me principle used in 3GPP LTE Rel-8 downlink spatial multiplexing. The trans- mission rank can be dynamically adapted. It is possible to configure the uplink single-user spatial- multiplexing transmission with or without the layer shifting. In the case of the layer shifting, shifting in time-domain is supported. If layer shifting is supported, the HARQ feedbacks for all transport blocks are bundled into a single HARQ feedback; thus, a one-bit ACK is transmitted to the UE, if all transport blocks are successfully decoded by the eNB; otherwise, a one-bit NACK is transmitted to the UE. If layer shifting is not configured, each transport block has its own HARQ feedback [114]. In FDD and TDD modes, precoding is performed according to a predefined codebook. If layer shifting is not used, precoding is applied after layer mapping. If layer shifting is configured, precoding is applied after the layer shifting operation. The 3GPP LTE-Advanced supports the application of a single precoding matrix per uplink component carrier. In the case of full-rank transmission, only an identity precoding matrix is supported. For uplink spatial multiplexing with two transmit-antennas, a 3-bit precoding codebook as shown in Table 10-36 is used. For uplink spatial multiplexing with four transmit-antennas, a 6-bit precoding codebook is used. A subset of the precoding codebook is used for single-layer and dual-layer transmission. For 3-layer transmission, the number of precoding matrices is 20; only BPSK or QPSK alphabets are used for non-zero elements in precoding matrices [114]. For UEs with multiple transmit antennas, an uplink single-antenna port mode is defined, where the UE's behavior is the same as having a single-antenna from the eNB's point of view. For a given UE, the uplink single antenna port mode can be independently configured for its PUCCH, PUSCH, and sounding reference signal transmissions. The uplink single antenna port mode is the default mode before the eNB is made aware of the UE's transmit antenna configuration. Table 10-