Summary based on the input provided by MediaTek Inc. in SP-220455 (merge of RP-221547 and CP-221272).
The "NB-IoT/eMTC support for Non-Terrestrial Networks (NTN)"
work item specifies enhanced features necessary for the support of Bandwidth reduced Low complexity (BL) UEs, UEs in enhanced coverage and NB-IoT UEs by Non-Terrestrial Networks (NTN).
SA and CT aspects of NB-IoT/eMTC Non-Terrestrial Networks in EPS provide minimum essential functionality for the Rel-17 UE and the network to support satellite E-UTRAN access in WB-S1 mode or NB-S1 mode with CIoT EPS optimization. The functionality is largely aligned with that of Rel-17 NR Non-Terrestrial Networks in 5GS, with the exception of discontinuous coverage that is addressed only within the present work item in Rel-17.
Overall architecture and general aspects:
E-UTRAN supports radio access over non-terrestrial networks for BL UEs, UEs in enhanced coverage and NB-IoT UEs. Non-terrestrial networks encompasses platforms that provide radio access through satellites in Geosynchronous orbits (GSO) as well as Non-Geosynchronous Orbit (NGSO), which includes Low-Earth Orbit (LEO) and Medium Earth Orbit (MEO).
As illustrated in Figure 5.2-1
, non-terrestrial access is provided by means of an NTN payload, i.e. a network node on-board a satellite, and an NTN Gateway interconnected by a feeder link, the UE accessing NTN network services through the NTN payload via a service link.
Three types of service links are supported:
Earth-fixed: provisioned by beam(s) continuously covering the same geographical areas all the time (e.g., the case of GSO satellites);
Quasi-Earth-fixed: provisioned by beam(s) covering one geographic area for a limited period of time and a different geographic area during another period of time (e.g., the case of NGSO satellites generating steerable beams);
Earth-moving: provisioned by beam(s) whose coverage area slides over the Earth surface (e.g., the case of NGSO satellites generating fixed or non-steerable beams).
With NGSO satellites, the eNB can provide either quasi-Earth-fixed cell coverage or Earth-moving cell coverage, while eNB operating with GSO satellites can provide Earth fixed cell coverage or quasi-Earth-fixed cell coverage.
Support for BL UEs, UEs in enhanced coverage and NB-IoT UEs over NTN is only applicable to E-UTRA connected to EPC.
Only BL UEs, UEs in enhanced coverage and NB-IoT UEs with GNSS capability are supported.
Timing and Synchronization:
The network broadcasts ephemeris information and common Timing Advance (common TA) parameters in each NTN cell. A UE shall acquire its GNSS position as well as the satellite ephemeris and common TA before connecting to an NTN cell. To achieve uplink synchronisation, before performing random access, the UE shall autonomously pre-compensate the Timing Advance, as well as the frequency doppler shift by considering the common TA, the UE position and the satellite position through the satellite ephemeris. In connected mode, the UE shall continuously update the Timing Advance and frequency pre-compensation, but the UE is not expected to perform GNSS acquisition. The UE does not perform any transmissions due to outdated satellite ephemeris, common TA or GNSS position based on timers. In connected mode, upon outdated satellite ephemeris and common Timing Advance, the UE re-acquires the broadcasted parameters and upon outdated GNSS position the UE moves to idle mode. The UEs may be configured to report Timing Advance at initial access or in connected mode. In connected mode triggered reporting of the Timing Advance is supported.
For downlink synchronization in case of NB-IoT, the two LSB of the ARFCN is signalled in MIB for bands for which a 200 kHz channel raster is not supported, and the legacy 100 kHz raster is used. Otherwise, for bands for which a 200 kHz channel raster is supported, there is no signalling of ARFCN information in MIB.
Downlink and uplink timings are frame aligned at the uplink time synchronization reference point (RP). To accommodate the long propagation delays in NTN, the timing relationships are enhanced by the support of two scheduling offsets: and as illustrated in Figure 5.2-2
Uplink segmented transmission is supported for uplink transmission with repetitions. The UE shall apply UE pre-compensation per segment of UL transmission of PUSCH/PUCCH/PRACH for BL UEs and UEs in enhanced coverage and NPUSCH/NPRACH for NB-IoT from one segment to the next segment. The configuration of uplink transmission segment is indicated on SIB for initial access and can be re-configured by RRC signalling.
Discontinuous coverage and assistance information:
As a satellite moves on a specified orbit, for example in case of a NGSO satellite, the satellite beam(s) coverage area may move and cover different portions of a geographical area due to the orbital movement of the satellite. As a consequence, a UE located in the concerned geographical area may experience a situation of discontinuous coverage, due to e.g., a sparse satellite constellation deployment.
The network may broadcast assistance information relating to the serving satellite and other satellites of the constellation to enable UEs to predict upcoming satellites fly-over periods and save power during periods of no coverage. The broadcast assistance information includes SGP4 ephemeris elements based on the TLE (Two-Line Elements) sets industry standard. Additional assistance information, such as coverage footprint parameters and cell radius, may also be optionally broadcast by the network.
Predicting out of coverage and in coverage periods is up to UE implementation. When out of coverage, the UE is not required to perform Access Stratum (AS) functions.
In the Core Network, discontinuous coverage is handled by means of Tracking Area- or RAT-specific configuration of the MME such that the MME is able, via existing functionality (namely periodic TAU timer, mobile reachable timer, implicit detach timer and high latency communication), to ensure that when the UE is unreachable, a) the UE does not trigger NAS transaction or detach from the network and b) mobile-terminated data destined to the UE can be stored in the network.
The network may broadcast more than one Tracking Area Code (TAC) per PLMN in a cell in order to reduce the signalling load at cell edge in NTN, in particular for Earth-moving cell coverage. The AS layer indicates all received TACs for the selected PLMN to the NAS layer. The network may update the UEs upon TAC removal. UEs may by UE implementation also check whether a TAC has been removed from the TACs broadcast by the network.
At the NAS layer, the UE need not trigger a Tracking Area Update due to mobility reason, if any of the broadcast TAC(s) in the cell where the UE is located is part of the UE's Tracking Area List.
For quasi-Earth-fixed cells, timing information on when the cell is going to stop serving the area may be broadcast by the network. This may be used by the UE to start measurements on neighbour cells before the broadcast stop time of the serving cell, while the exact start of the measurements is up to UE implementation.
Radio link failure and RRC connection re-establishment are supported in NTN. To enable mobility in NTN, the network provides target cell satellite parameters needed to access the NTN cell in the handover command. Conditional handover is supported for BL UEs and UEs in enhanced coverage.
Different RAT types are introduced that allow distinction by the Core Network between existing terrestrial accesses and new non-terrestrial accesses as well as, among non-terrestrial accesses, between the different types of satellite constellations (LEO, MEO, GEO, OTHERSAT) and radio access type (i.e. WB-EUTRAN, NB-IoT and LTE-M). This allows the Core Network nodes and the HSS to identify the access a UE is using such that they are able to adjust their behavior and that of the UE accordingly (e.g. setting of NAS timers, determination and enforcement of access restrictions, etc.).
The NTN Control function determines the point in time when a feeder link switch over between two eNBs is performed. For BL UEs and UEs in enhanced coverage, the transfer of the affected UE(s)' context between the two eNBs at feeder link switch over is performed by means of either S1 based or X2 based handover, and it depends on the eNBs' implementation and configuration information provided to the eNBs by the NTN Control function.
Network-interfaces signalling aspects:
The Cell Identity in NTN corresponds to a fixed geographical area identified by a Mapped Cell ID, irrespective of the orbit of the NTN payload or of the type of the service link. For a BL UE or a UE in enhanced coverage, the Cell Identity included within the target identification of the handover messages allows identifying the correct target cell. The mapping between Mapped Cell IDs and geographical areas is configured in the RAN and the Core Network (e.g. pre-configured depending on operator's policy, or based on implementation). For a BL UE or a UE in enhanced coverage or a NB-IoT UE that supports S1-U data transfer or User Plane CIoT EPS optimisation, the eNB is responsible for constructing the Mapped Cell ID based on the UE location information received from the UE, if available. The User Location Information may enable the MME to determine whether the UE is allowed to operate at its present location. Pre-configuration of special mapped cell identifiers may be used to indicate areas outside the serving PLMN's country.
The eNB reports the broadcasted TAC(s) of the selected PLMN to the MME. In case the eNB knows the UE's location information, the eNB may determine the TAI the UE is currently located in and provide that TAI to the MME.
MME(Re-)Selection by eNB:
For an RRC_CONNECTED UE, when the eNB is configured to ensure that the BL UE or the UE in enhanced coverage is using an MME that serves the country in which the UE is located. If the eNB detects that a BL UE or a UE in enhanced coverage is in a different country from that served by the serving MME, it should perform an S1 handover to change to an appropriate MME or initiate a UE Context Release Request procedure towards the serving MME (in which case the MME may decide to detach the UE).
For an RRC_CONNECTED NB-IoT UE, when the eNB is configured to ensure that the NB-IoT UE is using an MME that serves the country in which the UE is located. If the eNB detects that the UE is in a different country to that served by the serving MME, it should initiate a UE Context Release Request procedure towards the serving MME (in which case the MME may decide to detach the UE).
Verification of UE location:
The network may, according to regulatory requirements, need to enforce that the PLMN selected by the UE is allowed to operate in the geographical location where the UE is located. To this end, the MME may invoke the ULI (User Location Information) procedure during Mobility Management and Session Management procedures in order to determine the UE location. If the MME is able to determine with sufficient accuracy that it is not allowed to operate in the UE location it may reject and/or detach the UE.
The NTN related parameters shall be provided by O&M to the eNB providing non-terrestrial access, as specified in TS 38.300
for NR NTN.
Support for E-UTRAN:
For S1 and X2 interfaces, codepoints in the RAT Restriction Information IE (in the Handover Restriction List) allow the selection of different constellation types, i.e., "LEO"
for satellite access.
For S1 interface, additional codepoints are added to the RAT Type IE associated with a TAC, i.e., "NBIoT-LEO"
For S1 interface, the UE Context Reference at Source IE (eNB UE S1AP ID) is introduced in the Source eNB to Target eNB Transparent Container.
For S1 interface, a new cause value is added signalling that a UE is not within the serving area of its current PLMN.
For TAC reporting over S1, the LTE NTN TAI Information IE and the semantics description for the TAI IE are added in the User Location Information IE; the LTE NTN TAI Information IE and the semantics description for the TAI IE are added to the eNB CP RELOCATION INDICATION, HANDOVER NOTIFY, PATH SWITCH REQUEST, INITIAL UE MESSAGE, UPLINK NAS TRANSPORT, and LOCATION REPORT messages.
Other NAS protocol Aspects:
Enhancements to NAS signalling allow the UE to register to EPS core network using satellite E-UTRAN radio access technology. UICC-ME interface is extended to support network selection over satellite access and allowing to prioritize networks offering satellite access. EPS NAS re-transmission timers are extended to support longer propagation delays and response times due to extended distance between peer entities when satellite access is used. The UE supporting satellite E-UTRAN access supports also GNSS and potential uplink signalling delays to be considered in UE and network NAS implementations.
: Evolved Universal Terrestrial Radio Access (E-UTRA) and Evolved Universal Terrestrial Radio Access Network (E-UTRAN); Overall description; Stage 2 (RAN)
: NR; NR and NG-RAN Overall description; Stage-2 (RAN)
: E-UTRAN; User Equipment (UE) radio access capabilities (RAN)
: E-UTRAN; S1 Application Protocol (S1AP) (RAN)
: E-UTRAN; X2 Application Protocol (X2AP) (RAN)
: Policy and charging control architecture (SA2)
: Functional stage 2 description of Location Services (LCS) (SA2)
: General Packet Radio Service (GPRS) enhancements for Evolved Universal Terrestrial Radio Access Network (E-UTRAN) access (SA2)
: Architecture enhancements to facilitate communications with packet data networks and applications (SA2)
: Non-Access-Stratum (NAS) functions related to Mobile Station (MS) in idle model (CT1)
: Non-Access-Stratum (NAS) protocol for Evolved Packet System (EPS); Stage 3 (CT1)
: Organization of subscriber data (CT4)
: AT command set for User Equipment (UE) (CT1)
: Policy and Charging Control (PCC); Reference points (CT3)
: Evolved Packet System (EPS); Mobility Management Entity (MME) and Serving GPRS Support Node (SGSN) related interfaces based on Diameter protocol (CT4)
: 3GPP Evolved Packet System (EPS); Evolved General Packet Radio Service (GPRS) Tunnelling Protocol for Control plane (GTPv2-C); Stage 3 (CT4)
: Characteristics of the Universal Subscriber Identity Module (USIM) application (CT6)
: Universal Subscriber Identity Module (USIM) Application Toolkit (USAT) (CT6)