code word are mapped onto the different layers of the transmission. Symbols from the same code word can be mapped to one or several layers, as shown in Figure 3.26. Therefore, the number of layers is always greater than or equal to the number of code words. Precoding. One modulation symbol from each layer is extracted and the resulting vector is processed according to the applied multiantenna scheme. This stage includes the precoding process (as presented in Section 3.3.3.3) for spatial multiplexing and the mapping procedure defined by SFBC (Section 3.3.2.2) for transmit diversity. The output of the precoding stage is an M-size vector of complex values to be mapped on each of the M antenna ports. This stage can be seen as a means to create a set of virtual antennas (or layers) from physical antennas. Hence, the full base station power can be used irrespective of the number of transmission layers. An example of antenna mapping, including layer mapping and precoding, is illustrated in Figure 3.26 for spatial multiplexing with two code words, K = 3 layers and M = 4 transmit antennas. In this case, the first code word is directly mapped to the first layer, while the second code word is demultiplexed into the second and third layers. Finally, the precoding stage performs the conversion from three layers to the four antennas of the eNodeB. 3.3.4.2 LTE Multiantenna Configurations The different multiantenna schemes supported in LTE have been presented in Sections 3.3.2 and 3.3.3 for achieving diversity and multiplexing gains, respec- tively. However, these schemes are only the ingredients to be combined in order to © 2009 by Taylor & Francis Group, LLC Radio-Interface Physical Layer obtain a certain multiantenna configuration. Concretely, three configurations have been defined for LTE downlink: Transmit diversity consists of applying an SFBC scheme (as detailed in Section 3.3.2.2) according to the number of antennas of the eNodeB. There is only one code word and the number of layers is equal to the number of transmit ant