h, the reception of UE2 is blocked by the UE1 transmission. UE-to-UE interference depends heavily on the UE locations and on the UE transmission power levels. The worst-case interference can be challenging, but the probability of it occurring is very low. These interference scenarios can also occur if the cells are synchronized, but different asymmetries between downlink and uplink are used in adjacent cells. Therefore, the base stations need to be synchronized in TDD networks, and the same asymmetry must be used if we want to completely avoid these interference cases. 8.7.2 TDD Frame Configuration Options While LTE specifications limit the uplink and downlink frame configurations to seven different options, 5G specifications allow a large number of options for splitting the frame between the uplink and downlink: the TDD frame size can be selected, and the split between the downlink and uplink can be selected. Typical 5G frame configurations are shown in Figure 8.12 for the downlink, uplink, and special slots. The special slot includes a guard period (GP) for switching between the downlink and uplink. The spe- cial slot is typically configured mostly for the downlink with a GP of a few symbols. The 15-kHz option with the 5 ms frame is designed for coexistence with TD-LTE. There are a few different options for the 30 kHz case with 5.0, 2.5, and 2.0 ms frame sizes that are typically used for mid-band cases without TD-LTE coexistence. All cases have asymmet- ric allocation where the downlink has larger share of the time than the uplink. mmWave cases use a 120 kHz subcarrier spacing, and the TDD frame is normally 0.625 or 0.5 ms. 8.7.3 Cell Size and Random Access Channel TDD frame configurations have an impact on the relative capacity split between the downlink and uplink, on the latency, and also on the maximum cell radius. For example, if we have 30 kHz subcarrier spacing and just one uplink slot in the frame, only the short Physical Random Access Channel (PRACH) format can be utilized but not the long PRACH format