Wireless Ad Hoc and Sensor Networks

472 Wireless Ad Hoc and Sensor NetworksTABLE 10.1Selective Backoff PseudocodeIf reader is not in backoff modeIf P next == P maxchange reader to backoff modeinitialize wait time t wIf reader is in backoff modeSet P next = P mindecrease t wIf t w == 0reader exit backoff modeThe backoff policy will cause negative changes in interference, andhence does not adversely affect the performance of the adaptive powerupdate. A detailed pseudocode for implementing selective backoff isgiven in Table 10.1.10.3.3 DAPC ImplementationDAPC can be easily implemented at the MAC layer of the RFID reader;MAC implementation is not covered in detail here. The algorithm requirestwo parameters to be known initially. These are the desired range, r d , andrequired SNR, R required .Proposed DAPC can be seen as a feedback between the transmitter andreceiver units of a reader. A block diagram of the implementation is shownin Figure 10.3. The detailed description of the algorithm implementationfollows:1. Power update block at the receiver unit of a reader obtains sensedinterference I(l).2. In the power update block, based on r d , R required , and current powerP(l), the current SNR R i−rd (l) is calculated.< = CQ++1I(L)SNRCompareto R_requiredPerc_achievedPerc_achievedPower Output1ReceiverP(L)P(L + 1)Min==CTriggerTransmitterPower updateLimiterCompareto P_maxSelective back-offFIGURE 10.3Block diagram for DAPC implementation.