Wireless Ad Hoc and Sensor Networks

254 Wireless Ad Hoc and Sensor Networks6.7 Background on Rate AdaptationResource constraints as pointed out in Chapter 1 require that ad hocwireless and sensor networks are energy efficient during transmission andrate adaptation. In this chapter, we present two novel energy-efficient rateadaptation schemes from Zawodniok and Jagannathan (2005) to selectmodulation schemes online to maximize throughput based on channelstate while saving energy. These protocols use the DPC algorithm fromprevious sections to predict the channel state and determine the necessarytransmission power that optimizes the energy consumption. The firstproposed rate adaptation scheme heuristically alters the transmission rateusing energy efficiency as a constraint to meet the required throughput,which is estimated with the queue fill ratio. Moreover, the backoff schemeis incorporated to mitigate congestion and reduce packet losses due tobuffer overflows, thus minimizing corresponding energy consumption.The backoff scheme implemented recursively becomes a back-pressuresignal. Consequently, the nodes will conserve energy when the traffic islow, offer higher throughput when needed, and save energy during congestionby limiting transmission rates.The second rate adaptation scheme uses the burst mode described in the802.11 standard to provide a flow control mechanism. The dynamic programming(DP) principle is employed to provide an analytical method to selectthe modulation rate and a burst size to be transmitted over the radio link.The proposed quadratic cost function minimizes the energy consumption.Additionally, buffer occupancy is included in the cost function for the purposeof congestion control. The proposed DP solution renders a Riccati equationultimately providing an optimal rate selection. The simulation results, shownlater in this chapter, indicate that an increase in throughput by 96% andenergy-efficiency by 131% is observed when compared to the receiver-basedauto rate (RBAR) protocol (Holland et al. 2001).Because of the need for higher throughputs in the next generationwireless networks, modulation schemes that render higher data rateshave been introduced; for example 54 Mbps capability of the 802.11gstandard. However, the communication range decreases as the transmissionrate increases. Hence, connectivity is reduced for modulation schemesthat provide higher throughput. A simple remedy is to increase thetransmission power. However, a node’s energy is drained quickly andthe energy-efficiency of transmission, which is measured as the numberof bits transmitted per joule, decreases with rate reducing the overalllifetime of the nodes and the network.To address the rate adaptation in wireless networks based on the 802.11standard, several schemes were proposed in the literature (Holland et al.2001, Kamerman and Monteban 1997). However, these protocols focus