Wireless Ad Hoc and Sensor Networks

Distributed Power Control and Rate Adaptation 259SNR(t)R4Packet OK with optimal ratePacket OK but with sub-optimal ratePacket LOST because min. SNR not metRTS-DATA loop-delayR3R2R1RTS RTS RTSDATA(R2)optimalrateDATA(R3)suboptimalrateDATA(R4)packetlostFIGURE 6.13RBAR rate selection.By contrast, the RBAR protocol (Holland et al. 2001) improves thethroughput via suitable rate adaptation by measuring the SNR ratio, atthe receiver, for the RTS MAC frame. This information is piggybacked inthe CTS frame, which is sent to the transmitting node. Then a suitablerate is selected for the DATA frame transmission as shown in Figure 6.13.Because the RTS and DATA frames are sent one after the other, the rateselection is more accurate than in ARF. Additionally, a more appropriaterate is selected in RBAR, while in ARF, the rate changes one step at a time.Consequently, fewer packets are received incorrectly, and the achievedthroughput is always higher than the ARF protocol. However, the rateadaptation is determined using outdated information that was measuredone loop-delay prior to the DATA transmission. Hence, some packets willbe transmitted with an inaccurate rate that is either too low or too high.Moreover, the packets are transmitted at the maximum power. As a result,both the throughput and the energy consumption are not optimal.6.8 Heuristic Rate AdaptationThe proposed heuristic protocol uses the previously mentioned DPCalgorithm (Zawodniok and Jagannathan 2004) to predict the channel stateone step ahead, which is subsequently used to select the rate and the