36 3-bit Precoding Codebook for Uplink Spatial Multiplexing with Two Transmit Antennas [114] Number of Layers Codebook Index 10.10 Multi-antenna techniques in 3GPP LTE-advanced For uplink control channels with Rel-8 PUCCH format 1, 1a, and 1b, the Spatial Orthogonal- Resource Transmit Diversity (SORTD) scheme is supported for transmissions with two antenna ports. In this transmit diversity scheme, the same modulation symbol from the uplink channel is transmitted from two antenna ports, on two separate orthogonal resources. For the UEs with four transmit antennas, the two-transmit antenna transmit diversity scheme is applied [114]. The 3GPP LTE-Advanced extends LTE Rel-8 downlink spatial multiplexing with support for up to eight layers of spatial multiplexing. In the downlink, using single-user spatial multiplexing with eight transmit antennas, up to two transport blocks can be transmitted to a scheduled UE in a subframe per downlink component carrier. Each transport block is assigned its own modulation and coding scheme. For HARQ feedback on the uplink, one bit is used for each transport block. A transport block is associated with a codeword. For up to four layers, the codeword-to-layer mapping is the same as for 3GPP LTE Rel-8. For more than four layers, as well as the case of mapping one codeword to three or four layers, which is for re-transmission of one out of two codewords that were initially transmitted with more than four layers, layer mapping can be performed. Complex- valued modulation symbols for codewords q are mapped onto the layers, where vis the number of layers and is the number of modulation symbols per layer. Downlink coordinated multipoint transmission (CoMP) is a relatively general term referring to different types of coordination in downlink transmission from multiple geographically separated transmission points (TP). This includes coordination in scheduling, including any beam-forming functionality, between geographically separated transmission points and joint transmission from geographical