se-band sig- nal for each antenna is mapped to the carrier frequency (f), as depicted in Figure 3.8. In general, the use of OFDM in both downlink and uplink (in a DFT-spread OFDM form) is an adequate transmission scheme for several reasons: OFDM improves the spectral efficiency of the system. OFDM provides a high degree of robustness against channel frequency selectivity. Frequency-domain link adaptation and scheduling are allowed. LTE can easily support different spectrum allocations by varying the num- ber of OFDM subcarriers. Table 3.3 summarizes the main OFDM modulation parameters* for different spectrum allocations. As mentioned before, LTE supports a scalable transmission bandwidth that leads to a different number of occupied subcarriers (assuming the same subcarrier spacing). Two different CP lengths are allowed. A longer CP is more suitable for environments with very extensive delay spread (e.g., very large cells). Equivalently, for multimedia/multicast single-frequency network (MBSFN)- based transmission, all enhanced NodeBs are transmitting the same signal to the terminals, which increase the delay spread of the equivalent channel. In this par- ticular case, the extended cyclic prefix is therefore typically needed. However, a In FDD configuration, the uplink uses the same frame structure and modulation parameters as the downlink. © 2009 by Taylor & Francis Group, LLC Table 3.3 OFDM Modulation Parameters Transmission BW 1.4 MHz 3 MHz 5 MHz 10 MHz 15 MHz 20 MHz Slot duration 0.5 ms (Two-slot compound; 1-ms subframe) Subcarrier spacing 15 kHz Sampling frequency 1.92 MHz 3.84 MHz 7.68 MHz 15.36 MHz 23.04 MHz 30.72 MHz (1/2 X 3.84 MHz) (2 X 3.84 MHz) (4 X 3.84 MHz) (6 X 3.84 MHz) (8 X 3.84 MHz) DFT size Number of occupied subcarriers Number of OFDM Seven symbols (short CP) or six symbols (long CP) per slot symbols per subframe (short/long CP) (4.69/9) X 6, (4.69/18) X 6, (4.69/36) X 6, (4.69/72) X 6, (4.69/108) X 6, (4.69/144) X 6, CP length short (5.21/10) X 1 (5.21/20) X 1 (5.21/40) X 1 (5.21/80) X 1 (5.21