WO2022067639A1

WO2022067639A1 - Method and apparatus for time synchronization

Info

Publication number: WO2022067639A1
Application number: PCT/CN2020/119310
Authority: WO
Inventors: Jing HAN; Lianhai WU; Mingzeng Dai; Haiming Wang
Original assignee: Lenovo (Beijing) Limited
Priority date: 2020-09-30
Filing date: 2020-09-30
Publication date: 2022-04-07

Abstract

The present application relates to a method and an apparatus for time synchronization. One embodiment of the subject application provides a method performed by a User Equipment (UE), comprising: transmitting a request to a first Base Station (BS) for timing information; receiving a signal indicating a first timing information from the first BS, wherein the first timing information is determined based on a second timing information received from a second BS; and performing time synchronization with the second BS based on the first timing information when a handover procedure from the first BS to the second BS is successful.

Description

METHOD AND APPARATUS FOR TIME SYNCHRONIZATION

TECHNICAL FIELD

The subject application relates to wireless communication, and more particularly, the present disclosure relates to a method and an apparatus for time synchronization.

BACKGROUND OF THE INVENTION

During a handover procedure, it is important for the User Equipment (UE) to be time synchronized with the target BS. For the time sensitive networks (TSN) , this issue is especially important.

Therefore, it is desirable to provide a solution for maintaining 5G system (5GS) timing synchronization accuracy when the UE is in the handover procedure.

SUMMARY

It is desirable to provide a solution to method and an apparatus for time synchronization during a handover procedure.

One embodiment of the subject application provides a method performed by a User Equipment (UE) , comprising: transmitting a request to a first Base Station (BS) for timing information; receiving a signal indicating a first timing information from the first BS, wherein the first timing information is determined based on a second timing information received from a second BS; and performing time synchronization with the second BS based on the first timing information when a handover procedure from the first BS to the second BS is successful.

Another embodiment of the subject application provides a method performed by a User Equipment (UE) , comprising: maintaining a first time clock associated with a first Base Station (BS) and a second time clock associated with a second BS during a handover procedure from the first BS to a second BS; and abandoning the second time clock when the handover procedure fails; or abandoning the first time clock when the handover procedure is successful.

Yet another embodiment of the subject application provides a method performed by a first Base Station (BS) , comprising: receiving a request to from a User Equipment (UE) for timing information; determining a signal indicating a first timing information based on a second timing information received from a second BS; and transmitting the signal to the UE.

Still another embodiment of the subject application provides an apparatus, comprising: a non-transitory computer-readable medium having stored thereon computer-executable instructions; a receiving circuitry; a transmitting circuitry; and a processor coupled to the non-transitory computer-readable medium, the receiving circuitry and the transmitting circuitry, wherein the computer-executable instructions cause the processor to implement the method performed by a User Equipment (UE) , comprising: transmitting a request to a first Base Station (BS) for timing information; receiving a signal indicating a first timing information from the first BS, wherein the first timing information is determined based on a second timing information received from a second BS; and performing time synchronization with the second BS based on the first timing information when a handover procedure from the first BS to the second BS is successful.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 illustrates a schematic diagram of a wireless communication system in accordance with some embodiments of the subject disclosure.

Figure 2a illustrates a flow chart for time synchronization during the handover procedure according to some embodiments of the present disclosure.

Figure 2b illustrates a method for determining the timing information according to some embodiments of the present disclosure.

Figure 2c illustrates another method for determining the timing information according to some embodiments of the present disclosure.

Figure 3 illustrates another flow chart for time synchronization during the handover procedure according to some embodiments of the present disclosure.

Figure 4 illustrates a method performed by a UE for wireless communication according to a preferred embodiment of the present disclosure.

Figure 5 illustrates a method performed by a source BS for wireless communication according to a preferred embodiment of the present disclosure.

Figure 6 illustrates a block diagram of a UE according to the embodiments of the present disclosure.

Figure 7 illustrates a block diagram of a BS according to the embodiments of the present disclosure.

DETAILED DESCRIPTION

The detailed description of the appended drawings is intended as a description of the currently preferred embodiments of the present invention, and is not intended to represent the only form in which the present invention may be practiced. It should be understood that the same or equivalent functions may be accomplished by different embodiments that are intended to be encompassed within the spirit and scope of the present invention.

For TSN synchronization, the entire end to end (E2E) 5G system (5GS) can be considered as a time-aware system. The UEs, BSs, user plane function (UPF) , network (NW) TSN Translators (TT) , device-side (DS) TT, are synchronized with the 5G grand master (GM) clock, i.e. the 5G internal system clock, which keeps these network elements synchronized.

That is, the timing synchronization in the 5G system is important, and one goal of the present disclosure is to study mobility issue for time synchronization, i.e. how to maintain the accuracy of 5GS time synchronization when UE is during mobility e.g. handover.

Figure 1 illustrates a schematic diagram of a wireless communication system. In Figure 1, the wireless communication system includes a UE group, group 1, which includes UE 101, BSs 102, e.g. BS 102-1, and BS 102-2. Even though a specific number of UEs and BS are depicted in Figure 1, persons skilled in the art will recognize that any number of UEs and BSs may be included in the wireless communication system.

The UE 101 may include computing devices, such as desktop computers, laptop computers, personal digital assistants (PDAs) , tablet computers, smart televisions (e.g., televisions connected to the Internet) , set-top boxes, game consoles, security systems (including security cameras) , vehicle on-board computers, network devices (e.g., routers, switches, and modems) , or the like. According to an embodiment of the present disclosure, the UE 101 may include a portable wireless communication device, a smart phone, a cellular telephone, a flip phone, a device having a subscriber identity module, a personal computer, a selective call receiver, or any other device that is capable of sending and receiving communication signals on a wireless network. In some embodiments, the UE 101 includes wearable devices, such as smart watches, fitness bands, optical head-mounted displays, or the like. Moreover, the UE 101 may be referred to as a subscriber unit, a mobile, a mobile station, a user, a terminal, a mobile terminal, a wireless terminal, a fixed terminal, a subscriber station, a user terminal, or a device, or described using other terminology used in the art. The UE 101 may communicate directly with the BSs 102 via uplink (UL) communication signals.

The BSs 102 including BS 102-1 and BS 102-2, may be distributed over a geographic region. In certain embodiments, each of the BSs 102 may also be referred to as an access point, an access terminal, a base, a macro cell, a Node-B, an enhanced Node B (eNB) , a gNB, a Home Node-B, a relay node, or any device described using other terminology used in the art. The BSs 102 are generally parts of a radio access network that may include one or more controllers communicably coupled to one or more corresponding BSs 102.

The wireless communication system is compatible with any type of network that is capable of sending and receiving wireless communication signals. For example, the wireless communication system is compatible with a wireless communication network, a cellular telephone network, a Time Division Multiple Access (TDMA) -based network, a Code Division Multiple Access (CDMA) -based network, an Orthogonal Frequency Division Multiple Access (OFDMA) -based network, an LTE network, a 3rd Generation Partnership Project (3GPP) -based network, a 3GPP 5G network, a satellite communications network, a high altitude platform network, and/or other communications networks.

In some embodiments, the wireless communication system is compatible with the 5G new radio (NR) of the 3GPP protocol, wherein the BSs 102 transmit data using an orthogonal frequency division multiplexing (OFDM) modulation scheme on the downlink and the UEs transmit data on the uplink using a Discrete Fourier Transform-Spread-Orthogonal Frequency Division Multiplexing (DFT-S-OFDM) or a Cyclic Prefix-Orthogonal Frequency Division Multiplexing (CP-OFDM) scheme. More generally, the wireless communication system may implement some other open or proprietary communication protocols, for example, WiMAX, among other protocols.

In some other embodiments, the BSs 102 may communicate using other communication protocols, such as the IEEE 802.11 family of wireless communication protocols. Further, in some embodiments, the BSs 102 may communicate over licensed spectrums, whereas in other embodiments the BSs 102 may communicate over unlicensed spectrums. The present disclosure is not intended to be limited to the implementation of any particular wireless communication system architecture or protocol. In another embodiment, the BSs 102 may communicate with the UEs using 3GPP 5G protocols.

In Figure 1, the UE is in a handover procedure from the source BS 102-1 to the target BS 102-2. As emphasized above, the time synchronization is important during the handover procedure.

In step 201, the UE transmits a request to a source BS, i.e. the serving BS, for timing information. The timing information may include the timing information from the source BS and/or the target BS. After receiving this request, in step 202, the source BS indicates to the target BS that the UE is preferred being provisioned with the timing information. In one embodiment, the source BS may transmit the handover request which includes the UE's interest on timing information to the target BS.

In step 203, the target BS performs admission control, that is, the target BS evaluates whether it has sufficient resource for the UE. After the target BS determines that it can provide the service to the UE, in step 204, the target BS sends timing formation of the target BS to the source BS. The timing information may be transmitted on Xn interface via the handover request acknowledgement. Alternatively, the timing information of the target BS may be included in a container, which is transparent to the source BS.

The timing information of the target BS at least includes two parameters: i) a time field; and ii) a reference system frame number (SFN) , which may be represented as: referenceSFN. The value of the time field indicates the ending time of the system frame indicated by the reference SFN. For example, in Figure 2b, suppose the reference system frame number is 700, then the time field indicates the ending time of the system frame with the number of 700.

Back to Figure 2a, in step 205, after receiving the timing information from the target BS, the source BS determines to transmit this timing information to the UE. In step 206, the source BS sends the timing information of the target BS to the UE. In one embodiment, the timing information of the target BS may be transmitted via the RRC reconfiguration signaling. For example, it may be transmitted in a handover command, such as the handover command ReconfigurationWithSync, or in a conditional handover command, e.g. ConditionalReconfiguration.

After receiving the timing information, the UE stores it. In step 207, the UE accesses to the target BS, i.e. handover from the source BS to the target BS. After the handover procedure is successful, i.e. the UE switches from the source BS to the target BS, in step 208, the UE performs time synchronization with the target BS using the stored timing information. In another case, as long as the UE has downlink (DL) synchronized with the target BS and obtained the timing advance (TA) information from the target BS, the UE performs time synchronization with the target BS using the stored timing information. In the 5G systems, the UE may perform 5GS time synchronization with the target BS using the stored timing information. Then the UE may inform upper layers of the reference time.

In step 209, the UE confirms that the handover to the target BS is successful to the target BS by transmitting an RRCreconfigurationcomplete message to the target BS.

Figure 2b includes three periods of SFN. One period of SFN ranges from 0 to 1023. In Figure 2b, the timing information of the target BS includes the SFN 500. The system frame when the UE switches from the source BS is the system frame with SFN of 700. Since there are multiple periods of SFN, it means that there are multiple system frames with a frame number of 500. The UE then selects the system frame with a shortest the distance to the SFN 700 among the multiple SFN 500.

Although only three SFN 500 are displayed in Figure. 2b, it should be noted that any number of SFN 500 may be included.

The distance from SFN 700 to the first SFN 500, D1, is 200 frames, i.e. 2s; the distance from SFN 700 to the second SFN 500, D2, is 1023-700+501 = 824 frames, i.e. 8.24s; the distance from SFN 700 to the third SFN 500, D3, is 1023-700+1024+501 = 1848 frames, i.e. 18.24s. Therefore, the distance of D1 is shortest among the three distances in time domain, and the UE determines the first SFN 500 is the system framed indicated by the reference SFN, and the UE uses the time and the first SFN 500 to perform the time synchronization with the target BS. In conclusion, the UE determines the system frame indicated by the reference SFN has a shortest distance in time domain to the system frame among multiple system frames with the reference SFN of the target BS.

There is one specification condition, for example, in Figure 2b, the system frame when UE switches from the source BS is the system frame with SFN of 1012. Then based on calculation, D1 is 512 frames, and D2 is also 512 frames. Then further operations may be needed to perform the time synchronization with the target BS correctly. For example, the UE may request the timing information of the target UE for the second time. For another example, a default location is utilized, e.g. if there are two location has the same distance in time domain, then the first location is the location that the BS indicated.

During a conditional handover (CHO) procedure, the source base station may transmit a conditional handover command to the UE, indicating one or more handover execution conditions for a handover. The UE may execute the handover, e.g. transmit a random access preamble to a target BS, when one or more criteria of the handover execution conditions are met by a target BS. Therefore, there may be a long period from the time when the UE receives the CHO command, to the time when the UE performs the CHO and accesses the target BS. When the period is longer than a whole period of SFN, which is 1024 frames, 10.24s, or the period is longer than a half period of SFN, which is 512 frames, 5.12s, the system frame determined by the UE according to the method in Figure 2b, may not be the system frame actually indicated by the target BS.

For example, in Figure 2c, the first system frame with the frame number of 500 is indicated by the target BS, and the UE accesses to the target BS in the system frame with the frame number of 700. Based on the calculation method in Figure 2b, the UE determines the second system frame with the SFN 500 is the system frame indicated by the target BS. Therefore, the time synchronization between the UE and the target BS is incorrect.

The present discloser proposes to add to reference hyper frame number (HFN) into the timing information to overcome this objection. That is, in step 204 in Figure 2a, the timing information transmitted from the target BS to the source BS includes at least three parameters: i) a time field; ii) a reference SFN; and iii) a reference HFN, which may be represented as: referenceHFN. Accordingly, the system frame is determined by the second and third parameters: a reference SFN and a reference HFN.

In Figure 2c, the target BS indicates the timing information, which includes the reference SFN, e.g. 500, and the reference HFN, e.g. HFN1. After obtaining this timing information, the UE determines that the first system frame with a SFN of 500 is the indicated system frame, and uses the first system frame and the value of the time field to perform the time synchronization.

In the solution in Figure 3, the timing information transmitted by the target BS does not include a reference SFN or a reference HFN.

In step 301, the UE transmits a request to source BS, or serving BS, for timing information. After receiving this request, in step 302, the source BS indicates the target BS that the UE is preferred being provisioned with the timing information. In one embodiment, the source BS may transmit the handover request which includes the UE's interest on timing information to the target BS.

In step 303, the target BS performs admission control, that is, the target BS evaluates whether the target BS has sufficient resource for the UE. After the target BS determines that it can provide the service to the UE, in step 304, the target BS performs time stamping. The time stamped may be the time when the target BS performs the time stamping, or may be a future time, for example, the time when the target BS will transmit the timing information to the source BS.

In step 305, the target BS transmits the reference timing information, e.g. referenceTimeInfo, to the source BS, the reference timing information may be transmitted on Xn interface via the handover request acknowledgement. The reference timing information may indicate the time when the target BS will transmit the timing information to the source BS; alternatively, the reference timing information may indicate the time that the target BS performs time stamping.

For the next generation (NG) handover, the timestamp information may be included in a General Packet Radio Service (GPRS) Tunneling Protocol User Plane (GTP-U) message, a Stream Control Transmission Protocol (SCTP) protocol message. For example, the timestamp information may be added to the GTP-U header or GTP-U extension header, or SCTP header. The timestamp information may also be included in an Xn Application Protocol (XnAP) message, e.g. in the handover request acknowledge message. The timestamp information may still be included an inter radio access network (RAN) node transparent container, so that the timestamp information can be transparent to core network nodes.

After receiving the reference timing information, the source BS may use the reference timing information to determine the time offset component 1. The time offset component 1 may include a time period from the time when the target BS transmits the reference timing information of the target BS to the source BS, to the time when the source BS receives the reference timing information, in other words, the time offset component 1 includes the delay on the Xn interface, delay 1 in Figure 3. Alternatively, the time offset component 1 may include a time period from the time when the target BS performs the time stamping, to the time when the source BS receives the reference timing information. In this embodiment, the time offset component 1 includes two parts, the first part is a period from the time when the target BS performs the time stamping, to the time when the target BS transmits the reference timing information; the second part is a period from when the target BS transmits the reference timing information, to the time when the source BS receives the reference timing information, i.e. the delay on the Xn interface, also is delay 1 as shown in Figure 3.

In step 306, the source BS determines the time offset component 2, which is a residence period in the source BS. It is from the time when the source BS receives the reference timing information from target BS, to the time when the source BS transmits the updated timing information to the UE. The updated timing information may include a sum of the time offset component 1 and the time offset component 2 to the reference timing information. That is, after determining the time offset component 2, the source BS may add the time offset component 1 and the time offset component 2 to the reference timing information of the target BS, and transmit the updated reference timing information of target BS, namely, the sum, to the UE in step 307.

The updated timing information may include the time offset component 1, the time offset component 2, and the reference timing information of the target BS. That is, the source BS may directly transmit the time offset component 1, the time offset component 2, and the reference timing information of target BS to the UE. The updated timing information may be transmitted via the RRC reconfiguration signaling. For example, it may be transmitted in a handover command, such as the command ReconfigurationWithSync, or in a conditional handover command, e.g. ConditionalReconfiguration.

After receiving the updated timing information from the source BS, the UE determines the time offset component 3, which is a period from the time when the source BS transmits the updated timing information, to the time when the UE calculates the reference time. This period includes two parts, one part is a period from the time when the source BS transmits the updated timing information, to the time when the UE receives the updated timing information, i.e. the delay on the Uu interface, also is as shown the delay 2 in Figure 3. The other part is the residence time in UE, which is a period from the time when the UE receives the updated timing information, to the time when the UE calculates the reference time.

The UE may determine the delay on the Uu interface. Regarding the period from the time when the UE receives the updated timing information, the UE may introduce a timer to count it. The timer is started, or restarted, when the UE correctly receive updated timing information from the source BS, that is, the UE receives the updated timing information in the handover command, for example, it may be transmitted in a handover command, such as the command ReconfigurationWithSync, or in a conditional handover command, e.g. ConditionalReconfiguration.

When UE determines the reference time, the timers is stopped, and the value of the timer is the residence time of timing information.

The reference time is the sum of the above three time offset components, the UE then stores it. In step 307, the UE accesses to the target BS. After the accessing is successful, i.e. the UE switches from the source BS to the target BS, in step 309, the UE performs time synchronization with the target BS using the stored timing information. In another case, as long as the UE has DL synchronized with the target BS and obtained the TA information from the target BS, the UE performs time synchronization with the target BS using the stored timing information. Then UE may inform upper layers of the reference time.

In step 310, the UE confirms that the handover to the target BS is successful to the target BS by transmitting an RRCreconfigurationcomplete message to the target BS.

In one preferred embodiment, the UE may perform a Dual Active Protocol Stack (DAPS) handover procedure. In this case, the UE may be synchronized to both the source BS and the target BS simultaneously.

Specifically, the UE can maintain two time clocks at the same time, one time clock is synchronized with the source BS, and the other is synchronized with the target BS. The UE may calculate reference time of the source BS whenever UE receives the source BS timing information from the source BS, and may calculate reference time of the target BS whenever UE receives the target BS timing information from the source BS, thus the UE is synchronized to both the source BS and the target BS.

When the UE is switched from the source BS to the target BS successfully, i.e. after UE transmit RRCReconfigurationComplete message to target BS, the time clock associated with the source BS is abandoned.

When the handover procedure from the source BS to the target BS fails, the UE maintains the time clock is associated with the source BS, and abandons the clock is associated with the target BS.

Figure 4 illustrates a method performed by a UE for wireless communication according to a preferred embodiment of the subject disclosure.

In step 401, the UE transmit a request to the first BS for timing information. In step 402, the UE receives a signal indicating a first timing information from the first BS, wherein the first timing information is determined based on a second timing information received from a second BS. In step 403, the UE performs time synchronization with the second BS based on the first timing information when a handover procedure from the first BS to the second BS is successful. Or, the UE performs time synchronization with the second BS based on the first timing information when the UE has DL synchronized with the target BS and obtained the TA information from the target BS. The first BS may be the source BS, and the second BS may be the target BS.

The first timing information or the second timing information includes a reference SFN of the second BS and an ending time of a system frame of the second BS indicated by the reference SFN, that is, two parameters: i) a reference SFN, and ii) a time value of the end of the system frame indicated by the reference SFN. For example, as shown in Figure 2b, the reference SFN is 700, and the ending time is the time indicated by the "time field" .

After receiving this timing information, the UE determines the system frame indicated by the reference SFN has a shortest distance in time domain to the system frame when the handover procedure is successful, among multiple system frames with the reference SFN. For example, in Figure 2b, the UE determines that D1 has the shortest distance among the three distances in time domain, D1, D2, and D3, thus the first system frame which correspond to the distance D1 is the system frame indicated by the target BS.

The first timing information or the second timing information may further include a reference HFN of the second BS, in addition to the reference SFN of the second BS and the ending time of a system frame of the second BS indicated by the reference SFN. Therefore, there are there parameters: i) a reference SFN, and ii) a time value of the end of the system frame indicated by the reference SFN; and iii) a reference HFN. For example, in Figure 2c, the system frame indicated by the second BS has a reference HFN with a value of HFN1.

In one embodiment, the second timing information includes a time when the second BS transmits the second reference timing information to the first BS, or a time when the second BS performs time stamping. For example, in Figure 3, the target BS performs time stamping, and transmits the time to the source BS.

In another embodiment, the first timing information is further determined based on a delay from the first BS and the second BS and a period from a time when the first BS receives the second timing information to a time when the first BS transmits the first timing information to the UE. That is, the first timing information is determined based on the delay on the Xn interface and the residence time on the source BS.

The UE then performs time synchronization based on the first timing information, a delay from the first BS to the UE, and a period from a time when the UE receives the first timing information to a time when the UE processes the first timing information. Namely, the UE performs time synchronization based on the first timing information, a delay on the Uu interface and the residence time on the UE.

The UE may set a timer to determine the period from the time when the UE receives the first timing information to the time when the UE processes the first timing information.

One preferred embodiment of the present discloses relates to the case of DAPS. The UE maintains the first clock associated with the first BS, i.e. the source BS, and the second clock associated with the second BS, i.e. the target BS, during a handover procedure from the first BS to a second BS. When the handover procedure fails, the UE may abandon the second time clock, otherwise, the UE may abandon the first time clock.

Figure 5 illustrates a method performed by a source BS for wireless communication according to a preferred embodiment of the subject disclosure.

In step 501, the BS receives a request to from the UE for timing information, in step 502, the source BS determines a signal indicating a first timing information based on a second timing information received from a second BS, and in step 503, the BS transmits the signal to the UE. More specifically, the source BS receives a request from the UE, and the UE is preferred to be provided with the reference timing information of the target BS. The source BS then determines the second timing information based on the reference timing information from the target BS, and transmits it to the UE.

The second timing information is received in one of the following signals: a GTP-U message, a SCTP protocol message, a XnAP message, or an inter RAN node transparent container.

Figure 6 illustrates a block diagram of a UE according to the embodiments of the subject disclosure.

The UE may include a receiving circuitry, a processor, and a transmitting circuitry. In one embodiment, the UE may include a non-transitory computer-readable medium having stored thereon computer-executable instructions; a receiving circuitry; a transmitting circuitry; and a processor coupled to the non-transitory computer-readable medium, the receiving circuitry and the transmitting circuitry. The computer executable instructions can be programmed to implement a method (e.g. the method in Figure 4) with the receiving circuitry, the transmitting circuitry and the processor.

Figure 7 illustrates a block diagram of a BS according to the embodiments of the subject disclosure.

The BS may include a receiving circuitry, a processor, and a transmitting circuitry. In one embodiment, the BS may include a non-transitory computer-readable medium having stored thereon computer-executable instructions; a receiving circuitry; a transmitting circuitry; and a processor coupled to the non-transitory computer-readable medium, the receiving circuitry and the transmitting circuitry. The computer executable instructions can be programmed to implement a method (e.g. the method in Figure 5) with the receiving circuitry, the transmitting circuitry and the processor.

The method of the present disclosure can be implemented on a programmed processor. However, controllers, flowcharts, and modules may also be implemented on a general purpose or special purpose computer, a programmed microprocessor or microcontroller and peripheral integrated circuit elements, an integrated circuit, a hardware electronic or logic circuit such as a discrete element circuit, a programmable logic device, or the like. In general, any device that has a finite state machine capable of implementing the flowcharts shown in the figures may be used to implement the processing functions of the present disclosure.

While the present disclosure has been described with specific embodiments thereof, it is evident that many alternatives, modifications, and variations will be apparent to those skilled in the art. For example, various components of the embodiments may be interchanged, added, or substituted in other embodiments. Also, all of the elements shown in each figure are not necessary for operation of the disclosed embodiments. For example, one skilled in the art of the disclosed embodiments would be capable of making and using the teachings of the present disclosure by simply employing the elements of the independent claims. Accordingly, the embodiments of the present disclosure as set forth herein are intended to be illustrative, not limiting. Various changes may be made without departing from the spirit and scope of the present disclosure.

In this disclosure, relational terms such as "first, " "second, " and the like may be used solely to distinguish one entity or action from another entity or action without necessarily requiring or implying any actual such relationship or order between such entities or actions. The terms "comprises, " "comprising, " or any other variation thereof, are intended to cover a non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements does not include only those elements but may include other elements not expressly listed or inherent to such process, method, article, or apparatus. An element proceeded by "a, " "an, " or the like does not, without more constraints, preclude the existence of additional identical elements in the process, method, article, or apparatus that comprises the element. Also, the term "another" is defined as at least a second or more. The terms "including, " "having, " and the like, as used herein, are defined as "comprising. "

Claims

A method performed by a User Equipment (UE) , comprising:

transmitting a request to a first Base Station (BS) for timing information;

receiving a signal indicating a first timing information from the first BS, wherein the first timing information is determined based on a second timing information received from a second BS; and

performing time synchronization with the second BS based on the first timing information when a handover procedure from the first BS to the second BS is successful.
The method of Claim 1, wherein the first timing information or the second timing information includes a reference system frame number (SFN) of the second BS and an ending time of a system frame of the second BS indicated by the reference SFN.
The method of Claim 2, further comprising:

determining that the system frame indicated by the reference SFN has a shortest distance in time domain to the system frame when the handover procedure is successful, among multiple system frames with the reference SFN of the second BS.
The method of Claim 2, wherein the first timing information or the second timing information further includes a reference hyper frame number (HFN) of the second BS, and the ending time of the system frame is indicated by the reference SFN and the reference HFN.
The method of Claim 1, wherein the second timing information includes a time when the second BS transmits the second reference timing information to the first BS, or a time when the second BS performs time stamping.
The method of Claim 5, wherein the first timing information is further determined based on a delay from the first BS and the second BS and a period from a time when the first BS receives the second timing information to a time when the first BS transmits the first timing information to the UE.
The method of Claim 6, further comprising:

performing time synchronization based on the first timing information, a delay from the first BS to the UE, and a period from a time when the UE receives the first timing information to a time when the UE processes the first timing information.
The method of Claim 7, further comprising:

setting a timer to determine the period from the time when the UE receives the first timing information to the time when the UE processes the first timing information.
A method performed by a User Equipment (UE) , comprising:

maintaining a first time clock associated with a first Base Station (BS) and a second time clock associated with a second BS during a handover procedure from the first BS to a second BS; and

abandoning the second time clock when the handover procedure fails; or

abandoning the first time clock when the handover procedure is successful.
A method performed by a first Base Station (BS) , comprising:

receiving a request to from a User Equipment (UE) for timing information;

determining a signal indicating a first timing information based on a second timing information received from a second BS; and

transmitting the signal to the UE.
The method of Claim 10, wherein the first timing information or the second timing information includes a reference system frame number (SFN) of the second BS and an ending time of a system frame of the second BS indicated by the reference SFN.
The method of Claim 11, wherein the first timing information or the second timing information further includes a reference hyper frame number (HFN) of the second BS, and the ending time of the system frame is indicated by the reference SFN and the reference HFN.
The method of Claim 10, wherein the second timing information includes a time when the second BS transmits the second reference timing information to the first BS, or a time when the second BS performs time stamping.
The method of Claim 13, wherein the second timing information is received in one of the following signals: a General Packet Radio Service (GPRS) Tunneling Protocol User Plane (GTP-U) message, a Stream Control Transmission Protocol (SCTP) protocol message, a Xn Application Protocol (XnAP) message, or an inter radio access network (RAN) node transparent container.
The method of Claim 13, wherein the first timing information is further determined based on a delay from the first BS and the second BS and a period from a time when the first BS receives the second timing information to a time when the first BS transmits the first timing information to the UE.