1 Maximum Received Time Difference Versus Time Alignment Error The synchronization requirements in the 3GPP specifications are expressed either in terms of MRTD or TAE. For DC or CA use cases, excessive MRTD between the transmissions from the co- operating base stations to the UE can cause problems, because some of the information will arrive too early and/or the other will be too late for the UE to use. For example, according to the 3GPP TS 28.133, inter-band CA in FR1 is allowed a budget of up to 33 us MRTD. Higher frequencies (FR2) with shorter range have a lower budget. See the earlier section "Dual Connectivity" for more details. Any calculation of MRTD also needs to consider the propagation delay of approximately 3.3 us per km. Therefore, a difference of 9.5 km in distance between each cooperating base station and the UE accounts for around 31.5 us MRTD. This loss of 31.5 us of budget through the propagation delay means that the two radios must be phase synchronized to within 1.5 us to allow MRTD to work. This is a tighter requirement than the general 3 us allowed between two neighboring TDD radios. The intra-band CA use case between LTE and NR base stations, also known as EN-DC (E-ULTRA NR Dual Connectivity), is defined for co-located deployments. In co-located deployment, there is no need to consider the propagation differences between the UE and the cooperating base stations because it is the same for both radios. Table 10-19 summarizes the detailed phase and frequency requirements for 5G deployments for FR1 and FR2 frequency ranges when using CA or DC. Note that the values of +1.5 us, 260 ns, 130 ns, and 65 ns align with the requirements outlined in the RAN specifications from organizations like CPRI, eCPRI, IEEE 8021.CM, and O-RAN. Table 10-19 Time Synchronization Requirements for Various 5G Use Cases TAE or MRTD (Air Application Specification Interface) 5G NR TDD +1.5 us TAE 3GPP TS 38.133 Inter-band synchronous EN-DC (LTE & 5G NR Dual 33 us MRTD 3GPP TS 38.133 Connectivity) NR inter-band CA 33 us MRT