) measurement of the (reciprocal) reverse channel. Some MCSs and their MCRs (modulation coding rates) are listed in Table 2.1. In a system with AMC, users in favorable positions (close to the cell site) are typically assigned higher-order modulation with higher code rates (e.g., 64 QAM with R = 3/4 turbo codes), while users in unfavorable positions (close to the cell boundary) are assigned lower-order modulation with lower code rates (e.g., QPSK with R = 1/2 turbo codes). AMC can benefit the system from several aspects. A higher data rate is avail- able for communication under good channel conditions, which consequently © 2009 by Taylor & Francis Group, LLC Long Term Evolution: 3GPP LTE Radio and Cellular Technology Table 2.1 AMC Schemes and Modulation Coding Rates Modulation Coding MCR (bits/symbol) 0.667 16 QAM 16 QAM 64 QAM increases the average throughput of the system. Communication quality is guaran- teed in cases in which selection of MCSs is in accordance with channel condition. Interference variation is reduced due to link adaptation based on variations in the modulation/coding scheme instead of variations in transmission power. In systems with TDD mode, AMC is more convenient for application and the scheduling of various MCSs is flexible with low latency. Hybrid automatic repeat request (HARQ). Hybrid ARQ is an important link adaptation technique. In AMC schemes, C/I measurements or similar measure- ments are used to set the modulation and coding format; in HARQ, link layer acknowledgments are used for retransmission decisions. AMC by itself does pro- vide some flexibility to choose an appropriate MCS for the channel conditions according to measurements either based on AT measurement reports or network determined. However, an accurate measurement is required and there is a delay. Therefore, the ARQ mechanism is still required. HARQ automatically adapts to the instantaneous channel conditions and is insensitive to the measurement error and delay. Combining AMC with HARQ leads to the best of both fields: 