ross-layer design is required to support efficient and adaptive physical layer functionalities. 3.1.5 OFDM/SC-FDMA Signal Generation The LTE downlink transmission scheme is based on orthogonal frequency divi- sion multiple access (OFDMA), which is a multiuser version of the OFDM mod- ulation scheme. In the uplink, single carrier frequency division multiple access (SC-FDMA) is used, which can be also viewed as a linearly precoded OFDM scheme known as discrete Fourier transform (DFT)-spread OFDM. However, SC-FDMA has been selected for the uplink due to the lower peak-to-average power ratio (PAPR) of the transmitted signal compared to OFDM. Low PAPR values benefit the terminal in terms of transmit power efficiency, which also trans- lates into increased coverage. The processing sequence in the signal generation process is quite similar in downlink and uplink, as shown in Figure 3.7. The main difference comes from the elimination of the antenna mapping process and the addition of a DFT-spread block, which is the key process for the PAPR reduction. The following processing blocks are involved in the signal generation sequence: N-point DFT. This block performs the DFT-spread OFDM operation, which provides a high degree of similarity with the downlink OFDM scheme and the possibility to use the same system parameters. Resource element mapping. Modulation symbols for each antenna will be mapped to specific resource elements in the time-frequency resource grid. M-point inverse discrete Fourier transform (IDFT). OFDM symbols are con- verted into the time domain to be transmitted through the air interface. © 2009 by Taylor & Francis Group, LLC Radio-Interface Physical Layer cos(2nft) Re{s(t)} Split Filtering Im{s(t)} -sin(2ft) Figure 3.8 Modulation and up-conversion. Cyclic prefix (CP). For each OFDM symbol to be transmitted, the last Ncp samples of the symbol are prepended to the symbol as a cyclic prefix with the aim of improving the robustness to multipath effect. RF (radio frequency) up-conversion. Complex-valued OFDM ba