System Level Modeling and Optimization of the LTE Downlink

2. 3GPP Long Term EvolutionTable 2.6.: LTE codebook for CLSM mode and two transmit antennas for each of the possiblenumber of layers (ν) [39].Layers (ν)12[ ]1 1√ , 2 1Precoder codebook] ], ,[1√21−1[1 1 12 1 −1],1√2[1i12[ 1 1i −i[1 1√2 −i]]Although desirably the PMI and RI feedback combination would be sent for eachsubcarrier, one PMI value is sent per RB (12 subcarriers), while for the RI, just asingle wide-band value for the whole bandwidth is transmitted. These reductionswere imposed by the need of reducing signaling traffic.Table 2.6 lists the available precoders for the two-transmit-antenna case. For thefour-antenna case, the codebook size increases to sixteen precoders, supporting upto four layers.2.3. MAC LayerThe Medium Access Control (MAC) layer controls the access to the transmissionmedium. It provides data transfer and radio resource allocation services to upperlayers, while the physical layer provides it with lower level data transfer services,signaling (HARQ feedback and scheduling requests), as well as channel measurementssuch as PMI, RI, and CQI reports [55]. Implementation-wise, the MAC layeris realized by a scheduler, which discretionally decides the PHY resource allocationfor each UE according to its applied scheduling algorithm and the channel stateinformation received from the PHY layer, as shown in Figure 2.9.1 RBfrequency1 TTIcommon values per UE and TTI- code rate & modulation- number of spatial layers (rank)**: when applicabletimeUE 1 UE 2 UE 3Figure 2.9: Example time-and-frequency domain scheduling in LTE. On a given TTI, eachUE can be assigned a set of RBs, which is coded employing a common singleMCS. If SM is employed, while the precoder choice can be frequency-selective, acommon rank (number of layers) is employed over the whole set of RBs scheduledto the UE.21