 = 0,1, layer - 1 with M Superscript(a) symb = For transmission on four antenna ports, P E {0,1,2,3}, the output y(i) = [y(i) of the precoding operation is defined by Re(x()(i)) Re(x(2)(i)) Re(x(3)(i)) Im(x()(i)) Im(x(1)(i)) Im(x(2)(i)) Im(x(3)(i)) for i = 0,1, symb layer 1 with = 5.3 MU-MIMO In 3GPP LTE, MU-MIMO is still a topic in discussion. Per-user unitary and rate control (PU-RC) is a typical proposed MU-MIMO scheme [6,7]. Figure 5.5 shows the block diagram for PU2RC. In such a system, a set of precoder matrices, E = {E E(G-1)}, is used, where © 2009 by Taylor & Francis Group, LLC MIMO OFDM Schemes for 3GPP LTE Pre-coding matrix Group 0 corresponding to Multi-user the selected group stream User 1 Ant 0 stream User K Ant M-1 stream Group G-1 Selected Multi-user Multi-user stream stream Pre-Coder Preferred matrix index feedback from each user Controller M CQI feedback from each user Figure 5.5 PU2RC transmitter chain. matrix, and e m (g) is the mth precoding vector in the set. Each UE will calculate a channel quality indicator (CQI) value for every vector in every matrix in the set E. The system designer can trade off the amount of feedback overhead with the sched- uling flexibility at the node B by choosing an appropriate value of G and deciding the amount of information that the UE needs to feed back to the node B. For most flexibility, the UE can feed back every CQI value for every matrix in the set E and thus a total of GM CQI values for every UE. For lowest overhead, the UE needs to feed back only the best CQI value and the corresponding vector index, which would require log2 (GM) bits per UE in addition to the actual CQI value. This supports the restricted space division multiple access (SDMA)-only case because each user can be scheduled on only one specific beam. This also supports the transmit beam-forming-only case when a user is scheduled on the time-frequency unit. A trade-off of these two is that each UE feeds back the M CQI values corresponding to its preferred/best group. The node B can now sele