Patent Publication Number: US-2022240208-A1

Title: User equipment, base station, and method for time synchronization

Description:
BACKGROUND OF DISCLOSURE 
     1. Field of Disclosure 
     The present disclosure relates to the field of communication systems, and more particularly, to a user equipment, a base station, and method for time synchronization. 
     2. Description of Related Art 
     Wireless communication systems, such as the third-generation (3G) of mobile telephone standards and technology are well known. Such 3G standards and technology have been developed by the Third Generation Partnership Project (3GPP). The 3rd generation of wireless communications has generally been developed to support macro-cell mobile phone communications. Communication systems and networks have developed towards being a broadband and mobile system. In cellular wireless communication systems, user equipment (UE) is connected by a wireless link to a radio access network (RAN). The RAN comprises a set of base stations (BSs) that provide wireless links to the UEs located in cells covered by the base station, and an interface to a core network (CN) which provides overall network control. As will be appreciated the RAN and CN each conduct respective functions in relation to the overall network. The 3rd Generation Partnership Project has developed the so-called Long Term Evolution (LTE) system, namely, an Evolved Universal Mobile Telecommunication System Territorial Radio Access Network, (E-UTRAN), for a mobile access network where one or more macro-cells are supported by a base station known as an eNodeB or eNB (evolved NodeB). More recently, LTE is evolving further towards the so-called 5G or NR (new radio) systems where one or more cells are supported by a base station known as a gNB. 
     Technical Problem 
     Time sensitive communication (TSC), as defined in the technical specification (TS) 23.501 is a communication service that provides high reliability and availability to support deterministic communication with critical timing requirements, such as isochronous communication. Some examples of such services are cyber-physical control applications as described in TS 22.104 in the area of industrial internet of things (IoT). 
     According to 3GPP standard Release  16 , to support strict synchronization accuracy requirements of TSC applications, a gNB may signal 5G system reference time information (RTI) to a UE using unicast or broadcast radio resource control (RRC) signaling with a granularity of 10 nanoseconds (ns). An uncertainty parameter may be included in reference time information to indicate RTI accuracy. 
     Propagation delay is a travel time of a frame transmitted between a UE and a gNB, and may be calculated based on a timing advance (TA) value after performing downlink synchronization by decoding the PSS and SSS signal and the uplink PRACH preamble transmission. Time synchronization between a UE and a gNB makes an internal clock of the UE as identical as possible to an internal clock of the gNB based on the reference time information (RTI) provided by the BS and the propagation delay. Propagation delay should be compensated with respect to RTI to meet high synchronization accuracy requirements. 
     SUMMARY 
     An object of the present disclosure is to propose a user equipment, a base station, and method for time synchronization. 
     In a first aspect, an embodiment of the invention provides a time synchronization method executable in a user equipment (UE), comprising:
     obtaining reference time information (RTI) of a first grant master clock domain; and   transmitting the RTI of the first grant master clock domain to a first base station of a second grant master clock domain to allow the first base station to synchronize with the first grant master clock domain.   

     In a second aspect, an embodiment of the invention provides a user equipment (UE) comprising a transceiver and a processor. The processor is connected to the transceiver and configured to execute the following steps: obtaining reference time information (RTI) of a first grant master clock domain; and transmitting the RTI of the first grant master clock domain to a first base station of a second grant master clock domain to allow the first base station to synchronize with the first grant master clock domain. 
     In a third aspect, an embodiment of the invention provides a time synchronization method executable in a base station, comprising:
     receiving reference time information (RTI) of a first grant master clock domain from a user equipment (UE) to allow the base station to synchronize with the first grant master clock domain, wherein the base station is in a second grant master clock domain.   

     In a fourth aspect, an embodiment of the invention provides a base station comprising a transceiver and a processor. The processor is connected to the transceiver and configured to execute the following steps:
     receiving reference time information (RTI) of a first grant master clock domain from a user equipment (UE) to allow the base station to synchronize with the first grant master clock domain, wherein the base station is in a second grant master clock domain.   

     In a fifth aspect, an embodiment of the invention provides a time synchronization method executable in a base station, comprising:
     receiving RTI related information from a user equipment (UE); and   providing the RTI related information to a target base station during a handover operation of the UE from the base station to the target base station.   

     In a sixth aspect, an embodiment of the invention provides a base station comprising a transceiver and a processor. The processor is connected to the transceiver and configured to execute the following steps:
     receiving RTI related information from a user equipment (UE); and   providing the RTI related information to a target base station during a handover operation of the UE from the base station to the target base station.   

     The disclosed method may be implemented in a chip. The chip may include a processor, configured to call and run a computer program stored in a memory, to cause a device in which the chip is installed to execute the disclosed method. 
     The disclosed method may be programmed as computer executable instructions stored in non-transitory computer readable medium. The non-transitory computer readable medium, when loaded to a computer, directs a processor of the computer to execute the disclosed method. 
     The non-transitory computer readable medium may comprise at least one from a group consisting of: a hard disk, a CD-ROM, an optical storage device, a magnetic storage device, a Read Only Memory, a Programmable Read Only Memory, an Erasable Programmable Read Only Memory, EPROM, an Electrically Erasable Programmable Read Only Memory and a Flash memory. 
     The disclosed method may be programmed as a computer program product, that causes a computer to execute the disclosed method. 
     The disclosed method may be programmed as a computer program, that causes a computer to execute the disclosed method. 
     Advantageous Effects 
     The disclosed method may enable synchronization in a target cell and enhance continuity of the synchronization service, even in high mobility environments. The disclosed method may facilitate synchronization in wide areas, such as large automobile assembly factories. The disclosed method provides synchronization in a scenario where a grant master clock is attached to one of a plurality of UEs, or a grant master clock is attached to a gNB. A UE with a grant master clock may be applied in a factory environment. An embodiment of the disclosed method allows updating of timing advance (TA) value, preference of TA adjustment accuracy, and preference of reference time information (RTI) accuracy. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       In order to more clearly illustrate the embodiments of the present disclosure or related art, the following figures will be described in the embodiments are briefly introduced. It is obvious that the drawings are merely some embodiments of the present disclosure, a person having ordinary skill in this field may obtain other figures according to these figures without paying the premise. 
         FIG. 1  illustrates a schematic view of a telecommunication system. 
         FIG. 2  illustrates a schematic view showing signaling between a UE and a base station using synchronization-specific random access channel signals. 
         FIG. 3  illustrates a schematic view showing a time synchronization method according to an embodiment of the invention. 
         FIG. 4  illustrates a schematic view showing synchronization signaling between a UE and a base station including pre-compensated RTI and a compensation indication. 
         FIG. 5  illustrates a schematic view showing an embodiment of the time synchronization method using pre-compensated RTI and a compensation indication. 
         FIG. 6  illustrates a schematic view showing synchronization signaling between a UE and a base station including signaling of propagation delay (PD) compensation conditions. 
         FIG. 7  illustrates a schematic view showing an embodiment of the time synchronization method performing PD compensation according to PD compensation conditions. 
         FIG. 8  illustrates a schematic view showing synchronization signaling between a UE and a base station including signaling of timing advance (TA) validity conditions and TA update requests. 
         FIG. 9  illustrates a schematic view showing an embodiment of the time synchronization method for requesting TA update according to TA validity conditions. 
         FIG. 10  illustrates a schematic view showing periodic RTI delivery from a base station to a UE. 
         FIG. 11  illustrates a schematic view showing synchronization signaling during a handover of a UE from a source base station to a target base station. 
         FIG. 12  illustrates a schematic view showing an embodiment of the time synchronization method performing PD compensation for a target base station using TA of a source base station and a time difference of synchronization signals from the two base stations. 
         FIG. 13  illustrates a schematic view showing synchronization signaling during a handover of a UE from a source base station to a target base station, including time locations of reference signals of the target base station. 
         FIG. 14  illustrates a schematic view showing an embodiment of the time synchronization method performing PD compensation for a target base station using TA of the target base station obtained from the time locations of the reference signals. 
         FIG. 15  illustrates a schematic view showing synchronization signaling during a handover of a UE from a source base station to a target base station using geographic location and beam related information. 
         FIG. 16  illustrates a schematic view showing synchronization signaling during handover of a UE from a source base station to a target base station using RTI requests and RTI related information. 
         FIG. 17  illustrates a schematic view showing an embodiment of the time synchronization method using RTI requests and RTI related information. 
         FIG. 18  illustrates a schematic view showing RTI signaling from one clock domain to another clock domain. 
         FIG. 19  illustrates a schematic view showing an embodiment of the time synchronization method transmitting RTI from one clock domain to another clock domain through a UE. 
         FIG. 20  illustrates a schematic view showing pre-compensated RTI signaling from one clock domain to another clock domain through a UE having a grant master clock. 
         FIG. 21  illustrates a schematic view showing pre-compensated RTI signaling from one clock domain to another clock domain through a UE receiving grant master clock RTI from a base station. 
         FIG. 22  illustrates a schematic view showing an embodiment of the time synchronization method transmitting pre-compensated RTI from one clock domain to another clock domain through a UE. 
         FIG. 23  illustrates a schematic view showing signaling of a time reference point associated with RTI. 
         FIG. 24  illustrates a schematic view showing an embodiment of the time synchronization method using a time reference point associated with RTI. 
         FIG. 25  illustrates a schematic view showing a system for wireless communication according to an embodiment of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     Embodiments of the disclosure are described in detail with the technical matters, structural features, achieved objects, and effects with reference to the accompanying drawings as follows. Specifically, the terminologies in the embodiments of the present disclosure are merely for describing the purpose of the certain embodiment, but not to limit the disclosure. 
     With reference to  FIG. 1 , a telecommunication system including a UE  10   a,  a UE  10   b,  a base station (BS)  20   a,  and a network entity device  30  executes the disclosed method according to an embodiment of the present disclosure.  FIG. 1  is shown for illustrative not limiting, and the system may comprise more UEs, BSs, and CN entities. Connections between devices and device components are shown as lines and arrows in the FIGS. The UE  10   a  may include a processor  11   a,  a memory  12   a,  and a transceiver  13   a.  The UE  10   b  may include a processor  11   b,  a memory  12   b,  and a transceiver  13   b.  The base station  20   a  may include a processor  21   a,  a memory  22   a,  and a transceiver  23   a.  The network entity device  30  may include a processor  31 , a memory  32 , and a transceiver  33 . Each of the processors  11   a,    11   b,    21   a,  and  31  may be configured to implement proposed functions, procedures and/or methods described in the description. Layers of radio interface protocol may be implemented in the processors  11   a,    11   b,    21   a,  and  31 . Each of the memory  12   a,    12   b,    22   a,  and  32  operatively stores a variety of programs and information to operate a connected processor. Each of the transceivers  13   a,    13   b,    23   a,  and  33  is operatively coupled with a connected processor, transmits and/or receives radio signals or wireline signals. The UE  10   a  may be in communication with the UE  10   b  through a sidelink The base station  20   a  may be an eNB, a gNB, or one of other types of radio nodes, and may configure radio resources for the UE  10   a  and UE  10   b.    
     Each of the processors  11   a,    11   b,    21   a,  and  31  may include an application-specific integrated circuit (ASICs), other chipsets, logic circuits and/or data processing devices. Each of the memory  12   a,    12   b,    22   a,  and  32  may include read-only memory (ROM), a random access memory (RAM), a flash memory, a memory card, a storage medium and/or other storage devices. Each of the transceivers  13   a,    13   b,    23   a,  and  33  may include baseband circuitry and radio frequency (RF) circuitry to process radio frequency signals. When the embodiments are implemented in software, the techniques described herein may be implemented with modules, procedures, functions, entities, and so on, that perform the functions described herein. The modules may be stored in a memory and executed by the processors. The memory may be implemented within a processor or external to the processor, in which those may be communicatively coupled to the processor via various means are known in the art. 
     The network entity device  30  may be a node in a CN. CN may include LTE CN or 5G core (5GC) which includes user plane function (UPF), session management function (SMF), mobility management function (AMF), unified data management (UDM), policy control function (PCF), control plane (CP)/user plane (UP) separation (CUPS), authentication server (AUSF), network slice selection function (NSSF), and the network exposure function (NEF). 
     An example of the UE in the description may include one of the UE  10   a  or UE  10   b.  An example of the base station in the description may include the base station  20   a.  Uplink (UL) transmission of a control signal or data may be a transmission operation from a UE to a base station. Downlink (DL) transmission of a control signal or data may be a transmission operation from a base station to a UE. 
     To address the issues of uplink time synchronization, the invention provides a time synchronization method with flexible synchronization accuracy and propagation delay compensation with respect to received reference time information (RTI) under a large coverage range. The disclosed method may be applied to a UE in human to human (H2H) communication or a UE in machine to machine (M2M) or machine type communication (MTC), which may undergo frequent handover among gNBs in a larger operation area to meet synchronization accuracy requirements. The UE in MTC is referred to as a machine equipment (ME). 
     Based on an actual evaluation of time synchronization accuracy over Uu interface between a gNB and a single UE, a timing synchronization error between a gNB and a UE no worse than 540 nanoseconds (ns) is achievable. For small service areas with dense small cell deployments, compensation for propagation delay may not be needed. For larger areas with sparse cell deployments, e.g. a cell with a radius exceeding 200 meters, compensation for propagation delay is required. For moving robot or mobile machine equipment, the mobility issue must be involved in compensating for the delay of propagation. 
     When a TSN clock is located in a base station, such as a gNB, all UEs under coverage of the base station are synchronized with the TSN clock provided by the base station. However, for the case of an uplink synchronization scenario where a TSN clock is located in one of a plurality of UEs, a base station, such as a gNB, needs to receive the TSN clock from the UE and relay the TSN clock to other UEs to achieve UE-to-UE synchronization. In this case, maintaining synchronization accuracy is more challenging. Since two-hop synchronization may cause more synchronization error, propagation delay compensation is required to meet the synchronization accuracy requirement of 1 microsecond (us). 
     In an embodiment of the invention, a UE performs propagation delay compensation taking into account larger coverage and mobility issues while the TSN clock is located in the gNB as well as the scenario where the TSN clock is located in the UE. An embodiment of the invention provides an indication of compensation activation from a base station, such as a gNB. An embodiment of the invention allows a UE to transmit a conditional compensation request to a base station. An embodiment of the invention allows autonomous propagation delay compensation by a UE during handover. An embodiment of the invention allows RTI forwarding by a UE to extend clock synchronization domain. An embodiment of the invention allows a grant master clock in a UE, and the UE may provide clock information to serving base stations for uplink synchronization. In the description, propagation-delay-related value is a value of at least one of timing advance (TA), propagation delay (PD), and the specific value granularity is granularity of at least one of timing advance (TA), propagation delay (PD). Embodiments of the invention are provided in the following. 
     Embodiment 1 
     In an embodiment of the invention, either a UE or a base station performs propagation delay compensation or pre-compensation respectively with respect to RTI 
     Embodiment 1-1 
     A UE derives propagation-delay-related value based on a synchronization-specific random-access channel (RACH) procedure and performs PD compensation. The UE may obtain a propagation delay from a TA value indicated by a base station in a synchronization-specific RACH downlink signal after transmitting synchronization-specific PRACH. The obtained propagation delay between gNB  2  and UE  1  may be approximately half of the indicated timing advance, that is TA/2. The synchronization-specific PRACH transmission is an embodiment of a synchronization-specific uplink signaling. With reference to  FIG. 2  and  FIG. 3 , an embodiment of the invention is detailed in the following: 
     UE  1  receives reference time information (RTI)  100  from gNB  2  (step S 1102 ) via a system information block (SIB), such as SIB 9 , or a unicast radio resource control (RRC) message, such as DLlnformationTransfer message. 
     UE  1  derives a TA value of the corresponding serving gNB  2  via a synchronization-specific RACH procedure. In the synchronization-specific RACH procedure, the UE transmits a synchronization-specific PRACH  101  to the gNB  2  (step S 1103 ). The synchronization-specific PRACH  101  is an embodiment of a synchronization-specific uplink signal and may be allocated dedicated RACH resources or a dedicated preamble which may be provided with less synchronization timing error. The synchronization-specific uplink signal comprises a dedicated preamble, a sounding reference signal (SRS), or an uplink DMRS signal for propagation-delay-related signaling, the propagation-delay-related signaling is associated with at least one of timing advance (TA) and propagation delay (PD) between the UE and a serving base station of the UE  1 . The dedicated preamble, the sounding reference signal (SRS) or the uplink DMRS signal is transmitted based on a predetermined or previously acquired timing advance (TA) or propagation delay (PD). The dedicated preamble is used for a non-contention based RACH procedure during an RRC_CONNECTED state of the UE  1 . The synchronization-specific uplink signal may comprise a request for provision or update of a propagation-delay-related value or reference time information (RTI), and the propagation-delay-related value comprises a value of at least one of timing advance (TA) and propagation delay (PD)The UE  1  may generate the synchronization-specific PRACH  101  using the following example schemes:
     First scheme: The gNB  2  may allocate to the UE  1  dedicated RACH resources in PRACH for TA acquisition only, e.g., for acquisition of a random access response (RAR) message Msg 2  with only a TA field. For example, the UE  1  may generate and transmit the synchronization-specific PRACH  101  as Msgl in dedicated RACH resources to the gNB 2 .   

     Second scheme: The gNB  2  may allocate to the UE  1  dedicated preamble in PRACH for TA acquisition only, e.g., for acquisition of a random access response (RAR) message Msg 2  with only a TA field. For example, the dedicated preamble is for non-contention based PRACH, and UE  1  may generate and transmit the synchronization-specific PRACH  101  as Msgl with the dedicated RACH preamble to the gNB 2 .UE  1  receiving a synchronization-specific downlink signal in response to the synchronization-specific uplink signal in step S 1104 . The downlink signal comprises a type of timing advance (TA) associated with a source of reference time information (RTI) or associated with a time sensitive communication (TSC) traffic type. The synchronization-specific downlink signal may be transmitted in a random access response (RAR) or a medium access control (MAC) control element (CE). In an embodiment, the UE  1  receives a random-access response (RAR)  102  from the gNB  2  and obtains a value of a TA in the RAR  102  (step S 1104 ). For example, the UE  1  may generate and transmit the synchronization-specific PRACH  101  as Msgl in dedicated RACH resources to the gNB 2 , and receive the RAR Msg 2  with only a TA field from the gNB  2  in response to the Msgl without transmitting Msg 3 . Alternatively, the UE  1  may generate and transmit the TA-specific PRACH  101  as Msgl with the dedicated RACH preamble to the gNB 2 , and receive the RAR Msg 2  with only a TA field from the gNB  2  in response to the Msgl without transmitting Msg 3 . Alternatively, to enhance timing synchronization accuracy, after the UE having acquired a previous timing advance value given by the gNB  2  may adjust transmission time of a sounding reference signal (SRS) using the previous timing advance value, and send the SRS to the gNB  2  on the adjusted transmission time. The gNB  2  receiving the SRS can measure the SRS sent from the UE during the RRC_CONNECTED state, refine timing advance calculation by generating a refined TA based on the SRS measurement, and send the refined TA to the UE  1  in a medium access control (MAC) control element (CE). The UE  1  receives a medium access control (MAC) control element (CE)  150  as a timing advance (TA) command from the gNB  2  and obtains a TA value in the MAC CE  150  in step S 1104 . 
     UE  1  may receive an indication of propagation delay compensation  103  from gNB  2  and determine whether to perform propagation delay compensation or not according to the indication of propagation delay compensation  103  (step S 1105 ). The indication of propagation delay compensation  103  may be referred to as a UE-side propagation delay compensation indication carried in a downlink channel from the gNB  2  to the UE  1  using one of the following example schemes:
     First scheme: The UE-side propagation delay compensation indication may be located in a RAR, such as the   

     RAR  102 . For example, the UE-side propagation delay compensation indication may be jointly transmitted with the TA in the RAR  102 .
     Second scheme: The UE-side propagation delay compensation indication may be located in a MAC CE. For example, the UE-side propagation delay compensation indication may be jointly transmitted with the TA in the MAC CE  150 .   Third scheme: The UE-side propagation delay compensation indication may be carried in RRC signaling, e.g., jointly transmitted with RTI in a broadcast RRC message, such as a SIB 9  or another SIB, or a unicast RRC message, such as a DLInformationTransfer message.   

     UE  1  compensates propagation delay for the received reference time information based on the determination in step S 1105  (step S 1106 ). 
     When the UE-side propagation delay compensation indication is not available, the UE  1  may determine whether to perform propagation delay compensation or not based on a pre-determined rule, such as a PD compensation triggering condition stored in the UE  1  (step S 1107 ). The UE  1  may further determine whether to perform the UE-side propagation delay compensation based on whether propagation delay pre-compensation has been performed by serving base station. Whether the propagation delay pre-compensation has been performed may be indicated by a propagation delay pre-compensation indication. The propagation delay pre-compensation indication is transmitted from the base station  2  to the UE  1  in a random access response (RAR), a system information block (SIB), a medium access control (MAC) control element (CE), or a radio resource control (RRC) signal. 
     Embodiment 1-2 
     A gNB may pre-compensates propagation delay based on received PRACH. 
     With reference to  FIG. 4  and  FIG. 5 , the UE  1  may transmit a specific PRACH  101  as a TA request and/or a pre-compensation request (step S 1203 ). The specific PRACH  101  is an embodiment of the synchronization-specific uplink signal and may be allocated dedicated RACH resources or a dedicated preamble. The UE  1  may generate the specific PRACH  101  using following example schemes.
     First scheme: The gNB  2  may allocate to the UE  1  dedicated RACH resources in PRACH for TA acquisition and/or pre-compensation request, e.g., for acquisition of a random access response (RAR) message Msg 2  with only a TA field. For example, the UE  1  may generate and transmit the specific PRACH  101  as Msgl in dedicated RACH resources to the gNB 2 .   Second scheme: The gNB  2  may allocate to the UE  1  dedicated preamble in PRACH for TA acquisition and/or pre-compensation request, e.g., for acquisition of a random access response (RAR) message Msg 2  with only a TA field. For example, the UE  1  may generate and transmit the specific PRACH  101  as Msg l with the dedicated RACH preamble to the gNB 2 .   

     The gNB  2  derives TA value or propagation delay based on the received specific PRACH  101  (step S 1204 ). 
     Alternatively, the gNB  2  may also derive TA value based on the uplink reference signal transmitted from UE, e.g., a sounding reference signal (SRS) or demodulation reference signal (DMRS). 
     The gNB  2  pre-compensates the RTI according to the derived TA value (step S 1205 ) and transmits pre-compensated RTI  120  to UE (step S 1206 ). The UE  1  receives the pre-compensated RTI  120  from the gNB  2 . 
     The UE  1  may receive an indication  103  of the pre-compensation from gNB  2 (step S 1207 ), where indication  103  indicates whether the RTI has been pre-compensated or not (step S 1208 ). The indication  103  of the pre-compensation may be referred to as a propagation delay pre-compensation indication carried in a synchronization-specific downlink signal using one of the following example locations:
     First scheme: The propagation delay pre-compensation indication may be located in a RAR, such as the RAR  102 . For example, the propagation delay pre-compensation indication may be jointly transmitted with the TA in the RAR.   Second scheme: The propagation delay pre-compensation indication may be located in a MAC CE. For example, the propagation delay pre-compensation indication may be jointly transmitted with the TA in the MAC CE.   Third scheme: The propagation delay pre-compensation indication may be carried in RRC signaling, e.g., jointly transmitted with RTI in a broadcast RRC message, such as a SIB 9  or another SIB, or a unicast RRC message, such as a DLlnformationTransfer message.   

     The UE  1  determines whether the RTI has been pre-compensated or not using the propagation delay pre-compensation indication. When the RTI has been pre-compensated by gNB  2 , the UE  1  does not perform propagation delay compensation (step S 1209 ). When the RTI has not been pre-compensated by gNB  2 , the UE may determine whether to compensate or not by itself based on certain conditions and perform propagation delay compensation accordingly (step S 1210 ). For example, the UE  1  performs UE-side propagation delay compensation when the propagation delay pre-compensation has not been performed. 
     Embodiment 1-3 
     The gNB may provide pre-configured conditions that trigger PD compensation at the UE. The UE receives the pre-configured conditions and determines the necessity of performing propagation delay compensation based on pre-configured conditions. The pre-configured conditions may be referred to as PD compensation triggering conditions. The UE  1  may perform PD compensation in response to a PD compensation triggering event generated based on a PD compensation triggering condition. For example, the UE  1  may transmit the synchronization-specific uplink signal in response to a PD compensation triggering event generated based on a PD compensation triggering condition. The PD compensation triggering event may be an event that breaches the PD compensation triggering condition or meets the PD compensation triggering condition. 
     With reference to  FIG. 6  and  FIG. 7 , the UE  1  receives RTI  100  from gNB  2  (step S 1303 ). The gNB  2  may determine PD compensation triggering conditions that trigger PD compensation and send the conditions to the UE  1  to trigger PD compensation at the UE  1  based on the PD compensation triggering conditions. The UE  1  receives the PD compensation triggering conditions from the gNB  2  (step S 1304 ). The following are examples of the conditions:
     First scheme: One of the PD compensation triggering conditions indicate a range or a threshold of TA. The PD compensation triggering event may be an event that breaches the PD compensation triggering condition or meets the PD compensation triggering condition. For example, when the value of TA exceeds the range of TA value or is larger than a threshold, then the UE determines the propagation delay cannot be neglected and perform PD compensation in response to the event.   Second scheme: One of the PD compensation triggering conditions indicate a subcarrier spacing (SCS) value of the serving cell. The PD compensation triggering event may be an event that breaches the PD compensation triggering condition or meets the PD compensation triggering condition. For example, the propagation delay accumulates depends on SCS, and the larger SCS the larger the timing error. The UE  1  may perform PD compensation in response to an event representing SCS &gt;30 KHz.   Third scheme: One of the PD compensation triggering conditions indicate a range or a threshold of received signal strength. The PD compensation triggering event may be an event that breaches the PD compensation triggering condition or meets the PD compensation triggering condition. For example, the received signal strength comprises reference symbol received power (RSRP), reference signal received quality (RSRQ), or reference signal resource indicator (RSRI) of synchronization signal block (SSB), CSI-RS, or DMRS. The UE  1  may perform PD compensation in response to an event representing the received signal strength less than the range or the threshold of the required signal strength due to long distance from the gNB  2 .   Fourth scheme: One of the PD compensation triggering conditions indicate a geographic location, such as a GPS location, or a beam direction. The PD compensation triggering event may be an event that breaches the PD compensation triggering condition or meets the PD compensation triggering condition. For example, the UE  1  may perform PD compensation in response to an event representing a position of the UE  1  exceeding the range or the threshold of the geographic location. Alternatively, the UE  1  may perform PD compensation in response to an event that the UE  1  enters a range or a threshold of the geographic location.   

     gNB  2  may provide a PD compensation triggering condition to UE  104  via the following schemes:
     First scheme: PD compensation triggering condition may be jointly transmitted with RTI in a broadcast RRC, such as SIB 9  or another SIB, or a unicast RRC, such as a DLlnformationTransfer message.   Second scheme: PD compensation triggering condition may be transmitted using a new broadcast or unicast RRC message. Thus, the UE  1  may receive the PD compensation triggering condition from a SIB, a MAC CE, or an RRC signal.   

     The UE  1  transmits a specific PRACH  101  to the gNB  2  (step S 1305 ) and receives a RAR  102  containing a TA from the gNB  2  (step S 1306 ). After deriving TA from the gNB  2 , the UE  1  determines whether a PD compensation triggering event of one of the PD compensation triggering conditions is detected (step S 1307 ). The UE  1  performs PD compensation according to the derived TA in response to a PD compensation triggering event that is generated based on a PD compensation triggering condition (step S 1309 ). For example, the UE  1  performs PD compensation according to the derived TA when at least one of the conditions received from gNB  2  is satisfied. 
     Embodiment 1-4 
     During UE mobility, the propagation delay may change with the location of the UE  1 , and the TA value is determined based on the location of the UE  1  or a relative distance between the UE  1  and the gNB  2 . To ensure the previously derived TA value is still valid, the UE  1  may determine the validity of the TA value for propagation delay compensation based on the following conditions given by the gNB  2 . 
     With reference to  FIG. 8  and  FIG. 9 , after receiving RTI  100  from gNB  2  (step S 1403 ), the UE  1  receives one or more TA validity conditions  105  from the gNB  2  (step S 1404 ). The UE  1  transmits a specific PRACH  101  to the gNB  2  as a request for TA (step S 1405 ). The gNB  2  transmits a RAR  102  with the requested TA to the UE  1  in response to the specific PRACH  101 . The UE  1  receives the RAR  102  and obtains the TA in the RAR  102  (step S 1406 ). The gNB  2  may thus provide the TA validity conditions  105  that trigger UE  1  to determine the validity of TA for propagation delay compensation (step S 1407 ). The UE  1  may determine the validity of TA for propagation delay compensation based on the TA validity conditions  105 . Examples of the TA validity conditions  105  comprise:
     First scheme: A TA validity condition may comprise a location of the UE  1 . The TA validity event may be an event that breaches the TA validity condition or meets the TA validity condition. For example, the TA validity condition indicates a range or a threshold of the location of the UE  1 . A TA validity event of the TA validity condition may be a change of the location of the UE  1  relative to the serving gNB  2  which exceeds the range or the threshold of the location of the UE 1 .   Second scheme: A TA validity condition may comprise a beam direction. The TA validity event may be an event that breaches the TA validity condition or meets the TA validity condition. For example, the TA validity condition indicates a range or a threshold of a beam direction of the gNB  2 . A TA validity event of the TA validity condition may be a change of a beam from the gNB  2  to the UE  1 , which exceeds the range or the threshold of the beam direction.   Third scheme: A TA validity condition may comprise received signal strength, such as RSRP, RSRQ, or RSRI, from the gNB  2 . The TA validity event may be an event that breaches the TA validity condition or meets the TA validity condition. For example, the TA validity condition indicates a range or a threshold of the received signal strength from the gNB  2 . A TA validity event of the TA validity condition may be an event representing the received signal strength exceeding the range or the threshold of the signal strength. For example, a TA validity event of the TA validity condition may be an event representing (SrxlevRef −current Srxlev) &lt;SsearchdeltaP. Where:
       Srxlev is the current Cell selection RX level value of the serving cell (dB).   Srxlev Ref  is the reference Srxlev value of the serving cell (dB).   S searchdeltaP  is the Srxlev delta threshold (in dB) during relaxed monitoring.   
       Fourth scheme: A TA validity condition may comprise a predetermined period of time kept by a PD compensation timer. A TA validity event of the TA validity condition may be an event representing an expiration of the PD compensation timer. The UE  1  may set up the compensation timer and restart the timer whenever receiving a TA after a RACH procedure. The predetermined period of time comprises a time duration of a discontinuous reception (DRX) cycle.   

     The UE  1  may determine the TA is invalid or outdated based on a TA validity event of one of the TA validity conditions. The UE  1  may transmit a second TA-specific PRACH  106  to request a new TA value and/or the propagation delay pre-compensation indication in response to a TA validity event of one of the TA validity conditions (step S 1409 ). The second TA-specific PRACH  106  is another synchronization-specific uplink signal the UE  1  transmits in response to a TA validity event generated based on a TA validity condition. The another synchronization-specific uplink signal comprises a dedicated preamble, a sounding reference signal (SRS) or an uplink DMRS signal for propagation-delay-related signaling, and is transmitted based on a predetermined or previously acquired timing advance (TA) or propagation delay (PD). The TA validity condition is predefined in the UE  1  or received by the UE  1  in a system information block (SIB), a medium access control (MAC) control element (CE), or a radio resource control (RRC) signal from the gNB  2 . 
     The UE  1  may use a TA preference indication uplink message, such as a MAC CE or an RRC message, to indicate TA content preference  160  associated with TA to be requested by UE, and send the TA preference indication uplink message to the gNB  2 . For example, the TA preference indication uplink message may comprise accuracy of TA, the precision of TA, such as the granularity of TA, a type of TA associated with a selected source of RTI or a selected traffic type. The second TA-specific PRACH  106  is a request to indicate or update the granularity of the TA or the propagation delay; 
     In response to the second TA-specific PRACH  106 , the gNB  2  may report a second RAR  112  with a new TA to the UE  1  using different granularities based on the TA content preference  160  received from the UE  1  or a synchronization requirement of a time sensitive communication (TSC) traffic. For example, the TSC traffic synchronization requirement may be obtained from TSC assistance information given by a TSC server via the core network. The second RAR  112  is an embodiment of the synchronization-specific random access channel downlink signal which comprises a propagation-delay-related value of a specific value granularity. The specific value granularity is one of a plurality of propagation-delay-related value granularities supported by the UE  1 . The propagation-delay-related value is a value of at least one of timing advance (TA), propagation delay (PD), and propagation delay compensation (PDC), and the specific value granularity is the granularity of at least one of timing advance (TA), propagation delay (PD), and propagation delay compensation (PDC). The specific value granularity is selected from the plurality of propagation-delay-related value granularities based on TA content preference received from the UE  1  or based on a TSC traffic synchronization requirement. The UE  1  sends a request to indicate or update the granularity of the propagation-delay-related value, such as TA or the propagation delay. The gNB  2  provides to the UE  1  an update of the propagation-delay-related value with updated granularity in response to a request for indicating the updated granularity of the propagation-delay-related value. 
     Embodiment 1-5 
     For preventing overhead of transmitting request messages for propagation-delay-related values, such as RTI and TA, from UEs, the gNB  2  may perform periodic reporting of RTI or TA. 
     With reference to  FIG. 10 , the gNB  2  may perform periodic reporting of RTI and/or TA to UEs, including the UE  1 , without UE request. For example, the gNB  2  periodically transmits propagation-delay-related values, such as RTI  113  and/or TA  114  to the UE  1  based on a timer or configured grant configuration. The gNB  2  may periodically set up a timer and transmit RTI or TA in an RRC message upon expiration of the timer. The gNB  2  may periodically transmit RTI or TA through periodic DL transmission using semi-persistent scheduling (SPS). The gNB  2  can determine the periodicity of the periodic reporting based on UEAssistancelnformation message from the UE  1 . 
     The UE  1  may send a request message to the gNB  2  for an update of RTI preference when necessary. Embodiments of the update of RTI preference are detailed in the following:
     In an embodiment of the invention, the UE  1  may send a request to the gNB  2  for a single shot RTI delivery via a unicast RRC message in addition to the periodic transmission of RTI. The gNB  2  may receive the request for single-shot RTI delivery and send a message of RTI to the UE  1  as a single shot RTI in response to the request.   In an embodiment of the invention, the UE  1  may send a request to the gNB  2  to indicate or update granularity or uncertainty level of RTI that may be subsequently sent from the gNB  2  to the UE  1 . The gNB  2  may receive the request indicating or updating the granularity of the RTI and send a message of RTI with the granularity or uncertainty level to the UE  1  in response to the request.   In an embodiment of the invention, the UE  1  may send a request to the gNB  2  to indicate or update periodicity of RTI delivery or TA delivery, that is, periodicity of the periodic reporting of RTI and/or TA. The gNB  2  may receive the request indicating or updating the periodicity and send a message of RTI or TA with the periodicity to the UE  1  in response to the request.   In an embodiment of the invention, the UE  1  may send a request to the gNB  2  to indicate or update an RTI type of a corresponding traffic type so that the gNB  2  transmits RTI of the indicated RTI type to the UE  1 . The gNB  2  may receive the request indicating or updating the RTI type and send a message of RTI of the RTI type to the UE  1  in response to the request.   

     Propagation delay is a travel time of a frame transmitted between a UE and a gNB, and may be calculated based on a timing advance (TA) value after the UE  1  performs downlink synchronization by decoding a primary synchronization signal (PSS) and a secondary synchronization signal (SSS) and transmitting the uplink PRACH preamble transmission. The gNB  2  may transmit a downlink synchronization signal block (SSB), DMRS, or CSI-RS to the UE  1  as a downlink synchronization signal for the downlink synchronization. The UE  1  may transmit a request to indicate or to update the periodicity of the downlink synchronization signal, such as a downlink synchronization signal block (SSB), DMRS, or CSI-RS, to the gNB  2 . The gNB  2  receives the request for indicating or updating the periodicity or location of downlink synchronization signals sent from the UE  1 , and provides downlink synchronization signals with the periodicity in response to the request. An example of at least one of the downlink synchronization signals comprises downlink synchronization signal block (SSB), DMRS, or CSI-RS. 
     Embodiment 2 
     Embodiments of propagation delay compensation during handover are detailed in the following. 
     Embodiment 2-1 
     With reference to  FIG. 11  and  FIG. 12 , after deriving a TA value of a source gNB, such as gNB  2 , the UE  1  autonomously derives a TA value of a target gNB, such as gNB  3 , and makes PD compensation with respect to RTI without performing a random access channel (RACH) procedure toward the target gNB. 
     The UE  1  receives reference time information (RTI)  200  from the source gNB  2  (step S 2102 ). For example, the RTI  200  may be carried in SIB 9  or another SIB, or a unicast RRC, such as DLlnformationTransfer message, from the source gNB  2 . 
     The UE  1  performs a RACH procedure  201  with the source gNB  2  and derives a TA value of the source gNB  2  via the RACH procedure  201  (step S 2103 ). In some embodiments of the invention, the UE  1  may obtain a propagation-delay-related value in step S 2103 ). The propagation-delay-related value comprises a value of at least one of timing advance (TA), propagation delay (PD). 
     The UE  1  receives DL synchronization signals from the source gNB and the target gNB respectively and derives receiving time difference between a DL synchronization signal from the source gNB and a DL synchronization signal from the target gNB (step S 2105 ). For example, the UE  1  calculates a receiving time difference between a first synchronization signal  202  transmitted from the source gNB  2  and a second synchronization signal  203  transmitted from the target gNB  3 . Examples of each of the first synchronization signal  202  and the second synchronization signal  203  may comprise a reference signal, such as a synchronization signal block (SSB), DMRS and/or CSI-RS. Each of the first synchronization signal  202  and the second synchronization signal  203  may comprise the transmission time of the synchronization signal. The UE  1  may send a request to indicate or update the periodicity or location of a downlink synchronization signal block (SSB), DMRS, or CSI-RS 
     The UE  1  derives the timing advance (TA) value of the target gNB  3  based on the TA value of the source gNB  2  as well as the receiving time difference between the first synchronization signal  202  of the source gNB  2  and the second synchronization signal  203  of the target gNB  3  during the handover (step S 2106 ). In some embodiments of the invention, the UE  1  derives a propagation-delay-related value of the target gNB  3  based on the propagation-delay-related value of the source gNB  2  and the receiving time difference. 
     The UE  1  compensates for the effect of propagation delay in the RTI received from the source gNB  2  using the TA value of the target gNB  3  during or after the handover procedure (step S 2107 ). That is, the UE  1  performs propagation delay compensation on the RTI received from the source gNB  2  using the TA value of the target gNB  3 . Because the handover is RACH-less, the UE  1  need not obtain the TA of the target gNB  3  from a random access response (RAR) message transmitted from the target BS, thus reducing lost synchronization time during handover. 
     Embodiment 2-2 
     With reference to  FIG. 13  and  FIG. 14 , as illustrated in the previous embodiment, the UE  1  receives reference time information (RTI)  200  from the source gNB  2  (step S 2102 ). The UE  1  performs a RACH procedure  201  with the source gNB  2  and derives a TA value of the source gNB  2  via the RACH procedure  201  (step S 2103 ). 
     The source gNB  2  may provide UE assistance information to assist receiving of a downlink synchronization signal of the target gNB. 
     The target gNB  3  may share time location information  220  of reference signals of the target gNB  3  to the source gNB  2 .Tthe source gNB  2  is a serving base station of the UE  1 . The reference signals of the target gNB  3  are reference signals transmitted from the target gNB  3 , which may include such as SSB, DMRS and/or CSI-RS. The UE  1  may send a request to indicate or update periodicity of a downlink synchronization signal block (SSB), DMRS, or CSI-RS. 
     The source base station, such as the gNB  2 , receives a transmission time of a reference signal of the target base station, such as the gNB  3 , and provides the transmission time of the reference signal of the target base station to the UE  1 . The source gNB  2  may assist the UE  1  by sending a message, such as a handover message  204 , indicating the time location information  220  of reference signals of the target gNB  3  to the UE  1  (step S 2104 ). The handover massage  204  may include transmission time of a reference signals, such as SSB, DMRS and/or CSI-RS, as the time location information  220  of the reference signals. The UE  1  receives the time location information  220  of the reference signals in the message and calculates propagation delay associated with the target gNB  3  using the time location information  220 , such as a transmission time, of the reference signals. 
     The UE  1  detects the reference signal transmitted from target gNB (step S 2105 a). The UE  1  calculates a value of propagation delay of the target gNB based on a time difference between the reception time of the reference signal and the transmission time location of the reference signal (step S 2106   a ). The UE  1  performs propagation delay compensation for the received RTI based on the calculated TA/PD value of target gNB during or after handover (step S 2017 ). 
     Embodiment 2-3 
     The target gNB  3  may provide geography location and beam related information of the target gNB  3  to the source gNB  2 . The source gNB  2  receiving the geography location and beam related information may transmit a message, such as the handover message, which indicates the geographic location and beam related information of the target gNB  3  to the UE  1 . The geography location and beam related information may comprise a mapping between a geography location and a beam, and a mapping between the beam and a propagation-delay-related value, such that the UE  1  detecting the beam can determine the geographic location and the propagation-delay-related value associated with the beam. The propagation-delay-related value may be calculated and stored in a table maintained by the target gNB and shared to the UE  1 . 
     With reference to  FIG. 15 , the target gNB  3  may share geography location and beam related information  221  of the target gNB  3  to the source gNB  2 . 
     The source gNB  2  transmits to the UE  1  a message  210 , such as the handover message, which indicates the geographic location and beam related information of the target gNB  3 . 
     When receiving the geography location and beam related information  221 , the UE  1  may perform propagation delay compensation autonomously based on received RTI and the geographic location and beam related information  221  of the target gNB  3 . 
     Embodiment 2-4 
     The UE  1  may send a request for RTI to a gNB, such as the source gNB  2 , based on timing requirements of a corresponding traffic type operated by the UE  1 . The request for RTI may be referred to an RTI request in the description. An RTI request may be associated with RTI preference requested by the UE  1 , which may include RTI parameters, such as clock synchronization level and periodicity of RTI delivery. RTI preference associated with an RTI request may be included in the RTI request or not. The UE  1  may transmit RTI related information to a source gNB, such as the gNB  2 , so that the gNB  2  and the gNB  3  negotiating the RTI related information during a handover operation of the UE. The RTI related information may comprise an RTI request, RTI preference, and/or RTI parameters. 
     With reference to  FIG. 16  and  FIG. 17 , when the handover of the UE  1  is triggered (step S 2504 ), the source gNB  2  may forward RTI related information of the UE  1  to the target gNB  3  via Xn interface (step S 2505 ). The target gNB receives the RTI related information of the UE  1  from the gNB  2 . The target gNB may be referred to as a destination base station of the handover operation. Two embodiments of the forwarding RTI relevant information of the UE  1  between two base stations are detailed in the following: 
     First scheme: Upon receiving an RTI request  209  from the UE  1 , the source gNB  2  may forward UE&#39;s RTI request and related RTI preference  210   a  to the target gNB  3  via Xn interface, such that the source gNB  2  may share the RTI preference  210   a  with the target gNB  3  (step S 2505 ). The RTI preference  210   a  may comprise a preference of one of a plurality of RTI types. An RTI type is associated with a set of RTI parameters which may include either or both of a clock synchronization level requested by the UE  1  and the periodicity of RTI delivery requested by UE. Two different RTI types may be respectively associated with two sets of RTI parameters. The clock synchronization level is a level of synchronization accuracy between a UE and a gNB. 
     Second scheme: The RTI related information comprises a synchronization status. Whenever handover is triggered to hand over the UE  1  (step S 2504 ), the source gNB  2  sends a message including synchronization status of the UE  1  to the target gNB  3  through Xn interface. (step S 2505 ). The synchronization status of the UE  1  is a status of the UE  1  regarding ongoing synchronization proceedings between the UE  1  and the source gNB  2  and may include RTI parameters of the UE  1 .
     In an embodiment of the invention, the synchronization status comprises an indication that indicates whether the UE  1  has been synchronized with a clock of the source gNB  2  based on RTI delivery with a certain periodicity. As aforementioned, the RTI delivery may be realized using periodically transmitted broadcast and/or unicast downlink signals carrying RTI.   In an embodiment of the invention, the synchronization status indicates a required level of synchronization accuracy of the UE  1 . The synchronization accuracy may include the required RTI delivery periodicity necessary for the UE  1 .   In an embodiment of the invention, the synchronization status indicates a period of time for the UE  1  to be synchronized with a clock of the target gNB after the handover of the UE  1  from the source gNB  2  to the target gNB  3 .   

     Based on the received RTI related information, the target gNB  3   52601  may determine the RTI preference including RTI parameters for UE, and start delivering RTI  212  autonomously to the UE  1  without a request from the UE  1  for RTI  212  from the target gNB  3  (step S 2606 ). The UE may obtain TA  207  of the target gNB  3  based on previously mentioned embodiments (step S 2608 ). 
     Embodiment 3 
     With reference to  FIG. 18  and  FIG. 19 , a UE may send clock information, such as RTI, to gNBs in an uplink direction. 
     A grand master (GM) clock may be located at a UE side. For example, when a grand master clock  4  is located at the UE  1 , the UE  1  may send clock information, such as RTI, to one or more serving gNBs, such as the gNB  2 , for uplink synchronization. 
     Embodiment 3-1 
     Once UE  1  derives RTI  300  (step S 3103 ). The RTI  300  may be an RTI  200  received from a GM clock of the first gNB  2  based on the aforementioned embodiments or an RTI from the GM clock  4  attached to the UE  1 . In the previous case, the UE  1  may transmit RTI  300  to other base stations including the gNB  3 , during or after a handover procedure (step S 3104 ). 
     The UE  1  provides RTI  300  to the gNB  3  based on one of the following schemes (step S 3104 ):
     First scheme: MAC CE:
       Ex: The UE  1  may provide the RTI  300  in a MAC CE and transmit the MAC CE to the gNB  3 . The MAC CE carrying the RTI  300  may be a current MAC CE with a modified field for the RTI  300  or a new proposed MAC CE.   
       Second scheme: Physical layer uplink channel:
       The UE  1  may transmit the RTI  300  in a physical layer uplink channel, such as a physical random access channel (PRACH), a physical uplink shared channel (PUSCH) or a physical uplink control channel (PUCCH), to the gNB  3 .   The UE  1  may transmit the RTI  300  as aperiodic feedback in a physical layer uplink channel, such as a PUSCH or a PUCCH, assigned by downlink control information (DCI). The DCI may be uplink grant DCI.   The UE  1  may transmit the RTI  300  as periodic timing feedback or semi-persistent timing feedback in a physical layer uplink channel, such as a PUSCH or a PUCCH, assigned by DCI. The DCI may be uplink grant DCI.   The UE  1  may transmit the RTI  300  in a PRACH and using a preamble representing the RTI  300 . Each of the UE  1  and the gNBs may maintain a mapping table storing mappings between preambles and RTI values. Each of the mappings between preambles and RTI values is a mapping between a preamble in the preambles and an RTI value in the RTI values. The RTI value may be an absolute RTI value or a relative RTI value.   
       Third scheme: RRC message:
       The UE  1  may provide the RTI  300  in an RRC message and transmit the RRC message to the gNB  3 . The RRC message carrying the RTI  300  may be a current RRC message with a modified field for the RTI  300  or a new proposed RRC message.   For example, the RRC message carrying the RTI  300  may be a Msg 3  or a Msg 5  in a RACH procedure. For example, the RRC message carrying the RTI  300  may be a UE-specific RRC message, such as a measurement report. The UE  1  may, in addition to performing CQI, PMI, RI measurement and transmitting the measurement in measurement reports, carry the RTI  300  periodically or aperiodically to a serving gNB, such as the gNB  2  or gNB  3 , where CQI represents channel quality indicator, PMI represents precoding matrix indicator, and RI represents rank indication. Generally, PUSCH is used for measurement reports. For example, the UE  1  may use a boundary of a system frame number (SFN) as a reference point for transmission of RTI  300 . For example, the UE  1  transmits the RTI  300  to the gNB  3  in a certain SFN a RRC message carrying the value of RTI  300  corresponds to the reference point at the end of a specific SFN.   
       

     The UE  1  can provide more than one RTIs with respect to different GM clocks, i.e., different clock domains, to a set or different sets of gNBs. For example, the UE  1  may provide RTI of GM clocks  4  in a first clock domain and RTI of a GM clock  5  in a second clock domain to different sets of gNBs. The different set of gNBs belongs to different clock domains. 
     Embodiment 3-2 
     The UE  1  obtains reference time information (RTI) of a first grant master clock domain and transmits the RTI of the first grant master clock domain to a target base station, such as the gNB  3 , of a second grant master clock domain to allow the target base station synchronize with the first grant master clock domain. 
     The UE  1  may derive TA of second gNB  3  as detailed in the aforementioned embodiments and pre-compensates propagation delay in the RTI transmitted. The second gNB may be referred to as a target gNB. 
     With reference to  FIG. 20  to  FIG. 22 , UE  1  derives the RTI, e.g. from the GM clock  4  or another GM clock of a first gNB, such as the gNB  2  (step S 3203 ). The RTI in step S 3203  is from a first GM clock domain. In an embodiment of the invention, the UE  1  obtain the RTI of the GM clock  4 , and the GM clock  4  is in the first grant master clock domain. In an embodiment of the invention, the gNB  2  sends RTI of a GM clock of the gNB  2  in the first grant master clock domain to the UE  1  to allow the UE  1  to synchronize with the first grant master clock domain. 
     UE  1  derives TA of a second gNB, such as the gNB  3 , using a TA derivation procedure  302  (step S 3204 ). The TA derivation procedure  302  may be a procedure comprising steps for deriving TA as detailed in the aforementioned embodiments. In some embodiments of the invention, the TA in step S 3204  may be replaced by a propagation-delay-related value of the target base station. The propagation-delay-related value comprises a value of at least one of timing advance (TA) or propagation delay (PD). 
     The UE  1  pre-compensates propagation delay in the RTI using the derived TA (step S 3205 ). The RTI is the RTI obtained by the UE  1  in step S 3203  and may be from the GM clock  4  or another GM clock of the first gNB, such as the gNB  2 . For example, the UE  1  may pre-compensate PD in the RTI obtained from the GM clock  4  to generate pre-compensated master clock RTI  303 . Alternatively, the UE  1  may pre-compensate PD in the RTI obtained from the GM clock of the first gNB, such as the gNB  2 , to generate pre-compensated RTI  304 . 
     UE  1  transmits the pre-compensated RTI, such as the pre-compensated master clock RTI  303  or the pre-compensated RTI  304 , to the second gNB  3  allow the target base station, such as the second gNB  3 , to synchronize with the first grant master clock domain (step S 3206 ). In some embodiments of the invention, the UE  1  may pre-compensate the RTI of the first grant master clock for the target base station using the propagation-delay-related value of the target base station. The UE  1  transmits the pre-compensated RTI of the first grant master clock domain to the target base station to allow the target base station, such as the second gNB  3 , to synchronize with the first grant master clock domain. 
     In the embodiment, the second gNB  3  needs not to do propagation delay compensation. 
     Embodiment 3-3 
     In addition to the aforementioned embodiment, the UE  1  may transmit RTI and a time reference point of the RTI to a base station, such as the gNB  3 , so that the base station can estimate propagation delay using the time reference point and the transmitted master clock RTI to associated with the time reference point. 
     With reference to  FIG. 23  and  FIG. 24 , the UE  1  may obtain the RTI  300 , e.g. from the GM clock  4  or another GM clock of a first gNB, such as a GM clock of the gNB  2  (step S 3303 ). The UE  1  transmits preference  305  of a time reference point  306  associated with the master clock RTI  300  to other gNBs including the gNB  3  (step S 3304 ). The preference  305  may comprise an indication of one type of time reference point among a plurality of types of time reference points which may comprise time points of a PRACH preamble, a sounding reference signal (SRS), a DMRS, a CSI-RS, and a boundary of an SFN. The type of time reference point in preference  305  may be selected by the UE  1 . The boundary of an SFN may be the beginning or the end of the nearest SFN. 
     The UE  1  transmits RTI and a reference time point  306  of the RTI to other gNBs including gNB  3  (step S 3305 ). The RTI in step  53305  may be yet-compensated RTI. The yet-compensated RTI and a reference time point may be utilized to derive the first grant master clock domain at the other gNBs, such as the gNB  3 . One or more gNBs in the other gNBs that receive the RTI and a reference time point  306  may calculate propagation delay and synchronize with the first grant master clock domain based on the received RTI and the reference time point, such as a PRACH preamble, an SRS, or other reference signal (step S 3306 ). For example, a second gNB in the other gNBs, such as the gNB  3 , that receives the RTI and a reference time point  306  may derive the first grant master clock domain at the other gNBs based on the received RTI and the reference time point, such as a PRACH preamble, an SRS, or another reference signal (step S 3306 ). 
     One or more gNBs in the other gNBs that receive the RTI and a reference time point  306  may determine whether to perform PD compensation on the received RTI  300  associated with the reference time point  306  based on the propagation delay (step S 3307 ). For example, the second gNB in the other gNBs, such as the gNB  3 , that receives the RTI and a reference time point  306  may determine whether to perform PD compensation on the received RTI  300  associated with the reference time point  306  based on the propagation delay (step S 3307 ). If the RTI  300  received by the second gNB, such as the gNB  3 , has not been pre-compensated by UE  1 , the second gNB, such as the gNB  3 , may perform propagation delay compensation on the RTI  300 . 
       FIG. 25  is a block diagram of an example system  700  for wireless communication according to an embodiment of the present disclosure. Embodiments described herein may be implemented into the system using any suitably configured hardware and/or software.  FIG. 25  illustrates the system  700  including a radio frequency (RF) circuitry  710 , a baseband circuitry  720 , a processing unit  730 , a memory/storage  740 , a display  750 , a camera  760 , a sensor  770 , and an input/output (I/O) interface  780 , coupled with each other as illustrated. 
     The processing unit  730  may include circuitry, such as, but not limited to, one or more single-core or multi-core processors. The processors may include any combinations of general-purpose processors and dedicated processors, such as graphics processors and application processors. The processors may be coupled with the memory/storage and configured to execute instructions stored in the memory/storage to enable various applications and/or operating systems running on the system. 
     The baseband circuitry  720  may include circuitry, such as, but not limited to, one or more single-core or multi-core processors. The processors may include a baseband processor. The baseband circuitry may handle various radio control functions that enable communication with one or more radio networks via the RF circuitry. The radio control functions may include, but are not limited to, signal modulation, encoding, decoding, radio frequency shifting, etc. In some embodiments, the baseband circuitry may provide for communication compatible with one or more radio technologies. For example, in some embodiments, the baseband circuitry may support communication with 5G NR, LTE, an evolved universal terrestrial radio access network (EUTRAN) and/or other wireless metropolitan area networks (WMAN), a wireless local area network (WLAN), a wireless personal area network (WPAN). Embodiments in which the baseband circuitry is configured to support radio communications of more than one wireless protocol may be referred to as multi-mode baseband circuitry. In various embodiments, the baseband circuitry  720  may include circuitry to operate with signals that are not strictly considered as being in a baseband frequency. For example, in some embodiments, baseband circuitry may include circuitry to operate with signals having an intermediate frequency, which is between a baseband frequency and a radio frequency. 
     The RF circuitry  710  may enable communication with wireless networks using modulated electromagnetic radiation through a non-solid medium. In various embodiments, the RF circuitry may include switches, filters, amplifiers, etc. to facilitate communication with the wireless network. In various embodiments, the RF circuitry  710  may include circuitry to operate with signals that are not strictly considered as being in a radio frequency. For example, in some embodiments, RF circuitry may include circuitry to operate with signals having an intermediate frequency, which is between a baseband frequency and a radio frequency. 
     In various embodiments, the transmitter circuitry, control circuitry, or receiver circuitry discussed above with respect to the UE, eNB, or gNB may be embodied in whole or in part in one or more of the RF circuitries, the baseband circuitry, and/or the processing unit. As used herein, “circuitry” may refer to, be part of, or include an 
     Application Specific Integrated Circuit (ASIC), an electronic circuit, a processor (shared, dedicated, or group), and/or memory (shared, dedicated, or group) that execute one or more software or firmware programs, a combinational logic circuit, and/or other suitable hardware components that provide the described functionality. In some embodiments, the electronic device circuitry may be implemented in, or functions associated with the circuitry may be implemented by, one or more software or firmware modules. In some embodiments, some or all of the constituent components of the baseband circuitry, the processing unit, and/or the memory/storage may be implemented together on a system on a chip (SOC). 
     The memory/storage  740  may be used to load and store data and/or instructions, for example, for the system. The memory/storage for one embodiment may include any combination of suitable volatile memory, such as dynamic random access memory (DRAM)), and/or non-volatile memory, such as flash memory. In various embodiments, the I/O interface  780  may include one or more user interfaces designed to enable user interaction with the system and/or peripheral component interfaces designed to enable peripheral component interaction with the system. User interfaces may include, but are not limited to a physical keyboard or keypad, a touchpad, a speaker, a microphone, etc. Peripheral component interfaces may include, but are not limited to, a non-volatile memory port, a universal serial bus (USB) port, an audio jack, and a power supply interface. 
     In various embodiments, the sensor  770  may include one or more sensing devices to determine environmental conditions and/or location information related to the system. In some embodiments, the sensors may include, but are not limited to, a gyro sensor, an accelerometer, a proximity sensor, an ambient light sensor, and a positioning unit. The positioning unit may also be part of, or interact with, the baseband circuitry and/or RF circuitry to communicate with components of a positioning network, e.g., a global positioning system (GPS) satellite. In various embodiments, the display  750  may include a display, such as a liquid crystal display and a touch screen display. In various embodiments, the system  700  may be a mobile computing device such as, but not limited to, a laptop computing device, a tablet computing device, a netbook, an ultrabook, a smartphone, etc. In various embodiments, the system may have more or less components, and/or different architectures. Where appropriate, the methods described herein may be implemented as a computer program. The computer program may be stored on a storage medium, such as a non-transitory storage medium. 
     The embodiment of the present disclosure is a combination of techniques/processes that may be adopted in 3GPP specification to create an end product. 
     A person having ordinary skill in the art understands that each of the units, algorithm, and steps described and disclosed in the embodiments of the present disclosure are realized using electronic hardware or combinations of software for computers and electronic hardware. Whether the functions run in hardware or software depends on the condition of the application and design requirement for a technical plan. A person having ordinary skill in the art may use different ways to realize the function for each specific application while such realizations should not go beyond the scope of the present disclosure. It is understood by a person having ordinary skill in the art that he/she may refer to the working processes of the system, device, and unit in the above-mentioned embodiment since the working processes of the above-mentioned system, device, and unit are basically the same. For easy description and simplicity, these working processes will not be detailed. 
     It is understood that the disclosed system, device, and method in the embodiments of the present disclosure may be realized in other ways. The above-mentioned embodiments are exemplary only. The division of the units is merely based on logical functions while other divisions exist in realization. It is possible that a plurality of units or components are combined or integrated into another system. It is also possible that some characteristics are omitted or skipped. On the other hand, the displayed or discussed mutual coupling, direct coupling, or communicative coupling operate through some ports, devices, or units whether indirectly or communicatively by ways of electrical, mechanical, or other kinds of forms. 
     The units as separating components for explanation are or are not physically separated. The units for display are or are not physical units, that is, located in one place or distributed on a plurality of network units. Some or all of the units are used according to the purposes of the embodiments. Moreover, each of the functional units in each of the embodiments may be integrated into one processing unit, physically independent, or integrated into one processing unit with two or more than two units. 
     If the software function unit is realized and used and sold as a product, it may be stored in a readable storage medium in a computer. Based on this understanding, the technical plan proposed by the present disclosure may be essentially or partially realized as the form of a software product. Or, one part of the technical plan beneficial to the conventional technology may be realized as the form of a software product. The software product in the computer is stored in a storage medium, including a plurality of commands for a computational device (such as a personal computer, a server, or a network device) to run all or some of the steps disclosed by the embodiments of the present disclosure. The storage medium includes a USB disk, a mobile hard disk, a read-only memory (ROM), a random access memory (RAM), a floppy disk, or other kinds of media capable of storing program codes. 
     The disclosed method may enable synchronization in a target cell and enhance continuity of the synchronization service, even in high mobility environments. The disclosed method may facilitate synchronization in wide areas, such as large automobile assembly factories. The disclosed method provides synchronization in a scenario where a grant master clock is attached to one of a plurality of UEs. A UE with a grant master clock may be applied in a factory environment. An embodiment of the disclosed method allows updating of timing advance (TA), preference of TA, and preference of reference time information (RTI). 
     While the present disclosure has been described in connection with what is considered the most practical and preferred embodiments, it is understood that the present disclosure is not limited to the disclosed embodiments but is intended to cover various arrangements made without departing from the scope of the broadest interpretation of the appended claims.