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Wireless Ad Hoc and Sensor Networks

Wireless Ad Hoc and Sensor Networks

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<strong>Ad</strong>aptive <strong>and</strong> Probabilistic Power Control Scheme 4733. R i−rd (l) is compared to R required , <strong>and</strong> percentage of time required forachieving required SNR, r is calculated <strong>and</strong> recorded.4. Based on Equation 10.24, the channel is estimated for the nexttime step l + 1, <strong>and</strong> the power for P(l + 1) is also calculated usingthe feedback control from Equation 10.21.5. P(l + 1) is then limited to maximum power P max ; if P(l + 1) isgreater than P max , the selective backoff scheme is triggered,otherwise P(l + 1) is used as the output power for the nextcycle.6. The selective backoff block follows the algorithm provided in thepreceding subsection <strong>and</strong> restricts the final output power for thenext cycle.Simulation <strong>and</strong> results of the preceding implementation are discussedin Section 10.5 <strong>and</strong> Section 10.6, respectively, along with those of PPC.Next, the PPC is discussed.10.4 Probabilistic Power ControlThe idea of PPC comes from simple TDM algorithms. If a reader isassigned a time slot to transmit in full power while others are turnedoff, it will achieve its maximum range. A round-robin assignment oftime slots can ensure that all readers operate with no interference.However, this is inefficient in terms of average read range, readerutilization, <strong>and</strong> waiting periods. It is obvious that more than one readercan operate in the same time slot but at different power levels toaccomplish better overall read range. If the power levels at all readerschange in each time slot following a certain distribution, over time,every reader will be able to achieve its peak range while maintaininga good average.For a distributed solution, this would involve setting a probability distributionfor power to be selected for each time step. Such a distributionwould need to be adapted based on the density <strong>and</strong> other parameters ofthe reader network.10.4.1 Power DistributionEquation 10.9 states that the read range of a particular reader is dependenton its transmission power <strong>and</strong> the interference experienced, which is afunction of powers of all other readers. If reader powers follow a certain

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