Multi-Carrier and Spread Spectrum Systems: From OFDM and MC ...

3GPP Long Term Evolution (LTE) 227The first stage is the layer mapping, where the transport blocks are mapped on to one orseveral layers. In the second stage, the layers are pre-coded according to the selected multiantennatransmission scheme. This can be spatial multiplexing, beamforming, or transmitdiversity. The specified multi-antenna transmission schemes are detailed in the following.Spatial MultiplexingSpatial multiplexing is used to transmit in parallel two transmission blocks using thesame time–frequency resource. If the two transmission blocks belong to one user, thescheme is referred to as a single-user MIMO (SU-MIMO). Alternatively, the two transportblocks can belong to different users. This scheme is referred to as multi-user MIMO (MU-MIMO). The spatial multiplexing is based on a so-called codebook-based pre-coding. Thecodebook specifies a set of spatial pre-coding matrices that can be used at the transmitter.The actual pre-coding matrix is chosen based on channel quality information obtained bychannel measurements per antenna in the downlink.Spatial Multiplexing with Cyclic Delay Diversity (CDD)To improve the performance of spatial multiplexing further, the combination of codebookpre-coding with CDD (see Section 6.3.1.3) is specified. Two types of CDD are distinguished.The first type is CDD with large cyclic shifts in order to achieve frequencydiversity within one sub-band. The second type is CDD with small cyclic shifts wherethe channel remains flat within one sub-band but a frequency scheduling gain can beobtained by increased frequency selective fading over the total bandwidth.BeamformingCodebook pre-coding with only one layer, i.e. one transport block per time–frequencyresource, results in beamforming.Beamforming with Cyclic Delay Diversity (CDD)CDD can be combined with beamforming in the same way as already described for spatialmultiplexing.Transmit DiversitySpace frequency block codes (SFBCs, see Section 6.3.4) are applied as transmit diversityscheme. Mapping schemes are defined for two and four transmit antennas. A singlecodeword is mapped to the layers, which are SFBC pre-coded before transmission via theantennas.5.2.6 Uplink Transmission SchemeThe transmission scheme of the LTE uplink is localized DFT-spread OFDM(see Section 3.2.4). The block diagram for DFT-spread OFDM is shown in Figure 5-11.DFT-spread OFDM is a single-carrier FDMA scheme that guarantees a low PAPR. Thisis essential for the power efficiency of the mobile terminal transmitter. A certain amountof flexibility in resource allocation and scheduling is achieved with DFT-spread OFDMin the uplink. The block diagram of the LTE uplink is shown in Figure 5-12.