ennas. From a conceptual point of view, layer mapping is not performed because there is no spatial multiplexing. Hence, the layer mapping procedure acts as a complementary part of precoding to perform the SFBC processing. Both layer mapping and precoding functionalities are defined in a different way when transmit diversity is applied (3GPP 36.212). Beamforming is the closed-loop transmit diversity solution adopted for LTE downlink. It is based on codebook precoding as explained in Section 3.3.2.2. Regarding the antenna mapping procedure, beam forming is con- sidered as a particular case of spatial multiplexing with only one code word and one layer. Thus, the layer mapping stage is transparent. Spatial multiplexing employs a SU-MIMO approach with two code words and codebook-based precoding (see Section 3.3.3). In addition, CDD is applied in order to provide delay diversity. That is, once the precoding is performed, a different cyclic shift is applied to the signal transmitted over the different antennas. Therefore, spatial multiplexing configuration in LTE can be seen as a hybrid multiplexing-diversity transmission mode. Table 3.5 summarizes the multiantenna configurations defined for LTE down- link, together with their corresponding setup of code words, layers, and transmit antennas. Spatial multiplexing allows for enhancing peak data rates of users expe- riencing good channel conditions (i.e., high SNR in a rich scattering scenario). However, transmit diversity is applied for users experiencing a low SNR in order to enhance coverage or link robustness. Control channels such as PDCCH are Table 3.5 Multiantenna Configurations for LTE Downlink Multiantenna No. Code Configuration Applied Schemes Words Layers Antennas Transmit diversity Codebook-based beam Beam forming 2 or 4 forming SU-MIMO with codebook- Spatial based precoding, and multiplexing 2, 3, or 4 © 2009 by Taylor & Francis Group, LLC Long Term Evolution: 3GPP LTE Radio and Cellular Technology eNode B Channel measurements Reference signals (Pilots) higher