Wireless Ad Hoc and Sensor Networks

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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 However, the rateadaptation is determined using outdated information that was measuredone loop-delay prior to the DATA transmission.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
260 Wireless Ad Hoc and Sensor Networkstransmission power. This rate is adjusted according to the achieved energyefficiencylevels and local queue utilization. In consequence, a more accuraterate is selected while transmitting the DATA frame when compared to ARFor RBAR. Because of the DPC, minimal power required to transmit datapackets under the current channel state is used, thus saving energy. Finally,to minimize buffer overflows during network congestion, a backoff intervalis altered based on the buffer utilization of the receiver.6.8.1 OverviewFigure 6.14 presents the dataflow for the proposed rate adaptation scheme,which is applied at a transmitting node. First, the SNR thresholds are predefinedfor all modulation schemes in accordance with the desired BER value,thus reducing the computation overhead. During communication, the DPCalgorithm is used to continuously assess the channel state and to calculatethe transmission power for the target SNR 0 of the lowest supported rate. InSubsection 6.8.2, the details of the channel state estimation is given.Next, a set of possible rates is calculated. The upper limit of the set correspondsto the maximum rate that can be achieved due to the transmissionDPC(for lowest supported rate)Pt_0Pt_0SNR(t)Measured SNR(scaled to lowestsupported rate)SNR_0(target SNR forthe lowestsupported rate)Pt_0,SNR_0DetermineMIN rate (themost energyefficientoneMIN rate,SNR_1DetermineMAX rate (forthe currentchannel state)MAX rate,SNR_hSNR_0, SNR_1,..., SNR_NSNRthresholds(SNR_x =f(BER,Rate_x)Select rate fromRate K,SNR_KCalculatepower for theselected rate KUtilizationRate K,Pt_KQueueutilizationTransmitpacketFIGURE 6.14Energy-efficient rate adaptation data flow.
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Wireless Ad Hocand SensorNetworksPr
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18. Chaos in Automatic Control, edi
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CRC PressTaylor & Francis Group6000
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Table of Contents1 Background on Ne
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3 Congestion Control in ATM Network
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5.4.2 Distributed Power Controller
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7.3 Adaptive and Distributed Fair S
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9.2.2 Overview.....................
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the number of connections in each l
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y Maheswaran Thiagarajan, and tirel
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2 Wireless Ad Hoc and Sensor Networ
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10 Wireless Ad Hoc and Sensor Netwo
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2BackgroundIn this chapter, we prov
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Background 51u(k)x n (k + 1)x n (k)
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Background 53with x( 0)being the in
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Background 55with tr(.) the matrix
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Background 57nGiven a function ft (
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Background 592.3.2 Lyapunov Stabili
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Background 61as well as Jagannathan
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Background 63The subsequent example
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Background 65Evaluating this along
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Background 67Now, select the feedba
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Background 69is SISL if and only if
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Background 71L 1 (x)L(x, t)L 0 (x)0
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Background 73for some R > 0 such th
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Background 75Evaluating the first d
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Background 77Section 2.4Problem 2.4
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100 Wireless Ad Hoc and Sensor Netw
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4Admission Controller Design for Hi
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Admission Controller Design for Hig
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Page 326 and 327: 7Distributed Fair Scheduling in Wir
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Distributed Fair Scheduling in Wire
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8Optimized Energy and Delay-Based R
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Optimized Energy and Delay-Based Ro
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9Predictive Congestion Control for
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Predictive Congestion Control for W
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10Adaptive and Probabilistic Power
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Adaptive and Probabilistic Power Co
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Wireless Ad Hoc and Sensor Networks

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