 avoids having to receive the full slot before actual decoding can start. This, together with the efficient channel coding (see the later section on 5G channel coding with data transmission), allows fast processing to achieve low-latency operation (Figure 6.8). The frame structure depends also on the subcarrier spacing. If the subcarrier spacing is 15 kHz, then the sub-frame duration is 1 ms, as is the minimum scheduling interval. If the subcarrier spacing becomes shorter, then the slot duration (and the minimum scheduling interval) becomes shorter, as shown in Figure 6.9. The typical subcarrier spacings are either 15 kHz (for bands below 1 GHz), 30 kHz for bands such as 3.5 GHz, Figure 6.7 Example 5G TDD structures. 2 ms structure Switching slot 2.5 ms structure DL Data 6 5G Physical Layer LTE Reference Signals 5G Reference Signals Subcarriers/ Frequency domain Symbols/Time Domain Symbols/Time Domain Figure 6.8 5G and LTE reference symbols in the frame structure. 1 ms Subframe 0.125 ms Slot 120 kHz 1 ms Subframe 0.5 ms Slot 30 kHz 1 ms Subframe 1 ms Slot 15 kHz Figure 6.9 Typical frame durations expected in the first-phase 5G devices. and 120 GHz subcarrier spacing with 26/28 GHz bands. 60 GHz subcarrier spacing is not supported by the first-phase 5G devices (an optional UE feature). In the downlink direction, the first two symbols carry the PDCCH while the rest of the frame carries the PDSCH. After the PDCCH part follows the DM-RS. In the example shown in Figure 6.8, there is only one instance of DM-RS, the so-called front-loaded DM-RS. In an environment with high mobility, one would need to add additional DM-RS later in the slot (Figure 6.10). Besides the allocation of the 14 symbols with the "regular" frame structure, there is also the approach called mini-slot. With the mini-slot structure, instead of 14 symbols, one can also allocate 2, 4, or 7 symbols. This allows shorter latencies to be achieved even with the 15 kHz subcarrier spacing, which otherwise would use the 1 ms allocation period. With the very hi