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

3GPP Long Term Evolution (LTE) 2251 slot0.5 ms1 downlink resource block:(12 sub-carriers x 7 OFDM symbols)180 kHztotal number of sub-carriers N cFigure 5-8Resource block with normal cyclic prefix length in the downlinkoriginal set of pilots as in the case with a single transmit antenna. The second transmitantenna places its pilots also in the first and third last OFDM symbols, but shifts the pilotsymbol positions by three sub-carriers. In the case of four transmit antennas, the pilotsymbols for the first and second transmit antennas are the same as for the two transmitantenna case. For the third transmit antenna, two pilot symbols are inserted in the secondOFDM symbol with a distance of six sub-carriers. For the fourth transmit antenna, thepilot symbols are also inserted in the second OFDM symbols with a distance of six subcarriers.However, relative to the pilots for the third transmit antenna, the pilots for thefourth transmit antenna are shifted by three sub-carriers. For all pilot grids hold that whenone antenna transmits a pilot symbol in one resource element, the other antennas do nottransmit in this resource element in order to avoid interference. Moreover, as the pilotdensity in time direction is reduced for the third and fourth transmit antennas, the casewith four transmit antennas, applying for example spatial multiplexing, is mainly targetedfor low mobility scenarios.5.2.5.3 MAC SchedulerThe MAC scheduler in Figure 5-5 is responsible for scheduling and rate matching by controllingthe HARQ and selecting the modulation scheme, resource and power assignment,and antenna mapping.