6.7 us 33.3 us 16.7 us 8.33 us 4.17 us Nominal cyclic prefix 4.7 us 2.3 us 1.2 us 0.59 us 0.29 us Maximum bandwidth 50 MHz 100 MHz 100 MHz 400 MHz 400 MHz Slot length 1.0 ms 0.5 ms 0.25 ms 0.125 ms 0.125 ms subcarrier spacing and scheduling intervals depending on the bandwidth and on the latency requirements. Subcarrier spacings of 15-240 kHz are defined in Release 15. In later releases, higher subcarrier spacings can be accommodated. With higher subcarrier spacings, more symbols can be accommodated in a subframe, resulting in a lower acqui- sition time. The narrow spacings are used with narrow 5G bandwidths and are better for the extreme coverage. If we consider typical 5G deployment at the 3.5 GHz band, the bandwidth could be 40-100 MHz, the subcarrier spacing 30 kHz, and slot length 0.5 ms. The corresponding numbers in LTE are 20 MHz bandwidth, 15 kHz subcarrier spacing, and slot length 1 ms. 5G subcarrier spacing is designed to be 2N multiples of 15 kHz. If very low latency is required, then the so-called mini-slot can be used where the transmission time is shorter than one slot. It is also possible to combine multiple slots together. Most spectrum allocations globally below 2.5 GHz are limited to 20-40 MHz per oper- ator per band. Therefore, the LTE maximum bandwidth is defined as 20 MHz. Wider spectrum allocations are generally available at 2.5 and 3.5 GHz or higher frequencies. 5G radio is designed to support wider bandwidths like 100 MHz or even 400 MHz at higher frequencies. 3.4.2 Short Transmission Time and Mini-slot LTE Release 8 has the minimum transmission time of 1 ms, which leads to a minimum round trip time of 10-15 ms in practice. The transmission time is 1 ms in both directions, 3.4 Flexible Physical Layer and Protocols Minimum transmission time Round trip time 20-30 ms 10-15 ms 0.125 ms 1-2 ms Figure 3.9 Minimum transmission time and corresponding round trip time. Figure 3.10 Mini-slot in 5G with 15 kHz Mini-slot of subcarrier spacing. 2 symbols = 0.14 ms Slot of 14 symbols (1 ms) the buffering