plex-valued modulation symbols on each layer are pre-coded for transmission and are further mapped to resource elements for each antenna port. The complex-valued time-domain OFDMA signal for each antenna port is then gener- ated following these stages [30,97]. In the uplink, the baseband signal is processed by scrambling the 10.9 Multi-antenna techniques in 3GPP LTE Input Bit Stream Antenna (User Data) Layers Ports Modulation Resource OFDM Signal Scrambling Mapper Element Mapper Generation Layer Precoding Mapper Modulation Resource Scrambling OFDM Signal Mapper Element Mapper Generation Downlink PHY Processing Resource SC-FDMA Modulation Transform Scrambling Element Mapper Precoder Signal Mapper Generation Input Bit Stream (User Data) Uplink PHY Processing FIGURE 10-49 Overview of 3GPP LTE multi-antenna signal processing input coded bits and then by modulation of scrambled bits to generate complex-valued symbols. The complex-valued modulation symbols are transform-precoded (DFT-based precoding) to generate complex-valued symbols that are later mapped to resource elements. The complex-valued time- domain SC-FDMA signal for each antenna port is then generated. A 2 X 2 antenna configuration for MIMO is assumed as the baseline in the 3GPP LTE downlink; i.e., two transmit antennas at the base station and two receive antennas at the terminal side [107]. Configurations with four transmit or receive antennas are also supported in the specifications. Different downlink MIMO modes are supported in 3GPP LTE, which can be adapted based on channel conditions, traffic requirements, and UE capability. These include transmit diversity, open-loop spatial multiplexing (no UE feedback), closed-loop spatial multiplexing (with UE feedback), multi-user MIMO (more than one UE is assigned to the same resource block), and closed-loop rank-1 precoding. Multi-antenna transmission with two and four transmit antennas is supported. The maximum number of codewords is two, irrespective of the number of antennas with fixed mapping between codewo