Summary based on the input provided by M. Jean-Yves FINE, Thales in SP-220967, with the assistance of M. Amer Catovic, Qualcomm, for the "Terminal and Core Network aspects"
The "Integration of satellite components in the 5G architecture"
work item adds or enhances a number of features in 5GCore architecture in order to support Non-Terrestrial Networks (NTN), for several use cases:
Coverage extension: Many commercial activities, such as agriculture, mining, forestry take place outside inhabited areas. Coverage extension with satellite networks is useful to enable e.g. voice communication, video monitoring, and remote control in uncovered or under-covered areas.
Internet of Things: many Internet-of-Things applications relate to monitoring of assets (e.g. ships, trains, trucks), infrastructure (e.g. bridges, pipelines, railway track), or the environment (e.g. agriculture sensors). In many IoT applications, only small amounts of data are exchanged and communication is optimized for low power usage. Satellite communication should also be able to address these requirements.
Disaster communication: Public safety authorities have a responsibility to provide assistance in case of natural disasters. This requires communication, also in cases where because of that disaster the cellular infrastructure is damaged. Satellite communication can be used as fall back for these cases. Ideally the user equipment (UE) and way of working when cellular networks are available should also be usable with satellite access.
Global roaming: Applications like tracking and tracing of containers need to be available globally across satellite and terrestrial networks. When a container is in a harbour or transported on a truck, using a terrestrial cellular network is probably most efficient. However, when the container is on a ship in the middle of an ocean, only satellite communication is possible.
Broadcasting: Satellite communication is particularly suitable to broadcast the same information over a very wide area. This can also be used in context of 5G mobile edge applications (e.g. mobile gaming), where application content needs to be available in many different edge locations.
To address such use cases, 3GPP has set Key Performance Indicator (KPI) targets for satellite in TS 22.261
At 5G Core Network architecture level, in SA2, a dedicated study on architecture aspects for using satellite access in 5G (FS_5GSAT_ARCH) was conducted to select the solutions able to cope with satellite specific key issues. The outcome of the study (TR 23.737
) identifies the impacts of satellite integration in the 5GS and solutions to adjust the 5G system accordingly.
The 5GSAT_ARCH work item, following the study, updated architecture specifications (TS 23.501
, TS 23.502
, TS 23.503
) to implement the solutions identified.
In CT1, TR 24.821
studied "Non-Terrestrial Impact of PLMN selection procedure"
and, following 5GSAT_ARCH_CT, led to update TS 23.122
and TS 24.501
Furthermore, RAN has defined "3GPP defined radio access networks supporting non-Terrestrial Networks"
, described in the next clause.
A PLMN core network can be connected to a satellite NG-RAN. A satellite NG-RAN can be shared between more than one core networks.
Satellite NG-RAN can be used as a new RAN 3GPP access but also as backhaul between the core and terrestrial access network, providing a transport for the N1/N2/N3 reference points.
Multi-connectivity and URLLC over satellite are not considered in Rel-17. Basic assumptions are that UEs are equipped with GNSS, and transparent mode: satellites (LEO/MEO/GEO) are relaying the Uu interface only at physical layer level.
Impacts on 5GC of Satellite NG-RAN used as new RAN 3GPP access
In Rel-17, only direct access with transparent satellite is considered, as shown in following Figure:
Impacts of satellites onto 5GS are linked to the size of the cells (larger than the terrestrial ones), the fact that the satellite cells can be fix on earth, when beam is steerable, but also moving on earth, when beam is not steerable. This characteristics impacts 5GS mobility management, i.e. the management of the handover of radio bearer between nodes and the management of the reachability of a UE for downlink services (paging), that need to be adapted to take into account both the satellite beam size and fix or moving cells configuration.
A basic assumption in Re-17 is that tracking areas (TAs) and cell identities (cell IDs) refer to specific geographical areas, so that 5G services can use these identifiers as representation of a UE location.
To avoid Tracking Area Codes (TAC) fluctuations, in the moving cells case, it has been decided that the Radio Access Network will broadcast in the cell the list of Tracking Area Codes, corresponding to tracking areas that have been define on the earth surface through network planning, for the zone currently enlighten by the radio cell.
New Radio Access Technology types are introduced in the 5GC to distinguish between different satellite configurations (LEO, MEO, GEO, other).
The distance earth - satellite also introduces higher delay values than for terrestrial cells and new 5QI is also introduced in TS 23.501
to cope with this delay, depending on the satellite RAT type.
Impacts on 5GC of Satellite NG-RAN used as backhaul
Rel-17 only considers backhauling with constant delay. Here, the satellite operator is able to mask any delay changes in service/feeder links by exploiting the knowledge of the satellite position to calculate how much variable delay should be added to keep the overall delay constant. Connecting gNBs to 5GC via, e.g., a single GEO satellite or a single NGSO satellite without ISLs are examples of such backhauling as shown in following Figure.
Backhauling with constant delay minimizes the impact on the 3GPP network architecture. However, there are following new features introduced in TR 23.501
related to the QoS aspects of satellite backhauling:
Reporting of satellite backhaul category to the 5G Core Network.
New 5G QoS Indicator (5QI) defined for satellite backhaul and satellite access.
Terminal and Core Network aspects
PLMN selection procedure is updated for satellite integration in 5GS. Then, in the continuity of the study done in TR 24.821
, the following aspects of UE impact at the NAS layer (including PLMN selection) have ben specified by CT1:
New "NG-RAN satellite" RAT type in USIM
Extension of the NAS supervision timers over satellite access for GEO and MEO RAT types (LEO uses legacy timers)
Modification of the higher priority PLMN selection procedure to include shared MCC 9xx
New minimum periodic search timer for higher priority PLMN search over satellite access when PLMN uses shared MCC
New trigger for PLMN selection upon transition in/out international areas (based on UE implementation)
New Forbidden List of PLMNs not allowed to operate at UE location and its handling
New 5GMM cause value#78 and its handling (related to the list in the previous bullet)
Support for multiple TACs for the same PLMN broadcast in the radio cell, including corresponding logic for determining the "Current TAI" and impact on ME-USIM procedures
There are corresponding network impacts.
List of related CRs:
: "Service Requirements for the 5G System".
: "Study on architecture aspects for using satellite access in 5G";
: "Study on PLMN selection for satellite access in 5G";
: "System architecture for the 5G System (5GS), Stage 2";
: "Procedures for the 5G System (5GS), Stage 2";
: "Policy and charging control for the 5G System (5GS), Stage 2";
: "Non-Access-Stratum (NAS) functions related to Mobile Station (MS) in idle mode";
: "Non-Access-Stratum (NAS) protocols for the 5G System (5GS), Stage3"
RP-221946: "Summary for NR support for Non-Terrestrial Network (NTN)"
RP-221169: "Solutions for NR to support non-terrestrial networks (NTN)" / UID 860046
Summary based on the input provided by Thales in RP-221946.
This Rel-17 RAN WI "Solutions for NR to support Non-Terrestrial Networks (NTN)"
introduces support of non-terrestrial networks into the NR protocol and NG-RAN architecture. NTN refers to networks, or segments of networks, using a spaceborne vehicle or an airborne vehicle for transmission, defined as follows:
Spaceborne vehicles: Geosynchronous (GSO) and Non-Geosynchronous (NGSO) orbiting satellites. NGSO includes Low Earth Orbit at altitude approximately between 300 km and 1500 km and Medium Earth Orbit at altitude approximately between 7000 km and 25000 km.
Airborne vehicles: High Altitude Platforms (HAPs) encompassing Unmanned Aircraft Systems (UAS) including Lighter than Air UAS (LTA), Heavier than Air UAS (HTA), all operating in altitudes typically between 8 and 50 km, quasi-stationary.
This clause present the RAN aspects while the System and core network aspects ("5GSAT_ARCH"
) are covered in the next clause.
The addressed radio specifics of NTN compared to 5G terrestrial networks include
Delay variation, Doppler variation as well as possible Earth moving radio cells, due to the motion of space/airborne vehicles
Long latency due to the altitude of the space/airborne vehicles
Differential delay and possible multi country cell coverage due to larger radio cell size
Different propagation channel model (See TS 38.811)
Different radio unit performance due to specific payload performance
Overall architecture and general aspects
As illustrated in Figure 5.1.2-1
, non-terrestrial access is provided by means of an NTN payload, i.e. a network node on-board a satellite or HAPS, and an NTN Gateway interconnected by a feeder link, the UE accessing NTN network services through the NTN payload via a service link.
The NTN payload transparently forwards the radio protocol received from the UE (via the service link) to the NTN Gateway (via the feeder link) and vice-versa. A gNB may serve multiple NTN payloads while an NTN payload may be served by multiple gNBs.
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 and a different geographic area during another period (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 NTN gNB can provide either quasi-Earth-fixed cell coverage or Earth-moving cell coverage, while gNB operating with GSO satellite can provide Earth fixed cell coverage.
Timing, Synchronization and HARQ enhancements (RAN1)
The network broadcast ephemeris information and common Timing Advance (common TA) parameters in each NTN cell. Since NTN capable UE are expected to be all GNSS-capable, they shall acquire a valid 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 (information from the gNB), the UE position, the satellite position and satellite velocity through the satellite ephemeris. In connected mode, the UE shall continuously update the Timing Advance and frequency pre-compensation. If the UE does not have a valid GNSS position and/or valid satellite ephemeris, it does not communicate with the network until both are regained. 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.
While the pre-compensation of the instantaneous Doppler shift experienced on the service link is to be performed by the UE for the uplink, the management of Doppler shift experienced over the feeder link is left to the network implementation.
To accommodate the propagation delay in NTNs, several timing relationships are enhanced by a Common Timing Advance (Common TA) and two scheduling offsets and . is a configured offset that corresponds to the Round Trip Time (RTT) between the Reference Point (RP) and the NTN payload. is a configured scheduling offset that approximately corresponds to the sum of the service link RTT and the common TA. is a configured offset that approximately corresponds to the RTT between the RP and the gNB.
To mitigate the impact of HARQ stalling in NTN the HARQ feedback can be disabled in the presence of ARQ re-transmissions at the RLC layer (e.g., in GSO satellite systems) and/or the number of HARQ processes for re-transmissions at the MAC layer can be increased to 32 (e.g., in NGSO satellite systems).
Mobility Management (RAN2)
To enable mobility in NTN, the network provides serving cell's and neighbouring cell's satellite ephemeris needed to access the target serving NTN cell in the handover command.
UE supports mobility between NTN and Terrestrial Network (i.e. from NTN to Terrestrial Network (hand-in) and from Terrestrial Network to NTN (hand-out)), but is not required to connect to both NTN and Terrestrial Network at the same time. It may also support mobility between radio access technologies based on different orbit (GSO, NGSO at different altitude).
Triggering conditions upon which UE may execute Conditional Hand-Over (CHO) to a candidate cell, have been introduced: event A4, time-based trigger condition, location-based trigger condition. The two last conditions are configured together with one of the measurement-based trigger conditions. Location is defined by the distance between UE and a reference location. Time is defined by the time between T1 and T2, where T1 is an absolute time value and T2 is a duration started at T1.
For the measurements the network can configure multiple SS/PBCH Block Measurement Timing Configuration (SMTCs) in parallel per carrier and for a given set of cells depending on UE capabilities using propagation delay difference and ephemeris information. It can also configure measurement gaps based on multiple SMTC.
The adjustment of SMTCs is possible under network control based on UE assistance information if available for connected mode and under UE control based on UE location and satellite assistance information (e.g., ephemeris, common TA parameters) for idle/inactive modes.
In the quasi-earth fixed cell scenario, UE can perform time-based and location-based measurement in RRC_IDLE/RRC_INACTIVE. The timing and location information associated to a cell are provided via system information. They refer respectively to the time when the serving cell is going to stop serving a geographical area and to the reference location of serving cell.
A Tracking Area corresponds to a fixed geographical area. Any respective mapping is configured in the RAN. The network may broadcast multiple Tracking Area Codes (TAC) per PLMN in a NR NTN cell in order to reduce the signalling load at cell edge, in particular for Earth-moving cell coverage. A TAC change in the System Information is under network control and may not be exactly synchronised with real-time illumination of beams on ground.
Regarding the UE location aspects, upon network request, after AS security is established in connected mode, a UE should report its coarse UE location information (most significant bits of the GNSS coordinates, ensuring an accuracy in the order of 2 km) to the NG-RAN if available.
A service link switch refers to a change of serving satellite.
A feeder link switch over is the procedure where the feeder link is changed from a source NTN Gateway to a target NTN Gateway for a specific NTN payload. The feeder link switch over is a Transport Network Layer procedure. Both hard and soft feeder link switch over are applicable to NTN.
Service and feeder link switch overs apply mostly for the case of NGSO.
NG-RAN signalling (RAN3)
The Cell Identity, indicated by the gNB to the Core Network as part of the User Location Information corresponds to a Mapped Cell ID, irrespective of the orbit of the NTN payload or the types of service links supported. It is used for Paging Optimization in NG interface, Area of Interest and Public Warning Services.
The Cell Identity included within the target identification of the handover messages allows identifying the correct target radio cell as well as for RAN paging.
The mapping between Mapped Cell IDs and geographical areas is configured in the RAN and Core Network. The gNB is responsible for constructing the Mapped Cell ID based on the UE location info received from the UE, if available. The mapping may be pre-configured (e.g., up to operator's policy) or up to implementation.
The gNB reports the broadcasted TAC(s) of the selected Public Land Mobile Network (PLMN) to the Access and Mobility Management Function (AMF) as part of UE Location Information (ULI). In case the gNB knows the UE's location information, the gNB may determine the Tracking Area Indicator (TAI) the UE is currently located in and provide that TAI to the AMF as part of ULI.
AMF (Re-)Selection by gNB (RAN3)
For a RRC_CONNECTED UE, when the gNB is configured to ensure that the UE connects to an AMF that serves the country in which the UE is located. If the gNB detects that the UE is in a different country to that served by the serving AMF, then it should perform an NG handover to change to an appropriate AMF, or initiate an UE Context Release Request procedure towards the serving AMF (in which case the AMF may decide to de-register the UE).
O&M Requirements (RAN3)
The NTN related parameters, as listed in clause 16.14.7 of TS 38.300
, shall be provided by O&M to the gNB providing non-terrestrial access. Additional NTN related parameters in Annex B4 of TS 38.300
may be provided by O&M to the gNB for its operation.
RF performances and RRM requirements (RAN4)
Based on coexistence studies captured in TR 38.863
, the minimum RF and performance requirements in FR1 for respectively NR User Equipment (UE) supporting satellite access operation and NR Satellite Access Node (SAN) are defined in TS 38.101-5
and TS 38.108
illustrate the satellite access node which encompass on ground non-NTN infrastructure gNB functions, gateway and feeder link and the RF functions of the NTN payload.
The considered operating bands in frequency range FR1 are defined in Table 5.1.2-1:
RF requirements of an NTN capable UE (as defined in TS 38.101-5
) requires the same RF performance as UE operating with terrestrial network. This allows connectivity to both NTN or Terrestrial Network.
Note that RF requirements of the SAN as defined in TS 38.108
are lower compared to the BS RF requirements of a terrestrial network as defined in TS 38.104
Specific requirements for radio resource management in NTN are defined in TS 38.133
. They mostly relate to specific delay as well as timing and frequency errors in the different procedures.
In addition to SAN, RF requirements of HAPS were defined in TS 38.104
as HAPS BS class which refers to Wide Area BS class without additional changes.
NR operating band n1 can be applied for HAPS operation, as defined in TS 38.104
NR UEs as defined by current TS 38.101-1
can support HAPS deployments with no additional changes needed in TS 38.101-1