems is aimed at realizing this vision. The 3G cellularsystems are expected to provide the ability to communicate using thevoice over Internet protocol (VoIP), unparalleled network capacity, dataaccess with multiple users and high-speed data access even when subscribersare on the move.Researchers are now looking at the technologies that may constitute afourth-generation (4G) system. Different modulation technologies, suchas orthogonal frequency division modulation (OFDM), which is used inother wireless systems, for instance WirelessMAN (IEEE 802.16) anddigital video broadcast (DVB), are being investigated. Research is alsotaking place on mesh network technology, where each handset or terminalcan be used as a repeater, with the links routed and maintained in adynamic way. New standards and technologies are being implementedto replace the existing copper cable lines. Most of today’s cellular communicationsystems utilize second-generation (2G) cellular standardscommonly known as 2G technologies. A brief description of the wirelesscellular standards is provided below. Later, the IEEE 802.11 standard forad hoc networks is described in detail.1.3.1.1 Wireless Cellular Standards and ProtocolsFirst-generation cellular networks: The first-generation cellular systemsrelied on FDMA/FDD (frequency division multiple access/frequencydivision duplexing) multiple access techniques to efficientlyutilize the available bandwidth resources. Analog FM, an