Wireless Ad Hoc and Sensor Networks

Adaptive and Probabilistic Power Control Scheme 471Whenever the reader finds the target SNR is not achievable at maximumpower, implying that the interference level is too high in the network, itshould back off to a low output power for a period of time. As interferenceis a locally experienced phenomenon, multiple readers will face this situationand they will all be forced to back off. The rapid reduction of powerwill result in significant improvement of SNR at other readers. Afterwaiting for the backoff period, a reader will return to normal operationand attempt to achieve the target SNR. The process is repeated for everyreader in the network. To fairly distribute the channel access among allcongested readers, certain quality measurements must be ensured for allreaders in the backoff scheme. The selective backoff scheme uses thepercentage of time a reader has achieved desired range as the qualitycontrol parameter to ensure fairness.After backing off, each reader must wait for a time duration t w . Toillustrate the effect of backoff, t w is defined as a logarithmic function of thepercentage of time r a reader has attained the required SNR. A neglectedreader will exit backoff mode quickly and attain the required SNR, whereasother readers in the vicinity fall back. The calculation of t w is given byt w = 10 ⋅ [log 10 (r + 0.01) + 2] (10.25)Using the preceding equation, a reader with a r of 10% will wait for 10time intervals, whereas the waiting time for r of 100% equals 20. A plotof waiting time t w vs. r is presented in Figure 10.2.252015τw10500 0.1 0.2 0.3 0.4 0.5ρ0.6 0.7 0.8 0.9 1FIGURE 10.2Selective backoff function plot.