Wireless Ad Hoc and Sensor Networks

Distributed Power Control and Rate Adaptation 283and Rayleigh fading can attenuate the power of the signal at the receiver,and thus cause variations in the received SNR or SIR. In addition, otherwireless devices working in the same frequency band interfere, therebyincreasing the noise level at the receiver. The objective of the DPC is toovercome these channel uncertainties and maintain a desired SNR at thereceiver.In cellular networks, the uncertain dynamic wireless channel results inintermittent connectivity and high energy consumption. DAPC schemescan be applied between the transmit tower and mobile user to reduce theeffect of channel fluctuation and decrease the connection outage probability(Jagannathan et al. 2006). On the other hand, for wireless ad hocand sensor networks, DAPC schemes applied between paired connectionscan increase the energy efficiency of the nodes, and therefore extendthe lifetime of the network and improve the QoS (Zawodniok andJagannathan 2004).Similarly, in radio frequency identification (RFID) systems, the detectionrange and read rates will suffer from interference among high-powerreading devices. The problem grows severe and degrades system performancesin dense RFID networks. DAPC scheme can be deployed at theRFID reader to mitigate the interference issue among readers, and ensurethe overall coverage area of the system while maintaining a desired readrate (Cha et al. 2006).Work presented in this section focuses on the implementation ofDAPC protocol on a generic wireless test platform developed at theUniversity of Missouri — Rolla (UMR) (Cha et al. 2006). Hardwareexperiments are set up to evaluate the DAPC operation under variouschannel conditions in hardware because the authors are not aware ofany known hardware implementations in this area. The results fromthe experiments illustrate that the protocol performs satisfactorily, asexpected.For the sake of simplicity, only the mathematical equation used forimplementation is described here. In the discrete-time domain, the feedbackcontrol for the DAPC is selected as (Zawodniok and Jagannathan2004)p iIi()l( l + 1 ) =i lRi l Rrequiredg () l[ − θ ˆ () () + + k v( R i( l ) − R required)]ii(6.61)where θiˆ () l is the estimation of the unknown parameters defined as θ i () land is a control parameter.The mean channel estimation error along with the mean SNR errorconverges to zero asymptotically, if parameter updates are taken ask vˆ ( ) ˆ () ()( () )θ l+ 1 = θ l + σ R l R l −Ri i i i required(6.62)