Understanding Smart Sensors - Nomads.usp

Transceivers, Transponders, and Telemetry 179communication, commercial applications of spread spectrum include wirelessLANs, security systems, instrument monitoring, factory automation, remotebar code reading, automatic vehicle location, pollution monitoring, medicalapplications, and remote sensing of seismic and atmospheric parameters.8.2 Wireless Data and CommunicationsError-free wireless data is becoming possible due to digital data transmission. Anumber of protocols are vying for acceptance in RF signal transmission. Existingaccess methods include time-division multiple access (TDMA), frequencydivisionmultiple access (FDMA), and the newest method, code division multipleaccess (CDMA). Users are assigned a time slot, frequency, or code thatallows multiple users to share the available frequency spectrum. CDMA providesan additional dimension and option that allows more efficient utilizationof the available spectrum and therefore greater capacity (up to 20 times greater)with a given bandwidth [5]. That means an increased number of users canaccess the system and maintain the level of service—access, data rate, bit errorrate (BER), and so on—with only a slight degradation in process gain. The discontinuousnature of human speech is the key to one aspect of CDMA technology.Only one-third of the air time is used for speech; the rest is spent onlistening and pauses between words, syllables, and thoughts. CDMA uses avariable-rate voice encoder (vocoder), which performs a speech-encoding algorithmto transmit speech at the minimum data rate needed. Encoding rate isfrom 1 to 8 Kbps, depending on speech activity.Several techniques, including frequency shift keying (FSK), amplitudeshift keying (ASK), on-off keying (OOK), differential phase shift keying(DPSK), quadrature phase shift keying (QPSK), and Gaussian minimum shiftkeying (GMSK), are used to modulate the signal transmission. Techniques previouslyused for military and satellite communications such as differentialquadrature shift keying (DQSK) and GMSK maximize the information densityin a spectral bandwidth. The choice depends on cost and performance objectiveswithin the system.Cellular digital packet data (CDPD) technology divides data into smallpackets that can be sent in short bursts during the idle time between cellularvoice transmissions. The open specification is designed to support a variety ofinteroperable equipment and services and is backed by major cellular carriers.CDPD allows faster transmission (19.2 Kbps) of short messages, compared tothe existing circuit-switched system, which handles longer files [6]. CDPD alsoincorporates error correction protocols. Table 8.2 lists some of the competing