User equipment and base station involved in handover

Abstract

To provide a procedure for facilitating improving a handover procedure.SOLUTION: A receiving process by UE for receiving a common timing advance value for a radio cell in a procedure of handover from a source base station to a target base station receives the common timing advance value in a first handover message of the handover procedure. The first handover message includes a timing indication for transmitting a second handover message of the handover procedure from the UE to the target base station. The process also determines timing of a first uplink transmission to the target base station with respect to downlink transmissions from the target base station, based on the common timing advance value and the timing indication, and transmits a second handover message to the target base station based on the determined uplink timing.SELECTED DRAWING: Figure 19

Description

[Technical field]

[0001] The present disclosure is directed to methods, devices, and articles in communication systems, such as 3GPP communication systems. [Background technology]

[0002] Currently, the 3rd Generation Partnership Project (3GPP®) is working on technical specifications for the next generation of cellular technology, also known as the fifth generation (5G).

[0003] One objective is to provide a single technical framework that addresses all usage scenarios, requirements, and deployment scenarios (see, for example, Section 6 of Non-Patent Document 1), including at least enhanced mobile broadband (eMBB), ultra-reliable low-latency communications (URLLC), and massive machine type communication (mMTC). For example, deployment scenarios for eMBB may include indoor hotspots, dense urban areas, suburban areas, wide urban areas, and high-speed. Deployment scenarios for URLLC may include industrial control systems, mobile health management (remote monitoring, remote diagnosis, and remote treatment), real-time control of vehicles, and wide-area monitoring and control systems for smart grids. Deployment scenarios for mMTC may include scenarios using a large number of devices with low latency data transmission, such as smart wearables and sensor networks. eMBB and URLLC services are similar in that both require extremely high bandwidth, but differ in that URLLC services preferably require extremely low latency.

[0004] A second objective is to achieve forward compatibility: backward compatibility to Long Term Evolution (LTE, LTE-A) cellular systems is not required, which will facilitate the design of entirely new systems and / or the introduction of new features. [Prior art documents] [Non-patent literature]

[0005] [Non-Patent Document 1] TR 38.913 version 15.0.0 [Non-Patent Document 2] 3GPP TS 38.300 v15.6.0 [Non-Patent Document 3] 3GPP TR 38.801 v14.0.0 [Non-Patent Document 4] 3GPP TS 38.211 v15.6.0 [Non-Patent Document 5] TS 23.501 v16.1.0 [Non-Patent Document 6] 3GPP TS 38.321 v15.6.0 [Non-Patent Document 7] TS 38.213 version 15.6.0 [Non-Patent Document 8] TR 38.821 v0.3.0 [Non-Patent Document 9] TS 38.401 [Non-Patent Document 10] 3GPP standard TS 38.331 version 15.6.0 Summary of the Invention

[0006] One non-limiting exemplary embodiment facilitates providing procedures that facilitate improved handover procedures.

[0007] A main aspect of the present disclosure is an integrated circuit for controlling a process of a user equipment (UE), the process comprising: a receiving process for receiving a common timing advance value for a target radio cell from a source base station of a source radio cell, the UE being connected to the source radio cell and being involved in a handover procedure for handing over the UE from the source radio cell to the target radio cell, the common timing advance value being received from the source base station in a first handover message of the handover procedure, the first handover message of the handover procedure further including a timing indication for transmitting a second handover message of the handover procedure from the UE to a target base station; a determining process for determining a first uplink timing of an uplink transmission to the target base station with respect to a downlink transmission from the target base station based on the received common timing advance value and the timing indication; a sending process of sending the second handover message of the handover procedure to the target base station based on the determined uplink timing; the determining process determining a UE-specific timing advance value, specific to the UE and the target radio cell, to be used by the UE for performing uplink transmission in the target radio cell. It is an integrated circuit.

[0008] In one embodiment, the techniques disclosed herein feature a user equipment (UE) having a receiver, the receiver of the UE receiving a common timing advance value for a target radio cell from a source base station of a source radio cell. The UE is connected to the source radio cell and is involved in a handover procedure for handing over the UE from the source radio cell to the target radio cell. The common timing advance value is also received from the source base station in a first message of the handover procedure, the first message also including a timing indication for transmitting a second message from the UE to the target base station. A processor of the UE then determines a first uplink timing of an uplink transmission to the target base station for a downlink transmission from the target base station based on the received common timing advance value and the timing indication. A transmitter of the UE transmits the second message of the handover procedure to the target base station based on the determined uplink timing. The processor determines a UE-specific timing advance value, specific to the UE and the target radio cell, to be used by the UE for performing uplink transmission in the target radio cell.

[0009] It should be noted that the general or specific embodiments may be implemented as a system, a method, an integrated circuit, a computer program, a storage medium, or any selective combination thereof.

[0010] Further benefits and advantages of the disclosed embodiments and various implementations will become apparent from the specification and drawings. These benefits and / or advantages may be obtained individually by the various embodiments and features of the specification and drawings, however, they need not all be present in order to obtain one or more of such benefits and / or advantages. [Brief description of the drawings]

[0011] Exemplary embodiments will now be described in more detail with reference to the accompanying drawings.

[0012] [Figure 1] FIG. 1 illustrates an example architecture of a 3GPP NR system. [Diagram 2] FIG. 1 illustrates an example user and control plane architecture for LTE eNB, gNB, and UE. [Diagram 3] Schematic diagram showing the division of functions between NG-RAN and 5GC [Figure 4] Sequence diagram of RRC connection setup / reconfiguration procedure [Diagram 5] Schematic showing the usage scenarios for Enhanced Mobile Broadband (eMBB), Massive Machine Type Communications (mMTC), and Ultra-Reliable Low Latency Communications (URLLC) [Figure 6] Block diagram illustrating an exemplary 5G system architecture for a non-roaming scenario [Figure 7] FIG. 1 illustrates messages exchanged between an eNB and a UE when performing a contention-based RACH procedure. [Figure 8] FIG. 1 illustrates messages exchanged between an eNB and a UE when performing a contention-free RACH procedure. [Figure 9] FIG. 1 illustrates timing misalignment of uplink transmissions from two mobile terminals received at a base station when uplink timing alignment is not performed. [Figure 10] FIG. 1 illustrates the effect of performing uplink timing transmissions with synchronized uplink timing alignment for uplink transmissions from two mobile terminals. [Figure 11] FIG. 1 illustrates an exemplary transparent satellite based NG RAN architecture. [Figure 12] FIG. 1 illustrates an exemplary NG RAN architecture based on regenerative satellites. [Figure 13] Diagram showing regenerative satellite scenarios with corresponding propagation delays and reference points for common TA calculations. [Figure 14]Diagram showing transparent satellite scenarios with corresponding propagation delays and reference points for common TA calculations. [Figure 15] FIG. 1 illustrates signaling exchange for legacy handover procedures. [Figure 16] FIG. 1 shows signaling exchange for RACH-less handover procedure [Figure 17] FIG. 1 illustrates an exemplary simplified structure of a UE and a gNB. [Figure 18] FIG. 1 illustrates the structure of a UE according to an exemplary implementation of an improved handover procedure. [Figure 19] FIG. 1 is a flow diagram of a UE operation according to an exemplary implementation of an improved handover procedure. [Figure 20] FIG. 1 is a flow diagram of the operation of a base station according to an exemplary implementation of an improved handover procedure. [Figure 21] Signaling and Processing Diagram According to an Exemplary Implementation of an Improved Handover Procedure [Figure 22] Signaling and Processing Diagram According to an Exemplary Implementation of an Improved Handover Procedure [Figure 23] FIG. 1 shows a signaling and processing diagram according to an exemplary implementation of an improved handover procedure based on a first option on how the UE determines a UE-specific TA value of a target radio cell. [Figure 24] FIG. 1 shows a signaling and processing diagram according to an exemplary implementation of an improved handover procedure based on a second option on how the UE determines a UE-specific TA value of a target radio cell. [Diagram 25] FIG. 13 is a signaling and processing diagram according to an exemplary implementation of an improved handover procedure based on a third option on how a GNSS-equipped UE determines a UE-specific TA value of a target radio cell. [Figure 26] FIG. 1 illustrates a transparent satellite scenario and the feeder link and processing delays caused by the communication. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0013] 5G NR System Architecture and Protocol Stack

[0014] 3GPP is working on the next release of fifth-generation cellular technology (known simply as 5G), which includes the development of a new radio access technology (NR) that will operate in frequencies up to 100 GHz. The first version of the 5G standard will be completed at the end of 2017, which will allow smartphones compliant with the 5G NR standard to proceed to testing and commercial deployment.

[0015] In particular, the overall system architecture envisages an NG-RAN (Next Generation - Radio Access Network) with gNBs terminating NG radio access user plane (SDAP / PDCP / RLC / MAC / PHY) and control plane (RRC) protocols. The gNBs are interconnected with each other by an Xn interface. The gNBs are also connected to an NGC (Next Generation Core) by a Next Generation (NG) interface, more specifically to an AMF (Access and Mobility Management Function) (e.g., a specific core entity that runs the AMF) by an NG-C interface, and to a UPF (User Plane Function) (e.g., a specific core entity that runs the UPF) by an NG-U interface. The NG-RAN architecture is shown in Figure 1 (see, for example, Section 4 of Non-Patent Document 2).

[0016] Various different deployment scenarios can be supported (see, for example, Non-Patent Document 3). In this document, for example, a decentralized deployment scenario (see, for example, Section 5.2 of Non-Patent Document 3; a centralized deployment is shown in Section 5.4) is presented, in which a base station supporting 5G NR can be deployed. Figure 2 shows an exemplary decentralized deployment scenario (see, for example, Figure 5.2.-1 of Non-Patent Document 3), further showing an LTE eNB and a user equipment (UE), which is connected to both the gNB and the LTE eNB. The new eNB of NR 5G can be exemplarily called a gNB. The eLTE eNB is an evolution of the eNB and supports connectivity to the EPC (Evolved Packet Core) and NGC (Next Generation Core).

[0017] The user plane protocol stack in NR (see, for example, Section 4.4.1 of Non-Patent Document 2) includes a PDCP (Packet Data Convergence Protocol, see Section 6.4 of Non-Patent Document 2), a RLC (Radio Link Control, see Section 6.3 of Non-Patent Document 2), and a MAC (Medium Access Control, see Section 6.2 of Non-Patent Document 2) sublayer, which terminate at the gNB on the network side. In addition, a new sublayer (SDAP, Service Data Adaptation Protocol) of the access stratum (AS) is introduced on top of the PDCP (see, for example, Section 6.5 of Non-Patent Document 2). A control plane protocol stack is also defined in NR (see, for example, Section 4.4.2 of Non-Patent Document 2). An overview of the functions of Layer 2 is described in Section 6 of Non-Patent Document 2. The functions of the PDCP sublayer, the RLC sublayer, and the MAC sublayer are described in clauses 6.4, 6.3, and 6.2, respectively, of 3GPP TS 2011-010111. The functions of the RRC layer are described in clause 7 of 3GPP TS 2011-010111.

[0018] For example, the medium access control layer handles scheduling and scheduling-related functions, including multiplexing of logical channels, as well as handling of various numerologies.

[0019] The physical layer (PHY) is responsible for e.g. coding, PHY HARQ processing, modulation, multi-antenna processing, and mapping of signals to appropriate physical time-frequency resources. The physical layer also handles mapping of transport channels to physical channels. The physical layer provides services to the MAC layer in the form of transport channels. A physical channel corresponds to a set of time-frequency resources used for the transmission of a particular transport channel, and each transport channel is mapped to a corresponding physical channel. One physical channel is the PRACH (Physical Random Access Channel), which is used for random access.

[0020] NR use cases / deployment scenarios include enhanced Mobile Broadband (eMBB), ultra-reliable and low latency communications (URLLC), and massive machine type communications (mMTC), which have diverse requirements in terms of data rates, latency, and coverage. For example, eMBB is expected to support peak data rates (20 Gbps downlink and 10 Gbps uplink) and user-perceived data rates on the order of three times those offered by IMT-Advanced. In contrast, URLLC has more stringent requirements, including extremely low latency (user plane latency of 0.5 ms for UL and DL, respectively) and high reliability (1-10 Mbps within 1 ms). -5 ) and mMTC requires high connection density (1 km 2 1,000,000 devices per second), wide coverage in harsh environments, and extremely long battery life (15 years) to lower device costs may be preferably required.

[0021] Thus, an OFDM numerology (e.g., subcarrier spacing, OFDM symbol duration, cyclic prefix (CP) duration, number of symbols per scheduling interval) suitable for one use case may not work well for another use case. For example, low latency services may preferably require a shorter symbol duration (and therefore a larger subcarrier spacing) and / or fewer symbols per scheduling interval (also referred to as TTI) than mMTC services. Furthermore, deployment scenarios with large channel delay spreads may preferably require a longer CP duration than scenarios with small delay spreads. To maintain a comparable CP overhead, the subcarrier spacing should be optimized depending on the delay spread. In NR, more than one value of subcarrier spacing may be supported. Thus, currently, subcarrier spacings of 15 kHz, 30 kHz, 60 kHz, ... are considered. Symbol duration T u and the subcarrier spacing Δf is expressed by the formula (Δf=1 / T u ), as in LTE systems, the term "resource element" can be used to denote the smallest resource unit consisting of one subcarrier for the length of one OFDM / SC-FDMA symbol.

[0022] In the new radio system 5G-NR, for each numerology and carrier, a resource grid of subcarriers and OFDM symbols is defined in the uplink and downlink, respectively. Each element in the resource grid is called a resource element and is identified based on a frequency index in the frequency domain and a symbol position in the time domain (see Non-Patent Document 4).

[0023] Split of 5G NR functions between NG-RAN and 5GC

[0024] Figure 3 shows the division of functions between NG-RAN and 5GC. The logical nodes of NG-RAN are gNB or ng-eNB. The logical nodes of 5GC are AMF, UPF, and SMF.

[0025] In particular, the gNB and ng-eNB handle the following key functions: - Radio Resource Management functions, such as Radio Bearer Control, Radio Admission Control, Connection Mobility Control, dynamic resource allocation (scheduling) to UEs in both uplink and downlink directions - IP header compression, encryption, and data integrity protection - Selection of AMF at UE attach time when routing to AMF cannot be determined from information provided by the UE - Routing user plane data to the UPF - Routing control plane information to AMF - Establishing and releasing connections - Scheduling and sending paging messages - Scheduling and transmission of system broadcast information (sent from AMF or OAM) - Configuring measurements and measurement reporting for mobility and scheduling - Transport-level packet marking in the uplink - Session Management - Network slicing support - QoS flow management and mapping to data radio bearers - Support for UEs in RRC_INACTIVE state - NAS message delivery function - Wireless Access Network Sharing - Dual Connectivity - Tight interworking between NR and E-UTRA

[0026] The Access and Mobility Management Function (AMF) handles the following main functions: - Termination of Non-Access Stratum (NAS) signalling - NAS signaling security - Access Stratum (AS) security control - Core Network (CN) inter-node signaling for mobility between 3GPP access networks - Reachability for idle mode UEs (including control and execution of paging retransmissions) - Registration Area Management - Support for intra-system and inter-system mobility - Access Authentication - Access authentication, including roaming rights checks - Mobility management controls (subscriptions and policies) - Network slicing support - Selection of Session Management Function (SMF)

[0027] In addition, the User Plane Function (UPF) handles the following main functions: - Anchor points for intra-RAT / inter-RAT mobility (when applicable) - External PDU session points for interconnection with data networks - Packet routing and forwarding - User plane part of packet inspection and policy rule enforcement - Traffic usage reports - An uplink classifier to support routing of traffic flows to the data network - Branching points to support multi-homed PDU sessions - User plane QoS processing (e.g. packet filtering, gating, UL / DL rate enforcement) - Validation of uplink traffic (mapping of SDF to QoS flows) - Downlink packet buffering and downlink data notification triggering

[0028] Finally, the Session Management Function (SMF) handles the following major functions: - Session Management - UE IP address allocation and management - UP function selection and control - Configuring traffic steering in the User Plane Function (UPF) to route traffic to the correct destination - Policy enforcement and QoS control - Downlink data notification

[0029] RRC connection establishment and reconfiguration procedures

[0030] Figure 4 shows the interactions between the UE, gNB, and AMF (5GC entities) in the NAS portion when the UE transitions from RRC_IDLE to RRC_CONNECTED (see non-patent document 2).

[0031] RRC is a higher layer signaling (protocol) used for the configuration of the UE and the gNB. In particular, in this transition, the AMF prepares UE context data (including, for example, PDU session context, security keys, UE radio capabilities, and UE security capabilities, etc.) and sends it to the gNB by an INITIAL CONTEXT SETUP REQUEST. The gNB then activates AS security with the UE, which is performed by the gNB sending a SecurityModeCommand message to the UE and the UE responding with a SecurityModeComplete message to the gNB. The gNB then performs a reconfiguration to establish a signaling radio bearer 2 (SRB2) and a data radio bearer (DRB), which is performed by the gNB sending an RRCReconfiguration message to the UE and the gNB receiving an RRCReconfigurationComplete from the UE in response. In the case of a signaling-only connection, these steps related to RRCReconfiguration are skipped since SRB2 and DRB are not established. Finally, the gNB informs the AMF that the establishment procedure is complete via an INITIAL CONTEXT SETUP RESPONSE.

[0032] Therefore, in the present disclosure, a fifth generation core (5GC) entity (e.g., AMF, SMF, etc.) is provided that has a control circuit that, in operation, establishes a next generation (NG) connection with a gNodeB, such that a signaling radio bearer is established between the gNodeB and a user equipment (UE), and a transmitter that, in operation, transmits an initial context setup message to the gNodeB via the NG connection. In particular, the gNodeB transmits radio resource control (RRC) signaling including a resource allocation setting information element to the UE via the signaling radio bearer. The UE then performs uplink transmission or downlink reception based on the resource allocation setting.

[0033] Usage scenarios for IMT-2020 and beyond

[0034] Figure 5 shows some of the use cases for 5G NR. Three use cases are considered for 3GPP NR (3rd generation partnership project new radio) that are envisaged to support a wide variety of services and applications by IMT-2020. The specifications for Phase 1 of enhanced mobile broadband (eMBB) have been finalised. Current and upcoming work will include standardisation for ultra-reliable and low latency communications (URLLC) and large-scale machine-type communications, in addition to further extending eMBB support. Figure 5 shows some examples of envisaged usage scenarios for IMT beyond 2020.

[0035] URLLC use cases have stringent requirements on capabilities such as throughput, latency and availability, and are envisioned as one of the enablers of future vertical applications such as wireless control of industrial manufacturing or production processes, remote medical surgery, power distribution automation in smart grids, and transportation safety. The ultra-high reliability of URLLC is supported by identifying technologies to meet the requirements set by NR URLLC in Release 15. For NR URLLC in Release 15, key requirements include a user plane target latency of 0.5 ms for UL (uplink) and 0.5 ms for DL ​​(downlink). A typical URLLC requirement for a single transmission of a packet is a BLER (block error rate) of 1E-5 for a packet size of 32 bytes with a user plane latency of 1 ms.

[0036] From the RAN1 perspective, reliability can be improved in many possible ways. Current scope for improving reliability includes defining separate CQI tables for URLLC, more compact DCI formats, PDCCH repetition, etc. However, as NR becomes more stable and developed (for a key requirement of NR URLC), the scope for achieving ultra-high reliability may increase. Specific use cases for NR URLCC in Release 15 include augmented reality / virtual reality (AR / VR), e-health, e-safety, and mission-critical applications.

[0037] Furthermore, technology enhancements targeted by NR URLCC target latency improvement and reliability improvement. Technology enhancements for latency improvement include configurable numerology, non-slot-based scheduling with flexible mapping, grant-free (configured grant) uplink, slot-level repetition of data channel, and preemption for downlink. Preemption means that a transmission for which resources are already allocated is aborted and the already allocated resources are used for another transmission with smaller latency / higher priority requirement requested later. Thus, an already granted transmission is preempted by a later transmission. Preemption applies regardless of the specific service type. For example, a transmission of service type A (URLCC) can be preempted by a transmission of service type B (eMBB, etc.). Technology enhancements for reliability improvement include dedicated CQI / MCS tables for a target BLER of 1E-5.

[0038] The mMTC (Massive Machine Type Communication) use case is characterized by a very large number of connected devices transmitting relatively small amounts of data that are generally not sensitive to latency. The devices need to be low cost and have extremely long battery life. From an NR perspective, utilizing very narrow bandwidth portions is one possible solution to achieve power savings from the UE perspective to enable long battery life.

[0039] As mentioned above, it is expected that the range of reliability in NR will increase. One key requirement for all cases, especially for URLLC and mMTC, is high or ultra-high reliability. Several mechanisms can be considered to improve reliability from a radio perspective and from a network perspective. In general, there are several key areas that can help improve reliability. These areas include compact control channel information, data channel / control channel repetition, and diversity related to frequency, time, and / or spatial domains. These areas are generally applicable to reliability, regardless of the specific communication scenario.

[0040] For NR URLLC, further use cases with more stringent requirements have been identified, e.g. in factory automation, the transport industry, power supply, etc. Depending on the use case, the more stringent requirements are higher reliability (up to the level of 10-6), higher availability, packet sizes up to 256 bytes, time synchronization on the order of a few μs (values ​​range from 1 to a few μs depending on the frequency range), and low latency on the order of 0.5-1 ms (in particular the target latency for the user plane is 0.5 ms).

[0041] Furthermore, for NR URLLC, several technology enhancements are recognized from the RAN1 perspective. In particular, PDCCH (Physical Downlink Control Channel) enhancements related to compact DCI, PDCCH repetition, and increased PDCCH monitoring. Furthermore, UCI (Uplink Control Information) enhancements related to HARQ (Hybrid Automatic Repeat Request) enhancements and CSI feedback enhancements. Also, PUSCH enhancements related to minislot level hopping and retransmission / repetition are recognized. The term "minislot" refers to a Transmission Time Interval (TTI) that contains fewer symbols than a slot (a slot containing 14 symbols).

[0042] QoS Control

[0043] The 5G Quality of Service (QoS) model is based on QoS flows and supports both QoS flows that require guaranteed flow bitrates (GBR QoS flows) and QoS flows that do not require guaranteed flow bitrates (non-GBR QoS flows). Thus, at the NAS level, QoS flows are the finest granularity of QoS differentiation in a PDU session. Within a PDU session, QoS flows are identified by a QoS Flow Identifier (QFI) that is conveyed in the encapsulation header over the NG-U interface.

[0044] The 5GC establishes one or more PDU sessions for each UE. The NG-RAN establishes at least one Data Radio Bearer (DRB) for each UE together with the PDU sessions, and can then configure additional DRBs for the QoS flows of that PDU session (NG-RAN decides when to do so), e.g. as described above with reference to FIG. 4. The NG-RAN maps packets belonging to different PDU sessions to different DRBs. NAS-level packet filters in the UE and 5GC associate UL and DL packets with QoS flows, and AS-level mapping rules in the UE and NG-RAN associate UL and DL QoS flows with DRBs.

[0045] FIG. 6 shows the 5G NR non-roaming reference architecture (see section 4.23 of 3GPP 5). Application Functions (AFs) (e.g., external application servers handling 5G services exemplarily described in FIG. 5) interact with the 3GPP Core Network to provide services. For example, they support application influence on traffic routing, access Network Exposure Functions (NEFs), or interact with a policy framework (see Policy Control Function PCF) for policy control (e.g., QoS control). Based on the operator's deployment, application functions (AFs) that are considered trusted by the operator can be allowed to interact directly with the relevant Network Functions. Application Functions (AFs) that are not allowed by the operator to directly access network functions interact with the relevant Network Functions using an external exposure framework via the NEF.

[0046] Figure 6 shows further functional units of the 5G architecture: Network Slice Selection Function (NSSF), Network Repository Function (NRF), Unified Data Management (UDM), Authentication Server Function (AUSF), Access and Mobility Management Function (AMF), Session Management Function (SMF), and Data Network (DN) (e.g. operator services, internet access or third party services).

[0047] Random Access Procedure

[0048] Like LTE, 5G NR provides a Random Access Channel (RACH) procedure (or simply random access procedure). For example, the RACH procedure can be used by a UE to access a cell that the UE has found. The RACH procedure is not related to other contexts in NR, e.g. In case of handover, when synchronizing to a new cell, Re-establishing uplink synchronization to the current cell if synchronization is lost due to too long a period of time without uplink transmission from the device; Requesting uplink scheduling if a dedicated scheduling request resource is not configured for the device; It can also be used in

[0049] There are a number of events that may cause the UE to perform a random access procedure (see, for example, section 9.2.6 of 3GPP TS 2010-010361).

[0050] The RACH procedure is described in more detail below with reference to Figures 7 and 8. A mobile terminal can be scheduled for uplink transmission if its uplink transmission is time synchronized. Thus, the Random Access Channel (RACH) procedure serves as an interface between an unsynchronized mobile terminal (UE) and an orthogonal transmission of uplink radio access. For example, random access is used to achieve uplink time synchronization for user equipment that has not yet acquired or lost uplink synchronization. Once the user equipment achieves uplink synchronization, the base station can schedule uplink transmission resources for it. One scenario related to random access is when a user equipment in RRC_CONNECTED state that hands over from its current serving cell to a new target cell performs a random access procedure to achieve uplink time synchronization in the target cell.

[0051] There may be two types of random access procedures that allow access to be either contention-based, i.e., implying an inherent risk of collision, or contention-free (non-contention-based). An exemplary definition of a random access procedure can be found in 3GPP TS 2.0, Section 5.1.

[0052] The contention-based random access procedure is described in more detail below with respect to Fig. 7. This procedure consists of four "steps" and can therefore also be called a four-step RACH procedure. First, the user equipment transmits a random access preamble to the base station on the Physical Random Access Channel (PRACH) (i.e., message 1 of the RACH procedure). After detecting the RACH preamble, the base station transmits a Random Access Response (RAR) message (message 2 of the RACH procedure) on the Physical Downlink Shared Channel (PDSCH) addressed on the PDCCH with a (Random Access) RA-RNTI that identifies the time-frequency and slot where the preamble was detected. If multiple user equipments transmit the same RACH preamble on the same PRACH resource (also called collision), they will receive the same random access response message. The RAR message may convey the detected RACH preamble, a Timing Alignment Command (TA command) for synchronization of subsequent uplink transmissions based on the timing of the received preamble, an initial uplink resource allocation (grant) for transmitting the first scheduled transmission, and an allocation of a Temporary Cell Radio Network Temporary Identifier (T-CRNTI) that is used by the base station to address the mobile station for which the RACH preamble was detected until the RACH procedure is finished, since the "real" identity of the mobile station at this point in time is not yet known by the base station.

[0053] The user equipment monitors the PDCCH to receive a random access response message within a given time window (e.g., called the RAR reception window), which can be set by the base station. In response to the RAR message received from the base station, the user equipment first transmits a scheduled uplink transmission on the radio resources allocated by the grant in the random access response. This scheduled uplink transmission carries an actual message with a specific function, such as, for example, an RRC Connection Request, an RRC Resume Request, or a buffer status report.

[0054] If a preamble collision occurs in the first message of the RACH procedure, i.e., multiple user equipments transmit the same preamble on the same PRACH resource, the colliding user equipments receive the same T-CRNTI in the random access response and also collide on the same uplink resource when transmitting their scheduled transmissions in the third step of the RACH procedure. If the base station successfully decodes the scheduled transmission from one user equipment, the contention remains unresolved for the other user equipment. To resolve this type of contention, the base station transmits a contention resolution message (fourth message) addressed to the C-RNTI or Temporary C-RNTI. This ends the procedure.

[0055] Figure 8 shows a contention-free random access procedure, which is simplified compared to the contention-based random access procedure. In a first step, the base station provides the user equipment with a dedicated preamble to be used for random access, so that there is no risk of collision, i.e. no risk of multiple user equipments transmitting the same preamble. The user equipment then transmits the preamble signaled by the base station in the uplink on the PRACH resource. Since contention-free random access avoids the case where multiple UEs are transmitting the same preamble, essentially the contention-free random access procedure is terminated after successful reception of the random access response by the UE.

[0056] 3GPP is also considering a two-step (contention-based) RACH procedure for 5G NR, in which message 1 (which may also be referred to as msgA), corresponding to message 1 and message 3 in the four-step LTE RACH procedure, is transmitted first. The gNB then responds with message 2 (which may also be referred to as msgB), corresponding to message 2 and message 4 in the LTE RACH procedure. This msgB may include, for example, a Success Random Access Response (RAR), a Fallback RAR, and optionally a backoff indication. For the two-step RACH procedure, some further assumptions are made, for example, that after the UE determines the RACH type (e.g., two-step RACH), it keeps retrying the same RACH type until failure. However, it may be possible that the UE can fall back to the four-step RACH procedure after a certain time.

[0057] Furthermore, the network may semi-statically determine the mutually exclusive radio resources used to perform the two-step RACH procedure and the four-step RACH procedure. The radio resources used to transmit the first message in the RACH procedure include at least a RACH occasion and a preamble. For example, in the two-step RACH procedure, the first message msgA uses not only the PRACH resource (e.g., the RACH occasion and the preamble) but also the associated PUSCH resource.

[0058] One of the main objectives of the random access procedure is to obtain a timing advance value that is used by the UE to time its uplink transmissions to the base station. Uplink orthogonality is maintained by making the transmissions from different user equipments in a cell time-aligned at the base station receiver. This avoids the occurrence of intra-cell interference, both between user equipments assigned to transmit in consecutive subframes and between user equipments transmitting on adjacent subcarriers. Time alignment of uplink transmissions is achieved by applying a timing advance at the user equipment transmitter with respect to the received downlink timing, as illustrated in Figures 9 and 10. The main role of this is to counteract the different propagation delays between different user equipments in a cell.

[0059] 9 exemplarily illustrates misalignment of uplink transmissions from two UEs when a base station (here exemplarily assumed to be an eNodeB) receives respective uplink transmissions from the two UEs at different timings because uplink timing alignment is not performed. The propagation delays PD_eNB-UE1 and PD_UE1-eNB for UE1 and PD_eNB-UE2 and PD_UE2-eNB for UE2 are significantly different from each other, so that the uplink transmissions by UE1 and UE2 are received by the eNodeB at different times.

[0060] In contrast, Figure 10 shows synchronized uplink transmissions to two UEs. Uplink timing alignment (TA) is performed by each UE and applied to the uplink transmissions from the two UEs such that they arrive at approximately the same time at the base station.

[0061] Thus, the user equipment specific timing advance determines the uplink transmission timing from the user equipment perspective. The timing advance reference is the downlink subframe boundary timing from the user equipment perspective, as shown in Figure 10.

[0062] Transmission timing adjustment for uplink transmissions is described in detail in section 4.2 of non-patent document 7 and section 5.2 of non-patent document 6.

[0063] The Timing Advance Command MAC CE is identified by a MAC subheader with the LCID given in table 6.2.1-1 of non-patent document 6. It has a fixed size and consists of a single octet defined as follows: - TAG Identity (Timing Advance Group ID): This field indicates the TAG identity of the addressed TAG. The TAG containing the SpCell has a TAG Identity of 0. The length of the field is 2 bits. - Timing Advance Command: This field indicates the index value TA (0,1,2,...,63) that is used to control the amount of timing adjustment required by the MAC entity. The length of the field is 6 bits.

[0064] [Table 1]

[0065] During the random access procedure, if uplink time alignment is not performed, the UE transmits a preamble according to DL synchronization timing. The base station then responds with a timing advance value in an RAR message, where the timing advance value is, for example, 2×PD(P D =P U Assume that (P D is the downlink propagation time, and P U (where, is the uplink propagation time). The UE applies the received timing advance value to perform the uplink transmission.

[0066] Non-Terrestrial Network (NTN)

[0067] Satellites remain the most effective means to reach areas beyond terrestrial coverage, as well as passengers on trains, aircraft, and ships. Including satellite as an integral part of the 5G ecosystem therefore adds resilience. The satellite industry has been involved in various committees, including 3GPP, EC, and ITU-T, to ensure that satellite systems are integrated as an essential part of the 5G ecosystem. The goals are 1) to support highly available and reliable connectivity using satellites for use cases such as ubiquitous coverage, disaster relief, public safety requirements, emergency response, remote sensor connectivity, and broadcast services, 2) to support an air interface with one-way latency of up to 275 ms when involving satellite connections, and 3) to support seamless mobility between terrestrial and satellite-based networks with varying latencies. The role and benefits of satellite in 5G have been studied in 3GPP Release 14, leading to specific requirements to support satellite access.

[0068] Figure 11 shows an exemplary NG RAN architecture based on transparent satellite. According to one exemplary implementation (see section 5.1 of Non-Patent Document 8), the satellite payload implements frequency conversion and radio frequency amplifiers in both uplink and downlink directions. This corresponds to an analog RF repeater. Thus, the satellite relays the NR-Uu radio interface from the feeder link (between the NTN gateway and the satellite) to the service link (between the satellite and the UE) and vice versa. The satellite radio interface (SRI) on the feeder link is NR-Uu. That is, the satellite does not terminate the NR-Uu.

[0069] Figure 12 shows an exemplary NG RAN architecture based on regenerative satellites. According to one exemplary implementation (see section 5.2 of Non-Patent Document 8), the NG-RAN logical architecture described in Non-Patent Document 9 is used as a baseline for the NTN scenario. The satellite payload implements the regeneration of the signal received from the Earth. The NR-Uu radio interface is on the service link between the UE and the satellite. The Satellite Radio Interface (SRI) is on the feeder link between the NTN Gateway and the satellite. The SRI (Satellite Radio Interface) is the transport link between the NTN GW and the satellite.

[0070] The satellite payloads also provide the Inter-Satellite Links (ISLs) between the satellites. Inter-Satellite Links (ISLs) are transport links between satellites.

[0071] NTN Timing Advance

[0072] To support long delays in NTN scenarios, it is possible to command a complete timing advance based on two components: a common timing advance value and an individual (i.e., UE-specific) timing advance value. Together, the common TA value and the individual TA value constitute a total TA value. The common TA is used to compensate for the round trip delay (RTD) at the reference point location within the radio cell / beam (e.g., at the closest point of the cell) and may be determined to be twice the transmission delay between the satellite and the reference point. On the other hand, the individual TA is used to control the timing per UE and varies, for example, based on the location of the specific UE.

[0073] Two different satellite scenarios are shown in Figure 13 and Figure 14, where regenerative satellite deployment is shown in Figure 13 and transparent satellite deployment is shown in Figure 14. The delay corresponding to the illustrated delay d1 can be compensated by a common TA value, calculated at the illustrated reference point. Meanwhile, for a different position in the cell (with total delay d2), a differential delay corresponding to d3 (d3=d2-d1) can be further compensated by an individual TA. Thus, the common TA value is the same as the total TA value when the UE is located at the reference point.

[0074] It is exemplarily assumed that the gNB responds with an individual TA in a random access response message during the RACH procedure. The common TA value can be broadcast, for example, in the system information of the radio cell. Thus, a UE that has acquired both parameters can determine the total TA value.

[0075] Correspondingly, the UE can use a common TA when transmitting the random access preamble, and then obtains an individual TA in the RAR. Without applying a common TA when transmitting the RACH preamble, the guard period (GP) in the PRACH transmission must be long enough to accommodate the NTN RTD (which is very long), and thus may not be easily possible. This is different from the non-NTN scenario, in which, when performing the RACH procedure, the UE usually does not apply a timing advance when transmitting the RACH preamble, but rather follows the downlink timing.

[0076] In addition, the common TA may be indicated per radio cell or per beam. In the following, it is exemplarily assumed that the common TA is valid for the whole cell. However, if the common TA value is per beam of the radio cell, multiple common TA values ​​each corresponding to one of the multiple beams may be provided. Then, the UE selects one of the beam-specific common TA values ​​based on, for example, SSB selection (Synchronization Signal Block selection).

[0077] As a further exemplary implementation, the common TA should be understood as a parameter that can be signaled as a time offset value to compensate for the RTT (Round Trip Time), which may also be used for other purposes (e.g., not necessarily in the same form as "common TA") and may be referred to differently.

[0078] Handover Procedure

[0079] A typical simplified handover is illustrated in Figure 15 and briefly described below. Handover of a UE involves the source base station making a decision whether or not to handover the UE to a neighbouring radio cell. The decision to handover a UE from a source cell to a target cell is typically made by the source gNB, for example based on measurement results from the UE (conveyed in one or more measurement reports).

[0080] Then, between the two involved base stations (source base station and target base station), a handover is prepared (preparation phase) by the source gNB sending a handover request message to the target base station and the target gNB responding with a handover request acknowledgement message. Then, in the execution phase, the UE is instructed to switch from the source cell to the target cell through a handover command message (RRCReconfiguration message), which includes, for example, reconfiguring the UE's radio resources to establish a connection with the target base station of the target radio cell. The UE thus performs its reconfiguration and attaches to the new target base station. This includes performing synchronization and (contention-free) random access procedures. For example, synchronization may include the UE acquiring synchronization signals (e.g., Primary Synchronization Signal (PSS) and Secondary Synchronization Signal (SSS)) normally used to acquire the cell identity and frame timing of the target cell, thereby achieving time and frequency synchronization. A random access procedure is performed by the UE, for example to obtain a timing advance value and to acquire uplink resources for uplink transmission in an RAR message. This consists of at least transmitting a RACH preamble (along with what the preamble indicated in the contention-free RACH handover command message), receiving a random access response (e.g., including an uplink grant), and finally, as the third and final step of the random access procedure, the UE confirming that the reconfiguration and access to the target cell is complete by sending a handover confirmation message (RRCReconfigurationComplete message) to the target gNB.

[0081] An exemplary implementation of such a handover procedure is defined for 5G NR in clause 9.2.3 of 3GPP TS 2010-010521.

[0082] RACH-less Handover for NTN

[0083] Discussions are underway to enhance the mobility of NTN. The inventors have recognized that for non-terrestrial communications, the round trip delay (RTD) can be significantly larger than for terrestrial communications. For example, the maximum RTD of NTN is 541.1 ms for GEO (Geostationary Earth Orbiting, e.g., at an altitude of 35786 km) and 25.76 / 41.76 ms for LEO (Low Earth Orbiting, e.g., at an altitude of 600 / 1200 km). For terrestrial communications, the RTD can be, for example, up to 5 ms.

[0084] A long RTD may result in increased handover latency. Therefore, a specific solution is needed to minimize or reduce the latency during handover for mobile UEs, especially for real-time applications.

[0085] One possible solution is a RACH-less handover as described below.

[0086] One exemplary assumption for a RACH-less handover procedure is that the source and target cells are time synchronized such that the subframe boundaries between them are aligned. For example, according to the strictest time synchronization, the subframe boundaries are synchronized down to the symbol timing, e.g., following the same system frame number, same slot number, and same symbol number. However, less strict time synchronization is also possible, e.g., down to the same system frame number or down to the same slot number.

[0087] A RACH-less handover may be similar to the legacy handover described above in relation to, for example, FIG. 15, but with some important differences, as will be explained with reference to FIG. 16.

[0088] First, the UE needs to know whether to perform a RACH-less handover or another type of handover (such as, for example, the legacy handover procedure of FIG. 15). One possibility is to configure the UE accordingly to perform a RACH-less handover. In this case, for example, the source base station can provide the UE with corresponding configuration information. Another possibility is that the handover command message provides the UE with a corresponding indication of whether to perform a RACH-less handover procedure (or not, for example, a legacy handover including a random access procedure). For example, to coordinate the uplink transmission of a handover confirmation message to the target base station, the handover command message may include an indication of the uplink resources used by the UE for the transmission of the handover confirmation message. This resource indication is not necessarily required for legacy handover procedures, since an appropriate UL grant is usually conveyed by the Random Access Response message of the RACH procedure. Thus, if the UE determines that the handover command message includes a UL grant, the UE may determine that the source base station has instructed the UE to perform a RACH-less handover. Otherwise, if no UL grant is present in the handover command message, the UE may determine to perform another type of handover, such as, for example, the legacy handover procedure of Figure 15. Alternatively or additionally, the handover command message may simply include one or more bits that encode the type of handover procedure that the UE should perform.

[0089] The UL grant in the handover command message can be understood as indicating a specific time (e.g., a specific SFN) that can be mutually agreed upon between the source base station and the target base station. For example, the target base station provides the UL grant information in a handover confirm message to the source gNB.

[0090] After receiving the command to handover to the target cell, the UE may proceed to synchronize with the target gNB. On the other hand, assuming that the source and target gNBs are (fully) synchronized, the UE may not even need to acquire the PSS and SSS for time and frequency synchronization. Furthermore, the UE does not initiate a normal RACH procedure and therefore does not transmit a random access preamble. Correspondingly, the UE does not need to perform a random access procedure (or at least a full random access procedure).

[0091] Instead, at said mutually agreed time, the UE will handover from the source radio cell to the target radio cell, and therefore no RACH procedure is initiated and the handover procedure proceeds with the UE confirming the handover to the target cell by sending a HANDOVER CONFIRM message using the UL resources indicated in the HANDOVER COMMAND message.

[0092] Eliminating the RACH delay during the handover procedure can significantly reduce data interruption during handover and improve user experience significantly, which is especially important in NTN scenarios with long RTD assumptions.

[0093] However, one of the main purposes of the RACH procedure during handover is to match the target and uplink timing by performing a random access procedure and obtaining a target cell timing advance value (target cell TA value) to be used by the UE for uplink transmissions in the target cell. However, in the absence of a RACH procedure, the UE does not obtain a timing advance value in the RAR message. Therefore, the UE has to determine the target cell TA value in a different way.

[0094] According to one possible solution, the UE can estimate the required TA value of the gNB based on the satellite ephemeris and the UE location. For example, the UE must estimate the target cell TA value before sending the RRCReconfigurationComplete message to the target cell to ensure that the message can be correctly received at the target node. First, the UE observes the DL propagation delay difference between the source cell and the target cell, e.g., T2-T1 (or T1-T2), where T1 is the DL propagation delay between the UE and the source gNB and T2 is the DL propagation delay between the UE and the target gNB. Also, assume that the UL propagation delay is the same as the DL propagation delay. Under this assumption, the UE can derive the target cell timing advance based on the source cell timing advance as follows:

[0095]

number

[0096] However, the inventors have identified a need for improvements in time alignment and RACH-less handover procedures in general, particularly for NTN scenarios.

[0097] The estimation of the target TA value described above (see Equation 1) has drawbacks when applied to the RACH-less handover procedure in NTN.

[0098] Conventional RACH-less HO with normal TA estimation may cause the following problems due to the lack of timing advance correction (e.g., common and individual TA). First, the UE cannot perform RACH immediately after HO to the target cell because the common TA of the target cell is not yet obtained. Also, the UE may not be able to fall back to use contention-based HO if the contention-free HO is not successful because the common TA of the target cell is not yet known. Therefore, when the UE receives the updated common TA of the target cell (e.g., in the system information) after completing the handover, the UE does not know how to adjust the TA while performing UL transmission because the individual TA is still unknown to the UE.

[0099] Furthermore, the UE may not be able to observe the true DL timing difference between the source and target cells because there may be a "more difference than SFN" situation in the NTN due to the large RTD. For example, if the synchronization signal from the source cell is from SFN n and the synchronization signal from the target cell is from SFN n-1, these two synchronization signals arrive very close in time to the UE. The UE then thinks that the DL timing difference between these two cells is very small, even though the actual DL timing difference is already as large as 10 ms. This scenario is possible when the distance between the source cell and the UE is much longer than the distance between the target cell and the UE, or vice versa.

[0100] In addition, such a solution is not an optimal solution for GNSS-equipped UEs.

[0101] Additionally, feeder link delays and satellite processing delays in transparent satellite scenarios must be considered.

[0102] Furthermore, the assumption that the DL and UL propagation delays are the same may be incorrect, especially for the transparent satellite case, where the DL and UL propagation delays may be different, because the propagation delay covers both the feeder link delay and the satellite processing delay, which may be different for the DL and UL cases.

[0103] Accordingly, the inventors have identified a possibility to improve the handover procedure to be performed to hand over a UE from a source base station to a target base station. Such an improved handover procedure may facilitate overcoming one or more of the problems set forth above.

[0104] In the following, UEs, base stations, and procedures that meet such needs are described for new radio access technologies envisioned for 5G mobile communication systems, but which may also be used in LTE mobile communication systems. Various implementations and variations are also described. The following disclosure has been facilitated by, and may be based, for example, at least in part on, the above discussion and discoveries.

[0105] In general, it should be noted that many assumptions are made herein to allow the principles underlying the present disclosure to be explained in a clear and understandable manner. However, these assumptions should be understood as merely examples made herein for illustrative purposes, without limiting the scope of the present disclosure. Those skilled in the art will recognize that the principles of the following disclosure and claims can be applied to various scenarios and in ways not explicitly described herein.

[0106] Furthermore, some of the terms such as procedures, entities, layers, etc. used below are closely related to the terms used in the LTE / LTE-A system or in the current 3GPP 5G standardization, even though the specific terms used in the context of new radio access technologies for the upcoming 3GPP 5G communication system have not yet been fully determined or may eventually change. Therefore, the terms may be changed in the future without affecting the functionality of the embodiments. Therefore, those skilled in the art will recognize that the embodiments and their scope of protection should not be limited to the specific terms used illustratively in this specification due to the lack of other more recent or finally agreed upon terms, but should be more broadly understood in terms of the functions and concepts underlying the functions and principles of the present disclosure.

[0107] For example, a mobile station or mobile node or user terminal or user equipment (UE) is a physical entity in a communication network. A node may have several functional entities. A functional entity refers to a software or hardware module that implements and / or provides a given set of functions to other functional entities of the same or other nodes or networks. A node may have one or more interfaces that connect the node to communication facilities or media that allow the node to communicate. Similarly, a network entity may have logical interfaces that connect functional entities to communication facilities or media that allow the node to communicate with other functional entities or corresponding nodes.

[0108] The term "base station" or "radio base station" as used herein refers to a physical entity in a communication network. Similar to a mobile station, a base station may have several functional entities. A functional entity refers to a software or hardware module that implements and / or provides a given set of functions to other functional entities of the same or other nodes or networks. The physical entity performs several control tasks for the communication devices, including one or more scheduling and configuration. It should also be noted that base station functions and communication device functions may be integrated in a single device. For example, a mobile terminal may also implement the functions of a base station for other terminals. The term used in LTE is eNB (or eNodeB), while the term currently used in 5G NR is gNB.

[0109] 17 shows a general and simplified exemplary block diagram of a user equipment (also called a communication device) and a scheduling device (here illustratively assumed to be located in a base station, e.g., in an eLTE eNB (alternatively called ng-eNB) or in a gNB in ​​5G NR). The UE and eNB / gNB communicate with each other over (radio) physical channels using their respective transceivers.

[0110] A communication device may have a transceiver and a processing circuit. The transceiver may then have a receiver and a transmitter and / or function as a receiver and a transmitter. The processing circuit may be one or more hardware, such as, for example, one or more processors or any LSI. Between the transceiver and the processing circuit, there are input / output points (or nodes) through which the processing circuit can control the transceiver during operation, i.e., control the receiver and / or the transmitter and exchange receive / transmit data. The transceiver may include, as a transmitter and receiver, an RF (radio frequency) front including one or more antennas, amplifiers, RF modulators / demodulators, etc. The processing circuit may implement control tasks such as controlling the transceiver to transmit user data and control data provided by the processing circuit and / or to receive user data and control data that are further processed by the processing circuit. The processing circuit may also be responsible for performing other processes, such as, for example, judging, deciding, calculating, measuring, etc. The transmitter may be responsible for performing the transmitting process and other processes related thereto. The receiver may be responsible for performing the receiving process and other processes related thereto (eg, monitoring the channel, etc.).

[0111] In the following, an improved procedure on how to perform the handover procedure is described.

[0112] Figure 18 shows a simplified exemplary UE structure according to one solution of an improved handover procedure, which can be implemented based on the general UE structure described in relation to Figure 17. The various structural elements of the UE shown in this figure can be interconnected with each other, e.g., by means of corresponding input / output nodes (not shown), e.g., for exchanging control and user data and other signals. Although not shown for purposes of illustration, the UE may include further structural elements.

[0113] As can be seen from FIG. 18, the UE may include a common timing advance value receiver, a handover procedure circuit, a first uplink timing determination circuit, a second message timing indication receiver, a UE-specific timing advance value determination circuit, and a second handover message transmitter.

[0114] Thus, in this case, as will become apparent from the disclosure below, the processing circuitry may be exemplarily configured to at least partially perform one or more of: determining a first uplink timing for an uplink transmission to the target base station; determining a UE-specific timing advance value for the target cell; determining a second uplink timing for transmitting a message of the random access procedure; and determining a third uplink timing for transmitting uplink data, etc.

[0115] Thus, the receiver may be exemplarily configured to at least partially perform one or more of: receiving a first message from a source base station of the handover procedure, the first message including a common timing advance value and a timing instruction for transmitting a second message of the handover procedure; receiving a third message from a target base station of the handover procedure, the third message including information regarding a UE-specific timing advance value for the UE and the target cell;

[0116] Thus, the transmitter may be exemplarily configured to at least partially perform one or more of: transmitting a second message of the handover procedure to the target base station based on a predetermined uplink transmission timing; transmitting a message of the random access procedure to the target base station at the second uplink timing; and transmitting uplink data to the target base station at a third uplink timing.

[0117] One solution disclosed in more detail below is implemented by a UE including: a receiver of the UE receives a common timing advance value for a target radio cell from a source base station of a source radio cell. The UE is connected to the source radio cell and is involved in a handover procedure for handing over the UE from the source radio cell to the target radio cell. The common timing advance value is also received from the source base station in a first message of the handover procedure, the first message of the handover procedure also including a timing indication for transmitting a second message from the UE to the target base station. A processor of the UE then determines a first uplink timing of an uplink transmission to the target base station for a downlink transmission from the target base station based on the received common timing advance value and the timing indication. A transmitter of the UE transmits a second message of the handover procedure to the target base station based on the determined uplink timing. The processor determines a UE-specific timing advance value, specific to the UE and the target radio cell, used by the UE to perform an uplink transmission in the target radio cell.

[0118] As a further example implementation, the common TA may be signaled as a time offset value to compensate for the RTT (Round Trip Time), which may be used for other purposes (e.g., not necessarily in the same form as the "common TA").

[0119] Correspondingly, the UE already has knowledge of the common TA that it must use for communication in the target cell while still connected to the source BS. This facilitates overcoming the above-mentioned problems related to the fact that the UE does not obtain the common TA value if it does not read the system information of the target base station. For example, subsequently transmitted messages (e.g., of the handover procedure) are more likely to be correctly received by the target base station if they are time-aligned based on the common TA value. Since the common target TA has already been obtained during the handover procedure, the UE can perform the random access procedure immediately after completing the handover procedure.

[0120] Additionally, the solution includes determining a UE-specific TA value for the UE to be used in the target cell, which facilitates communication in the target cell, and allows the total TA to be adjusted and updated since the individual TA value is known.

[0121] Overall, the advantages of the RACH-less handover procedure of speeding up handover and thus significantly reducing handover interruption time by eliminating the random access procedure are maintained, while utilizing the obtained common TA value and the determined UE-specific TA value to align uplink communications in the target cell, thereby maintaining a high handover success rate even in the absence of RACH.

[0122] In the following, the above solution as well as variations and further improvements of the above solution will be explained in more detail.

[0123] For the following description of the improved handover procedure solution, some basic assumptions are exemplarily made. First, an NTN scenario is assumed, where the UE communicates via a satellite in a source radio cell and the neighboring radio cells as possible targets of the handover are non-terrestrial networks as described above. Furthermore, it is assumed that the UE supports handovers without performing a random access procedure and performs such a handover, e.g. a RACH-less handover as described above.

[0124] An exemplary implementation of this solution is described with reference to Figures 19, 20 and 21.

[0125] FIG. 19 is a sequence diagram of an exemplary UE operation, FIG. 20 is a sequence diagram of an exemplary source base station operation, and FIG. 21 shows message exchanges between the UE, the source BS, and the target BS implementing an improved handover procedure. The handover procedure is improved by providing a common TA value of the target cell in a first handover message transmitted from the source base station to the UE. The common TA value is valid in the target cell to which the UE is handed over. For example, the common TA value is common to all UEs in the target cell and is calculated with respect to a reference point (e.g., the shortest distance to a satellite or the center of the radio cell) in the target radio cell. On the other hand, a UE-specific TA value (also called an individual TA value), which will be described later, is specific only to a particular UE and its location and should be considered in addition to the common TA value. Exemplarily, the common TA value and the individual TA value should be understood as described above in connection with FIG. 13 and FIG. 14.

[0126] Furthermore, the first handover message also conveys a timing instruction for transmitting a second handover message by the UE to the target base station, thus providing the UE with a specific time to transmit the second handover message, thus proceeding with the execution of the handover procedure to the target base station. The timing instruction may be provided in the form of an uplink grant that schedules the UE an uplink resource to be used for transmitting the second handover message. Both the timing instruction and the common TA value are used to determine the actual time at which the UE transmits the second handover message. For example, the time point according to the timing instruction for transmitting the second handover message is corrected based on the common TA value to (at least partially) align the UE's uplink transmission with other uplink transmissions from other UEs to the target base station.

[0127] Thus, by providing a common TA value and uplink transmission timing for transmitting the second handover message, the handover procedure facilitates successful reception of the second handover message at the target base station.

[0128] For example, the timing indication may be implemented the same as or similar to the uplink grant provided in the random access response message (see FIGS. 7 and 8) during the random access procedure.

[0129] The UE may then send a second handover message to the target base station at a predetermined uplink transmission timing.

[0130] Following the transmission of the second handover message, the UE may determine a UE-specific TA value for the target cell in order to be able to more closely align the uplink transmission timing for the next uplink transmission. The UE-specific TA value is more important for correct uplink transmission time alignment when the UE is located far away from the reference point for calculating the common TA value. Conversely, when the UE is located at or near the reference point for calculating the common TA value, the UE may not even need to determine a UE-specific TA value for the target cell, because the UE-specific TA value is very small or zero and the common TA value is almost the same as the total TA value in the target cell. Thus, the UE may decide not to determine a UE-specific TA value, for example, when it is informed that it is located at or close enough to the reference point for calculating the common TA value in the target cell. The corresponding information may be provided, for example, by the source base station. However, in the following, it is assumed that the UE determines a UE-specific TA value in order to further improve the timing advance alignment of its uplink transmission.

[0131] In either case, by determining a timing advance value in this manner (e.g., determining a common TA value and a UE-specific TA value), the UE can align its uplink transmission timing and therefore improve uplink transmissions during the handover procedure (using only the common TA value) as well as after the handover procedure (using the common TA value and possibly also the UE-specific TA value).

[0132] More specifically, the UE may perform a random access procedure with the target base station at some point after completing the handover procedure. To enable the target base station to properly determine the required UE-specific timing advance value, the UE may transmit messages for the RACH procedure based only on the common timing advance value, e.g., without using a previously determined UE-specific TA value.

[0133] Furthermore, the UE may use the common TA value and the UE-specific TA value in determining an uplink transmission timing associated with transmitting uplink data to the target base station. For example, the target base station schedules uplink radio resources to the UE that the UE may use to transmit data on the uplink. The UE then aligns the transmission times provided by the scheduled UL radio resources based on the common TA value and the UE-specific TA value.

[0134] In the above description, the first handover message and the second handover message are mentioned as part of the solution presented with reference to Figures 19, 20, and 21. The first handover message can be considered as a handover command message that instructs the UE how to reconfigure the radio resource configuration of the UE to establish a connection to the target radio cell. From the UE's point of view, the handover command message is therefore the first message of the handover procedure. In an exemplary implementation of the solution in a 5G NR standardization scenario, the handover command message can be implemented as an RRCReconfiguration message defined in the corresponding 3GPP TS 2010-010621 (see also Figure 15 and the corresponding description). In case of reusing the already defined RRCReconfiguration message, its content has to be extended to be able to carry additional information regarding the timing indication (uplink grant) and the common TA value. For example, the RRCReconfiguration message can be extended to carry the above-mentioned Timing Advance Command MAC CE, whose value is then taken by the UE as the common TA value of the target cell. For example, the value of the TAC MAC CE may be placed in the secondaryCellGroup information element of the RRC Reconfiguration message, and the uplink grant may also be placed in the secondaryCellGroup information element of the RRC Reconfiguration message.

[0135] Meanwhile, the second handover message sent from the UE to the target base station can be considered as a handover confirm message indicating to the target base station that the establishment of the UE's connection to the target base station is complete. In an exemplary implementation of the solution in a 5G NR standardization scenario, the handover confirm message can be implemented as an RRCReconfigurationComplete message defined in the corresponding non-patent document 10. No additional modifications to the already defined RRCReconfigurationComplete of the 3GPP standard appear to be necessary for the implementation of the handover confirm described as part of the above solution.

[0136] Above it was explained that the UE receives the common TA value of the target cell from the source cell in a handover message. The source base station can then determine the common TA value of the target cell. This can be done in several ways. According to one solution, the target base station provides its neighbouring base stations with information about the common TA value it is using and broadcasting in its radio cell. For example, the target BS can provide the common TA value when it is asked to be a handover target, for example in a handover acknowledgement message sent by the target base station to the source base station in response to the handover request message. According to another solution, the target base station periodically and in advance broadcasts its common TA to its neighbouring base stations as part of a background information exchange.

[0137] In a further exemplary implementation of the improved handover procedure described above, the handover can be decided by the source base station based on measurements performed by the UE in the same or similar manner as described above for the legacy handover procedure of FIG. 15 and the RACH-less handover of FIG. 16. This exemplary variant of the improved handover procedure is reflected in FIG. 22, which shows a measurement report from the UE to the source base station and a subsequent decision by the source base station to handover the UE to the target base station. In response, the source base station transmits a handover request message to the target base station. It is assumed that the target base station approves the handover and responds with a handover acknowledgement message. In this exemplary implementation of FIG. 22, it is assumed that the common TA value is transmitted by the target base station to the source base station in the handover acknowledgement message. In addition, the handover acknowledgement message also indicates the radio resources that the UE is to use to transmit a second handover message to the target base station (see UL grant). These radio resources are to be understood as the mutually agreed times at which the UE will transmit a handover confirm message to the target base station, after which these radio resources are transferred by the source base station to the UE in a handover command message, as already described above.

[0138] The operation of the source base station shown in Fig. 20 shows that the source base station takes part in the improved handover procedure, as already clear from the above description. In particular, the source base station determines the common timing advance value and the UL timing at which the UE transmits the second handover message based at least on corresponding information, e.g. contained in the handover acknowledgement message received from the target base station. The source base station then subsequently forwards the information to the UE in the first handover message, as already mentioned above.

[0139] As can be seen from Figure 22, and in line with what has already been described in relation to Figure 21, the source base station proceeds with the handover by sending a handover command message to the UE, which causes the UE to switch its connection to the target cell.

[0140] Furthermore, the improved handover procedure may also exemplarily include a further step in which the UE decides whether to perform the improved handover procedure or another type of handover (such as, for example, the legacy handover procedure of FIG. 15). This may be implemented, for example, in a similar manner as described with respect to the RACH-less handover procedure of FIG. 16, i.e., based on the contents of the handover command message. For example, the UE decides to perform the improved handover procedure if the handover command message includes one or more of a UL grant and a common TA value. On the other hand, the UE may decide to perform the legacy handover procedure (see FIG. 15) if the handover command message includes neither a UL grant nor a common TA value. According to yet another implementation, the UE may decide to perform the RACH-less handover procedure (see FIG. 16) if the handover command includes a UL grant but does not include a common TA value.

[0141] Thus, although not shown in Figure 20, the source base station can control which type of handover procedure the UE should perform and then adapt the content of the handover command message according to the decision. The source base station may decide to perform an improved handover procedure or a legacy handover procedure, for example, depending on the location of the UE. If the UE is far from the reference point where the common TA value is calculated, the source base station may decide to perform a legacy handover procedure instead, because the source base station wants to ensure that the target base station can correctly decode the handover confirmation message.

[0142] The source base station can determine the location of the UE based on one or more of the base station location information, the UE's current serving beam, the beam angle of the UE serving beam, and the UE's measurement report.

[0143] The UE operation after the handover procedure may also include the UE updating the common TA value, for example, based on system information broadcasted in the target radio cell. Thus, when the UE acquires system information in the target cell with an updated common TA value, the UE performs further operations using the updated common TA value instead of the previously acquired common TA value. For example, when performing a random access procedure, the UE may use the updated common TA value, and when transmitting uplink data, the UE may use the updated common TA value + UE-specific TA value.

[0144] According to further exemplary implementation forms of the improved handover procedure, several different solutions are presented as to how the UE implements the process of determining a UE-specific timing advance value specific to the UE and to the target radio cell (see, for example, the last steps of Figures 19, 21 and 22).

[0145] According to a first solution shown in FIG. 23, the UE obtains information on the UE-specific TA value directly from the target base station in a further message of the handover procedure after the UE establishes a connection to the target base station. Correspondingly, the UE can determine the UE-specific TA value based on the received information. According to one exemplary solution, the UE-specific TA value can be conveyed in an additional message of the improved handover procedure, for example, called an RRCReconfigurationCompleteACK message. The RRCReconfigurationCompleteACK message is a message sent by the target base station to the UE after receiving an RRCReconfigurationComplete message from the UE. For example, the RRCReconfigurationCompleteACK message can be sent by the target base station to acknowledge receipt of the RRCReconfigurationComplete message from the UE and convey the individual TA value. According to an exemplary implementation of the improved handover procedure in the 5G NR standard, the RRCReconfigurationCompleteACK message includes the above-mentioned Timing Advance Command MAC CE, thereby terminating the improved handover procedure. TAC MAC The value indicated to the CE is the individual TA (ITA target )

[0146] Before transmitting the UE-specific TA value to the UE, the target base station needs to determine the UE-specific TA value. It should be noted that the RRCReconfigurationComplete message is transmitted by the UE based on the previously obtained common TA value and using the radio resource indicated by the UL grant. According to one exemplary implementation, if the target base station can decode the RRCReconfigurationComplete message and find that the starting position of the RRCReconfigurationComplete message is Tms later than the starting position indicated to the UE by the UL grant, the target base station knows that the UE-specific TA value is equal to Tms.

[0147] Furthermore, according to another example implementation, the target base station can configure a UL grant that is larger (in the time domain) than the size required to transmit the RRCReconfigurationComplete message, thereby facilitating the target base station to ensure that the RRCReconfigurationComplete message does not overlap with other messages of other UEs in the time domain, thereby facilitating that the message can be correctly decoded by the target base station.

[0148] As is evident from the explanation given in relation to Figures 13 and 14 above, the individual TA value becomes more relevant the further away from the reference point where the UE calculates the common TA value. Thus, if the UE is located close to the reference point, the individual TA value may not even be necessary to perform the handover successfully, and the RRCReconfigurationCompleteACK message may be optional. On the other hand, if the UE is located far away from the reference point, the individual TA value is necessary to correct the uplink time alignment and facilitate the further messages to be correctly received at the target base station. In such a case, the RRCReconfigurationCompleteACK message is indeed necessary.

[0149] Compared to the second solution (see below), the first solution has the advantage that the UE does not need to observe the DL timing difference (DL propagation time difference) between the source and target cells and does not need to estimate an individual timing advance for the target cell based on the observed timing difference, which would be time and processor consuming.

[0150] According to a second solution for determining the UE-specific TA value in the UE, the UE does not receive information on the UE-specific TA value from the target base station, but makes some assumptions and calculates the UE-specific TA value of the target cell taking into account the information available in the UE. For this purpose, the UE uses the following parameters: Common timing advance value of the target radio cell UE-specific timing advance value of the source radio cell The common timing advance value of the source radio cell Downlink propagation time difference between the source and target base stations

[0151] Based on these parameters, the UE can determine a UE-specific TA value of the target cell. The common TA value of the target cell is obtained, for example, in a handover command message from the source base station, as described above. Also, the common TA value and the UE-specific TA value of the source radio cell are known to the UE from previous communication in the source radio cell. Furthermore, the downlink propagation time difference between the source base station and the target base station can be observed by the UE based on the signals received from the source base station and the target base station, respectively. Based on the interrelationship of these parameters, the above-mentioned Equation 1(TA target =TA source Similarly, the UE can determine a UE-specific TA value. target is CTA_target+ITA_target, and TA source is CTA_source + ITA_source.

[0152] In one exemplary embodiment, the calculation of the UE-specific timing advance value for the UE and the target radio cell is based on the following equation:

[0153]

number

[0154] The parameter ITA_target is a UE-specific timing advance value for the UE and the target radio cell being calculated. The parameter CTA_source is a common timing advance value for the source radio cell, which is known to the UE from communication with the source base station.

[0155] ITA_source is the UE-specific timing advance value of the source radio cell, CTA_target is the received common timing advance value of the target radio cell, and (T1-T2) is the downlink propagation time difference between the source base station and the target base station.

[0156] The second solution for determining the UE-specific timing advance value for the target cell can be performed, for example, as shown in Figures 21 and 22, i.e. after the transmission of the handover confirmation message. However, since this second solution does not depend on the transmission of a message from the target base station as in the first solution, the second solution allows the UE-specific TA value to be determined at a much earlier time. According to a further variant of the improved handover procedure shown in Figure 24, the UE performs a step of determining the UE-specific TA value according to the second solution before the UL transmission time of transmitting the handover confirmation message. Thus, the UE can determine the UL transmission timing of the second handover message (here, for example, the handover confirmation message) based on the UE-specific TA value in addition to the UL grant information and the common TA value of the target cell.

[0157] The second solution has the advantage that the final TA applied to the handover complete message transmission (RRCReconfigurationComplete message in another exemplary implementation) can be very close to the actual TA experienced by the UE, since the determination of the individual TA value is based on detailed measurements performed at the UE and can already be applied to determine a time-aligned uplink transmission timing. Thus, the handover complete message transmitted by the UE with this time-aligned uplink transmission timing is likely to be successfully decoded by the target base station.

[0158] One possible drawback of the second solution is that the observation of the DL propagation time difference may not provide correct results: for example, the DL timing difference between the source and target cells may be larger than the SFN (system frame number) difference (e.g., larger than 10 ms), and in such a case, the UE may misinterpret the actual DL timing difference because it does not know the SFN of the target cell when performing the corresponding measurement (or observation).

[0159] A possible way to avoid the above drawback is for the source base station to instruct the UE to perform a legacy handover procedure in such a case, instead of performing an improved handover procedure. The source base station can determine whether this "greater than SFN difference" situation occurs, for example according to the approximate distance from the source base station to the UE and the approximate distance from the target base station to the UE. These approximate distances can be obtained approximately based on the satellite and UE positions, and possibly also on the beam angle information of the beams used by the source base station to serve the UE. Thus, when the source base station checks such a situation, it provides a corresponding instruction in a handover command message to the UE to perform a legacy handover procedure (e.g. one of those described in connection with Figure 15 or Figure 16) without performing an improved handover procedure. This can be done by appropriately generating the contents of the handover command message, so that the UE can derive which type of handover to perform. As mentioned above, this can be coded in the handover command message based on one or more bits, or it can be coded by the presence or absence of an uplink grant and / or a common TA value.

[0160] Furthermore, the following exemplary variants of the improved handover procedure facilitate overcoming the above drawbacks by enabling the UE to determine whether such a "greater than SFN difference" situation exists. According to a first variant, the UE can compare the common TA values ​​of the source and target cells to determine whether such a "greater than SFN difference" situation exists. For example, if the common TA value of the source radio cell is greater than the common TA value of the target radio cell but the observed DL time difference is negative (i.e., CTA_source>CTA_target and T1-T2<0), the observed DL propagation difference is likely to be incorrect, because the common TA value indicates that the source radio cell is further away than the target radio cell, while the observed DL timing difference indicates that the target radio cell is further away than the source radio cell.

[0161] According to a second variant of determining whether there is a "greater than SFN difference" situation, the UE acquires system information from a target radio cell (e.g., one or more of SIB, MIB, PBCH) to acquire a system frame number (SFN) of the target radio cell. Based on the acquired SFN_target, the UE can determine that there is a "greater than SFN difference" situation if the SFN_target is not the same as the SFN_source. Otherwise, there is no "greater than SFN difference" situation.

[0162] According to this second variant, the UE obtains system information from the target cell. Thus, assuming a solution in which the common TA value is broadcast in the system information by the base station in its radio cell, the UE may similarly obtain the common TA value of the target cell in this manner in addition to or instead of obtaining the common TA value of the target cell in the handover command message from the source base station. In such a case, the handover command message sent from the source base station to the UE may not need to include the common TA value of the target cell.

[0163] If the UE determines such a "greater than SFN difference" situation, the UE may decide not to perform the improved handover procedure (perhaps despite being instructed to do so), but to use another type of handover procedure (e.g., one of the legacy handover procedures described in connection with Figures 15 and 16).

[0164] For the third solution on how to determine a UE-specific TA value by the UE, it is assumed that the UE is equipped with Global Navigation Satellite System hardware (GNSS-equipped UE). As mentioned above, the improved handover procedure solution does not utilize the capabilities of the GNSS-equipped UE. However, the GNSS hardware allows the UE to obtain its own terrestrial position information, so that the UE can determine the total timing advance of the target cell (herein referred to as GNSS_TA_target, for example) based on the UE's position and the ephemeris information of the target satellite that is known to the UE.

[0165] It is further assumed that the source base station provides a common TA value of the target radio cell in the first handover message (as fully explained in the previous solution). The UE can then determine the UE-specific TA value of the target radio cell based on these two parameters, e.g. by ITA_target=GNSS_TA_target-CTA_target. As presented in connection with the second solution, said determination of GNSS_TA_target and thus of the UE-specific TA of the target cell can be performed before the indicated UL transmission timing of the handover confirm message. Thus, the UE can time align the UL transmission timing based on the GNSS_TA_target more accurately than only by the CTA_target.

[0166] Furthermore, although the ITA_target value is not used for determining a time-aligned UL transmission timing of the handover confirm message, by determining the ITA_target value in this way, the UE has sufficient information to determine an updated total TA value when it receives an update of the common TA value of the target radio cell (e.g. via system information) after completing the handover to the target radio cell. Said determined ITA_target value can subsequently be updated, for example by performing a RACH procedure and receiving an updated ITA_target value in a random access response message.

[0167] A particular implementation is shown in Figure 25. As exemplarily assumed in the figure, the UE obtains a common TA value of the target cell. Then, exemplarily, the UE may determine a GNSS_TA_target value (see above). Based on the GNSS_TA_target and the common TA value of the target cell, the UE can determine a UE-specific TA value of the target radio cell. In this exemplary implementation, the UE uses the previously determined GNSS_TA_target and UL grant to determine a time-aligned uplink transmission timing for transmitting a second handover message (here, a handover confirmation message) to the target base station.

[0168] However, determining the GNSS_TA_target value may not always be possible for a GNSS-equipped UE. For example, if there are no visible satellites (e.g., less than four satellites are visible), the UE cannot accurately determine the GNSS_TA_target value. If this situation is identified by the UE, the UE may resort to determining the ITA_target in a different way than the GNSS-based third solution described above, for example by using a second option that involves observing the DL propagation time difference and calculating the ITA_target using Equation 2.

[0169] In the improved handover procedure described above, no distinction was made as to whether the handover is performed in a regenerative or transparent satellite scenario. As explained in more detail with reference to Figs. 11-14, in a regenerative satellite, the gNB is located on the satellite, whereas in a transparent satellite scenario, the gNB is located on the ground and the satellite acts as a repeater. Therefore, for timing advance alignment, the transparent satellite scenario must also take into account the feeder link delay (FD_target) between the satellite (repeater) and the terrestrially located gNB, and the satellite processing delay (PD_target) for processing the incoming UL message from the UE and further forwarding such message to the terrestrial gNB. The additional delays (FD_target and PD_target) to be considered for timing advance are shown in Fig. 26.

[0170] Correspondingly, the target gNB broadcasts in its system information a common TA value that also takes these two additional delays into account. In such a transparent satellite scenario, the common TA value is not only based on the delay d1, but also takes into account the processing delay PD_target and the feeder link delay FD_target, i.e., CTA_target=2×(d1+PD_target+FD_target).

[0171] In other words, the common TA value for the transparent satellite scenario is twice the common TA value for the regenerative satellite case and the additional delays FD_target and PD_target for the transparent satellite case.

[0172]

number

[0173] Therefore, in all of the above improved handover procedures, the common TA value to be used depends on the specific satellite scenario. For example, in a transparent satellite scenario such as in FIG. 26, the common TA value (e.g., CTA_target or CTA_source sent to the UE in the handover command message) further takes into account the processing delay and the feeder link delay. In other words, the parameters CTA_target_transS and CTA_source_transS specific to the transparent satellite (transS) scenario should be used instead of the CTA_target_regS and CTA_source_regS specific to the regenerative satellite (regS) scenario, respectively.

[0174] Meanwhile, in an alternative variant, the timing advance for the transparent satellite scenario is indicated to the UE based on three different parameters, namely the common TA value (taking into account only the d1 delay, called CTA_source_regS or CTA_target_regS above), the feeder link delay (FD_source or FD_target) and the satellite processing delay (PD_source or PD_target). These three parameters for the target cell can then be provided to the UE by the source base station in a handover command message, during which the source base station determines these three parameters, for example from information received in a HO Required Acknowledgement message sent by the target base station to the source base station.

[0175] Correspondingly, the UE then needs to calculate the common TA value for the transparent satellite scenario by itself, for example using the following formula already mentioned above:

[0176]

number

[0177] For some of the above solutions for improved handover procedures, it was assumed that the DL delay (e.g., gNB → satellite → UE) is the same as the UL delay (UE → satellite → gNB). For example, Equation 1 and Equation 2 above are based on this assumption. However, this is not necessarily the case for several reasons, e.g., the DL delay and the UL delay are not the same. For example, the downlink and uplink transmissions may use different frequency bands and / or the satellite processing may be different for the uplink and downlink.

[0178] Therefore, such a solution based on the assumption that "DL delay = UL delay" cannot be used when the UL delay and the DL delay are different. To overcome this limitation, according to a variant of the improved handover procedure described above, the difference between the downlink delay and the uplink delay is further taken into account. For example, in Equation 1(TA target =TA source -2×(T1-T2)) needs to be adjusted.

[0179] The parameter Off_source is a time offset between the DL delay and the UL delay in the source radio cell. This parameter can be obtained by the UE in the source radio cell, for example by system information in an appropriate System Information Block. A positive value of Off_source can be understood to mean, for example, that the UL delay is longer than the DL delay in the source radio cell by an indication of Off_source. Conversely, a negative value of Off_source can be understood to mean, for example, that the UL delay is shorter than the DL delay in the source radio cell by an indication of Off_source.

[0180] Similarly, the parameter Off_target is a time offset between DL delay and UL delay in the target radio cell. This parameter can be obtained by the UE in the source radio cell, for example, by a first handover message (for example, in some implementations, a Handover Command message, in other implementations, an RRC Reconfiguration message), for example together with a common TA value of the target radio cell. A positive value of Off_target can be understood to mean, for example, that the UL delay is longer than the DL delay in the target radio cell by an indication value of Off_target. Conversely, a negative value of Off_target can be understood to mean, for example, that the UL delay is shorter than the DL delay in the target radio cell by an indication value of Off_target.

[0181] Based on the additional parameters Off_source and Off_target, Equation 1 can be modified to obtain Equation 3 below.

[0182]

number

[0183] Correspondingly, in a scenario where the uplink delay and the downlink delay are not the same in a radio cell, instead of using Equation 1, Equation 3 should be used.

[0184] Therefore, Equation 2 is modified to Equation 4, where the parameters TA_target and TA_source in Equation 3 are replaced with (CTA_target+ITA_target) and (CTA_source+ITA_source), respectively.

[0185]

number

[0186] In the above solution based on Equation 2, the UE can use Equation 4 instead of Equation 2 to determine the UE-specific TA value of the target cell. As already explained with respect to Equation 2, the UE must be able to obtain all parameters of Equation 4 to determine the UE-specific TA value ITA_target of the target radio cell. For example, Off_target can be obtained by the UE through the first handover message, as already mentioned with respect to Equation 3.

[0187] Correspondingly, the UE may first determine whether the downlink propagation delay in the source radio cell is the same as the uplink propagation delay in the source radio cell, and whether the downlink propagation delay in the target radio cell is the same as the uplink propagation delay in the target radio cell. According to one exemplary implementation, the UE is informed thereto by the source base station. For example, if the parameters Off_source and Off_target are included in the handover command message, the UE determines that the assumption of UL delay=DL delay is not true and a different formula must be used.

[0188] In particular, depending on the result of the determination, the appropriate formula 1, formula 2, formula 3 or formula 4 is used as described above. In particular, formula 1 and formula 2 can be used when the UL delay and DL delay in the source radio cell and the target radio cell are substantially the same, respectively. On the other hand, formula 3 and formula 4 can be used when the UL delay and DL delay in the source radio cell or the target radio cell are different from each other.

[0189] Further Aspects

[0190] According to a first aspect, a user equipment (UE) comprises a receiver for receiving a common timing advance value for a target radio cell from a source base station of a source radio cell. The UE is connected to the source radio cell and is involved in a handover procedure for handing over the UE from the source radio cell to the target radio cell. The common timing advance value is also received from the source base station in a first message of the handover procedure, the first message of the handover procedure further including a timing indication for transmitting a second message from the UE to a target base station. A processor of the UE then determines a first uplink timing of an uplink transmission to the target base station for a downlink transmission from the target base station based on the received common timing advance value and the timing indication. A transmitter of the UE transmits the second message of the handover procedure to the target base station based on the determined uplink timing. The processor determines a UE-specific timing advance value, specific to the UE and the target radio cell, to be used by the UE for performing an uplink transmission in the target radio cell.

[0191] According to a second aspect provided in addition to the first aspect, the processor determines a second uplink timing for transmitting a random access procedure message to the target base station based on the common timing advance value and not based on the UE-specific timing advance value, and determines a third uplink timing for transmitting uplink data to the target base station based on the common timing advance value and the UE-specific timing advance value.

[0192] According to a third aspect provided in addition to the first or second aspect, the receiver receives a third message of the handover procedure from the target base station, the third message including information regarding the UE-specific timing advance value for the UE and the target radio cell, and the processor determines the UE-specific timing advance value for the UE and the target radio cell from the received information regarding the UE-specific timing advance value.

[0193] According to a fourth aspect provided in addition to any one of the first to third aspects, the processor comprises: the received common timing advance value of the target radio cell, a UE-specific timing advance value of said source radio cell, a common timing advance value of said source radio cell, a downlink propagation time difference between the source base station and the target base station; and determining the UE-specific timing advance value for the UE and the target radio cell based on a calculation using: Also, the calculation is performed prior to determining the first uplink timing of an uplink transmission to the target base station, and the determination of the first uplink timing is further based on the calculated UE-specific timing advance value. In an optional implementation, the calculation of the UE-specific timing advance value for the UE and the target radio cell is based on the following equation:

number

[0194] According to a fifth aspect provided in addition to the fourth aspect, if a downlink propagation delay between the UE and the source base station is not the same as an uplink propagation delay between the UE and the source base station, the determination of the UE-specific timing advance value further takes into account a time offset between the downlink delay and the uplink delay in the source radio cell and a time offset between the downlink delay and the uplink delay in the target radio cell. In an optional implementation, the calculation of the UE-specific timing advance value for the UE and the target radio cell is based on the following formula:

number

[0195] According to a sixth aspect provided in addition to any one of the first to fifth aspects, the UE includes a Global Navigation Satellite System (GNSS) circuit configured to determine a total timing advance value of the target radio cell. Determining the UE-specific timing advance value for the target radio cell is based on the total timing advance value for the target radio cell and the received common timing advance value for the target radio cell. In an optional implementation, the UE-specific timing advance value for the target radio cell is the total timing advance value for the target radio cell minus the received common timing advance value for the target radio cell.

[0196] According to a seventh aspect provided in addition to the sixth aspect, the processor determines whether it is possible to determine the total timing advance value of the target radio cell using the GNSS circuitry, and if not, the processor decides to determine the UE-specific timing advance value of the target radio cell according to the third or fourth aspect, rather than using the GNSS circuitry to determine the UE-specific timing advance value for the target radio cell based on the total timing advance value of the target radio cell.

[0197] According to an eighth aspect provided in addition to any one of the first to seventh aspects, a common timing advance value is common to all UEs in the source / target radio cell and is calculated with respect to a reference point in the source / target radio cell, and a UE-specific timing advance value is specific to one UE in the source / target radio cell and is based on a position of the UE in the source / target radio cell.

[0198] According to a ninth aspect provided in addition to any one of the first to eighth aspects, the processor determines that the first message of the handover procedure instructs the UE to participate in the handover without performing a random access procedure with the target base station. In an optional implementation, the processor determines to perform the handover without performing the random access procedure if the first message includes the timing indication.

[0199] According to a tenth aspect provided in addition to any one of the first to ninth aspects, the first message of the handover procedure including the common timing advance value and the timing indication of the target radio cell is a handover command message of the handover procedure. The handover command message instructs the UE how to reconfigure a radio resource configuration of the UE to establish a connection to the target radio cell. The second message of the handover procedure is an RRCReconfigurationComplete message indicating to the target base station that the radio resource reconfiguration of the UE to establish a connection of the UE to the target base station is completed.

[0200] According to an eleventh aspect provided in addition to any one of the first to tenth aspects, the UE is connected to the source base station located on the ground via a source satellite, and the common timing advance value of the source radio cell takes into account a first delay that occurs in a communication while it is being transmitted from the source satellite and the source base station located on the ground, and takes into account a second delay that occurs in the communication while it is being processed by the source satellite.

[0201] According to a twelfth aspect, there is provided a base station comprising a transmitter for transmitting a common timing advance value for a target radio cell to a user equipment (UE), the UE being connected to a radio cell of the base station, the base station being involved as a source base station in a handover procedure for handing over the UE from the radio cell as a source radio cell to the target radio cell, the common timing advance value being transmitted by the base station as the source base station in a first message of the handover procedure, the first message of the handover procedure further comprising a timing indication for transmitting a second message from the UE to a target base station.

[0202] According to a thirteenth aspect provided in addition to the twelfth aspect, the base station has a processor for determining the common timing advance value of the target radio cell and for determining the timing indication for transmitting the second message from the UE to the target base station. In an optional implementation form, the base station has a receiver for receiving, from the target base station of the target radio cell, information on the common timing advance value of the target radio cell and information on the timing indication for transmitting the second message from the UE to the target base station.

[0203] According to a fourteenth aspect, provided in addition to the twelfth or thirteenth aspects, the base station, as a target base station, is involved in another handover procedure for handing over another UE from another source radio cell to the radio cell as the target radio cell. The base station, in operation as the target base station, comprises the transmitter for transmitting a common timing advance value of the radio cell of the base station to the other source radio base station controlling the other source radio cell in a first message of the other handover procedure. The first message of the other handover procedure further comprises a timing indication for transmitting another message from the other UE to the base station. The base station, as the target base station, comprises a receiver for receiving the other message of the other handover procedure transmitted from the other UE based on the common timing advance value of the radio cell and the timing indication for transmitting the other message.

[0204] According to a fifteenth aspect, there is provided a method, executed by a user equipment (UE), comprising the steps of: receiving a common timing advance value for a target radio cell from a source base station of a source radio cell, the UE being connected to the source radio cell and being involved in a handover procedure for handing over the UE from the source radio cell to the target radio cell, the common timing advance value being received from the source base station in a first message of the handover procedure, the first message of the handover procedure further including a timing indication for transmitting a second message from the UE to the target base station; determining a first uplink timing of an uplink transmission to the target base station relative to a downlink transmission from the target base station based on the received common timing advance value and the timing indication; sending the second message of the handover procedure to the target base station based on the determined uplink timing; determining a UE-specific timing advance value, specific to the UE and the target radio cell, to be used by the UE for performing uplink transmissions in the target radio cell; A method is provided, comprising:

[0205] Hardware and Software Implementations of the Disclosure

[0206] The present disclosure can be realized by software, hardware, or software in cooperation with hardware. Each functional block used in the description of each embodiment above can be realized, in part or in whole, by an LSI such as an integrated circuit, and each process described in each embodiment can be controlled, in part or in whole, by the same LSI or a combination of LSIs. The LSI may be formed as an individual chip, or one chip may be formed to include some or all of the functional blocks. The LSI may include a data input and a data output connected thereto. Here, the LSI may be called an IC, a system LSI, a super LSI, or an ultra LSI depending on the degree of integration. However, the technology for realizing the integrated circuit is not limited to the LSI, and may be realized using a dedicated circuit, a general-purpose processor, or a dedicated processor. In addition, a field programmable gate array (FPGA) that can be programmed after the LSI is manufactured, or a reconfigurable processor that can reconfigure the connection and settings of the circuit cells arranged inside the LSI may be used. The present disclosure can be realized as digital processing or analog processing. If future integrated circuit technologies replace LSI as a result of advances in semiconductor technology or other derived technologies, the future integrated circuit technologies can be used to integrate the functional blocks. Biotechnology is also applicable.

[0207] The present disclosure may be implemented by any type of apparatus, device, or system having communication capabilities, referred to as a communications apparatus.

[0208] Some non-limiting examples of such communications devices include telephones (e.g., cell phones, smartphones), tablets, personal computers (PCs) (e.g., laptops, desktops, notebooks), cameras (e.g., digital still / video cameras), digital players (digital audio / video players), wearable devices (e.g., wearable cameras, smart watches, tracking devices), game consoles, digital book readers, telehealth / telemedicine devices, vehicles (e.g., automobiles, airplanes, ships) that provide communications capabilities, and various combinations thereof.

[0209] Communications devices are not limited to being portable or mobile, but may include any type of apparatus, device, or system that is non-portable or fixed, such as, for example, smart home devices (e.g., appliances, lighting, smart meters, control panels), vending machines, or any other "things" in an "Internet of Things" network.

[0210] Communications may include, for example, the exchange of data via cellular systems, wireless LAN systems, satellite systems, and the like, as well as various combinations thereof.

[0211] A communications device may have devices such as controllers and sensors connected to the communications device to perform the communications functions described in this disclosure. For example, a communications device may have a controller or a sensor that generates control or data signals used by the communications device to perform the communications functions of the communications device.

[0212] Communications equipment may also include infrastructure facilities, such as base stations, access points, or any other equipment, device, or system that communicates with or controls the equipment in the non-limiting examples above.

[0213] Furthermore, the various embodiments may be implemented by means of software modules, which are executed by a processor or directly in hardware. Also a combination of software modules and hardware implementation is possible. The software modules can be stored on any kind of computer-readable storage medium, for example RAM, EPROM, EEPROM, flash memory, registers, hard disks, CD-ROM, DVD, etc. It should be noted further that individual features of the different embodiments may be the subject of another embodiment, individually or in any combination.

[0214] It will be apparent to those skilled in the art that various changes and / or modifications may be made to the present disclosure as set forth in the specific embodiments, and the present embodiments are therefore to be considered in all respects as illustrative and not restrictive.

Claims

1. An integrated circuit for controlling processing of a user equipment (UE), the processing comprising: a receiving process for receiving a common timing advance value for a target radio cell from a source base station of a source radio cell, the UE being connected to the source radio cell and being involved in a handover procedure for handing over the UE from the source radio cell to the target radio cell, the common timing advance value being received from the source base station in a first handover message of the handover procedure, the first handover message of the handover procedure further including a timing indication for transmitting a second handover message of the handover procedure from the UE to a target base station; a determining process for determining a first uplink timing of an uplink transmission to the target base station with respect to a downlink transmission from the target base station based on the received common timing advance value and the timing indication; a sending process of sending the second handover message of the handover procedure to the target base station based on the determined uplink timing; the determination process determining a UE-specific timing advance value, specific to the UE and the target radio cell, to be used by the UE for performing uplink transmissions in the target radio cell. Integrated circuits.

2. The determining process determines a second uplink timing for transmitting a message of a random access procedure to the target base station based on the common timing advance value and not based on the UE-specific timing advance value; the determining process determines a third uplink timing for transmitting uplink data to the target base station based on the common timing advance value and the UE-specific timing advance value.

10. The integrated circuit of claim 1.

3. the receiving process receives from the target base station a third message of the handover procedure, the third message including information on the UE-specific timing advance value for the UE and the target radio cell, and the determining process determines the UE-specific timing advance value for the UE and the target radio cell from the received information on the UE-specific timing advance value.

10. The integrated circuit of claim 1.

4. The process comprises: the received common timing advance value of the target radio cell, a UE-specific timing advance value of the source radio cell, a common timing advance value of said source radio cell, - downlink propagation time difference between the source base station and the target base station; determining the UE-specific timing advance value for the UE and the target radio cell based on a calculation using the calculation is performed prior to determining the first uplink timing of an uplink transmission to the target base station, the determination of the first uplink timing being further based on the calculated UE-specific timing advance value; Optionally, the calculation of the UE specific timing advance value for the UE and the target radio cell is based on the following formula: [0010] where ITA_target is the UE-specific timing advance value for the UE and the target radio cell, CTA_source is the common timing advance value of the source radio cell, ITA_source is the UE-specific timing advance value of the source radio cell, CTA_target is the received common timing advance value of the target radio cell, and (T2-T1) is the downlink propagation time difference between the source base station and the target base station.

10. The integrated circuit of claim 1.

5. if a downlink propagation delay between the UE and the source base station is not the same as an uplink propagation delay between the UE and the source base station, the determination of the UE-specific timing advance value further takes into account a time offset between the downlink propagation delay and the uplink propagation delay in the source radio cell and a time offset between the downlink propagation delay and the uplink propagation delay in the target radio cell, Optionally, the calculation of the UE specific timing advance value for the UE and the target radio cell is based on the following formula: [0025] where Off_source is a time offset between the downlink propagation delay and the uplink propagation delay in the source radio cell, and Off_target is a time offset between the downlink propagation delay and the uplink propagation delay in the target radio cell.

5. The integrated circuit of claim 4.

6. the UE comprises a Global Navigation Satellite System (GNSS) circuit for determining, during operation, a total timing advance value of the target radio cell; determining the UE-specific timing advance value for the target radio cell is based on the total timing advance value for the target radio cell and the received common timing advance value for the target radio cell, and optionally the UE-specific timing advance value for the target radio cell is the total timing advance value for the target radio cell minus the received common timing advance value for the target radio cell.

10. The integrated circuit of claim 1.

7. The determination process determines whether it is possible to determine the total timing advance value of the target radio cell using the GNSS circuitry; If not possible, the determination process determines to determine the UE-specific timing advance value for the target radio cell according to claim 3 or 4, instead of determining the UE-specific timing advance value for the target radio cell based on the total timing advance value of the target radio cell using the GNSS circuit.

7. The integrated circuit of claim 6.

8. a common timing advance value is common to all UEs in the source / target radio cell and is calculated with respect to a reference point in the source / target radio cell; a UE-specific timing advance value is specific to one UE in the source / target radio cell and is based on a position of the UE in the source / target radio cell; 10. The integrated circuit of claim 1.

9. The determination process determines that the first handover message of the handover procedure instructs the UE to participate in the handover without performing a random access procedure with the target base station; Optionally, the decision process decides to perform the handover without performing the random access procedure if the first handover message includes the timing indication.

10. The integrated circuit of claim 1.

10. the first handover message of the handover procedure, comprising the common timing advance value and the timing indication of the target radio cell, is a handover command message of the handover procedure, the handover command message instructing the UE how to reconfigure a radio resource configuration of the UE to establish a connection to the target radio cell; The second handover message of the handover procedure is an RRCReconfigurationComplete message, indicating to the target base station that the radio resource reconfiguration of the UE to establish a connection of the UE to the target base station is completed.

10. The integrated circuit of claim 1.

11. The UE is connected to the source base station located on the ground through a source satellite; the common timing advance value of the source radio cell takes into account a first delay incurred by a communication while being transmitted from the source satellite and the source base station located on the ground, and takes into account a second delay incurred by a communication while being processed by the source satellite; 10. The integrated circuit of claim 1.

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