 The net PHY through- put is obtained by multiplying by the coding rate. For LTE, coders have 1/3, 1/2, 2/3, and 5/6 [15]. The preceding approximation formula has been validated with tabular simulation results and differs from the simulation-measured data by at most 0.2 dB. © 2009 by Taylor & Francis Group, LLC Multihop Extensions to Cellular Networks QPSK-1/3 QPSK-1/2 QPSK-2/3 QAM16-1/2 QAM16-2/3 QAM16-5/6 QAM64-2/3 QAM64-5/6 QAM256-5/6 Shannon SINR (bB) Figure 8.6 Mutual information as a function of the SINR. Here, an error correc- tion coding using turbo codes of rates 1/3 to 5/6 is used. On the MAC layer, bit trains become packets. The CRC unit detects packets with errors. The probability of a packet error is the PER. It depends on the type of channel coder used, the coding rate, and the packet length. This mapping must be taken from tabular results of simulations. With ARQ, these dropped packets are retransmitted. Retransmissions always come with an overhead because capacity is wasted. The resulting net rate is given by aboveARQbelowARQ(1-PER Taking these effects into account, the resulting VMAC = f(SINR) can be derived. Figure 8.6 shows this rate function for the 3G-LTE PhyModes. The mode QAM 256-5/6 was added because it can be used for the quasi-static BS-to-RN links to improve the relaying performance. The optimum switching points can be determined from the last result; Table 8.1 shows the suitable PhyModes and their best SINR intervals. 8.2.4 Ad Hoc versus Fixed Relays Ad hoc networks have a different approach to multihop operation. As in IEEE 802.11 WiFi systems in ad hoc mode, user terminals can take over the role as access points. With extensions, all terminals can self-organize themselves as a mesh © 2009 by Taylor & Francis Group, LLC Long Term Evolution: 3GPP LTE Radio and Cellular Technology Table 8.1 LTE PhyModes and Their Required SINR min PhyMode Modulation Code Rate (in Decibels) QAM 16 QAM 16 QAM 16 QAM 64 QAM 64 network [34]. Due to the lack of centralized coordination and planning, this is 