Wireless Ad Hoc and Sensor Networks

244 Wireless Ad Hoc and Sensor NetworksNAV (G)(no pulses)Tx powerG does not sense any signal,starts transmission, whichcauses collision with (A->B)MaxDPC levelRTS CTSDATATimeACKFIGURE 6.3Transmission using DPC.Therefore, F can initiate transmission, which will collide with DATA frameat the receiving node B. In Figure 6.2, nodes located inside a markedregion, called “hidden terminals’ area,” are potential sources for this problem.The DPC schemes, generally, use much lower power for the transmissionof frames, and therefore result in increased collisions, as shownin Figure 6.3. A solution similar to that found in other schemes (Jung andVaidya 2002) is proposed.Generally, when a lower transmission power is used, the transmissionand sensing range of a node will decrease as shown in Figure 6.2. NodeG will fail to detect transmission from node A and it will initiate transmissionby assuming that the channel is idle. If node G uses maximumpower, then a collision will occur at receiver node B. Hence, the probabilityof hidden-terminal problem occurring in the network will increase whena low transmitter power is used.To overcome this problem, in our proposed methodology, a train ofshort pulses with increased transmitted power, periodically, is used duringtransmission. The RTS, CTS, DATA, and ACK frames are transmittedusing the power dictated by the proposed DPC, along with the train ofpulses. The pulses use maximum transmission power defined for thenetwork. This ensures that all the nodes in the sensing range of thetransmitter will detect the pulses and update their NAV vectors accordingly.Thus, the nodes in the sensing range of the transmitter node willnot cause collision. Figure 6.3 and Figure 6.4 display the difference inhandling of NAV vector in the available DPC schemes with and withoutthe train of pulses.The generation of the train of pulses should be implemented in thehardware. In details, the circuitry of RF amplifier should periodicallyincrease the transmission power for every transmission. Additionally, thissolution is simpler to implement than the one proposed in Jung andVaidya (2002), because an amplifier module in this scheme does not needto know the type of a transmitted frame.