 radio resource, the term space division multiple access (SDMA) is adopted. This scheme, commonly referred to as multiuser MIMO (MU-MIMO) (Jiang and Hanzo 2007, 1430), is under consideration in LTE. Figure 3.24 shows an example of MU-MIMO with two code words being trans- mitted by two antennas. The same time-frequency resource (or channel) is shared by users A and B (i.e., transmission on different antennas is allocated to different users). The way in which different code words are allocated to users is determined by the medium access control (MAC) layer according to different scheduling algo- rithms (Jiang and Hanzo 2007, 1430). MU-MIMO transmission allows increase of system capacity or, equivalently, support of a larger number of users per cell. © 2009 by Taylor & Francis Group, LLC Radio-Interface Physical Layer Frequency Channel 1 Channel 2 Code word #1 AAAABBBE eNode Code word #2 BBBBAAAA Channannel Figure 3.24 Example of multiuser MIMO with two code words. 3.3.3.3 Precoding-Based Spatial Multiplexing In general, as discussed earlier, the number of multiplexed signals (K) will be limited to the rank of the channel matrix (R), which is less than or equal to the number of transmit antennas (M). With some information about the downlink channel at the transmitter, precoding can be applied in order to improve isolation among parallel channels. Hence, interfering signals will be suppressed and effec- tive SINR for each of the data streams will rise, thus increasing system capacity. Precoding can be seen as a means to achieve a diagonal effective channel matrix D and thus to have R independent channels for transmission over M transmit anten- nas. Precoding is based on the singular-value decomposition (SVD) of the MIMO channel (Goldsmith 2005): H= UDVH (3.2) where U (NxR) and V (M x R) are matrices whose columns are orthonormal* and D is a (Rx1 R) diagonal matrix. Additionally, the diagonal elements of D are the R strongest eigenvalues of HHH. By applying the precoding matrix V prior to trans- mission and the shapin