naled to the UE. For a QPSK scheme, the UE cannot rely on the knowledge of the power ratio, but for higher modulation (64 QAM and 16 QAM) it is assumed that the UE is informed about the power ratio. The UEs can only be simultaneously scheduled if their preferred beams are spatially separated. This is an additional constraint on the scheduler that needs to group the UEs that have data to send and have sufficiently high geometry (SINR distribution). Thus, in order for MU-MIMO to be useful, the system load should be high with many active UEs requesting data in each subframe. This enables the scheduler to find a group of UEs that can be concurrently scheduled on beams which will limit intra- cell interference [99]. Dedicated beamforming is supported for improving data coverage when the UE supports data demodulation using the UE-specific reference signal. The eNB generates a beam using the array of antenna elements (e.g., an array of eight antenna elements), and then applies the same precoding to both the data sub-carriers and the UE-specific reference signals. It is noted that the UE-specific reference signal is transmitted such that its time-frequency location does not overlap with the cell- specific reference signal [101]. 10.10 MULTI-ANTENNA TECHNIQUES IN 3GPP LTE-ADVANCED In 3GPP LTE-Advanced, the existing SU-MIMO technologies are extended to support configurations with up to eight transmit antennas in the downlink, and up to four transmit antennas in the uplink. In addition, multi-cell Coordinated Multipoint (CoMP) transmission is also under discussion and evaluation. In the case of uplink single-user spatial multiplexing, up to two transport blocks can be transmitted from a scheduled UE in a subframe per uplink component carrier. Each transport block has CHAPTER 10 The IEEE 802. 16m Physical Layer (Part II) its own modulation and coding scheme. Depending on the number of transmission layers, the modulation symbols associated with each of the transport blocks are mapped onto one or two layers according to the sa