Same-pci cell switching in a non-terrestrial network

Abstract

A user equipment (UE) receives, in a serving cell, (i) an indication of a time when the serving cell stops serving a geographic area of the UE and (ii) an indication related to a new cell; performs, at the indicated time, one or more actions associated with expiration of a time alignment (TA) timer for the serving cell; and starts synchronization with the new cell.

Description

SAME-PCI CELL SWITCHING IN A NON-TERRESTRIAL NETWORKCROSS-REFERENCE TO RELATED APPLICATION

[0001] This application claims priority to and the benefit of the filing date of provisional U.S. Patent Application No. 63 / 515,834 entitled “NTN Mobility Enhancements in the Same PCI Cell Switch Scenario,” filed on July 26, 2023. The entire contents of the provisional application are hereby expressly incorporated herein by reference.FIELD OF THE DISCLOSURE

[0001] This disclosure relates generally to wireless communications and, more particularly, to enabling satellite communication, i.e., non-terrestrial network (NTN) communication, for a user equipment (UE) in the connected state to perform cell switch among the cells having the same physical cell identity (PCI).BACKGROUND

[0002] This background description is provided for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.

[0003] The objectives behind developing the fifth generation (5G) technology include providing a unified framework for such types of communication as enhanced mobile broadband (eMBB), ultra-reliable low-latency communications (URLLC), and massive machine type communication (mMTC).

[0004] The 5G technology relies primarily on legacy terrestrial networks. However, the 3rd Generation Partnership Project (3GPP) organization has proposed to extend 5G communications to non-terrestrial networks (NTNs) with 5G new radio (NR) technologies, or with the Long-Term- Evolution (LTE) technologies tailored for the Narrowband Intemet-of-Thing (NB-IoT) or the enhanced Machine Type Communication (eMTC) scenarios. In an NTN, an RF transceiver is amounted on a satellite, an unmanned aircraft systems (UAS) also called drone, balloon, plane, or another suitable apparatus. For simplicity, the discussion below refers to all such apparatus assatellites. In addition to satellites, an NTN can include the sat-gateways that connect the NonTerrestrial Network to a public data network, feeder links between sat-gateways and satellites, service links between satellites, and inter- satellite links (ISL) when satellites form constellations.

[0005] A satellite can belong to one of several types based on altitude, orbit, and beam footprint size. The types include Low-Earth Orbit (LEO) satellite, Medium-Earth Orbit (LEO) satellite, Geostationary Earth Orbit (GEO) satellite, UAS platform (including High Altitude Platform Station, HAPS), and High Elliptical Orbit (HEO) satellite. GEO satellites are also known as the Geosynchronous Orbit (GSO) satellites, and LEO / MEO satellites are also known as the non-GSO (NGSO) satellites.

[0006] A GSO satellite can communicate with one or several sat-gateways deployed over a satellite targeted coverage area (e.g. a region or even a continent). A non-GSO satellite at different times can communicate with one or several serving sat-gateways. An NTN is designed to ensure service and feeder link continuity between successive serving sat-gateways, with sufficient time duration to proceed with mobility anchoring and hand-over.

[0007] A satellite can support a transparent or a regenerative (with on board processing) payload, and typically generates several beams for a given service area bounded by the field of view. The footprints of the beams typically have an elliptic shape and depend on the on-board antenna configuration and the elevation angle. For a transparent payload implementation, a satellite can apply RF filtering and frequency conversion and amplification, and not change the waveform signal. For a regenerative payload implementation, a satellite can apply RF filtering, frequency conversion and amplification, demodulation and decoding, routing, and coding / modulation. This approach is effectively equivalent to implementing most of the functions of a base station, e.g., a gNB.

[0008] In these and other scenarios, numerous UEs may go through frequent handovers from one cell to another cell due to the rapid satellite movement. To mitigate the impact of the long service interruption time and signaling overheads caused by these kinds of handovers, different satellites can use the same PCI when serving the same geographical area, which can prevent UEs from triggering handover procedures due to the change of service-link (i.e., the link between the satellite and UE). When the current cell and an upcoming cell use the same PCI, these cells appear to be the same single cell to the UE. However, because the satellites in fact change even if these satellites usethe same PCI, the UE needs to update at least the Timing Advance (TA) value before attempting to communicate with the new satellite, to make sure the base station will be able to receive the UE data / signal from the UE. In addition, due to the potential gap period during which neither the outbound satellite nor by the upcoming satellite serves the UE, the UE may continue receiving the out-of-sync indication and eventually detect the radio link failure, if the UE is not aware of the service-link switch. Detecting radio link failure causes the UE to transition to the idle state (which can prolong the service interruption time and result in extra signaling overhead).SUMMARY

[0009] An example embodiment of the techniques of this disclosure is a synchronization method implemented in a user equipment (UE). The method comprises receiving, in a serving cell, (i) an indication of a time when the serving cell stops serving a geographic area of the UE and (ii) an indication related to a new cell; performing, at the indicated time, one or more actions associated with expiration of a time alignment (TA) timer for the serving cell; and starting synchronization with the new cell.

[0010] Another example embodiment of these techniques is a user equipment (UE) comprising a transceiver; and processing hardware. The UE is configured to implement a method of any of the preceding claims.BRIEF DESCRIPTION OF THE DRAWINGS

[0011] Fig. 1A is a block diagram of an example wireless communication system in which a user device and a base station of this disclosure can implement the same-PCI cell switching techniques of this disclosure;

[0012] Fig. IB is a block diagram of an example base station in which a centralized unit (CU) and a distributed unit (DU) that can operate in the system of Fig. 1 A;

[0013] Fig. 2A is a block diagram of an example protocol stack according to which the UE of Fig. 1A communicates with base stations;

[0014] Fig. 2B is a block diagram of an example protocol stack according to which the UE of Fig. 1A communicates with a CU and a DU;

[0015] Fig. 3A is a block diagram of an example NTN node with transparent payload implementation;

[0016] Fig. 3B is a block diagram of an example NTN node with transparent payload implementation, in which a base station connects to multiple satellites via the same sat-gateway;

[0017] Fig. 4A illustrates an exemplary user plane protocol stack for use with the architecture of Fig. 3A;

[0018] Fig. 4B illustrates an exemplary control plane protocol stack for use with the architecture of Fig. 3A;

[0019] Figs. 5A-B illustrate a same-PCI cell switching procedure, in which a stationary UE switches from one cell to another cell without incurring a HO procedure;

[0020] Fig. 6 is a messaging diagram of an example scenario in which a UE in the connected state switches from a first cell to a second cell having the same PCI as the first cell, and transmits the PRACH resource configured by the first cell in the second cell;

[0021] Fig. 7 is a messaging diagram of an example scenario in which a UE in the connected state switches from a first cell to a second cell having the same PCI as the first cell, and transmits the PRACH resource configured by the first cell in the second cell, with a random delay;

[0022] Fig. 8 is a messaging diagram of an example scenario in which a UE in the connected state switches from a first cell to a second cell having the same PCI as the first cell, and performs the contention-based random access (CBRA) procedure in the second cell;

[0023] Fig. 9 is a messaging diagram of an example scenario in which a UE in the connected state switches from a first cell to a second cell having the same PCI as the first cell, and performs the CFRA procedure in the second cell before the TA timer expires, based on a PDCCH order issued by the second cell;

[0024] Fig. 10 is a messaging diagram of an example scenario in which a UE in the connected state switches from a first cell to a second cell having the same PCI as the first cell, and performs the CFRA procedure in the second cell after the TA timer expires, based on a PDCCH order issued by the second cell;

[0025] Fig. 11 is a messaging diagram of an example scenario in which a UE in the connected state switches from a first cell to a second cell having the same PCI as the first cell, and transmits an UL message or an UL data in the second cell, using the PUSCH resource allocated / configured by the second cell;

[0026] Fig. 12 is a flow diagram of an example method that can be implemented by a UE in the connected state, for suspending and resuming the radio link monitoring during a same-PCI cell switch procedure;

[0027] Fig. 13A is a flow diagram of an example method that can be implemented by a UE in the connected state, for performing the CFRA procedure and receiving a Radom Access Response (RAR) message as the response, upon switching from a first cell to a second cell having the same PCI as the first cell;

[0028] Fig. 13B is a flow diagram of an example method that can be implemented by a UE in the connected state, for performing the CFRA procedure and receiving a PDCCH as the response, upon switching from a first cell to a second cell having the same PCI as the first cell;

[0029] Fig. 14 is a flow diagram of an example method that can be implemented by a UE in the connected state, for delaying performing the CFRA procedure by a random time units, after switching from a first cell to a second cell having the same PCI as the first cell;

[0030] Fig. 15A is a flow diagram of an example method that can be implemented by a UE in the connected state, for stopping the TA value, upon switching from a first cell to a second cell having the same PCI as the first cell;

[0031] Fig. 15B is a flow diagram of an example method that can be implemented by a UE in the connected state, for performing the CFRA procedure upon receiving a PDCCH order in a second cell, after switching from a first cell to the second cell having the same PCI as the first cell;

[0032] Fig. 16 is a flow diagram of an example method that can be implemented by a UE in the connected state, for pausing the TA timer upon switching from a first cell to a second cell having the same PCI as the first cell;

[0033] Fig. 17 is a flow diagram of an example method that can be implemented by a UE in the connected state, for switching from a first cell to a second cell having the same PCI as the first cell, without performing a RA procedure;

[0034] Fig. 18 is a flow diagram of an example method that can be implemented by a BS, for providing a UE with the time and PRACH resource configuration, for the UE to switch from the serving cell to another cell having the same PCI as the serving cell;

[0035] Fig. 19 is a flow diagram of an example method that can be implemented by a BS, for determining the UE context and C-RNTI of a UE that is switched from another cell having the same PCI as the serving cell; and

[0036] Fig. 20 is a flow diagram of an example method that can be implemented by a BS, for providing a UE with a PDCCH order, where the UE is switched from another cell having the same PCI as the serving cell.DETAILED DESCRIPTION OF THE DRAWINGS

[0037] Generally speaking, the techniques of this disclosure allow a connected UE to switch among the cells sharing the same PCI in an energy / signal-efficient way. As discussed in more detail below, a user equipment (UE) and / or a network node of a radio access network (RAN) can use the techniques of this disclosure for managing early data communication and transitioning a UE between states of a protocol for controlling radio resources between the UE and the RAN.

[0038] Referring first to Fig. 1A, an example wireless communication system 100 includes a UE 102, a base station (BS) 104, a base station 106, and a core network (CN) 110. The base stations 104 and 106 can operate in a RAN 105 connected to the core network (CN) 110. The CN 110 can be implemented as an evolved packet core (EPC) 111 or a fifth generation (5G) core (5GC) 160, for example. The CN 110 can also be implemented as a sixth generation (6G) core in another example.

[0039] The base station 104 covers a cell 124, and the base station 106 covers a cell 126. If the base station 104 is a gNB, the cell 124 is an NR cell. If the base station 104 is an ng-eNB or eNB, the cell 124 is an evolved universal terrestrial radio access (E-UTRA) cell. Similarly, if the base station 106 is a gNB, the cell 126 is an NR cell, and if the base station 106 is an ng-eNB or eNB, the cell 126 is an E-UTRA cell. The cells 124 and 126 can be in the same Radio Access Network Notification Areas (RNA) or different RNAs. In general, the RAN 105 can include any number of base stations, and each of the base stations can cover one, two, three, or any other suitable number of cells. The UE 102 can support at least a 5G NR (or simply, “NR”) or E-UTRA air interface tocommunicate with the base stations 104 and 106. Each of the base stations 104, 106 can connect to the CN 110 via an interface (e.g., SI or NG interface). The base stations 104 and 106 also can be interconnected via an interface (e.g., X2 or Xn interface) for interconnecting NG RAN nodes.

[0040] Among other components, the EPC 111 can include a Serving Gateway (SGW) 112, a Mobility Management Entity (MME) 114, and a Packet Data Network Gateway (PGW) 116. The SGW 112 in general is configured to transfer user-plane packets related to audio calls, video calls, Internet traffic, etc., and the MME 114 is configured to manage authentication, registration, paging, and other related functions. The PGW 116 provides connectivity from the UE to one or more external packet data networks, e.g., an Internet network and / or an Internet Protocol (IP) Multimedia Subsystem (IMS) network. The 5GC 160 includes a User Plane Function (UPF) 162 and an Access and Mobility Management Function (AMF) 164, and / or Session Management Function (SMF) 166. Generally speaking, the UPF 162 is configured to transfer user-plane packets related to audio calls, video calls, Internet traffic, etc., the AMF 164 is configured to manage authentication, registration, paging, and other related functions, and the SMF 166 is configured to manage PDU sessions.

[0041] As illustrated in Fig. 1A, the base station 104 supports a cell 124, and the base station 106 supports a cell 126. The cells 124 and 126 can partially overlap, so that the UE 102 can select, reselect, or hand over from one of the cells 124 and 126 to the other. To directly exchange messages or information, the base station 104 and base station 106 can support an X2 or Xn interface. In general, the CN 110 can connect to any suitable number of base stations supporting NR cells and / or EUTRA cells.

[0042] As discussed in detail below, the UE 102 and / or the RAN 105 may utilize the techniques of this disclosure when the radio connection between the UE 102 and the RAN 105 is suspended, e.g., when the UE 102 operates in an inactive or idle state of the protocol for controlling radio resources between the UE 102 and the RAN 105. For clarity, the examples below refer to the RRCJN ACTIVE or RRCJDLE state of the RRC protocol.

[0043] The base station 104 is equipped with processing hardware 130 that can include one or more general-purpose processors (e.g., CPUs) and a non-transitory computer-readable memory storing instructions that the one or more general-purpose processors execute. Additionally or alternatively, the processing hardware 130 can include special-purpose processing units. The processing hardware 130 in an example implementation includes a processor 132 to process datathat the base station 104 will transmit in the downlink direction, or process data received by the base station 104 in the uplink direction. The processing hardware 130 can also include a transmitter 136 configured to transmit data in the downlink direction. The processing hardware further can include a receiver 134 configured to receive data in the uplink direction. The base station 106 can include generally similar components. In particular, components 140, 142, 144, and 146 of the base station 106 can be similar to the components 130, 132, 134, and 136, respectively.

[0044] The UE 102 is equipped with processing hardware 150 that can include one or more general-purpose processors such as CPUs and non-transitory computer-readable memory storing machine-readable instructions executable on the one or more general-purpose processors, and / or special-purpose processing units. The processing hardware 150 in an example implementation includes a processor 152 to process data that the UE 102 will transmit in the uplink direction, or process data received by UE 102 in the downlink direction. The processing hardware 150 can also include a transmitter 156 configured to transmit data in the downlink direction. The processing hardware further can include a receiver 154 configured to receive data in the uplink direction.

[0045] Fig. IB depicts an example distributed or disaggregated implementation of any one or more of the base stations 104, 106. In this implementation, the base station 104, 106 includes a central unit (CU) 172 and one or more distributed units (DUs) 174. The CU 172 includes processing hardware, such as one or more general-purpose processors (e.g., CPUs) and a computer- readable memory storing machine-readable instructions executable on the general-purpose processor(s), and / or special-purpose processing units. For example, the CU 172 can include a PDCP controller, a RRC controller and / or a RRC inactive controller. In some implementations, the CU 172 can include a radio link control (RLC) controller configured to manage or control one or more RLC operations or procedures. In further implementations, the CU 172 does not include an RLC controller.

[0046] Each of the DUs 174 also includes processing hardware that can include one or more general-purpose processors (e.g., CPUs) and computer-readable memory storing machine-readable instructions executable on the one or more general-purpose processors, and / or special-purpose processing units. For example, the processing hardware can include a MAC controller configured to manage or control one or more MAC operations or procedures (e.g., a random access procedure), and / or an RLC controller configured to manage or control one or more RLC operations orprocedures. The process hardware can also include a physical layer controller configured to manage or control one or more physical layer operations or procedures.

[0047] In some embodiments, the RAN 105 supports Integrated Access and Backhaul (IAB) functionality. In some implementations, the DU 174 operates as an lAB-node, and the CU 172 operates as an lAB-donor. In some embodiments, the RAN 105 supports Non-Terrestrial Network (NTN) functionality.

[0048] In some implementations, the CU 172 can include a logical node CU-CP 172A that hosts the control plane part of the PDCP protocol of the CU 172. The CU 172 can also include logical node(s) CU-UP 172B that hosts the user plane part of the PDCP protocol and / or Service Data Adaptation Protocol (SDAP) protocol of the CU 172. The CU-CP 172A can transmit control information (e.g., RRC messages, Fl application protocol messages), and the CU-UP 172B can transmit the data packets (e.g., SDAP PDUs or Internet Protocol packets).

[0049] The CU-CP 172A can be connected to multiple CU-UP 172B through the El interface. The CU-CP 172A selects the appropriate CU-UP 172B for the requested services for the UE 102. In some implementations, a single CU-UP 172B can connect to multiple CU-CP 172A through the El interface. The CU-CP 172A can connect to one or more DU 174s through an Fl-C interface. The CU-UP 172B can connect to one or more DU 174 through the Fl-U interface under the control of the same CU-CP 172A. In some implementations, one DU 174 can connect to multiple CU-UP 172B under the control of the same CU-CP 172A. In such implementations, the connectivity between a CU-UP 172B and a DU 174 is established by the CU-CP 172A using Bearer Context Management functions.

[0050] Fig. 2A illustrates, in a simplified manner, an example protocol stack 200 according to which the UE 102 can communicate with an eNB / ng-eNB or a gNB (e.g., one or more of the base stations 104, 106).

[0051] In the example stack 200, a physical layer (PHY) 202A of EUTRA provides transport channels to the EUTRA MAC sublayer 204A, which in turn provides logical channels to the EUTRA RLC sublayer 206A. The EUTRA RLC sublayer 206A in turn provides RLC channels to an EUTRA PDCP sublayer 208 and, in some cases, to an NR PDCP sublayer 210. Similarly, the NR PHY 202B provides transport channels to the NR MAC sublayer 204B, which in turn provideslogical channels to the NR RLC sublayer 206B. The NR RLC sublayer 206B in turn provides data transfer services to the NR PDCP sublayer 210. The NR PDCP sublayer 210 in turn can provide data transfer services to Service Data Adaptation Protocol (SDAP) 212 or a radio resource control (RRC) sublayer (not shown in Fig. 2A). The UE 102, in some implementations, supports both the EUTRA and the NR stack as shown in Fig. 2A, to support handover between EUTRA and NR base stations and / or to support DC over EUTRA and NR interfaces. Further, as illustrated in Fig. 2A, the UE 102 can support layering of NR PDCP 210 over EUTRA RLC 206 A, and SDAP sublayer 212 over the NR PDCP sublayer 210.

[0052] The EUTRA PDCP sublayer 208 and the NR PDCP sublayer 210 receive packets (e.g., from an Internet Protocol (IP) layer, layered directly or indirectly over the PDCP layer 208 or 210) that can be referred to as service data units (SDUs), and output packets (e.g., to the RLC layer 206A or 206B) that can be referred to as protocol data units (PDUs). Except where the difference between SDUs and PDUs is relevant, this disclosure for simplicity refers to both SDUs and PDUs as “packets.”

[0053] On a control plane, the EUTRA PDCP sublayer 208 and the NR PDCP sublayer 210 can provide signaling radio bearers (SRBs) or RRC sublayer (not shown in Fig. 2A) to exchange RRC messages or non-access-stratum (NAS) messages, for example. On a user plane, the EUTRA PDCP sublayer 208 and the NR PDCP sublayer 210 can provide Data Radio Bearers (DRBs) to support data exchange. Data exchanged on the NR PDCP sublayer 210 can be SDAP PDUs, Internet Protocol (IP) packets or Ethernet packets.

[0054] Fig. 2B illustrates, in a simplified manner, an example protocol stack 250, which the UE 102 can communicate with a DU (e.g., DU 174) and a CU (e.g., CU 172). The radio protocol stack 200 is functionally split as shown by the radio protocol stack 250 in Fig. 2B. The CU at any of the base stations 104 or 106 can hold all the control and upper layer functionalities (e.g., RRC 214, SDAP 212, NR PDCP 210), while the lower layer operations (e.g., NR RLC 206B, NR MAC 204B, and NR PHY 202B) are delegated to the DU. To support connection to a 5GC, NR PDCP 210 provides SRBs to RRC 214, and NR PDCP 210 provides DRBs to SDAP 212 and SRBs to RRC 214.

[0055] Fig. 3A illustrates a certain type of NTN deployment referred to as transparent payload architecture, which involves a satellite gateway 302 and a “transparent” satellite 304 for extendingthe range of the Uu interface. The satellite 304 implements a frequency conversion and a Radio Frequency (RF) amplifier in both the uplink and downlink directions. The satellite function is similar to that of an analogue RF repeater. As a result, the satellite 304 repeats the Uu radio interface from the feeder link (between the NTN gateway and the satellite) to the service link (between the satellite and the UE) in the downlink direction and vice versa in the uplink direction. The Satellite Radio Interface (SRI) on the feeder link is the Uu, and the NTN gateway 302 supports all necessary functions to forward the signal of the Uu interface. The NTN gateway 302 can be placed at the same site where the base station (e.g., eNB, gNB) 104 locates, or be connected to the base station 104 at a distance via a wired link. It is also possible to connect more than one NTN gateway to a base station. Different transparent satellites may be connected to the same base station on the ground, via the same NTN gateway, or via different NTN gateways. Fig. 3B illustrates the case where two different satellites (304 and 306) are connected to the same base station 104 via the same NTN gateway 302, and these two satellites (304 and 306) are covering the Earth surface using two different Physical Cell IDs (PCIs).

[0056] The NTN user plane protocol stack involving the UE 102, the satellite 304, the NTN gateway 302, the NR base station (i.e., gNB) 104, and the UPF 114 is illustrated in Fig. 4A. The diagram of the NTN user plane protocol stack is similar to that of the terrestrial network (TN), with the addition of two new nodes, the satellite 304 and the NTN gateway 302, being placed in the middle of the NR-Uu interface. Similarly, the NTN control plane protocol stack illustrated in Fig. 4B is also similar to that of the terrestrial network.

[0057] In terms of the satellite moving pattern, there are three types of service links that are supported in NTN: (i) Earth-fixed: provisioned by beam(s) continuously covering the same geographical areas all the time (e.g., the case of GEO / GSO satellites); (ii) 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 LEO / MEO satellites capable of using steerable beams); and (iii) Earth-moving: provisioned by beam(s) whose coverage area slides over the Earth surface (e.g., the case of LEO / MEO satellites using fixed or non-steerable beams).

[0058] With LEO / MEO satellites, the eNB can provide either quasi-Earth-fixed cell coverage or Earth-moving cell coverage. With GEO satellites, the eNB can provide Earth fixed cell coverage.

[0059] Although the transparent pay load architecture illustrated in Figs. 3A / 3B is the current focus of the 3GPP development, the regenerative payload architecture that installs the BS functions on the satellite is also a possible NTN deployment in the future. In such an architecture, the Uu only exists between the satellite and the UE. In general, the techniques of this disclosure can apply to the transparent payload architecture as well as the regenerative payload architecture.

[0060] Due to the long propagation delay in NTN, the HO interruption time in NTN is usually much longer than that in the terrestrial network (TN). Moreover, as certain satellites (e.g., LEO or MEO satellites) are constantly moving and so do the cells these satellites project onto the surface of the Earth, there could be a large number of UEs that require a handover to another cell at any time. As a result, the HO signaling “storm” may occur more frequently in NTN than in TN. To resolve the problems of the long HO interruption time and HO signaling storm in NTN, it is possible to reduce the number of overall handover attempts by allowing certain deployment scenarios. In these deployment scenarios, an upcoming (also referred to as “target” below) cell of a LEO or MEO satellite can use the same PCI as the cell previously serving the same geographic area, which implies that a LEO / MEO cell may need to change its PCI while moving from one geographic area to another.

[0061] For simplicity, the discussion below refers to the NTN cell 124 or the NTN cell 125 performing certain actions such as configuring a UE with certain parameters, rather than base stations providing coverage in these cells performing these actions.

[0062] Figs. 5A and 5B illustrate an example scenario in which the UE 102, which may be stationary, switches from one cell to another cell without performing a HO procedure. The UE 102 forgoes the HO procedure because the source cell and the target cell use the same PCI. The UE thus performs a same-PCI cell switch.

[0063] At time T / and as illustrated in Fig. 5A, satellites 304 and 306 initially serve or cover different areas on the ground (e.g., Area X and Y, respectively) with steerable beam(s) and connect to the same gateway (e.g., GW1). Both satellites are constantly moving in the directions indicated by the arrows and, at time Ti, are about to stop serving the Areas X and Y, respectively. At time T2 and as illustrated in Fig. 5B, the satellite 304 moves away from the Area X and accordingly stops serving this area, while satellite 306 approaches the Area X and starts serving this area using the same PCI (i.e., PCI k) the satellite 304 previously used to serve the Area X.

[0064] The UE 102 still operates in the Area X and does not trigger a mobility procedure because the UE 102 detects the same PCI. However, the UE 102 may experience a service interruption in a time interval between Ti and T2.

[0065] The same-PCI cell switch scenario does not require the UE 102 in the Area X to trigger the HO procedure between Ti and T2, and thus prevents a “signaling storm.” However, there remains a question of how the UE 102 determines the timing of Ti and T2, or at least the gap length during which the UE 102 will experience no coverage / service. Without knowing Ti and T2, or the gap duration between Ti and T2, the UE 102 cannot determine when to start synchronizing or resynchronizing with the upcoming cell, and may also detect a Radio Link Failure (RLF) if the UE 102 continues monitoring the radio link (e.g., performing the Radio link Monitoring (RLM) procedure). As a result, the UE 102 may consume more power and spend more time when reconnecting to the network, and may even transition to the idle state to search for a cell from an initial state. Spending more power and time would contradict the objective of the same-PCI cell switch scenario.

[0066] Further, although the UE 102 does not need to perform the HO procedure in the same-PCI cell switch scenario, the UE 102 should re-acquire / re-calculate the TA value to perform the UL transmission in a new cell, before or after the UE 102 has switched to the new cell (with the same PCI). Otherwise, the network may fail to receive the UL data / signal the UE 102 transmits in the new cell, which makes the same-PCI cell switch procedure less practical. To obtain an updated TA value, the UE 102, after switching to the new cell, can perform a Random Access (RA) procedure in the new cell. However, there can be significant collisions when accessing the RA resources due to the potentially large number of UEs performing the cell switch simultaneously. Thus, it is desirable to distribute the RA attempts evenly.

[0067] Next, several example scenarios in which the UE 102 and / or the RAN 105 perform the techniques of this disclosure for cell switching between the cells sharing the same cell identity (in these examples, PCI) in the connected state are discussed with reference to Figs. 6-11. Generally speaking, similar events in Figs. 6-11 are labeled with similar reference numbers (sharing two least significant digits), with differences discussed below where appropriate. For example, event 601 is similar to events 701, 801, 901, 1001, and 1101; event 704 is similar to event 804, 904, 1004, and 1104, and 1104; event 650 is similar to events 750, 850, 950, and 1050; etc. Thus, where adiscussion of a certain event is omitted, it will be understood that the discussion of another event with similar two least significant digits can apply to the event. To simplify the following description, the term “idle state” is used and can represent the RRC_IDLE or the RRC_INACTIVE state, and the term “connected state” is used and can represent the RRC_CONNECTED state.

[0068] Referring first to Fig. 6, in an example scenario 600, a UE in the connected state switches from a first cell to a second cell having the same PCI as the first cell and transmits, in the second cell, the PRACH resource the UE received in the first cell.

[0069] In particular, the UE 102 initially connects to the NTN Cell 124 managed by the BS 104 via the satellite 304. While operating 601 in the connected state, the UE 102 receives 603, in the NTN Cell 124, a system information message including a t-Service value of the first cell (i.e., the NTN Cell 124), where t-Service indicates the time at which the first cell will stop serving the area. The UE 102 also receives 610, in the NTN Cell 124, a dedicated layer 3 message (e.g., DCCH message) or a dedicated layer 1 signal (e.g., PDCCH) including: (i)a t-ServiceStart of the second cell (i.e., NTN Cell 125), and (ii) the dedicated PRACH resource(s) the UE 102 can use in the second cell, where t-ServiceStart indicates the time at which the second cell will start serving the area. In one implementation, instead of receiving t-ServiceStart in the event 610, the UE 102 receives a value indicating a gap length during which the neither the first cell nor the second cell serves the UE 102. In this implementation, the UE 102 can derive the time at which the second cell will start serving the area (i.e., t-ServiceStart) based on the t-Service value received in the event 603 and the gap length received in the event 610 (e.g., t-ServiceStart = t-Service + gap length). In one implementation, the UE 102 receives the t-ServiceStart value or the gap length value via the system information instead of via the dedicated L3 / L1 signaling.

[0070] At the beginning of a time interval that begins at t-Service, the NTN Cell 124 stops 620 serving the area due to the movement of the satellite 304. At the same time, the UE 102 suspends or stops 621 the RLM as well, to prevent the UE 102 from encountering an RLF. At a later time, at t-ServiceStart, the NTN cell 125 starts 630 serving the area. The UE 102 resumes 631 the RLM by monitoring the synchronization / reference signal (SS / RS) configured RLM. When resuming the RLM in the second cell (i.e., the NTN cell 125), the UE 102 can resume (re-apply) the RLM resource configuration the UE 102 applied in the first cell (i.e., NTN cell 124), or may apply the new RLM resource configuration the first cell configured in the event 610.

[0071] After resuming 631 the RLM, the UE 102 receives 640 the synchronization signal and PBCH blocks in the NTN Cell 125 and synchronizes with the NTN Cell 125. The UE 102 also receives 642 the system information including (i)the ephemeris information of the serving cell (i.e., NTN Cell 125) and (ii) the common TA parameters of the serving cell. Based on the position information of the UE ' 102 and the ephemeris information of the serving cell, the UE 102 can determine (e.g., calculate) the UE-specific TA value, which is the round-trip-time between the UE 102 and the satellite 306. Using the common TA parameters and the UE-specific TA value, the UE 102 determines 650 the total TA value (i.e., common TA value + UE-specific TA value) to apply when transmitting the PRACH preamble in the NTN cell 125.

[0072] After the UE 102 has determined the total TA value to apply when transmitting the PRACH preamble, the UE 102 transmits 662 the dedicated PRACH preamble in the NTN Cell 125. As indicated above, the NTN cell 124 configured 610 the dedicated PRACH preamble. In response to receiving the dedicated PRACH preamble, the BS 104 can either respond 664 (via NTN Cell 125) with a RAR message (i.e., MSG2 payload) containing a TA command, or respond 663 with a PDCCH addressed to the C-RNTI of the UE 102 (assuming the C-RNTI of the UE 102 remains valid after the cell switch). These implementations are labeled in Fig. 6 as alternative 1 and alternative 2, respectively.

[0073] In case the BS 104 responds 664 to the PRACH preamble with a RAR message, the UE 102 may acknowledge the reception of the RAR message by transmitting 666 an UL DCCH message in the NTN Cell 125. To this end, the UE 102 can use the UL grant provided in the RAR message. The UE 102 may determine that the same-PCI cell switch is completed successfully, as long as the UE 102 has successfully decoded the RAR message responsive to the transmitted 662 PRACH preamble, or has successfully received a PDCCH addressed to the C-RNTI of the UE 102. If the UE 102 does not receive a TA command after transmitting 642 the dedicated PRACH preamble in the NTN cell 125, UE 102 may consider the current NTA value to be ‘0’ or the last NTA value used in the NTN cell 124, and apply this value in the next UL transmission.

[0074] Next, Fig. 7 illustrates an example scenario 700 in which a UE in the connected state switches from a first cell to a second cell having the same PCI as the first cell and transmits, in the second cell, the PRACH resource the UE received in the first cell, similar to the scenario 600, butwith a random delay. The messaging diagram of Fig. 7 is similar to that of Fig. 6, with the differences discussed below.

[0075] In the scenario 700, the UE 102 receives 704 the t-ServiceStart value (or the gap length value) in the system information instead of a dedicated layer 3 message / layer 1 signal (see the scenario 600), and the NTN cell 124 transmits 711, in the dedicated L3 / L1 signal, only the configuration of the dedicated PRACH resource for the UE 102 to use in the second cell (i.e., NTN Cell 125).

[0076] After the NTN cell 125 starts serving the area at t-ServiceStart, the UE 102 does not immediately synchronize with the NTN cell 125 or resume the RLM. Instead, the UE 102 starts synchronizing with the NTN cell 125 and resumes 732 the RLM at (t-ServiceStart + X time units), where the value X is a random value the UE 102 selects. The value X can range from Xmin to Xmax. Xmin and Xmax can be configurable values which the network can provide to the UE 102 using a common / dedicated RRC message. In another implementation X or Xmin, Xmax)^Q fixed values defined in a relevant specification. When UEs autonomously select the value X, the times when the UEs start synchronizing with the NTN cell 125 are evenly distributed among the UEs switching from the NTN cell 124 to the NTN cell 125. Consequently, the times when the UEs transmit the PRACH preamble are also evenly distributed, which effectively mitigates the RACH congestion.

[0077] Fig. 8 illustrates an example scenario 800 in which a UE in the connected state switches from a first cell to a second cell having the same PCI as the first cell, and performs the CBRA procedure in the second cell. The scenario 800 is generally similar to in the scenario 600 of Fig. 6, with the differences discussed below.

[0078] In the scenario 800, at the beginning of t-Service, the UE 102 does not only suspend / stop 922 the RLM to prevent the UE 102 from detecting an RLF, but also stops 922 the TA timer (e.g., artificially expires the TA timer). After the UE 102 has received 840 synchronization signals with and received 842 the system information from the NTN cell 125, the UE 102 detects 849 UL traffic pending at the UE 102. The pending UL traffic could come from the upper layer, or could be an UL DCCH message with which the UE 102 informs the network of completing the same-PCI cell switch procedure.

[0079] Because the UE 102 has UL traffic pending, and the TA tinier has expired, the UE 102 determines to trigger a CBRA procedure to align the UL transmission timing (i.e., NTA) with the serving cell (i.e., the NTN cell 125), before transmitting the pending UL traffic. Accordingly, the UE 102 first determines 850 the initial TA (i.e., UE-specific TA + common TA), for applying when transmitting the CBRA PRACH preamble, and then transmits 861 the CBRA PRACH preamble in the NTN cell 125. In response to the CBRA PRACH preamble, the BS 104 transmits 864 a RAR message (via the NTN cell 125) including a TA command and an UL grant to the UE 102. The UE 102, after receiving 864 the RAR message, updates its NTA and TTA values, and then transmits 870 a C-RNTI MAC CE in the NTN cell 125. The UE 102 may also transmit a BSR MAC CE along with the C-RNTI MAC CE, during the event 870.

[0080] In response to receiving 870 the C-RNTI MAC CE, the BS 104 transmits (via NTN Cell 125) 871 a PDCCH addressed to the C-RNTI of the UE 102. The PDCCH may indicate a UL grant the UE 102 can use. The UE 102, after receiving 871 the UL grant, may either transmit 872 a UL DCCH in the NTN cell 125, to inform the network of completing the same-PCI cell switch procedure, or transmit any pending UL traffic in the NTN cell 125.

[0081] Now referring to Fig. 9, in a scenario900,a UE in the connected state switches from a first cell to a second cell having the same PCI as the first cell, and performs the CFRA procedure in the second cell before the TA timer expires, based on a PDCCH order issued in the second cell. The scenario 900 is generally similar to the scenario 800 of Fig. 8, with the differences discussed below.

[0082] In the scenario 900, after the UE 102 has received 940 synchronization signals and has received 942 the system information from the NTN cell 125, the UE 102 further receives 943, from the NTN cell 125, a PDCCH order (DCI format l_0 addressed to the C-RNTI of the UE 102) indicating the PRACH resource the UE 102 can use. After receiving 943 this information, the UE 102 determines 650 the initial TA (i.e., UE-specific TA + common TA) to apply when transmitting the PRACH preamble configured in the PDCCH order, and then transmits 962 the PRACH preamble in the NTN cell 125. In response to receiving 962 the PRACH preamble, the BS 104 may respond 964 (via NTN Cell 125) with a RAR message (i.e., MSG2 payload) containing a TA command, which the UE 102 can interpret as an indication of successful completion of the same- PCI cell switch procedure.

[0083] Fig. 10 illustrates an example scenario 1000 in which a UE in the connected state switches from a first cell to a second cell having the same PCI as the first cell, and performs the CFRA procedure in the second cell after the TA timer expires, based on a PDCCH order issued in the second cell. The scenario of Fig. 10 is generally similar to that of Fig. 9, with the differences discussed below. In Fig. 10, at the beginning of t-Service, the UE 102 does not only suspend / stop the RLM to prevent the UE 102 from declaring the RLF, but also pauses 1023 the TA timer. After the NTN cell 125 starts 1030 serving the area, the UE 102 resumes 1033 the RLM as well as the TA timer. ,The UE 102 later receives 1040 synchronization signals, receives 1042 the system information from the NTN cell 125, and determines 1045 its TA timer gets expired. The UE 102 then receives 1043, from the NTN cell 125, a PDCCH order (DCI format l_0 addressed to the C- RNTI of the UE 102) indicating the PRACH resource the UE 102 should use.

[0084] Next, the UE 102 determines 1050 the initial TA (i.e., UE-specific TA + common TA) to apply when transmitting the PRACH preamble configured in the PDCCH order, and then transmits 1062 the PRACH preamble in the NTN cell 125. In response receiving 1062 the PRACH preamble, the BS 104 may respond 1064 (via NTN Cell 125) with a RAR message (i.e., MSG2 payload) containing a TA command, which the UE 102 can interpret as an indication of success of the same- PCI cell switch procedure. In this example, because the RAN 105 and the UE 102 released, at the expiry 1045 of the TA timer, the UL resources (e.g., CG configuration, PUCCH, SRS, and etc.) previously configured to the UE 102, the BS 104 transmits 1080 an RRC Reconfiguration message to the UE 102 (via the NTN cell 125), to re-configure the UE 102 with the UL resources. In response to receiving 1080 the RRC Reconfiguration message, the UE 102 transmits 1082 an RRC Reconfiguration Complete message to the BS 104, to acknowledge the reception of the RRC Reconfiguration message.

[0085] Next, Fig. 11 is illustrates an example scenario 1100 in which a UE in the connected state switches from a first cell to a second cell having the same PCI as the first cell, and transmits an UL message or an UL data in the second cell, using the PUSCH resource allocated / configured in the second cell. The scenario of Fig. 11 is generally similar to that of Fig. 6, with the differences discussed below.

[0086] In the scenario 1100, after receiving 1104 the system information including the t-Service of the NTN cell 124 and the t-ServiceStart of the NTN cell 125, the UE 102 further receives 1106,from the NTN cell 124, a dedicated RRC message including: (i) a dynamic PUSCH grant or a CG configuration, and (ii) an NTA value or an indication of the NTA value, indicating whether the UE 102, after switching to the NTN cell 125, should use the value ‘O’, or the NTA value the UE 102 previously used in the NTN cell 124.

[0087] After the UE 102 has synchronized 1140 with the NTN cell 125 and received 1142 the system information from the NTN cell 125, the UE 102 determines 1151 the complete TA (i.e., common TA + UE-specific TA + NTA) to apply when transmitting the UL data / message in the NTN cell 125. The UE 102 receives 1142 the common TA is as part ofthe system information, calculates the UE-specific TA using the position of the UE 102 and the ephemeris information the UE received 1142 as part of the system information, and determines the NTA value based on the NTA value or the indication of the NTA value the UE 102 received 1106 earlier.

[0088] After determining the complete TA, the UE 102 applies the complete TA and transmits 1173 an UL DCCH message, a C-RNTI MAC CE, a BSR MAC CE, or a PHR MAC CE in the NTN cell 125, using the dynamic UL grant (i.e., a dynamic PUSCH resource) or the CG resource provided / configured in the dedicated RRC message in the event 1106. In response to receiving 1173 the UL message, the BS 104 may respond 1175 with a layer 3 message (e.g., DL DCCH message), a layer 2 signal (e.g., Contention Resolution Identity MAC CE), or a layer 1 signal (e.g., HARQ feedback / ackno wledgement) .

[0089] Next, several methods which a UE can implement to support same-PCI cell switching are discussed with reference to Figs. 12-17, and several methods which a RAN node can implement to support same-PCI cell switching are discussed with reference to Figs. 18-20. Each of these methods can be implemented as a set of instructions stored on a non-transitory computer-readable medium and executable by one or more processors, for example.

[0090] Fig. 12 is a flow diagram of an example method 1200 that can be implemented by a UE (e.g., UE 102 in this disclosure) in the connected state, for suspending and resuming the radio link monitoring during a same-PCI cell switch procedure. Initially, at block 1204, the UE receives, in a first cell, via the system information or via a dedicated RRC message, an indication of a first time when the first cell will stop serving the area (see, e.g., events 603, 704, 804, 904, 1004, 1104). The UE also receives, at block 1205, in the first cell, via the system information or via a dedicatedL3 / L1 signal, an indication of a second time when the second cell will start serving the area, wherethe dedicated L3 signal can be a DL DCCH message (i.e., a DL RRC message) and the dedicated LI signal can be a Downlink Control Information (DCI) transmitted through the PDCCH (see, e.g., events 610, 704, 804, 904, 1004, 1104).

[0091] The flow then proceeds to the block 1221, where the UE stops or suspends, at the first time instance, the radio link monitoring function, (see, e.g., events 621, 721, 822, 922, 1023, 1121). At a later time, at block 1231, the UE resumes, the radio link monitoring function, at the beginning of the second time instance (see, e.g., events 631, 831, 931, 1033, 1131).

[0092] Fig. 13A is a flow diagram of an example method 1300A that can be implemented by a UE (e.g., UE 102 in this disclosure) in the connected state, for performing the CFRA procedure and receiving a RAR message as the response, upon switching from a first cell to a second cell having the same PCI as the first cell. At block 1305, the UE receives, in a first cell via the system information or via a dedicated L1 / L3 signal, an indication of a time when a second cell will starts serving the area (see, e.g., events 610, 704, 804, 904, 1004, 1104). The UE also receives, at block 1310, in the first cell via a dedicated L1 / L3 signal, a PRACH resource configuration indicating a dedicated PRACH resource for use in the second cell (see, e.g., events 610, 711).

[0093] The flow then proceeds to the block 1330, where the UE starts synchronizing with the second cell at the time indicated at block 1305 (see, e.g., events 631 and 640, 831 and 840, 931 and 940, 1033 and 1040, 1131 and 1140). Next, the UE receives, at block 1342, in the second cell via the system information or via a dedicated RRC message, the ephemeris and common TA information of the second cell (see, e.g., events 642, 742, 842, 942, 1042, 1142). After receiving the system information at block 1342, the UE determines, at block 1350, a TA value to be applied by the UE while performing the UL transmission in the second cell, based on the ephemeris and common TA information of the second cell (see, e.g., events 650, 750, 850, 950, 1050, 1151). The UE then transmits, at block 1362, in the second cell, a PRACH preamble based on the PRACH configuration provided by the first cell at block 1310 (see, e.g., events 662,762).

[0094] The flow then proceeds to the decision block 1364A, where the UE determines whether the UE has received a RAR containing the RAPID matching the preamble index of the PRACH preamble sent by the UE at block 1362 (see, e.g., events 664, 764, 864, 964, 1064). If yes, the flow proceeds to block 1365, where the UE determines that the same-PCI cell switch procedure has completed successfully. The flow then may proceed to optional block 1366, where the UEtransmits, in the second cell, an acknowledgement (e.g., an UL DCCH message) of the completion of the same-PCI cell switch (see, e.g., events 666, 766, 872, 175). On the other hand, if the determination at the decision block 1344 is negative (i.e., the ‘NO’ branch), the flow returns to the block 1362.

[0095] Fig. 13B is a flow diagram of an example method 1300B that can be implemented by a UE (e.g., UE 102 in this disclosure) in the connected state, for performing the CFRA procedure and receiving a PDCCH as the response, upon switching from a first cell to a second cell having the same PCI as the first cell. The method 1300B is similar to the method 1300A, with the differences discussed below. According to the method 1300B, after the UE transmits 1362 a PRACH preamble in the second cell, the flow proceeds to the decision block 1364B, where the UE determines whether the UE has received a PDCCH addressed to the C-RNTI of the UE.

[0096] If the UE determines a match at block 1364B, the flow further proceeds to the block 1365, where the UE considers the same-PCI cell switch procedure is completed successfully. The flow then may proceed to optional block 1366, where the UE transmits, in the second cell, an acknowledgement (e.g., an UL DCCH message) of the completion of the same-PCI cell switch (see, e.g., events 666, 766, 872, 175). On the other hand, if the determination at the decision block 1344 is negative (i.e., the ‘NO’ branch), the flow returns to the block 1362.

[0097] Fig. 14 is a flow diagram of an example method 1400 that can be implemented by a UE (e.g., UE 102 in this disclosure) in the connected state, for delaying performing the CFRA procedure by a random time units, after switching from a first cell to a second cell having the same PCI as the first cell. The method 1400 is generally similar to method 1300A or 1300B, with the differences discussed below.

[0098] At block 1404, the UE receives, in a first cell via the system information (rather than via the dedicated L3 / L1 signal), an indication of a time when a second cell will start serving the area. The UE also receives, at block 1410, in the first cell via a dedicated L1 / L3 signal, a PRACH resource configuration indicating a dedicated PRACH resource to be used in the second cell. The UE then adjusts, at block 1417, the time for starting the monitoring by adding a random value to the time at block 1404, where the random value ranges from a pre-defined or configurable minimal value to a pre-defined or configurable maximum value. Next, the UE starts 1431 synchronizing with the second cell at the adjusted time (i.e., the indicated time increased by the random value). Theflow then completes similar to the methos 1300A or 1300B: blocks 1442, 1450 1462, 1465, and 1466 are similar to blocks 1342, 1350, 1362, 1365, and 1366, respectively.

[0099] Fig. 15A is a flow diagram of an example method 1500A that can be implemented by a UE (e.g., UE 102 in this disclosure) in the connected state, for stopping the TA value, upon switching from a first cell to a second cell having the same PCI as the first cell. At block 1504, the UE receives, in a first cell via the system information or via a dedicated RRC message, an indication of a first time when the first cell will stop serving the area. The UE also receives, at block 1505, in the first cell via the system information or via a dedicated L1 / L3 signal, an indication of a second time when a second cell will start serving the area. At block 1522, at the first indicated time, the UE stops the TA timer, considers the TA timer expired, and in some cases performs actions associated with the expiry of the TA timer (e.g., flush all HARQ buffers, release PUCCH / SRS / CG resources for all serving cells). At block 1531, at the time indicated by the second time instance, the UE starts synchronizing with the second cell. At block 1542, the UE receives, in the second cell via the system information or via a dedicated RRC message, the ephemeris and common TA information of the second cell.

[0100] The flow then proceeds to the decision block 1564A, where the UE determines whether the UE has any pending UL traffic. If no (i.e., UE has no pending UL traffic), the flow returns to the decision block 1564A. Otherwise (i.e., UE has pending UL traffic), the flow proceeds to block 1550, where the UE determines a TA value to apply performing UL transmission in the second cell, based on the ephemeris and common TA information of a second cell. After determining the TA value, the UE performs, at block 1561, a CBRA procedure for obtaining an UL grant and updating the TA value. If the RAR message including an UL grant is received by the UE during the CBRA procedure, the UE transmits, at block 1572, in the second cell, the pending UL traffic (e.g., the UL DCCH message acknowledging the completion of the same-PCI cell switch). Then, the UE may restore, at block 1590, the RRC configurations regarding the PUCCH / SRS / CG resources (which were released previously due to the expiry of the TA timer), upon the successful completion of the CBRA procedure.

[0101] Fig. 15B is a flow diagram of an example method 1500B that can be implemented by a UE (e.g., UE 102 in this disclosure) in the connected state, for performing the CFRA procedure upon receiving a PDCCH order in a second cell, after switching from a first cell to the second cellhaving the same PCI as the first cell. The flow diagram in Fig. 15B is similar to that of Fig. 15 A, with the differences discussed below.

[0102] At the first indicated time, the UE may stop the TA timer, consider the TA timer expired, and perform actions upon the expiry of the TA timer (e.g., flush all HARQ buffers, release PUCCH / SRS / CG resources for all serving cells), as shown in block 1522. Alternatively, the UE may continue running the TA timer instead of stopping it at the time indicated by the first time instance.

[0103] After the UE has switched to the second cell, and received the ephemeris and common TA information of the second cell in the second cell (block 1542), the flow proceeds to the decision block 1564B, where the UE determines whether the UE has received a PDCCH order in the second cell. If no, the flow returns to block 1564B. If yes, the flow proceeds to the block 1550, where the UE determines the TA value to be applied by the UE while performing UL transmission in the second cell, based on the ephemeris and common TA information of a second cell. After the TA value is determined, the UE performs, at block 1562, a CFRA procedure using the PRACH resource configured in the received PDCCH order, for updating the TA value. Then, the UE may restore, at block 1591, the RRC configurations regarding the PUCCH / SRS / CG resources, upon the successful completion of the CFRA procedure. The UE can choose not to perform the actions at block 1591, if the UE has not released the PUCCH / SRS / CG resources earlier.

[0104] Fig. 16 is a flow diagram of an example method 1600 that can be implemented by a UE (e.g., UE 102 in this disclosure) in the connected state, for pausing the TA timer upon switching from a first cell to a second cell having the same PCI as the first cell. The flow diagram in Fig. 16 is similar to that in Fig. 15A, with the differences discussed below. In Fig. 16, the UE may pause the TA timer if the TA timer is still running, at the time indicated by the first time instance, as shown in block 1612. Pausing the TA timer would prevent the UE from releasing the PUCCH / SRS / CG resources due to the expiry of the TA timer. Then, at the time indicated by the second time instance, the UE starts synchronizing with the second cell, and may resume the TA timer (if the TA timer has been paused previously), at block 1530 and block 1622, respectively.

[0105] After the UE has determined, at block 1340, a TA value (i.e., UE-specific TA plus the common TA) to be applied by the UE while transmitting the pending UL traffic in the second cell, the flow proceeds to another decision block 1634, where the UE determines whether the TA timerhas expired or not. If the determination at the decision block 1634 is positive (i.e., the TA timer has expired), the flow proceeds to the block 1542A, where the UE performs a CBRA procedure for obtaining an UL grant and updating the TA value, before sending the pending UL traffic. On the other hand, if the determination at the decision block 1634 is negative (i.e., the TA timer has not expired yet), the flow proceeds to the block 1666, where the UE transmits the pending UL traffic using the UL resource (e.g., CG) available to the UE.

[0106] Fig. 17 is a flow diagram of an example method 1700 that can be implemented by a UE (e.g., UE 102 in this disclosure) in the connected state, for switching from a first cell to a second cell having the same PCI as the first cell, without performing a RA procedure. Initially, at block 1306A, the UE receives, in a first cell via the system information or via a dedicated L1 / L3 signal, a time instance upon which a second cell will start serving the area. The UE also receives, at block 1706, in the first cell via a dedicated L1 / L3 signal, a dynamic grant or a CG configuration to be used in the second cell, and a NTA value or an indication of the NTA value that UE needs to apply while transmitting UL data in the second cell. The indication of the NTA value may indicate whether the UE shall use the value ‘O’, or the NTA value previously applied by the UE in the first cell, after switching to the second cell.

[0107] At a later time, at the time indicated by the time instance, the UE starts, at block 1530, synchronizing with the second cell, and then receives, at block 1332, in the second cell via the system information or via a dedicated RRC message, the ephemeris and common TA information of the second cell. After obtaining the ephemeris and common TA information of the second cell, the UE determined, at block 1740, the TA value to be applied by the UE while performing UL transmission in the second cell, based on the ephemeris and common TA information of the second cell, and also based on the NTA value or the indication of the NTA value received at block 1706. In one implementation, the UE determines the TA value to be applied in the second cell equals to the sum of the UE-specific TA, common TA, and NTA, where the UE-specific TA equals to the round- trip-time (RTT) between the UE and the satellite offering the second cell.

[0108] At block 1756, the UE transmits, in the second cell, a UL DCCH message notifying the completeness of the same-PCI cell switch, by utilizing the dynamic grant or the CG resource provided in 1706. Instead of transmitting an UL DCCH message, the UE may transmit a C-RNTI MAC CE, a BSR MAC CE, or a PHR MAC CE at block 1756. Then, the UE may receive, at block1758, in the second cell, an explicit (e.g., a DL DCCH message paired with the UL DCCH message transmitted at block 1756) or an implicit indication (e.g., the presence or absence of the HARQ feedback) acknowledging the reception of the information transmitted at block 1756.

[0109] Fig. 18 is a flow diagram of an example method 1800 that can be implemented by a BS (e.g., BS 104 in this disclosure), for providing a UE with the time and PRACH resource configuration, for the UE to switch from the serving cell to another cell having the same PCI as the serving cell. Initially, at block 1870, the BS transmits, to another BS of a neighbor cell, a Cell Switch Request message including the UE RRC configuration per UE in the connected state. The BS then receives, at block 1872, from the BS of the neighbor cell, a Cell Switch Response message including a time instance upon which the neighbor cell will start serving the area, and a PRACH resource configuration per connected UE, which indicates the dedicated PRACH resource to be used by the UE.

[0110] Then, the BS transmits (or broadcasts), at block 1804, to each UE, via the system information or via a dedicated RRC message, a first time instance upon which the current cell will stop serving the area. The BS also transmits (or broadcasts), at block 1806A, to each UE, via the system information or via a dedicated L3 / L1 signal, a second time instance upon which the neighbor cell will start serving the area. Last, at block 1806B, the BS transmits, to each UE, via a dedicated L1 / L3 signal, a PRACH resource configuration indicating the dedicated PRACH resource to be used by the UE in the neighbor cell.

[0111] Fig. 19 is a flow diagram of an example method 1900 that can be implemented by a BS (e.g., BS 104 in this disclosure), for determining the UE context and C-RNTI of a UE that is switched from another cell having the same PCI as the serving cell. Initially, at block 1970, the BS receives, from a BS of a neighbor cell, a Cell Switch Request message including the RRC configurations and UE contexts of the UEs connected to the neighbor cell. In response to the Cell Switch Request message, the BS transmits, at block 1972, to the BS of the neighbor cell, a Cell Switch Response message including a time instance upon which the cell will start serving the area, and a PRACH resource configuration per connected UE, which indicates the dedicated PRACH resources to be used by the UE.

[0112] Then, the BS receives, at block 1942, from a UE, a PRACH preamble belonging to the PRACH resources configured at block 1972. Upon receiving the PRACH preamble, the BSdetermines, at block 1943, the C-RNTI of the UE based on the UE contexts received at block 1970 and the PRACH resource configurations provided at block 1972. In response to the reception of the PRACH preamble, the BS transmits, to the UE, a PDCCH addressed to the RA-RNTI calculated based on the PRACH resource used by the UE, or a PDCCH addressed to the C-RNTI of the UE, and considers, at block 1949, the same-PCI cell switch procedure has been successfully conducted by the UE.

[0113] Fig. 20 is a flow diagram of an example method 2000 that can be implemented by a BS (e.g., BS 104 in this disclosure), for providing a UE with a PDCCH order, where the UE is switched from another cell having the same PCI as the serving cell. Initially, at block 1970, the BS receives, from a BS of a neighbor cell, a Cell Switch Request message including the RRC configurations and UE contexts of the UEs connected to the neighbor cell. In response to the Cell Switch Request message, the BS transmits, at block 2072, to the BS of the neighbor cell, a Cell Switch Response message including a time instance upon which the cell will start serving the area.

[0114] Then, the BS transmits, at block 2041, to a UE that was originally connecting to the neighbor cell, a PDCCH order, after the time instance has passed. Shortly, the BS receives, at block 2042, from the UE, a PRACH preamble aligning the PRACH resource configuration provided in the PDCCH order, and therefore determines, at block 2043, the C-RNTI of the UE based on the UE contexts received at block 1970. The rest of the procedure (i.e., block 1948 and 1949) is the same as that in Fig. 19.

[0115] The following list of examples reflects a variety of the embodiments explicitly contemplated by the present disclosure.

[0116] Example 1. A method for radio link monitoring implemented in a UE and comprising: receiving, in a serving cell, a timing indication related to a target cell; suspending radio link monitoring when the serving cell stops serving a geographic area of the UE; and resuming the radio link monitoring based on the timing indication related to the target cell.

[0117] Example 2. The method of example 1, wherein both the serving cell and the target cell are associated with a same cell identifier.

[0118] Example 3. The method of example 2, wherein the cell identifier is a physical cell identity (PCI).

[0119] Example 4. The method of any of the preceding examples wherein: the serving cell is associated with a first NTN transceiver; and the target cell is associated with a second NTN transceiver.

[0120] Example 5. The method of example 4, wherein: the first NTN transceiver is implemented in a first satellite; and the second NTN transceiver is implemented in a second satellite.

[0121] Example 6. The method of any of the preceding examples, further comprising: receiving, in the serving cell, a timing indication related to the serving cell; and determining when the serving cell stops serving the geographic area of the UE based on the timing indication related to the serving cell.

[0122] Example 7. The method of any of the preceding examples, further comprising: stopping a Timing Advance (TA) timer when the serving cell stops serving a geographic area of the UE.

[0123] Example 8. The method of any of the preceding examples, further comprising: resuming the TA timer based on the timing indication related to the target cell.

[0124] Example 9. The method of any of the preceding examples, wherein: the timing indication related to the target cell indicates a time at which the target cell starts serving a geographic area of the UE.

[0125] Example 10. The method of claim of example 9, wherein the resuming of the radio link monitoring occurs at the time at which the target cell starts serving the geographic area of the UE.

[0126] Example 11. The method of claim of example 9, wherein the resuming of the radio link monitoring occurs at the time at which the target cell starts serving the geographic area of the UE, offset by a random value.

[0127] Example 12. The method of any of examples 9-11, wherein the timing indication related to the target cell is a t-ServiceStart IE.

[0128] Example 13. The method of any of examples 1-8, wherein the timing indication related to the target cell indicates a time interval during which neither the serving cell nor the target cell services the geographic area of the UE.

[0129] Example 14. The method of any of the preceding examples , wherein the receiving of the timing indication related to the target cell includes receiving a dedicated Layer 3 (L3) signal.

[0130] Example 15. The method of example 14, wherein the dedicated L3 signal is a dedicated control channel (DCCH) message.

[0131] Example 16. The method of any of examples 1-13, wherein the receiving of the timing indication related to the target cell includes receiving a dedicated Layer 1 (L3) signal.

[0132] Example 17. The method of example 16, wherein the dedicated LI signal is a physical dedicated control channel (PDCCH) message.

[0133] Example 18. The method of any of examples 1-5, wherein the receiving of the timing indication related to the target cell includes: receiving a system information for the serving cell, the system information including (i) a timing indication related to the serving cell and (ii) the timing indication related to the target cell.

[0134] Example 19. The method of any of the preceding examples, further comprising: subsequently to the resuming of the radio link monitoring, receiving, in the target cell, a common TA value; and transmitting a Physical Random Access Channel (PRACH) preamble using the common TA value.

[0135] Example 20. The method of example 19, further comprising receiving, in the serving cell, an indication of the PRACH preamble.

[0136] Example 21. The method of example 19, further comprising subsequently to the resuming of the radio link monitoring, receiving, in the target cell, a PDCCH order indicating the PRACH preamble.

[0137] Example 22. The method of any of examples 1-18, further comprising: receiving, in the serving cell, an RRC message indicating a dynamic PUSCH grant for transmitting an uplink message in the target cell.

[0138] Example 23. The method of any of examples 1-18, further comprising: receiving, in the serving cell, an RRC message indicating a configured grant (CG) configuration for transmitting an uplink message in the target cell.

[0139] Example 24. The method of any of examples 1-18, further comprising: receiving, in the serving cell, an RRC message indicating a TA value for calculating a TA prior to transmitting an uplink message in the target cell.

[0140] Example 25. A UE comprising: a transceiver; and processing hardware configured to implement a method of any of the preceding claims.

[0141] The following description may be applied to the description above.

[0142] Generally speaking, description for one of the above figures can apply to another of the above figures. Examples, implementations and methods described above can be combined, if there is no conflict. An event or block described above can be optional or omitted. For example, an event or block with dashed lines in the figures can be optional. In some implementations, “message” is used and can be replaced by “information element (IE)”, and vice versa. In some implementations, “IE” is used and can be replaced by “field”, and vice versa. In some implementations, “configuration” can be replaced by “configurations” or “configuration parameters”, and vice versa. In some implementations, “some” means “one or more”. In some implementations, “at least one” means “one or more”.

[0143] A user device in which the techniques of this disclosure can be implemented (e.g., the UE 102) can be any suitable device capable of wireless communications such as a smartphone, a tablet computer, a laptop computer, a mobile gaming console, a point-of-sale (POS) terminal, a health monitoring device, a drone, a camera, a media-streaming dongle or another personal media device, a wearable device such as a smartwatch, a wireless hotspot, a femtocell, or a broadband router. Further, the user device in some cases may be embedded in an electronic system such as the head unit of a vehicle or an advanced driver assistance system (ADAS). Still further, the user device can operate as an intemet-of-things (loT) device or a mobile-internet device (MID). Depending on the type, the user device can include one or more general-purpose processors, a computer-readable memory, a user interface, one or more network interfaces, one or more sensors, etc.

[0144] Certain embodiments are described in this disclosure as including logic or a number of components or modules. Modules may can be software modules (e.g., code, or machine-readable instructions stored on non-transitory machine -readable medium) or hardware modules. A hardware module is a tangible unit capable of performing certain operations and may be configured or arranged in a certain manner. A hardware module can comprise dedicated circuitry or logic that is permanently configured (e.g., as a special-purpose processor, such as a field programmable gate array (FPGA) or an application- specific integrated circuit (ASIC), a digital signal processor (DSP), etc.) to perform certain operations. A hardware module may also comprise programmable logic orcircuitry (e.g., as encompassed within a general-purpose processor or other programmable processor) that is temporarily configured by software to perform certain operations. The decision to implement a hardware module in dedicated and permanently configured circuitry, or in temporarily configured circuitry e.g., configured by software) may be driven by cost and time considerations.

[0145] When implemented in software, the techniques can be provided as part of the operating system, a library used by multiple applications, a particular software application, etc. The software can be executed by one or more general-purpose processors or one or more special-purpose processors.

[0146] Upon reading this disclosure, those of skill in the art will appreciate still additional and alternative structural and functional designs for handling mobility between base stations through the principles disclosed herein. Thus, while particular embodiments and applications have been illustrated and described, it is to be understood that the disclosed embodiments are not limited to the precise construction and components disclosed herein. Various modifications, changes and variations, which will be apparent to those of ordinary skill in the art, may be made in the arrangement, operation and details of the method and apparatus disclosed herein without departing from the spirit and scope defined in the appended claims.

Claims

What is claimed is:

1. A synchronization method implemented in a user equipment (UE), the method comprising: receiving, in a serving cell, (i) an indication of a time when the serving cell stops serving a geographic area of the UE and (ii) an indication related to a new cell; performing, at the indicated time, one or more actions associated with expiration of a time alignment (TA) timer for the serving cell; and starting synchronization with the new cell.

2. The method of claim 1, wherein: the indication related to the new cell is related to a timing advance (TA).

3. The method of claim 1 or 2, wherein: the indication related to the new cell includes a time when the new cell starts serving the geographic area of the UE.

4. The method of any of the preceding claims, further comprising: flushing a Hybrid Automatic Repeat Request (HARQ) buffer at the indicated time5. The method of any of the preceding claims, further comprising: flushing all HARQ buffers at the indicated time.

6. The method of any of the preceding claims, further comprising: receiving, in the new cell and after the starting of the synchronization, ephemeris information of the new cell.

7. The method of claim 6, wherein the ephemeris information is received in a system information message.

8. The method of any of the preceding claims, wherein: the indication related to the new cell includes a dedicated PRACH resource.

9. The method of any of the preceding claims, wherein: the indication of a time when the serving cell stops serving the geographic area and thendication related to the new cell are received in a system information block.

10. The method of any of the preceding, wherein: the indication of the time when the serving cell stops serving the geographic area is a t-ervice information element (IE).

11. The method of any of the preceding claims wherein: the serving cell is associated with a first non-terrestrial network (NTN) transceiver; and the new cell is associated with a second NTN transceiver.

12. The method of claim 11, wherein: the first NTN transceiver is implemented in a first satellite; and the second NTN transceiver is implemented in a second satellite.

13. The method of any of the preceding claims, wherein: the serving cell and the new cell are associated with a same cell identifier.

14. The method of claim 13, wherein the cell identifier is a physical cell identity (PCI).

15. A user equipment (UE) comprising: a transceiver; and processing hardware; the UE configured to implement a method of any of the preceding claims.

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