Wireless Ad Hoc and Sensor Networks

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