Section: 4.13.8.2 Mobility Management and UE Power Saving Optimization
Content: For satellite access that provides discontinuous network coverage, and in case both the UE and the network support Enhanced Discontinuous Coverage, then the Mobility Management and UE Power Saving Optimization functionality may be used. If both the UE and the network indicate support for "Enhanced Discontinuous Coverage Support" and if the UE determines it will lose coverage and will become unavailable, and decides to remain in no service during that time, then: - 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. 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. - The UE triggers a TAU procedure and includes an indication of upcoming loss of coverage in the TAU Request message to the MME. If the UE is able to determine a Start of Unavailability Period and/or Unavailability Period Duration it also includes them in the TAU Request message. - The UE should trigger the TAU procedure early enough such that the procedure, under normal conditions, is able to complete before the start of the unavailability period. - The UE and the MME re-negotiate unavailability at every TAU procedure, if it is required. If Start of Unavailability Period and/or Unavailability Period Duration is not included in a TAU Request message any pending loss of coverage configuration stored in the UE context at MME is discarded. - If the UE determines an upcoming loss of coverage to the network no longer applies or determines a new Start of Unavailability Period or Unavailability Period Duration related to the upcoming loss of coverage, the UE sends a new TAU Request to the MME to update the Start of Unavailabililty Period and/or Unavailability Period Duration. Upon receiving a TAU Request message from the UE including an indication of upcoming loss of coverage: - If the UE did not include a Start of Unavailability Period, the MME considers implicitly the Start of Unavailability Period to be the time at which it has received the TAU Request message from the UE. If the UE included a Start of Unavailability Period, the Start of Unavailability Period indicates the time at which the UE determines it expects to lose coverage, i.e. time until which the UE determines it is available. - The MME may determine, if not provided by the UE, or update the Unavailability Period Duration and/or the Start of Unavailability Period. If the MME knows an Unavailability Period Duratio and/or the Start of Unavailability Periodn (e.g. based on information available to the MME as described in clause 4.13.8.4) for the UE, and the UE not include an Unavailability Period Duration and/or the Start of Unavailability Period or included an Unavailability Period Duration and/or the Start of Unavailability Period different to the Unavailability Period Duration and/or the Start of Unavailability Period known to the MME, the MME may use either the Unavailability Period Duration and/or the Start of Unavailability Period known to the MME or the Unavailability Period Duration and/or the Start of Unavailability Period from the UE as the Unavailability Period Duration and/or the Start of Unavailability Period. The MME should include the Unavailability Period Duration and/or the Start of Unavailability Period known to the MME in the TAU Accept. How the UE treats the MME provided Unavailability Period Duration and/or the Start of Unavailability Period is up to UE implementation e.g. to help to determine when to return to coverage after a discontinuous coverage period, whether to listen to paging in eDRX, not to initiate any NAS signalling (including Service Request for MO data) within the discontinuous coverage period in case of any UL signalling/data request or the UE may deactivate its Access Stratum functions for satellite access in order to optimise power consumption until coverage returns, etc. The MME indicates to the UE in the Attach Accept or TAU Accept whether the UE is not required to perform a TAU procedure when the unavailability period has ended. - The MME stores the information that the UE is unavailable at the Start of Unavailability Period in the UE context, and considers the UE is unreachable from then until the UE enters ECM_CONNECTED state. If the UE requests power saving features the MME uses procedures defined in other clauses to provide the UE with timers (e.g. periodic TAU timer, extended idle mode DRX (see clause 5.13a), and PSM mode configuration (see clause 4.3.22)), and may also consider the Unavailability Period Duration (if available) and Start of Unavailability Period (if available). Unless the MME indicated that the UE is not required to perform a TAU procedure when the unavailability period has ended, then once the event which makes the UE unavailable is completed in the UE, the UE triggers a TAU procedure. If the event which makes the UE unavailable is delayed to a future time or cancelled or unavailability period deviates from negotiated value in the UE, the UE triggers TAU procedure. The MME should adjust the mobile reachable timer or Implicit Detach timer or both such that the MME does not implicitly detach the UE while the UE is out of coverage, see clause 3,4.2. Features described for High latency communication in clause 4.3.14.7 may be used to handle mobile terminated (MT) communication with UEs being unreachable due to satellite access with 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. The UE may send Tracking Area Update request message to inform the network of its UE unavailability period even if Mobility Management back-off timer is running.