 layers Pre-coding Matrix PUCCH Rank and Precoding CQI + PMI + Rank Indication (RI) Matrix Selection PDCCH Multiantenna Scheme + RI + PMI 1. PDCCH de-coding Mode of operation User Data 2. Data de-coding PDSCH Figure 3.27 Rank adaptation and precoding selection procedures. transmitted with transmit diversity configuration because a correct reception is the goal rather than an increased data rate for these channels. On the other hand, both spatial multiplexing and beamforming configurations are based on precoding, and thus the UE needs to know the precoding matrix (or vector) for properly recovering transmitted data. The applied configuration, includ- ing number of layers and precoding matrix, is decided at the eNodeB, which takes into account the information reported by the UE (i.e., suitable rank and preferred precoding matrix). However, the final eNodeB decision may differ with respect to the UE preferred values; therefore, the UE must be informed about the applied configuration (see Figure 3.27). As a consequence, closed-loop configurations can- not be used to transmit PDCCH (which contains L1 signaling) and will only be used for user data transmission (PDSCH). The aforementioned antenna mapping procedure, as well as multiantenna con- figurations, is not defined for uplink transmission at this stage of the LTE specifica- tions. However, future LTE releases may include the use of uplink MU-MIMO as collaborative or distributed MIMO. That is, the same time-frequency resources are shared by two UEs, which transmit on a single antenna port to the multiple receive antennas of the eNodeB. 3.3.4.3 Multiantenna Adaptation Procedures The time-variant channel matrix and user-specific conditions (e.g., speed) imply that several adaptation or switching procedures are needed in order to apply the appropriate multiantenna configuration for each user. The number of layers or spa- tial multiplexing order is dynamically adapted to the rank of the channel matrix. The UE reports the rank of the channel matrix to the eNodeB, which 