ignal mapper mapper generation Figure 3.7 Overview of downlink physical channel processing. Reproduced with permission from C 2009 3GPP [13] is such that each physical layer cell identity is part of one and only one physical layer cell-identity group. A physical layer cell identity is thus uniquely defined by a number in the range of 0-167, representing the physical layer cell identity group, and a number in the range of 0-2, representing the physical layer identity within the physical layer cell identity group. 3.5.3.4 General Structure for Downlink Physical Channels The smallest time-frequency unit for downlink transmission is denoted a resource element which is equivalent to one complex-valued modulation symbol. The generation of downlink transmission that is, baseband signal representing a downlink physical channel is shown in Figure 3.7 First the coded bits of each code word that need to be transmitted on a physical channel are scrambled. Then modulation (QPSK, 16-QAM, 64-QAM) of scrambled bits is performed to create complex-valued modulation symbols. Then these symbols are mapped on one or more transmission layers which are then precoded for transmission on the antenna ports. After precoding these symbols are mapped to resource elements on the downlink resource grid and there is one resource grid per antenna port. Finally, a complex-valued time-domain OFDM signal is generated for each antenna port. 3.5.3.5 Procedures The following physical layer procedures are supported: AMC: Adaptive modulation and coding (AMC) with various modulation schemes and channel coding rates is applied to the shared data channel. The same coding and modulation is applied to all groups of resource blocks belonging to the same L2 (layer 2) PDU (protocol data unit) 10 scheduled to one user within one TTI and within a single stream. Power Control: Downlink power control can be used. Cell Search: Cell search is the procedure by which a UE acquires time and frequency synchronization with a cell and detects the Cell ID of that cell. E-UT