Wireless Ad Hoc and Sensor Networks

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Distributed Power Control and Rate Adaptation 239THEOREM 6.3.1 (CONVERGENCE ANALYSIS)Given the DPC scheme above with channel uncertainties for a wireless ad hocnetwork and under the assumption of persistence of excitation of input signals,if the feedback from the DPC scheme is selected as Equation 6.4, then the meanchannel estimation error along with the mean SIR error converges to zero asymptotically,if the parameter updates are taken as Equation 5.64.PROOF See Zawodniok and Jagannathan (2004).6.4 DPC ImplementationA preliminary version of the proposed DPC algorithm was implementedfor cellular networks (Jagannathan et al. 2004), which was relativelystraightforward. Full duplex links and time-synchronized communicationsimplifies the estimation of channel gain. Additionally, the packets aresent at predefined intervals. This allows calculating the change of channelconditions between packets accurately, with a small estimation error.Moreover, the power for all connections is updated synchronously assumingthat the interference from other users is experienced by a particularconnection at the same time. Hence, the interference will not changeduring the packet transmission.On the other hand, the proposed MAC protocol is derived from 802.11standards, where the CSMA/CA random access method is used. Thisintroduces additional uncertainties with different challenges. Thereforethe proposed DPC algorithm has to be modified and has to be implementedcarefully in the MAC protocol.6.4.1 DPC FeedbackThe basic communication of a user packet employs a four-way handshakeexchange that consists of four frames: request to send (RTS) — from sourceto destination; clear to send (CTS) — from destination to source; DATAframe — from source to destination; acknowledgment (ACK) — fromdestination to source. All these frames are transmitted over a single radiochannel for wireless ad hoc networks. Hence, the communication betweenany two nodes is carried out over a shared half-duplex medium. Additionally,a particular handshake can be initiated at any time instance. Inconsequence, the interference experienced by a particular connection canchange at any time even during the transmission of a particular frame.Hence, the estimated power feedback has to overcome such an interferenceuncertainty.Moreover, the time interval between any consecutive transmissionsoccurring in the same direction will vary from frame to frame because of
240 Wireless Ad Hoc and Sensor NetworksNode1Node 2Tx frameDPC(Tx power update)Rx frameUpdate Tx Power usedtoward Node 2Calculate new Tx Powerto be used by Node 1FIGURE 6.1DPC feedback loop.the following reasons. First, the frames within the four-way handshakeare of different sizes — ranging from few bytes for ACK to over 2500bytes for DATA. Second, the delay between two consecutive handshakescan vary significantly because of packet transmission intervals and channelcontention. As a result, the channel estimation error will vary dependingon these time gaps, if feedback from the receiver is used to determinethe transmitter power for any DPC scheme. Therefore, the target SIR hasto be chosen suitably to overcome the worst-case scenario due to theseuncertainties. In our implementation, the target SIR value is obtained byusing a certain safety factor.6.4.2 DPC Algorithm for 802.11 Type Ad Hoc NetworksIn the proposed implementation shown in Figure 6.1, a feedback loop isused between the transmitter and the receiver to successfully implementthe DPC. The detailed description of the proposed DPC algorithm implementationis presented in Subsection 6.4.4.6.4.3 Retransmissions and Power ResetIn real scenarios, it is expected that the channel conditions at certain timeinstants can change too quickly to be correctly estimated by the DPCalgorithm, by violating the Assumption 6.3.1. Then frame losses will occur.Two mechanisms are introduced to overcome and mitigate such problems:increase of transmitter power in case of retransmission and reset of powerfor a long idle connection.A retransmission implies that the transmitted signal strength is notsufficient for decoding. A simple approach would be to retransmit theframe with the same power as in first transmission, and hope that channelstate will improve. However, the channel attenuation and/or interferences
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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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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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