patial multiplexing mode, the transmit diversity scheme is applied when the transmission rank is set to one [101]. The 3GPP LTE supports MU-MIMO in the uplink and downlink. In the uplink, the eNB can schedule a number of UEs to transmit over the same time-frequency resource, which forms a MU-MIMO transmission configuration. However, in order for the eNB to correctly separate and CHAPTER 10 The IEEE 802. 16m Physical Layer (Part II) Table 10-34 Large-Delay Cyclic Delay Diversity [107] Number of Layers e-j2/2 e-j2i/2 e-j2/3 e-j2/3 e-j4i/3 e-j4T/4 -j2ni/4 e-j8/4 e-j4ri/4 e-j12/4 e-j18/4 -j6i/4 Table 10-35 Codebook for Transmission on Antenna Ports 0 and 1 [107] Number of Layers N1 Codebook Index 1/1 / i demodulate the UEs' signals, the eNB needs to assign orthogonal reference signals for these UEs scheduled for the MU-MIMO transmission. In the 3GPP LTE uplink slot structure, the reference signal is transmitted using the fourth SC-FDMA symbol and the data is transmitted using the other symbols. For a given slot and subframe in each cell, a Zadoff-Chu sequence is defined as the base sequence for uplink reference signals. The cyclically-shifted versions of a given Zadoff-Chu sequence form an orthogonal set of sequences. Each UE scheduled for MU-MIMO transmission is assigned a distinctive cyclic-shift value. The UE combines this cyclic-shift value with the knowledge of the base Zadoff-Chu sequence to form a reference signal sequence that is orthogonal to other UEs' reference signal sequences. It is noted that the cyclic shift value is always contained in the control signaling, which the UE has to receive for data transmission on uplink, regardless of whether the MU-MIMO is used. In the downlink, if a UE is configured to use the MU-MIMO transmission mode, only rank-1 transmission can be scheduled for the UE. The eNB can schedule multiple UEs, which are configured to operate in the MU-MIMO transmission mode, in the same time-frequency resource using different rank-1 precoding matrices from Table 10-35 for two transmit ant