on in open radio access networks Time accuracy For radios that cooperate in a MIMO or carrier aggregation environment, TS 38.104 defines differ- ent time accuracy requirements depending on the type of carrier aggregation supported and the supported bands (sub-6 GHz or millimeter wave). Table 8-1 shows the time alignment required between radios. In an O-RAN environment, such radios will all be connected to the same O-DU. Additionally, 3GPP TS 38.133 (requirements for support of radio resource management) defines time alignment requirements between any radios that are within transmission range of each other to support TDD operation. These limits are 3 us for small radius cells (<3 km) and 10 us for wider coverage macrocells ( > 3 km). For any given radio, both the TS 38.104 and TS 38.133 time alignment requirements must be met. This is shown in Fig. 8-23. Table 8-1 3GPP carrier aggregation sync requirements. Carrier aggregation type Sub-6 GHz Millimeter wave 65 ns 65 ns Intraband contiguous 260 ns 130 ns Intraband noncontiguous 260 ns Interband O-RAN Configuration LLS-C2 802.1CM Case 1.1 Category B Local Synchronization Local PRTC/ T-TSC PRTC/ Remote Sync Local Sync Midhaul Fronthaul T-TSC |TE <95ns (T-TSC Class B in RU) |TE <140ns (enhanced T-TSC in RU) |TE|<130ns (Cat B) +/- 100ns +/- 1325ns (O-RAN WG4) +/- 1500ns (ITU G.8271 Accuracy Level 4) FIGURE 8-23 3GPP synchronization requirements. 292 Open Radio Access Network (O-RAN) Systems Architecture and Design Synchronization in O-RAN Now that we know how to synchronize a network and know what the synchronization requirements at the O-RU outputs are, we can finally get to the crux of the matter-how do we achieve what is required in practice? This will be split into two discussions. The first looks at O-RAN synchronization at the network level, whereas the second covers what is required of the equipment itself and the implementation details. O-RAN network-level synchronization In order to meet the air interface synchronization requirements of both 3GPP TS 38.104 and