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

224 Applications10 msRadio frame (= 20 slots): 1 2 3 4 201 msSub-frame (= 2 slots):1 20.5 msSlot (= 6 or 7 OFDM symbols): 1 2 3 6 or 7T CP66.7 µsOFDM symbol:CPFigure 5-7LTE downlink frame structurefor the remaining OFDM symbols. In the case of the extended cyclic prefix the length isT cp = 17.7 µs. The extended cyclic prefix is used in scenarios with a long delay spread andfor multicast and broadcast services in the single-frequency network mode. The OFDMsymbol duration without a guard interval is always 66.7 µs. A resource block has theduration of one slot and consists of 12 adjacent sub-carriers, as illustrated in Figure 5-8.Thus, for the normal cyclic prefix length, 84 (12 × 7) resource elements are transmittedin one resource block while in the case of the extended cyclic prefix, 72 (12 × 6) resourceelements are transmitted. A resource element corresponds to a symbol transmitted on onesub-carrier in one OFDM symbol. The sub-carrier distance in the downlink is 15 kHz sothat a resource block has a total bandwidth of 180 kHz.Pilot symbols are inserted in each resource block for channel estimation at the receiver,which is required for coherent data detection. Figure 5-9 shows the pilot grid of a resourceblock with normal cyclic prefix length for different numbers of transmit antenna.The basic pilot grid for the single antenna scheme has pilots in the first and third lastOFDM symbols. The pilot symbols have a distance of six sub-carriers in the frequencydirection and are shifted by three sub-carriers between the first and third last OFDM symbol.As described in Section 4.3, the channel estimator has to interpolate/filter the channelin the time and frequency directions. To improve the channel estimation accuracy, thechannel estimator can, especially in the downlink, interpolate/filter over several resourceblocks in time and frequency.The receiver must be able to estimate the channel from each antenna in the case ofmultiple transmit antennas. To avoid interference between the pilot symbols from differenttransmit antennas, orthogonal sets of pilot symbols are assigned to the transmit antennas.As shown in Figure 5-9 for the case with two transmit antennas, the first antenna uses the