System Level Modeling and Optimization of the LTE Downlink

2. 3GPP Long Term EvolutionSince the CDD matrix cycles with a period of two, D k can be expressed as D k mod 2for the two transmit antenna case. For ν = 1, OLSM is not defined.In the case of four transmit antennas, instead of a fixed W matrix, a different precoderis applied after ν vectors, as well as D k mod 4 instead of D k mod 2 . AppropriateU, D k , and W matrices are defined for ν = 3, 4 in [39].2.2.1.3. Closed Loop Spatial MultiplexingUnless the feedback is invalidated by a rapidly changing channel, gains can be obtainedin comparison to OLSM by signaling the eNodeB an optimum precodingmatrix W in combination with the number of desired layers ν instead of employingCDD. Thus, expressing the output symbol vector y as y = Wx.In order to simplify signaling, instead of feedbacking the actual optimum precodermatrix [44], a precoder is chosen from a predefined codebook, the index of whichis sent to the eNodeB as feedback. For N TX = 2, the LTE codebook is comprisedof four (ν = 1) and two (ν = 2) precoders, which are listed in Table 2.6. For fourtransmit antennas, the codebook spans 15 precoding choices for ν ∈ {1, 2, 3, 4}.2.2.2. Layer-to-Codeword MappingThe LTE standard allows for up to two parallel data streams, termed codewords,to be simultaneously transmitted. As seen in Section 2.2.1, up to four symbolscan be simultaneously transmitted when using either OLSM or CLSM. While theprecoding-related procedures work on a per-layer basis, the channel coding andchannel quality reporting procedures work on a per-codeword basis.1 or 2 codewords layers of equal size: 1, 2, 3, or 4 layerschannelcodingscramblingscramblingmodulation mappermodulation mapperlayermapperRE mapperOFDM signal generationinsert CPprecodingRE mapperOFDM signal generationinsert CP1, 2, 3, or 4 layers antenna ports: 1, 2, or 4 antennasFigure 2.6: Modulation and layer mapping procedures [39]. The one or two codewords outputby the channel coding procedures are scrambled, mapped to complex symbols,distributed in ν layers (layer mapping), mapped to N TX transmit antennaports and converted to the time domain.17