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

230 Applications1 uplink resource block:(7 DFTS-OFDM symbols)0.5 msdata symbolpilot symbol180 kHzFigure 5-14Uplink pilot allocation within one resource blocksix DFT-spread OFDM symbols with an extended cyclic prefix length. The DFT-spreadOFDM symbols with normal cyclic prefix have a cyclic prefix length of T cp = 5.2 µsfor the first DFT-spread OFDM symbol and of T cp = 4.7 µs for the other DFT-spreadOFDM symbols. In the case of the extended cyclic prefix the length is T cp = 17.7 µs. Theextended cyclic prefix if used in scenarios with a long delay spread and for multi-castand broadcast services in the single-frequency network mode. The DFT-spread OFDMsymbol duration without a guard interval is always 66.7 µs.A resource block is defined for one slot and consists of 12 adjacent sub-carriers. Thesub-carrier distance in the uplink is 15 kHz so that a resource block has a bandwidthof 180 kHz. In contrast to the downlink, in the uplink resource blocks assigned to onemobile terminal in one OFDM symbol have to be adjacent to each other in the frequencydomain.One pilot symbol is inserted in each resource block for channel estimation at thereceiver, which is required for coherent data detection. Figure 5-14 shows the pilot allocationwithin a resource block with seven DFT-spread OFDM symbols.With DFT-spread OFDM, the pilot symbols are not scattered in frequency but aretime multiplexed with the data symbols. Every fourth DFT-spread OFDM symbol withina resource block is a pilot symbol occupying the whole transmission bandwidth of theresource block. The pilot symbols are chosen such that they have nearly constant amplitudeand good auto- and cross-correlation properties. This is achieved with Zadoff–Chusequences (see Section 2.1.4.1). The correlation properties are important for separatingthe pilots of the resource block at the same positions between neighboring cells. DifferentZadoff–Chu sequences are assigned to resource blocks of different cells.Additionally, broadband channel sounding is required to enable channel-dependentscheduling in the uplink. This requires channel sounding over a much larger bandwidththan that of one resource block per user. While for the downlink pilot symbols are availableover a large bandwidth for channel estimation, this is not always the case in the uplink.The uplink channel sounding is achieved by reserving blocks within one sub-frame forchannel sounding. Data transmission is not possible in these blocks. The blocks withsounding signals occupy a bandwidth much larger than that of one resource block. Theseblocks are shared between different users either in the time, frequency, or code domain.