. Short PRACH limits the cell radius. The maximum cell radius with short PRACH with a 30 kHz subcarrier spacing is less than 5 km and with a 15 kHz subcarrier spacing less than 10 km. A larger cell radius may be needed, especially sub-urban and rural cases where high base station antennas and open propagation can enable larger cells from the propagation point of view. Long PRACH allows very large cell sizes. Long PRACH is possible with two consecutive uplink slots, for example, with the DDDSUDDSUU frame shown in Figure 8.13. 8 Deployment Aspects DDDSU frame DDDSUDDSUU frame 2.5 ms 2.5 ms 2.5 ms DDDSU DDDSUDDSUU Only short Also long PRACH PRACH Figure 8.13 Frame configurations with 30 kHz subcarrier spacings and PRACH options. 8.7.4 Guard Period and Safety Zone Radio waves travel at the speed of light, and hence from the BTS-to-BTS interference point of view, it is also worth noting that the duration of the GP defines the safety zone distance beyond which the downlink and uplink will start overlapping even if the net- work is perfectly synchronized. The duration of the GP, measured in microseconds, defines the interference safety zone distance. At the speed of light, in 1 us, the radio waves travel 300 m. To achieve, for example, a 15 km safety range, about a 50 us GP would be required from when transmitter switches off to when receiver is switched on. If a 40 km safety zone is required, a 134 us GP would be necessary but this also emphasizes the requirement for good-quality physical layer design and the importance of reducing overshooting. In LTE, the GP is defined by the selection of the Special Sub- Frame (SSF), of which nine different options are specified in [4]. In 5G, there are no SSFs as in TD-LTE; however, the slot formats defined in [5] (Tables 4.3.2-3) can contain a certain number of guard symbols defined by the networks. Typical settings for guard symbols are two or four in the case of a 30 kHz subcarrier spacing. Calculation of the safety distance should also consider factors like the transit period f