Wireless Ad Hoc and Sensor Networks

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10Adaptive and Probabilistic Power ControlScheme for RFID Reader NetworksIn Chapter 6, a power control scheme for wireless ad hoc and sensornetworks is presented. In this chapter, the distributed power control (DPC)for a different type of network is discussed wherein by controlling power,detection range and read rates can be enhanced.In radio frequency identification (RFID) systems (Finkenzeller andWaddington 2000), the detection range and read rates will suffer frominterference among high-power reading devices. This problem becomessevere and degrades system performance in dense RFID networks. Consequently,DPC schemes and associated medium access protocols (MACs)are needed for such networks to assess and provide access to the channelso that tags can be read accurately. In this chapter, we investigate a suiteof feasible power control schemes (Cha et al. 2006) to ensure overallcoverage area of the system while maintaining a desired read rate. Thepower control scheme and the MAC protocol dynamically adjust the RFIDreader power output in response to the interference level seen during tagreading and acceptable signal-to-noise ratio (SNR). The distributed adaptivepower control (DAPC) and probabilistic power control (PPC) fromCha et al. (2006) are introduced as two possible solutions. A suitablebackoff scheme is also added with DAPC to improve coverage. Both themethodology and implementation of the schemes are presented, simulated,compared, and discussed for further work.10.1 IntroductionThe advent of radio frequency identification (RFID) technology has broughtwith it increased visibility into manufacturing process and industry(Finkenzeller and Waddington 2000). From supply chain logistics toenhanced shop floor control, this technology presents many opportunitiesfor process improvement or reengineering. The underlying principle of RFIDtechnology is to obtain information from tags by using readers through radio461
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
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18. Chaos in Automatic Control, edi
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CRC PressTaylor & Francis Group6000
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Table of Contents1 Background on Ne
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3 Congestion Control in ATM Network
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5.4.2 Distributed Power Controller
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7.3 Adaptive and Distributed Fair S
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9.2.2 Overview.....................
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the number of connections in each l
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y Maheswaran Thiagarajan, and tirel
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10 Wireless Ad Hoc and Sensor Netwo
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2BackgroundIn this chapter, we prov
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Background 51u(k)x n (k + 1)x n (k)
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Background 53with x( 0)being the in
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Background 55with tr(.) the matrix
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Background 57nGiven a function ft (
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Background 592.3.2 Lyapunov Stabili
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Background 61as well as Jagannathan
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Background 63The subsequent example
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Background 65Evaluating this along
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Background 67Now, select the feedba
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Background 69is SISL if and only if
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Background 71L 1 (x)L(x, t)L 0 (x)0
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Background 73for some R > 0 such th
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Background 75Evaluating the first d
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Background 77Section 2.4Problem 2.4
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Admission Controller Design for Hig
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7Distributed Fair Scheduling in Wir
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Distributed Fair Scheduling in Wire
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8Optimized Energy and Delay-Based R
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