Understanding Smart Sensors - Nomads.usp

178 Understanding Smart SensorsNarrowband signalSpread-spectrum signal25 kHz26 MHzFigure 8.2 Spread spectrum versus narrowband.Direct sequence is a spread spectrum approach that creates an instantaneousRF signal consisting of many frequencies spread over a portion of the frequencyspectrum. The information signal, which is usually digital, is combinedwith a much faster stream of pseudorandom binary code that is repeated continuously.This technique is difficult to detect, and recapturing the informationrequires duplicating the spreading code at the receiver end.Another spread spectrum approach is frequency hopping. Both base andsubscriber (handset and base) “hop” from frequency to frequency in a simultaneousfashion [4]. The theory states that noise tends to occur at different frequenciesat different times. Therefore, even though a part of the transmissionmay be lost due to interference, enough of the message will be received by hoppingthrough the interference to create a noticeably better output compared tofixed-frequency systems. See Table 8.1 for a variety of applications and theirfrequencies. The 1.8- to 2.4-GHz band is open for data collection in Europe,which at present is the second largest market for RF data.Spread spectrum’s ability to put a greater number of carriers in a specificbandwidth and to increase the security of the transmitted data is enablingseveral new wireless applications for sensing. Spread spectrum techniquescombined with new frequency ranges in the unlicensed ISM bands have madenew applications not only possible but easy to implement. In addition to