Section: 9.1 gNB Synchronization
Content: The gNB shall support a logical synchronization port for phase-, time- and/or frequency synchronization. Logical synchronization port for phase- and time-synchronization shall provide: 1) accuracy that allows to meet the gNB requirements on maximum relative phase difference for all gNBs in synchronized TDD-unicast area; 2) continuous time without leap seconds traceable to common time reference for all gNBs in synchronized TDD-unicast area. In the case the TDD-unicast area is not isolated, the common time reference shall be traceable to the Coordinated Universal Time (UTC). A logical synchronization port for phase- and time-synchronization may also be provided for e.g., all gNBs in FDD time domain inter-cell interference coordination synchronization area. Furthermore common SFN initialization time shall be provided for all gNBs in synchronized TDD-unicast area. In case of non isolated networks, the start of the radio frame on the output shall be synchronous with the input time reference, i.e., when an UTC traceable reference is required, the start of the radio frame shall be aligned with the start time of the UTC second. Unless otherwise mutually agreed by the operators of the cells in non isolated networks and/or unless different SFN initialization offsettings do not affect operators’ networks in the same area, the common SFN initialization time should be 1980-01-06T00:00:19 International Atomic Time (TAI). Based on this information, the gNB may derive the SFN according to the following formula: , where: time time adjusted by the common SFN initialization time, in units of 10 ms to match the length of radio frame and accuracy accordingly; period(SFN) SFN period. In case gNB is connected via TDM interface, it may be used to frequency synchronize the gNB. The characteristics of the clock in the gNB shall be designed taking into account that the jitter and wander performance requirements on the interface are in accordance with network limits for output wander at traffic interfaces of either ITU-T Recommendation G.823 [8], ITU-T Recommendation G.824 [9] or network limits for the maximum output jitter and wander at any hierarchical interface of ITU-T Recommendation G.825 [10], whichever is applicable. In case gNB is connected via Ethernet interface and the network supports Synchronous Ethernet, the gNB may use this interface to get frequency synchronization. In this case the design of the gNB clock should be done considering the jitter and wander performance requirements on the interface are as specified for output jitter and wander at EEC interfaces of ITU-T Recommendation G.8261/Y.1361 [11], defined in clause 9.2.1. Further considerations on Synchronous Ethernet recommendations and architectural aspects are defined in clause 12.2.1 and Annex A of ITU-T Recommendation G.8261/Y.1361 [11]. A configurable LTE TDD-offset of start frame shall be supported by all gNBs in synchronized TDD-unicast areas in order to achieve interoperability in coexistence scenarios.