) Common to all diversity schemes is that they perform some kind of trans- formation on the data stream prior to transmission in order to map the different transforms of the signal to the different antennas and send them out accordingly. This transformation operation is often called precoding. The spatial multiplexing and diversity schemes can also be combined, meaning that a precoding operation can be used in the case of multistream transmission as well. For example, it is also possible to use two transmit antennas for beam forming toward two receiving users, while two other antennas spatially multiplex two streams of data to a third receiv- ing user equipment. The general structure of the channel transmission processing chain (applicable also for the LTE downlink) is shown in Figure 4.16. In LTE, at most two code words can be transmitted at the same time; this means that even in the case of four MIMO streams, only two code words are involved (i.e., one code word is split into two parts to result in two streams). The bits of each code word are first mapped to modulation symbols according to the modulation scheme employed (QPSK [quadrature phase shift keying], 16 QAM [quadrature amplitude modulation], 64 QAM), and then the modulation symbols are mapped to layers by the layer mapper. The layers are the input to the precoder and the number of layers (L) is always less than or equal to the number of transmit antennas (i.e., L > NV. The layer mapping differs depending on whether the transmission will be spatial multiplexing or transmit diversity. Let us denote the first and second code words after the modulation operation with the row vectors of d(1) and d(2), respectively, each with length of D. The layer mapper generates R number of consecutive output vectors of length L from the code words as input. Let us denote the output of the layer mapper at the ith epoch Antenna port #1 Code word #1 Modulation Resource OFDM Signal Mapper Generation Mapper Layer Mapper coding Code word #2 Modulation Resource OFDM Signal Anten