framework. This subsection provides an overview of the different multiantenna configura- tions and multiantenna-related procedures defined in LTE. First, antenna mapping procedures will be introduced for the sake of clarity, and then the defined multi- antenna configurations and different modes of operation will be presented. Finally, switching between modes of operation and adaptation procedures will be detailed. 3.3.4.1 Antenna Mapping Multiantenna processing is applied at the eNodeB in order to transmit data accord- ing to some specific multiantenna configuration. In LTE, the downlink physical channel processing chain includes a multiantenna processing stage referred to as antenna mapping (see Section 3.1.4). The complex modulation symbols of each code word are taken as inputs, and the complex values to be mapped on each antenna are the result after the antenna mapping procedure. The way antenna mapping is car- ried out is configured by higher layers and explicitly determines the multiantenna configuration that will be applied to each physical channel. Thus, antenna mapping represents the multiantenna transmission framework in LTE and is composed of the following procedures: Layer mapping. The purpose of this stage is to map the different code words (independently coded streams) onto the multiplexed streams (layers*) that Conceptually, a layer corresponds to a spatially multiplexed channel. Strictly speaking, the concept of layer should not be used when no spatial multiplexing is applied (e.g., transmit diversity). © 2009 by Taylor & Francis Group, LLC Long Term Evolution: 3GPP LTE Radio and Cellular Technology 2 Code words K Layers M Antennas Layer Pre-coding b3 b2 b1 bo Mapping [x = v . x] Complex modulation symbols Figure 3.26 Antenna mapping for spatial multiplexing with two code words, three layers, and four antennas. will be finally transmitted. A maximum of two code words is supported, whereas the number of layers is limited to the rank of the channel matrix. Hence, complex modulation symbols from each 