 LoS between each other. 8.7.7 Synchronization Requirements in 3GPP There are a number of requirements defined in 3GPP that lead to the necessity for phase synchronization in 5G TDD networks. 8.7 Network Synchronization and Coordination Requirements Adjacent channel selectivity, TXP = 0.2 W, f = 3500 MHz, ACS = 33 dBm, BW = 100 MHz Function of pathloss Function of distance, FSPL, ant gain = 0 dB UE to UE isolation (dB) UE to UE distance (km) Figure 8.23 Adjacent channel receiver selectivity in UE. 8.7.7.1 Cell Phase Synchronization Accuracy This is probably the most important requirement because most of the early 5G systems will be based on TDD frequency bands where the downlink and uplink share the same frequency. The 3GPP specification [10] defines the cell phase synchronization accuracy for TDD as the maximum absolute deviation in the frame start timing between any pair of cells on the same frequency that have overlapping coverage areas. The mini- mum requirements of the cell phase synchronization accuracy measured at base station antenna connectors is defined as better than 3 us. The cell phase synchronization accu- racy was defined based on the different downlink/uplink interference scenarios men- tioned in the aforementioned chapters, where several important factors such as the cell size, timing advance, BTS switching time, UE switching time, and the GP duration were taken into consideration. The value of 3 us is applicable for all subcarrier spacings defined in 3GPP Release 15, that is, 15, 30, 60, and 120 kHz. It is important to note that due to the short symbol duration of 60 and the 120 kHz subcarrier spacing, a minimum of two GP symbols is required to avoid downlink-uplink interference. 8.7.7.2 Maximum Receive Timing Difference (MRTD) for LTE-5G Dual Connectivity The 3GPP specification [10] defines the MRTD as the relative timing difference between the subframe timing boundary of an E-UTRA Pcell and the slot timing boundaries of a PScell to be aggregated for EN-DC (E-UTRA-NR Dual Connectivity) that the