nt physical cell IDs. The PSS and the SSS are transmitted together with the PBCH as shown in Figure 6.4. The so-called SS Block, also referred to as the sweeping sub-frame in connection with beamforming, has a duration of four symbols. The Sounding Reference Signals (SRSs) are used in the uplink direction to enable uplink frequency domain scheduling. SRS is especially useful in the Frequency Division Duplex (FDD) case when there is not channel reciprocity available, but it is also useful in the TDD case as allocation does not often cover the full band anyway. 6.4 Basic Structures for 5G Frame Structure Basic Structures for 5G Frame Structure The frame structure in 5G differs from the LTE structure. The one major difference is larger flexibility in terms of symbol allocation between uplink and downlink. The symbols can be configured to be basically downlink, uplink, or flexible in the case of TDD operation. While in the ideal situation everything would be dynamic all the time, in real 5G networks there needs to be alignment on the uplink downlink split with the adjacent cells; also, in Release 15, the assumption is that operators in the same graphical area would use the aligned uplink downlink allocation when operating in the same TDD band. Figure 6.7 shows some examples of configurations when dealing with a 5G-only band. If LTE is used in the same TDD band, the alignment requires a more downlink-heavy configuration. 5G naturally also supports the FDD operation, which is needed with low frequency bands. In this case, uplink and downlink resources are always available. Besides flexibility, another driver for differences has been the use of pipeline pro- cessing and the assumption that full carrier bandwidth should not be used for control signaling as the total carrier bandwidth might be too much in some cases for all UE types. Pipeline processing is enabled in 5G with the placement of the reference signals early in the frame, for example, in the second symbol (the first symbol is used to control information), which