In 3GPP Release 17, the Non-Terrestrial Network (NTN) work item was introduced. The NTN refers to a network or segment of networks using radio frequency (RF) resources on board a satellite (or Unmanned Aerial System (UAS) platform). The key concept in the NTN is that the NTN cells are provided from Non-Geostationary Orbit (NGSO) satellites, where the NGSO satellites are revolving around the earth periodically. The period of revolution is few hours. In other words, an NGSO satellite appears to a user equipment (UE) every few hours and an NTN service from the NGSO satellite is available for a limited time.
Each NGSO satellite has its own orbit of revolution, which is included in satellite ephemeris information. Based on the satellite ephemeris information, the network can predict the feeder link switchover and manage the UE mobility (idle and connected mode) and the radio resource control. For example, since the cell coverage of NTN cells over time is predictable, the network can determine, based on the satellite ephemeris, which NTN cells should be included in the mobility configuration (e.g., cell reselection or Conditional Handover (CHO) candidates).
In a terrestrial network, a UE can determine that the UE is near a cell edge or at cell center based on reference signal received power (RSRP) due to clear difference between RSRPs measured at cell edge and cell center. In FIG. A, the received signal strength of UE 1 at cell center is clearly different from that of UE 2 near cell edge. Thus, the UE mobility in the terrestrial network is performed based on measured RSRP/RSRQ (reference signal received quality) of UEs. On the contrary, such an effect may not occur in NTN deployment scenarios resulting in small-received signal strength difference between cell center and cell edge, as illustrated in FIG. B. The reason of such a small difference is that the distance from the UE to the satellite does not vary much at cell center and cell edge. This is called “near-far-effect” in NTN.
As a result of the near-far-effect, the UE may not be able to distinguish which cell is better for the UE mobility. If the cell reselection or handover in the NTN is based on only RSRP/RSRQ as in legacy procedure for the terrestrial network, the UE may locate near cell edge after performing the cell reselection or handover. Therefore, in R17 NTN, new parameters, such as time and location, were introduced for UE mobility in idle, inactive, and connected mode.
3GPP Release 17 supports UE in idle/inactive mode to perform location-based measurement initiation and time-based measurement initiation. For location-based measurement initiation, the UE may choose not to perform neighbor cell measurement if the distance between the UE and the serving cell reference location is shorter than a threshold. In this case, how to obtain UE location information is up to UE implementation. For time-based measurement initiation, if a quasi-earth fixed serving cell broadcasts a service stop time information when to stop serving the area that it is currently covering, the UE shall perform neighbor cell measurements before the service stop time regardless of UE location and serving cell quality. Before 3GPP Release 17, the UE may choose not to perform neighbor cell measurements if serving cell quality is good enough (e.g., Srxlev > SIntraSearchP and Squal > SIntraSearchQ, or Srxlev > SnonIntraSearchP and Squal > SnonIntraSearchQ), but this time-based measurement initiation should be performed regardless of the serving cell quality. Based on the time-based measurement initiation, the UE should perform cell reselection to another neighbor cell before the service stop time of the serving cell.
3GPP Release 17 supports time-based CHO triggering condition, condEventT1, and location-based CHO triggering condition, condEvent D1. The condEvent T1 includes a time interval as a time-based CHO triggering condition, and the UE can execute the CHO to the corresponding CHO candidate cell only within the time interval. For the condEvent D1, the UE can execute the CHO to the CHO candidate cell if the distance between the UE and a reference location of the PCell is longer than a distance threshold1 and the distance between the UE and a reference location of the CHO candidate cell is shorter than a distance threshold2.
The described new features introduced in 3GPP R17 NTN enable the network to configure the cell reselection or CHO configuration, considering satellite ephemeris information. Based on the satellite ephemeris information, the UE can move on to the NTN cell which can provide longer NTN service time. Longer NTN service time may bring less frequent UE mobility. Less frequent UE mobility is important in NTN deployment scenarios because performing handover takes longer due to long propagation delays between UE and the base stations and the UE may consume more power to transmit an uplink signal.
 3GPP TR 38.821 V16.1.0, “Solutions for NR to support non-terrestrial networks (NTN)” May. 2021
 3GPP TS 38.331 V17.2.0, “Radio Resource Control (RRC) protocol specification” Sep. 2022
 3GPP TS 38.304 V17.2.0, “User Equipment (UE) procedures in Idle mode and RRC Inactive state” Sep. 2022
ABOUT THE AUTHOR
Oanyong Lee focuses on 3GPP Release 18 standard technologies. His main interests are NR NTN and IoT-NTN which is for enabling the direct wireless communication between satellites and mobiles.
Prior to joining Ofinno, he worked as 3GPP RAN2 delegate since 2017 and mainly worked on control plane (Radio Resource Control; RRC) issues. In 3GPP Release 16, he worked mainly on DC/CA enhancements, UE power saving, and study on NR NTN. In 3GPP Release 17, his main parts were NR NTN, reduced capabilities, and SON/MDT. During the five years of 3GPP delegation, he contributed more than 170 contributions (discussion paper, CRs).