 to changing PhyModes and probably different PhyModes on hop one and hop two of a multihop connection, a PDU frequently takes more or fewer resources on hop two than on hop one. This is the reason for buffers required to © 2009 by Taylor & Francis Group, LLC Long Term Evolution: 3GPP LTE Radio and Cellular Technology Effect of the FTFading using the FTtdlm Strategy (bwthSubCar = 180 kHz, Tapdelay = 0.00000150 ms, DopplerShift = 10 Hz) Figure 8.11 Typical frequency- and time-dependent behavior of a broadband OFDM channel. store PDUs in flight and a proper resource partitioning SO that an adequate resource set is always available on both hops [36]. 8.4.2.3 Adaptive Power Allocation (APA) Within certain bounds, the transmit power can be adjusted, even per OFDM sub- channel. Transmissions to UTs nearby need less power than to UTs in the distance, SO this decision is made locally. 8.4.3 Closed-Loop Control Figure 8.12 shows that the methods for adaptivity are nothing less than functional blocks within a control loop. The value to be controlled (optimized) is the through- put, w.r.t. the least possible transmit power possible. This can be transformed into controlling the received SINR value to be about 20 dB, which is just enough to support the highest PhyMode. This control happens per subchannel because each one may be subject to fading (Figure 8.11). The bound of the total available RF power (maxPower) gives a global limiter block, which proportionally affects each C 2009 by Taylor & Francis Group, LLC Multihop Extensions to Cellular Networks desired maxPower PhyMode(f,t), TxPower(f,t) RxPower Interference RxSINR + Noise (20dB) SINR(f,t) Adaptive Power Adaptive Modulation Physical channel Control and Coding (Path loss + fading) Measurement RxSINR with nominal noise TxPower Channel quality SINR Estimation with measurement nominal txPower Special procedures to Interpolation get downlink channel Dynamic Subchannel quality at BS Assignment Filtering Send filtered info back to BS Prediction of path loss Prediction of inte