e current channel state at the time of transmission. In addition, an inexact channel estimation, quantification errors, or potential errors introduced in the feedback channel may lead to an erroneous CQI at the transmitter end. 3.2.1 Adaptive Modulation and Coding Different-order quadrature amplitude modulation (QAM) modulations have been chosen for LTE, ranging from binary phase key shifting (BPSK) to 64 QAM (see Table 3.2). Higher-order modulations allow for achieving higher data rates at the expense of decreasing robustness (i.e., they are suitable under good channel conditions). The adaptive modulation technique is aimed to improve data throughput while keeping the bit error rate (BER) below a predefined target value (BERT). In order to achieve that, the transmitted signal is modified according to the instantaneous channel quality. Additionally, the coding scheme may be dynamically modified to match the instantaneous channel conditions for each user, then being denoted as AMC. In this case, both modulation and coding schemes are jointly changed by the transmitter to © 2009 by Taylor & Francis Group, LLC Radio-Interface Physical Layer adapt the transmitted signal to the varying channel conditions in both time and fre- quency domains. The coding scheme establishes the amount of redundancy that is added to the transmitted data in order to increase transmission reliability. The ratio between original data length and coded data length establishes the effective coding rate R. Hence, good link quality conditions lead to the use of a high-order modula- tion scheme (more bits per symbol) as well as a low coding rate (less redundancy on transmitted data). In contrast, lower modulation schemes together with higher cod- ing rates are needed under poor link conditions in order to maintain the requested QoS level. In LTE specifications (3GPP 36.213), a predefined set of modulation and coding schemes (MCSs) corresponding to each CQI level is defined. 3.2.2 Channel Coding Since Claude Shannon published his landmark paper (19