B are aware of the predefined set of precoding matrices; consequently, only a matrix identifier called PMI (precoding matrix indicator) is reported (see Figure 3.25). Precoding in LTE is used for two purposes: In general, orthogonal transmission on the parallel channels is achieved and thus the interference among transmitted signals at the receiver is sig- nificantly reduced. The precoding stage at eNodeB also serves for mapping the K multiplexed data streams to the M transmit antennas. The application of precoding in combination with extra antennas (K < M) provides beam forming in addi- tion to spatial multiplexing. Actually, the same precoding framework is used for beam forming and precoded spatial multiplexing in LTE. In beam © 2009 by Taylor & Francis Group, LLC Radio-Interface Physical Layer forming, only one data stream is transmitted and the generic (M X K) precoding matrix becomes an (M X 1) beam-forming vector. It should be noted that the larger the codebook size is, the higher the beam-forming gain that will be achieved. 3.3.4 LTE Multiantenna Procedures LTE multiantenna transmission relies on the use of the aforementioned transmit diversity and spatial multiplexing schemes. Both open-loop and closed-loop solu- tions based on these schemes will be applied to achieve diversity or multiplexing gains. The LTE standard defines a set of possible multiantenna configurations for data transmission over the MIMO radio interface. The multiantenna configuration to be applied will be determined by the number of available transmit antennas, specific user capabilities, and current MIMO channel conditions. Therefore, multi- antenna transmission must be dynamically configured by upper layers for each phys- ical channel and corresponding user, according to the particular user information such as channel state, speed, or QoS issues. The procedure to apply a specific multi- antenna configuration for transmission of a physical channel is defined by so-called antenna mapping, which can be considered as the LTE multiantenna 