5.4.13.1 Mobility Management and Power Saving Optimization

For NR satellite access that provides discontinuous network coverage the Mobility Management and Power Saving Optimization functionality may be used.

If both the UE and the network support "Unavailability Period Support" and if the UE determines it will lose coverage and will become unavailable and the UE decides to remain in no service during that time, then:

• the UE triggers the Mobility Registration Update procedure to inform the network of its unavailability, as described in clause 5.4.1.4.
• The UE may be able to determine, including considering current and expected future locations of the UE, a Start of Unavailability Period and/or Unavailability Period Duration for when it expects to be out of coverage and include them in the Mobility Registration Update procedure, as described in clause 5.4.1.4.
• The UE should trigger the Mobility Registration Update procedure early enough such that the procedure, under normal conditions, is able to complete before the start of the unreachability period.

NOTE 1: A UE based on implementation can combine successive periods of no satellite coverage into a single continuous period that is notified to the network if the UE does not require network access during this period.

NOTE 2: UE informing the network of coverage gaps would increase signalling and UE power consumption if coverage gaps are more frequent than UE's communication period.

In case the UE requests power saving features the AMF uses the procedures defined in other clauses to provide the UE with timers (e.g. Periodic Registration Update timer), extended DRX in CM-IDLE configuration (see clause 5.31.7.2) and MICO mode configuration (see clause 5.4.1.3) and to provide the NG-RAN with Extended Connected Time (see clause 5.31.7.3) and may also consider the Unavailability Period Duration and Start of Unavailability Period (if available). This is to keep UE in power saving mode and avoid the network attempting to page the UE if it is out of satellite network coverage.

The AMF should adjust the mobile reachable timer or Implicit Deregistration timer or both such that the AMF does not implicitly deregister the UE while the UE is unavailable, see clause 5.4.1. Features described for High latency communication in clause 5.31.8 may be used to handle mobile terminated (MT) communication with UEs being unreachable due to NR satellite access discontinuous coverage and the Unavailability Period Duration (if available) and Start of Unavailability Period (if available) may be used when determining the Estimated Maximum Wait Time.

Tracking Area or RAT specific configuration in the AMF may be used to set timer values based on typical coverage periods of a satellite system.

NOTE 3: For example, if a satellite system only provides coverage to a UE for 20 minutes when a satellite passes and the maximum time before a satellite passes any point on the earth is 10 hours, the AMF could configure the periodic registration timer and Mobile Reachable timer to be just greater than 20 minutes and the Implicit Deregistration timer to be greater than 10 hours to avoid unintended implicit detach due to coverage gap. Such configuration does not require AMF to be aware of detailed coverage times for each UE or for different locations.

The UE may send Mobility Registration Update procedure to inform the network of its UE unavailability period (see clause 5.4.1.4) even if Mobility Management back-off timer is running.