Wireless Ad Hoc and Sensor Networks

Distributed Power Control and Rate Adaptation 25716001200SNR threscholds and energy-efficiency indexSNREnergy-efficiency indexSRN80040001 2 3 4 5 6 7 8Modulation rate (Mbps)FIGURE 6.11SNR thresholds and energy-efficiency index.the BER and the SNR threshold is well defined for a given modulationscheme (Holland et al. 2001). In general, a SNR threshold increases withthe rate as illustrated in Figure 6.11 for the set of modulation rates usedin simulations (1, 2, 4, 6, and 8 Mbps). Consequently, the minimal requiredtransmission power has to be increased with the rate. On the other hand,the power value is limited by the hardware capabilities. Thus, there is amaximum SNR value and corresponding modulation scheme that couldbe selected for a given power constraint. Given this maximum powerconstraint, the rate will result in highest throughput and higher energyefficiency as explained next. If the maximum power will yield SNR = 800,then the maximum possible rate that can be achieved is equal to 6 Mbps,as illustrated in Figure 6.11. It is important to note that channel stateimpacts the rate of transmission because the power will vary with interferencesand signal attenuation. Hence, it is important for the rate adaptationscheme to accurately assess the channel state to identify a suitablerate that results in the highest throughput under the channel conditions.Figure 6.11 depicts an energy-efficiency index that results in the lowestmodulation rate. It shows the SNR levels for all rates assuming a constantenergy-efficiency value. Comparing the energy-efficiency index and theactual SNR levels, it can be noticed that the energy-efficiency decreaseswith an increase in modulation rate.6.7.2 Protocol ComparisonIn the previous works (Holland et al. 2001, Kamerman and Monteban1997), the problem of rate adaptation was attempted either by considering