Wireless Ad Hoc and Sensor Networks

462 Wireless Ad Hoc and Sensor Networksfrequency (RF) links. The RFID technology basics and current standardscan be found at the EPC Global Web site (http://www.epcglobalinc.org/).In passive RFID systems, tags harvest energy from the carrier signal,which is obtained from the reader to power internal circuits. Moreover,passive tags do not initiate any communication but only decode modulatedcommand signals from the readers and respond accordinglythrough backscatter communication (Rao 1999). The nature of RF backscatterrequires high power ouput at the reader, and theoretically higheroutput power offers farther detection range with a desirable bit error rate(BER). For 915 MHz ISM bands, the output power is limited to 1 W (FCCCode 2000). When multiple readers are deployed in a working environment,signals from one reader may reach others and cause interference.This RFID interference problem was explained in Engels (2002) as readercollision.The work by Engels (2002) suggested that RFID frequency interferenceoccurs when a signal transmitted from one reader reaches another andjams its ongoing communication with tags in range. Studies also showthat interrogation zones among readers need not overlap for frequencyinterference to occur, the reason being that power radiated from onereader needs to be at the level of the tag backscatter signal (µW) (Karthausand Fischer 2003) to cause interference when reaching others. For a desiredcoverage area, readers must be placed relatively close to one another,forming a dense reader network. Consequently, frequency interferencenormally occurs, which results in limited read range, inaccurate reads,and long reading intervals. Placement of readers to mimize the interferenceand maximize the read range is an open problem.To date, frequency interference has been described as “collision,” as ina yes or no case in which a reader in the same channel at a certain distancecauses another reader not to read any of its tags in its range. In fact, higherinterference only implies that the read range is reduced significantly, butnot to zero. This result is mathematically given in Section 10.2. Previousattempts (Waldrop et al. 2003, Tech Report 2005) to solve this channelaccess problem were based on either spectral or temporal separation ofreaders. Colorwave (Waldrop et al. 2003) and “listen before talk” implementedas per CEPT regulations (Tech Report 2005) rely on time-basedseparation, whereas frequency hopping spread spectrum (FHSS) implementedas per the FCC regulations (FCC Code 2000) utilizes multiplefrequency channels. The former strategy is inefficient in terms of readertime and average read range, whereas the latter is not universally permittedby regulations. The work from Cha et al. (2006) is specifically targetedfor RFID networks to overcome these limitations.In this chapter, two power control schemes from Cha et al. (2006) thatemploy reader transmission power as the system control variable toachieve a desired read range and read rates are discussed. Degree of