4.23 NAS over Non-Terrestrial Network

24.5013GPPNon-Access-Stratum (NAS) protocol for 5G System (5GS)Release 18Stage 3TS

Tools: ARFCN - Frequency Conversion for 5G NR/LTE/UMTS/GSM

4.23.1 General

A 5GS can support 3GPP satellite NG-RAN access technology (see 3GPP TS 23.501 [8]). This clause describes NAS-specific aspects of the 5GS features to support 3GPP satellite NG-RAN access technology.

4.23.2 List of "PLMNs not allowed to operate at the present UE location"

For 3GPP satellite NG-RAN the UE shall store a list of "PLMNs not allowed to operate at the present UE location". Each entry consists of:

a) the PLMN identity of the PLMN which sent a message including 5GMM cause value #78 "PLMN not allowed to operate at the present UE location" via satellite NG-RAN access technology; and

b) the geographical location, if known by the UE, where 5GMM cause value #78 was received on satellite NG-RAN access technology; and

c) if the geographical location exists, a UE implementation specific distance value.

Before storing a new entry in the list, the UE shall delete any existing entry with the same PLMN identity. Upon storing a new entry, the UE starts a timer instance associated with the entry with an implementation specific value that shall not be set to a value smaller than the timer value indicated by the network in the Lower bound timer value IE, if any. If the Lower bound timer value IE was not provided by the network, the value of the timer shall be set based on the UE implementation.

The UE is allowed to attempt to access a PLMN via satellite NG-RAN access technology which is part of the list of "PLMNs not allowed to operate at the present UE location" only if:

a) the current UE location is known, a geographical location is stored for the entry of this PLMN, and the distance to the current UE location is larger than a UE implementation specific value; or

b) the access is for emergency services (see 3GPP TS 23.122 [5] for further details).

NOTE: When the UE is accessing network for emergency services, it is up to operator and regulatory policies whether the network needs to determine if the UE is in a location where network is not allowed to operate.

The list shall accommodate three or more entries. The maximum number of entries is an implementation decision. When the list is full and a new entry has to be inserted, the oldest entry shall be deleted.

Each entry shall be removed if for the entry:

a) the UE successfully registers via satellite NG-RAN access technology to the PLMN stored in the entry except when the UE registers for emergency services; or

b) the timer instance associated with the entry expires.

The UE may delete the entry in the list, if the current UE location is known, a geographical location is stored for the entry of this PLMN, and the distance to the current UE location is larger than a UE implementation specific value.

If the UE is in 5GMM-DEREGISTERED.LIMITED-SERVICE state and an entry from the list of "PLMNs not allowed to operate at the present UE location" is removed, the UE shall perform PLMN selection according to 3GPP TS 23.122 [5].

When the UE is switched off, the UE shall keep the list of "PLMNs not allowed to operate at the present UE location" in its non-volatile memory. The UE shall delete the list of "PLMNs not allowed to operate at the present UE location" if the USIM is removed.

If the UE is switched off when the timer instance associated with the entry in the list is running, the UE shall behave as follows when the UE is switched on and the USIM in the UE remains the same:

let t1 be the time remaining for timer instance associated with the entry in the list to timeout at switch off and let t be the time elapsed between switch off and switch on. If t1 is greater than t, then the timer shall be restarted with the value t1 – t. If t1 is equal to or less than t, then the timer need not be restarted and considered expired. If the UE is not capable of determining t, then the UE shall restart the timer with the value t1.

4.23.3 5GS mobility management via a satellite NG-RAN cell

For 5GS mobility management via a satellite NG-RAN cell the UE shall apply the value of the applicable NAS timer indicated in table 10.2.1 for access via a satellite NG-RAN cell.

NOTE 1: The applied NAS timer values are based on the current satellite NG-RAN access RAT type determined based on information from lower layers.

The NAS timer value obtained is used as described in the appropriate procedure subclause of this specification. The NAS timer value shall be calculated at start of a NAS procedure, and shall not be re-calculated until the NAS procedure is completed, restarted or aborted.

The access via a satellite NG-RAN cell by a UE is indicated to the AMF by lower layers and shall be stored by the AMF. When an AMF that supports access via satellite NG-RAN cells performs NAS signalling with a UE via satellite NG-RAN cells, the AMF shall calculate the value of the applicable NAS timer indicated in table 10.2.2 for access via a satellite NG-RAN cell.

NOTE 2: The applied NAS timer values are based on the current satellite NG-RAN access RAT type determined based on information from lower layers.

The NAS timer value obtained is used as described in the appropriate procedure subclause of this specification. The NAS timer value shall be calculated at start of a NAS procedure and shall not be re-calculated until the NAS procedure is completed, restarted or aborted.

4.23.4 5GS session management via a satellite NG-RAN cell

For 5GS session management via a satellite NG-RAN cell the UE shall apply the value of the applicable NAS timer indicated in table 10.2.1 for access via a satellite NG-RAN cell.

NOTE 1: The applied NAS timer values are based on the current satellite NG-RAN access RAT type determined based on information from lower layers.

If the use of extended NAS timer for access via a satellite NG-RAN cell is indicated by the AMF (see 3GPP TS 23.501 [8] and 3GPP TS 23.502 [9]), the SMF shall calculate the value of the applicable NAS timer indicated in table 10.3.2 for access via a satellite NG-RAN cell.

The NAS timer value obtained is used as described in the appropriate procedure subclause of this specification. The NAS timer value shall be calculated at start of a NAS procedure and shall not be re-calculated until the NAS procedure is completed, restarted or aborted.

4.23.5 Handling multiple tracking area codes from the lower layers

When a UE camps on a satellite NG-RAN cell, the UE may receive multiple TACs from the lower layers. The UE shall construct TAIs from the multiple TACs (i.e. concatenate the identity of the current PLMN and each of the TACs) and select a TAI as follows:

a) if at least one TAI belongs to the current registration area of the UE, the UE shall select a TAI which belongs to the current registration area of the UE according to the followings.

If there are multiple TAIs which belong to the current registration area of the UE, the UE shall select a TAI as follows:

1) if there is a TAI which belongs to the list of "allowed tracking area" (if any) and does not belong to the list of "non-allowed tracking areas" (if any), the UE shall select a TAI which belongs to the list of "allowed tracking area" (if any) and does not belong to the list of "non-allowed tracking areas" (if any). In this case, if there are multiple TAIs which belong to the list of "allowed tracking area" (if any) and does not belong to the list of "non-allowed tracking areas" (if any), then the UE shall consider each of these TAIs equal and select a TAI in an implementation-specific way (e.g. taking into account LADN service area information). If these multiple TAIs contain the previous current TAI, the current TAI can be left unchanged.

2) if there is no TAI which belongs to the list of "allowed tracking area" (if any) and does not belong to the list of "non-allowed tracking areas" (if any) or neither the list of "allowed tracking area" nor the list of "non-allowed tracking areas" is available, then the UE shall consider each of these TAIs equal and select a TAI in an implementation-specific way (e.g. taking into account LADN service area information). If these multiple TAIs contain the previous current TAI, the current TAI can be left unchanged.

b) if the current registration area is not available in the UE or no TAI belongs to the current registration area of the UE and:

1) there is a TAI which belongs to neither the list of "5GS forbidden tracking areas for roaming" nor the list of "5GS forbidden tracking areas for regional provision of service", the UE shall select a TAI which belongs to neither the list of "5GS forbidden tracking areas for roaming" nor the list of "5GS forbidden tracking areas for regional provision of service". In this case, if there are multiple TAIs which belong to neither the list of "5GS forbidden tracking areas for roaming" nor the list of "5GS forbidden tracking areas for regional provision of service", then the UE shall consider each of these TAIs equal and select a TAI in an implementation-specific way.

2) all TAIs belong to the list of "5GS forbidden tracking areas for roaming" or the list of "5GS forbidden tracking areas for regional provision of service", then the UE shall consider each of these TAIs equal and select a TAI in an implementation-specific way.

The UE shall consider the selected TAI as the current TAI. The UE shall select a TAI when:

a) the UE receives multiple TACs from the lower layers; or

b) the UE has received multiple TACs from the lower layers upon starting to camping on the current cell and the registration area, the list of "allowed tracking areas", the list of "non-allowed tracking areas", the list of "5GS forbidden tracking areas for roaming", or the list of "5GS forbidden tracking areas for regional provision of service" is updated.

Handling of the list of "5GS forbidden tracking areas for roaming" and the list of "5GS forbidden tracking areas for regional provision of service" is specified in clause 5.3.13.