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Wireless Ad Hoc and Sensor Networks

Wireless Ad Hoc and Sensor Networks

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184 <strong>Wireless</strong> <strong>Ad</strong> <strong>Hoc</strong> <strong>and</strong> <strong>Sensor</strong> <strong>Networks</strong>u () las vi() il = , where u is the input to each subsystem, which dependsIi() li( l)only on the total interference produced by the other users. To maintainthe SIR of each link above a desired target <strong>and</strong> to eliminate any steadystateerrors, SIR error ei() l = Ri()l − γ i is defined as the difference betweenactual SIR <strong>and</strong> its target value γ i . Therefore, defining e() l = x(),l we haveiix( l+ 1) = J x( l) + Hv()li i i i i(5.12)where J i = 1, <strong>and</strong> H i = 1. According to the state space theory (Lewis1999), the equation in the preceding text represents a first-order linearstate space system. Our goal is to maintain a target SIR for each networklink while the transmitter power is adjusted so that the least possiblepower is consumed.THEOREM 5.2.1Given the SIR system as Equation 5.12, <strong>and</strong> if the feedback for the ith transmitteris chosen as vi() l =− kixi()l +ηiwith k i representing the feedback gains,η i representing the protection margin <strong>and</strong> the associated power update, P i( l+ 1) = ( vi( l) Ii( l) + pi( l),given in Table 5.1 is used, then the closed-loop systemis stable for each link, <strong>and</strong> the actual SIRs will converge to their correspondingtarget values.PROOF Applying the feedback into Equation 5.12 the closed-loop errorsystem in SIR Is got asx( l+ 1) = ( J − H k ) x( l)+ Hηi i i i i i i(5.13)TABLE 5.1State-Space-Based Distributed Power ControllerSIR system state equationSIR errorFeedback controllerPower updateR( l+ 1) = R( l) + v () li i ix () l = R()l −γi i iv () l =− k x () l +ηi i i ip( l+ 1) = ( v ( l) I ( l) + p( l))i i i iwherek i, γ i , <strong>and</strong>ηi are design parameters

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