KR20200045952A - Method and apparatus for handover in mobile communication system - Google Patents

Abstract

Disclosed is a transferring method between a source base station and a target base station. An operation method of a terminal includes the steps of: receiving a transfer command from the source base station; determining whether to perform a single connection-based transfer or a simultaneous connection-based transfer with the source base station and the target base station based on the transfer command; performing connection release with a source base station and performing connection establishment with a target base station, when a single connection-based transfer is performed; and performing connection release with the source base station after completion of connection establishment with the target base station without disconnecting from the source base station when simultaneous connection-based transfer is performed.

Description

Method and apparatus for handover in mobile communication system

The present invention relates to a handover method and apparatus applied to a mobile communication system, and more specifically, a handover method using simultaneous connectivity (simultaneous connectivity) capable of eliminating data interruption (interruption) time during handover, and It is about the device.

When handover occurs due to the movement of the terminal in the 3 ^rd Generation Partnership Project (3GPP) Long-Term Evolution (LTE) system, the terminal disconnects from the source base station (or cell), and the target (target) During the connection with the base station (or cell), a disconnect time occurs during which data cannot be normally received. That is, in the case of the LTE system, a break-before-make handover is applied.

In order to reduce the disconnection time, it is necessary to maintain a connection with the source base station during handover, and a handover scheme (ie, make-before-break handover) that disconnects the source base station after the connection to the target base station is completed. Are being discussed. However, in the conventional single power down connection handover method, since the source base station does not know the handover operation status of the terminal, it operates in a manner of transmitting data to the terminal while forwarding data to the target base station.

In this case, since the source base station does not know when the terminal handovers to the target base station, there may be a problem of wasting resources between base stations by forwarding unnecessary data. In addition, the source base station may have a problem of wasting radio resources by transmitting data to the terminal even though the terminal is already handed over to the target base station.

An object of the present invention for solving the above problems is to provide a handover method using a simultaneous connection, which can eliminate the data disconnection time.

Another object of the present invention for solving the above problems is to provide a configuration of a base station and a terminal that performs handover using a simultaneous connection, which can eliminate data disconnection time.

An embodiment of the present invention for achieving the above object is a method of operating a terminal performing handover between a source base station and a target base station, the method comprising: receiving a handover command (handover command) from the source base station; Determining whether to perform a single connectivity handover or a simultaneous connectivity handover with the source base station and the target base station based on the handover command; When the single connection-based handover is performed, performing connection release with the source base station and performing connection establishment with the target base station; And when the simultaneous connection-based handover is performed, performing connection disconnection with the source base station after completion of connection establishment with the target base station without disconnection from the source base station.

According to an embodiment of the present invention, the handover operation step performed by the terminal is notified to the source base station so that only necessary data is forwarded from the source base station to the target base station. In addition, after the source base station knows that the terminal has completed handover to the target base station, data is not transmitted to the terminal so that radio resources are not wasted.

1 is a state transition diagram of a terminal in an LTE mobile communication system.
2 is a flowchart illustrating a handover method and a data disconnection time in an LTE mobile communication system.
3 is a state transition diagram of a terminal supporting a handover method according to an embodiment of the present invention.
4 is a flowchart illustrating a handover method according to an embodiment of the present invention.
5 shows an example of a data forwarding and PDCP retransmission procedure in a handover method of an LTE mobile communication system.
6 illustrates an example of data forwarding and PDCP retransmission procedures in a simultaneous connection-based handover method according to an embodiment of the present invention.
7 illustrates another example of a data forwarding and PDCP retransmission procedure in a simultaneous connection-based handover method according to an embodiment of the present invention.
8 is a block diagram illustrating a structure of a device to which embodiments of the present invention can be applied.

The present invention can be applied to various changes and may have various embodiments, and specific embodiments will be illustrated in the drawings and described in detail in the detailed description. However, this is not intended to limit the present invention to specific embodiments, and should be understood to include all modifications, equivalents, and substitutes included in the spirit and scope of the present invention. In describing each drawing, similar reference numerals are used for similar components.

Terms such as first, second, A, and B can be used to describe various components, but the components should not be limited by the terms. The terms are used only for the purpose of distinguishing one component from other components. For example, the first component may be referred to as a second component without departing from the scope of the present invention, and similarly, the second component may be referred to as a first component. The term and / or includes a combination of a plurality of related described items or any one of a plurality of related described items.

When an element is said to be "connected" or "connected" to another component, it is understood that other components may be directly connected to or connected to the other component, but other components may exist in the middle. It should be. On the other hand, when a component is said to be "directly connected" or "directly connected" to another component, it should be understood that no other component exists in the middle.

The terms used in this application are only used to describe specific embodiments, and are not intended to limit the present invention. Singular expressions include plural expressions unless the context clearly indicates otherwise. In this application, the terms "include" or "have" are intended to indicate the presence of features, numbers, steps, actions, components, parts or combinations thereof described herein, one or more other features. It should be understood that the existence or addition possibilities of fields or numbers, steps, operations, components, parts or combinations thereof are not excluded in advance.

Unless defined otherwise, all terms used herein, including technical or scientific terms, have the same meaning as commonly understood by a person skilled in the art to which the present invention pertains. Terms such as those defined in a commonly used dictionary should be interpreted as having meanings consistent with meanings in the context of related technologies, and should not be interpreted as ideal or excessively formal meanings unless explicitly defined in the present application. Does not.

Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings.

The present invention can be applied to various changes and may have various embodiments, and specific embodiments will be illustrated in the drawings and described in detail in the detailed description. However, this is not intended to limit the present invention to specific embodiments, and should be understood to include all modifications, equivalents, and substitutes included in the spirit and scope of the present invention. In describing each drawing, similar reference numerals are used for similar components.

Terms such as first, second, A, and B can be used to describe various components, but the components should not be limited by the terms. The terms are used only for the purpose of distinguishing one component from other components. For example, the first component may be referred to as a second component without departing from the scope of the present invention, and similarly, the second component may be referred to as a first component. The term and / or includes a combination of a plurality of related described items or any one of a plurality of related described items.

When an element is said to be "connected" or "connected" to another component, it is understood that other components may be directly connected to or connected to the other component, but other components may exist in the middle. It should be. On the other hand, when a component is said to be "directly connected" or "directly connected" to another component, it should be understood that no other component exists in the middle.

The terms used in this application are only used to describe specific embodiments, and are not intended to limit the present invention. Singular expressions include plural expressions unless the context clearly indicates otherwise. In this application, the terms "include" or "have" are intended to indicate the presence of features, numbers, steps, actions, components, parts or combinations thereof described herein, one or more other features. It should be understood that the existence or addition possibilities of fields or numbers, steps, operations, components, parts or combinations thereof are not excluded in advance.

Unless defined otherwise, all terms used herein, including technical or scientific terms, have the same meaning as commonly understood by a person skilled in the art to which the present invention pertains. Terms such as those defined in a commonly used dictionary should be interpreted as having meanings consistent with meanings in the context of related technologies, and should not be interpreted as ideal or excessively formal meanings unless explicitly defined in the present application. Does not.

Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings.

Embodiment according to the present invention are described below on the assumption that the application for convenience ^{3GPP (3 rd Generation Partnership Project)} LTE (Long-Term Evolution) , or NR (New Radio) mobile communication system of the description. However, embodiments according to the present invention can be applied to other mobile communication systems other than 3GPP LTE or NR mobile communication systems.

In addition, hereinafter, embodiments according to the present invention are mainly described as a method of operating a terminal performing handover. However, even when a method performed in the terminal (for example, transmission or reception of a signal or a message) is described, the corresponding base station corresponds to a method performed in the terminal (for example, reception of a signal or message or Transmission). That is, when the operation of the terminal is described, the corresponding base station may perform an operation corresponding to the operation of the terminal. Conversely, when the operation of the base station is described, the corresponding terminal may perform an operation corresponding to the operation of the base station. Therefore, it should be understood that the embodiments according to the present invention are applicable not only to a terminal but also to a base station.

1 is a state transition diagram of a terminal in an LTE mobile communication system.

Referring to FIG. 1, in the LTE system, when the RRC connection with the base station is successfully set through the RRC connection establishment procedure (S101) in the RRC idle state (RRC_IDLE state, 100), It may transition to the RRC connected state (RRC_CONNECTED state, 110).

The terminal in the RRC connection state 110 may transition back to the RRC idle state 100 when the RRC connection with the base station is released through the RRC connection release procedure (S102). In addition, when the terminal in the RRC connection state 110 performs handover to change the serving base station (or cell) to reset the changed serving base station (ie, target base station) and the RRC connection, the RRC connection state with the changed serving base station To maintain.

Meanwhile, in a conventional handover procedure, a data interruption time may occur in a process of changing a connection from a current serving base station (ie, a source base station) to a new serving base station (ie, a target base station). .

2 is a flowchart illustrating a handover method and a data disconnection time in an LTE mobile communication system.

The handover procedure illustrated in FIG. 2 is briefly reconstructed based on procedures necessary to describe the handover method of the LTE mobile communication system, and detailed procedures or subsequent procedures are omitted. In addition, the names of the messages used in the following description are used according to the intended purpose of the messages, and may be different from the names defined in the actual standard.

Referring to FIG. 2, the source base station 210 may perform measurement configuration so that the terminal 200 can perform measurements on neighboring base stations including the target base station 220. Next, the terminal 200 performs measurement on the signal strength of the neighboring base station including the source / target base station, and reports the measurement report to the source base station 210 when the measurement result satisfies a predetermined condition. It can be done (S210). At this time, the reported measurement result may include a signal measurement result of neighboring base stations.

The source base station 210 determines the handover to the target base station 220 based on the received measurement result, and transmits a handover preparation request message to the target base station 220 to the target base station 220 Handover preparation is instructed (S220). In this case, the source base station 210 may include the measurement result of the target base station 220 received from the terminal 200 in the handover preparation request message and transmit it to the target base station.

On the other hand, the target base station 220 determines whether to accept the handover of the terminal through admission control (S230), and when it is determined to accept the handover of the terminal, the hand to the source base station 210 A handover preparation acknowledgment message may be transmitted (S231).

The source base station 210 may transmit a handover command message to the terminal 200 (S240). In this case, the handover command message may be transmitted as an 'RRCConnectionReconfiguration' message, for example, an RRC signaling message. Also, the handover command message may be transmitted by including information on a resource to be used in a random access procedure for a target supporting station, which is received and included in a handover preparation approval message.

Upon receiving the handover command message, the terminal immediately performs handover. Therefore, the terminal 200 acquires downlink synchronization with the target base station 220 (S250), disconnects the connection with the source base station 110 (S251), and connects to the target base station 120. You can try (S260). Meanwhile, the source base station 210 may forward data to be transmitted to the terminal 200 to the target base station 220 (S241).

Subsequently, the terminal 200 attempts random access (RA) based on the information on the resource to be used in the random access procedure provided by the target base station included in the handover command message (S261), and the target base station Sends a random access response (RAR: RA Response) in response to the RACH (SS261). When the access procedure to the target base station 220 is completed based on the above procedure, the terminal 200 may transmit a handover complete message to the target base station 220 (S280). Upon receiving the handover completion message from the terminal 200, the target base station 220 may initiate data transmission to the terminal (S280).

Meanwhile, in the conventional handover method, the terminal completes handover to the target base station 220 from the time before and after the source base station 210 forwards data to the terminal base station 220 to receive data from the target base station. Until the point in time at which it can be done, a disconnection time (ie, data disconnection time) in which data cannot be received occurs. The handover disconnection time is generally analyzed to be about 49.5 ms.

Handover method according to the invention

Hereinafter, a handover method capable of minimizing or eliminating (eg, 0 ms) data disconnection time according to an embodiment of the present invention will be described. In order to describe a handover method according to an embodiment of the present invention, a concept of a state transition and a specific handover procedure of a terminal in consideration of simultaneous connection-based handover are described.

On the other hand, the handover method according to the present invention, compared to the break-before-make handover method in the prior art, the 'enhanced make-before-break handover method' It can be referred to as

3 is a state transition diagram of a terminal supporting a handover method according to an embodiment of the present invention.

Referring to FIG. 3, a terminal supporting a handover method according to an embodiment of the present invention performs RRC with a base station through an RRC connection establishment procedure (S301) in an RRC idle state 300 as in the prior art illustrated in FIG. 1 When the connection establishment is normally completed, it may transition to the RRC connection state 310.

In the RRC connection state 310, when the UE releases the RRC connection establishment with the base station through the RRC connection release procedure (S302), the terminal transitions to the RRC idle state 300. In the RRC connection state 310, when the UE performs handover to change the serving base station and the RRC connection is reset, the RRC connection state 310 is maintained. That is, the description so far is the state transition of the terminal when the single connection-based handover described in FIG. 2 is applied, and is the same as in the prior art.

Meanwhile, when a handover based on simultaneous connection according to an embodiment of the present invention is applied (that is, when the terminal maintains a connection with two base stations-a source base station and a target base station-and performs handover), the RRC of the terminal The connection state may be divided into an RRC_CONNECTED with simultaneous connectivity state (320) and a conventional RRC_CONNECTED with single connectivity state (310).

That is, the RRC simultaneous connection state 320 is a state in which a terminal maintains an RRC connection simultaneously with a source base station and a target base station, and the RRC single connection state 310 allows a terminal to establish a single RRC connection with a source base station or a target base station. It is maintained.

4 is a flowchart illustrating a handover method according to an embodiment of the present invention.

The handover method illustrated in FIG. 4 is different from the conventional handover method illustrated in FIG. 1, when the source base station transmits a handover command to the terminal, the terminal determines the actual handover execution time. Determine and transmit a handover indication (handover indication) message to the source base station. When conditional handover is applied, the handover command means a conditional handover command.

In addition, the handover method illustrated in FIG. 4 is briefly reconstructed based on procedures necessary to describe embodiments of the present invention, and detailed procedures or subsequent procedures are omitted. In addition, the names of the messages used in the following description are used according to the intended purpose of the messages, and may be different from the names defined in the actual standard.

Referring to FIG. 4, as in the conventional handover method described based on FIG. 2, the source base station (or cell) 410 has a terminal 400 around the target base station (or cell) 420. Measurement configuration may be performed to perform measurements on base stations. Next, the terminal 400 performs measurement on the signal strength of the neighboring base station including the source / target base station, and reports a measurement report to the source base station 410 when the measurement result satisfies a predetermined condition. It can be done (S410). At this time, the reported measurement result may include a signal measurement result of a neighboring base station.

The source base station 410 determines the handover to the target base station 420 based on the received measurement result, and transmits a handover preparation request message (handover preparation) to the target base station 420 to prepare the handover to the target base station Instructs (S420). At this time, the source base station 410 may include the measurement result of the target base station 420 received from the terminal 400 in the handover preparation request message and transmit it to the target base station 420.

The target base station 420 determines whether to accept the handover of the terminal through admission control (S430), and when it is determined to accept the handover of the terminal, prepares a handover to the source base station 410 A handover preparation acknowledgment message may be transmitted (S431). The source base station 410 may transmit a handover command message to the terminal 400 (S440). In this case, the handover command message may be transmitted as an 'RRCConnectionReconfiguration' message, for example, an RRC signaling message.

In the example of FIG. 2, unlike the terminal that receives the handover command message immediately performs handover, in the handover method of the present invention, the terminal that receives the handover command message is the first to target the data disconnection time to a minimum. The time to transmit the uplink may be determined as a handover execution event, and in this case (S441), a handover execution procedure is performed. And, in the handover method of the present invention based on the conditional handover procedure, the terminal receiving the handover command message proceeds with the handover execution procedure when a handover execution event occurs (S441).

On the other hand, the terminal maintains a connection with only one base station to perform a handover (i.e., a single connection-based handover) or maintains a connection with two base stations to perform a handover (i.e., a simultaneous connection-based handover). The source base station may determine and inform the target base station through a handover preparation request message, and may inform the terminal through a handover command message (eg, RRCConnectionReconfiguration message) (S440). Alternatively, the target base station determines whether the terminal performs the single connection-based handover or the simultaneous connection-based handover and informs the source base station through a handover preparation approval message, and the source base station performs a handover command message ( For example, the RRCConnectionReconfiguration message) may be notified to the terminal (S440). That is, in the handover method according to an embodiment of the present invention, a single connection-based handover and a simultaneous connection-based handover may be selected according to a situation and environment based on a determination of a source base station or a target base station.

Upon receiving the handover command from the source base station, the terminal may perform downlink synchronization with the target base station 420 when an event that requires actual handover occurs (S441) (S450). Thereafter, the terminal may transmit a first handover indication message to the source base station 420 (S451), and the source base station 410 may transmit the received first handover message to the target base station 420. It can be (S452).

At this time, the first handover indication message is a message for indicating the actual handover execution time determined by the terminal in the present invention, and starts data forwarding from the source base station to the target base station, and transmits data from the single connection-based handover to the terminal. It can be used to determine when to stop, or to attempt a connection connection from the terminal to the target base station, and to release the connection with the source base station in a single connection-based handover. This will be described later.

The terminal may determine whether to perform a single connection-based handover or a simultaneous connection-based handover based on the handover command message received from the source base station (S460). When instructed to perform a single connection-based handover, the terminal 400 may release the connection with the source base station in the same manner as in the conventional procedure (S461), and may perform a random access procedure for the target base station (S470).

On the other hand, when instructed by a source base station to perform a simultaneous connection-based handover based on the handover command message, a random access procedure with a target base station may be performed without releasing the connection with the source base station (S470). . In this case, handover (RACH-less handover) without transmission of a random access channel (RACH) may be performed, but in the handover method of the present invention, regardless of whether or not RACH-less handover is performed, Since the connection with the source base station is maintained, no data disconnection time occurs. This will be described later.

Subsequently, when the connection with the target base station 420 is successfully completed, the terminal 400 transmits a handover complete message to the target base station 420 (S480), and to the source base station 410 A second handover indication message instructing to release the connection to the source base station is transmitted (S491). Thereafter, the terminal 400 releases the connection with the source base station 410 (S492). Meanwhile, the target base station 420 may start data transmission to the target base station 420 after receiving the handover completion message from the terminal 400 (S480) (S490).

In this case, the second handover indication message may be used to determine when the data transmission from the source base station to the terminal is stopped in the case of simultaneous connection-based handover or when the terminal releases the connection with the serving cell in the simultaneous connection-based handover. You can. This will be described later.

Referring to FIG. 4, a data disconnection time when a handover method according to an embodiment of the present invention is applied may be described.

When the handover method according to the prior art is executed, since the random access procedure and the DL optimization procedure with the target base station must be performed after disconnection from the source base station, the data disconnection time is at least 14.5 ms. .

In addition, when the handover method according to the prior art is executed in the form of a RACH-less handover (ie, a handover in which a random access procedure with a target base station can be omitted), the target base station after disconnection from the source base station Although the random access procedure can be omitted, the downlink optimization procedure still needs to be performed, so the data disconnection time is at least 6 ms.

On the other hand, when the handover method according to the present invention is executed, while establishing a connection with the target base station, since the connection with the source base station is maintained, no data disconnection time occurs (ie, 0 ms). In addition, in the handover method according to the present invention, the data disconnection time does not occur regardless of whether the RACH-less handover is used. That is, since disconnection from the source base station (S492) is performed after the target base station 420 receives the handover completion message from the terminal 400 (S480), no data disconnection time is generated for the terminal.

On the other hand, in the handover method according to an embodiment of the present invention, ① when the source base station starts data forwarding to the target base station and in the case of a single connection handover, the source base station stops transmitting data to the terminal, ② the target at the terminal Attempting to connect to the base station and disconnecting from the source base station in the case of single-connection handover, ③ When the source base station stops transmitting data to the terminal in the case of single-connection handover, ④ Multi-connection handover In some cases, methods are needed to determine when a terminal releases a connection with a source base station. Hereinafter, examples of specific methods for determining the respective viewpoints will be described.

① How to determine the data forwarding time of the source base station

To determine when to start data forwarding from a source base station to a target base station and stop transmitting data to a terminal in the case of a single connection-based handover, one or a combination of two or more of the following methods may be used.

(First method) After receiving the first handover indication message (target cell access) message from the terminal (the terminal can determine the handover time point and inform it as a handover indication message)

-The first method has an advantage of obtaining the shortest handover disconnection time performance and efficiently using radio resources.

(Second method) After the source base station receives the X2 handover indication message (target cell access) message through the X2 interface from the target base station (if the terminal accesses the target base station, the target base station directs the X2 handover to the source base station) Message can tell)

-The second method has the disadvantage of having the largest handover disconnection time, but it can determine the time point most accurately.

(3rd method) After the source base station determines the handover time of the terminal itself by estimation

-The third method has the smallest signaling overhead, but is the most inaccurate method, and the inaccurate decision has a problem that the handover disconnection time can be increased.

② How to determine when the terminal connects to the target base station

In order to determine a point in time at which a terminal attempts to connect to a target base station and releases a connection with a serving base station in a single connection-based handover, one or a combination of two or more of the following methods may be used.

(First method) After transmitting the first handover indication message to the source base station (the terminal can determine the handover time point and inform the first handover indication message)

-Handover execution time is relatively short, and handover disconnection time can be reduced.

(Second method) After receiving the ACK for the first handover indication message from the source base station (ACK may be HARQ ACK or RLC (Radio Link Control) ACK)

-It is possible to accurately synchronize the execution time of handover between the terminal and the source base station.

(3rd method) After the terminal itself determines the handover point

The handover execution time can be shortened, but if the source base station and the handover execution time synchronization do not match, the handover disconnection time may be increased.

③ How to determine when the source base station stops transmitting data

In the case of simultaneous connection-based handover in the source base station, one or a combination of two or more of the following methods may be used to determine when to stop data transmission to the terminal.

(First method) After receiving the second handover indication message (old cell release) from the terminal (the terminal can determine when the source base station disconnects, and can inform the source base station with the second handover indication message)

(Second method) After receiving the X2 handover indication message (old cell release) through the X2 interface from the target base station (the terminal determines when the source base station is disconnected and sends it to the target base station as the second handover indication message) Can tell)

(3rd method) After the source base station determines its own successful handover completion time to the target base station of the terminal by estimation

(4th method) After receiving the X2 handover indication message (target cell access) through the X2 interface from the target base station

(5th method) After receiving the UE context release message from the target base station

④ How to determine when the terminal disconnects from the source base station

In order to determine a point in time when a terminal releases a connection with a source base station in a simultaneous connection-based handover, one or a combination of two or more of the following methods may be used.

(First method) After transmitting the second handover indication message (old cell release) to the source base station (the terminal can determine when the source base station is disconnected and notify the source base station with the second handover indication message) )

(Second method) After receiving the ACK for the second handover indication message (old cell release) from the source base station (ACK may be HARQ ACK or RLC ACK)

(3rd method) After transmitting the second handover indication message (old cell release) to the target base station (the terminal can determine when the source base station is disconnected and notify the target base station with the second handover indication message) )

(Fourth method) After receiving the ACK for the second handover indication message (old cell release) from the target base station (ACK may be HARQ ACK or RLC ACK)

(Fifth method) After receiving the source base station disconnection message (Source Cell Release) from the source base station (The source base station receives the second handover indication message (old cell release) or through its own source through estimation) Base station disconnect message can be sent)

(6th method) After receiving the source base station disconnection message (Source Cell Release) from the target base station (The target base station automatically sources itself after receiving the second handover indication message (old cell release) or through estimation, etc. Base station disconnect message can be sent)

(Seventh method) After the T3xx timer expires to disconnect from the source base station

The terminal manages a T3xx timer that triggers a disconnection with the source base station. The UE starts a T3xx timer when an event such as a physical layer problem occurs in the source base station or a handover is completed to the target base station. When the T3xx timer expires, it disconnects from the source base station. When an event occurs, such as when the physical layer problem is cleared at the source base station, the terminal stops the T3xx timer. When the T310 timer for the source base station is driven at the time of handover execution, the T310 timer value is set to the T3xx timer value, and the T310 timer is stopped. The T3xx timer value may be 0 ms.

When the terminal receives the source base station disconnect message from the base station and performs disconnection with the source base station, if the T3xx timer is running, the T3xx timer is stopped.

Method of data forwarding and UL data transmission between base stations

5 shows an example of a data forwarding and PDCP retransmission procedure in a handover method of an LTE mobile communication system.

Referring to FIG. 5, the source base station 210 transmits a handover command to the terminal 200 and then allocates a next packet packet data convergence protocol (PDCP) SN (sequence number) to be allocated by the target base station 220. Tells. In addition, the source base station 210 delivers the PDCP source data unit (SDU) that the terminal has not received to the target base station 220 according to the DL PDCP reception state of the terminal 200 together with the SN. Meanwhile, the source base station 210 transmits the PDCP SDU of fresh data received from the gateway 230 to the target base station 220 without SN. When the 'end marker' is received from the gateway 230, it is delivered to the target base station 320.

Meanwhile, in the handover method according to an embodiment of the present invention, the data forwarding procedure may be configured as one of the following procedures.

Handover data forwarding procedure 1

6 illustrates an example of data forwarding and PDCP retransmission procedures in a simultaneous connection-based handover method according to an embodiment of the present invention.

The source base station 410 may provide data duplication in consideration of the channel status of the terminal 400. The source base station 410 transmits data to the terminal 400 while simultaneously forwarding the same data to the target base station 420 so that the same data is transmitted from the target base station, thereby increasing reliability. The source base station 410 delivers the PDCP SDU that the terminal 400 has not received to the target base station 420 according to the DL PDCP reception state of the terminal 400. The PDCP SDU of fresh data received from the gateway 430 is transmitted to the target base station together with the SN. When the 'end marker' is received from the gateway 430, it is delivered to the target base station 420. The source base station 410 informs the next DL (next) PDCP SN to be allocated by the target base station 420.

Handover data forwarding procedure 2

7 illustrates another example of a data forwarding and PDCP retransmission procedure in a simultaneous connection-based handover method according to an embodiment of the present invention.

The source base station 410 may provide data split in consideration of the channel status of the terminal 400. The source base station 410 may transmit some data to the terminal 400 and forward some data to the target base station 420 without transmitting it to the terminal 400 to increase user data throughput. . The source base station 410 delivers the PDCP SDU that the terminal has not received to the target base station 420 according to the DL PDCP reception state of the terminal 400. Among the fresh data received from the gateway 430, the PDCP SDU of the data transmitted from the source base station to the terminal is transmitted to the target base station 420 together with the SN. Among the new data received from the gateway 430, data that is not transmitted from the source base station 410 to the terminal 400 transmits the PDCP SDU to the target base station 420 without SN. When the 'end marker' is received from the gateway 430, it is delivered to the target base station 420. The source base station 410 informs the next DL PDCP SN to be allocated by the target base station 420.

On the other hand, when the simultaneous connection-based handover is used in the handover method according to an embodiment of the present invention, the terminal may be in a state of being simultaneously connected to the source cell and the target cell. In this case, the procedure for the UE to transmit the uplink data may be configured as one of the following procedures.

UL data transmission procedure 1

In an embodiment of the present invention, in a simultaneous connection-based handover, a terminal may simultaneously transmit uplink (UL) data to a serving base station and a target base station. In one method, the terminal may transmit all UL data to the target base station after handover to the target base station. In this case, reordering of UL data is performed in the target base station PDCP.

UL data transmission procedure 2

In an embodiment of the present invention, in a simultaneous connection-based handover, a UE may simultaneously transmit UL data to a serving base station and a target base station. Alternatively, after accessing the target base station, the UE may transmit the same PDCP SDU to the serving base station simultaneously with the target base station. The terminal PDCP may have a separate PDCP entity for each serving base station and a target base station, or a separate PDCP sub-entity for each serving base station and target base station from one PDCP entity. The PDCP entity or PDCP sub-entity of the target base station becomes the master entity, and the PDCP entity or PDCP sub-entity of the serving base station becomes the secondary entity.

PDCP SDU duplication can be set at the request of the base station, or the UE can be implicitly set in a simultaneous connection-based handover. When a separate PDCP entity is set or a separate PDCP sub-entity is set in one PDCP entity, when PDCP SDU replication is activated, the primary entity duplicates the PDCP SDU and transmits it to each PDCP entity or PDCP sub-entity. At this time, the sequence number (SN) is assigned by the main entity, and security functions such as ciphering are performed by each entity, and each entity is also added to the PDU header. If PDCP SDU replication is not activated, all UL data is transmitted to the target base station in the same manner as in Method 1. When the PDCP SDU replication is set at the request of the base station, the UE RRC layer sets the PDCP SDU replication function of the PDCP layer according to the request of the base station. If necessary, the PDCP SDU replication function can be activated or deactivated. When activated, the PDCP SDU replication function is activated. Activation or deactivation depends on the order of the base station.

UL data transmission procedure 3

In an embodiment of the present invention, in a simultaneous connection-based handover, a UE may simultaneously transmit UL data to a serving base station and a target base station. Alternatively, after the UE accesses the target base station, different PDCP SDUs may be transmitted to the target base station or the serving base station. The terminal PDCP may have a separate PDCP entity for each serving base station and a target base station, or a separate PDCP sub-entity for each serving base station and target base station from one PDCP entity. The PDCP entity or PDCP sub-entity of the target base station becomes the primary entity, the PDCP entity or PDCP sub-entity of the serving base station becomes the secondary entity. The PDCP SDU split can be set at the request of the base station, or the UE can be implicitly set in a simultaneous connection-based handover. When a separate PDCP entity is configured or when a separate PDCP sub-entity is configured in one PDCP entity, when PDCP SDU splitting is activated, the primary entity may transmit a request for transmission if the data to be transmitted is greater than ul-DataSplitThresholdinSCMBB (radio link) control) Send data to the PDCP entity or PDCP sub-entity corresponding to the entity. At this time, the SN (sequence number) is assigned by the main entity, security functions such as encryption are performed by each entity, and the PDU header is also added by each entity. If ul-DataSplitDRB-ViaOldServingCell is set in another way, data is transmitted to the PDCP entity or PDCP sub-entity corresponding to the RLC entity that received the transmission request. At this time, the sequence number is assigned by the main entity, security functions such as encryption are performed by each entity, and the PDU header is also added by each entity. Otherwise, in the same manner as in Method 1, all UL data is transmitted to the target base station. When the PDCP SDU segmentation is set by the request of the base station, the UE RRC layer sets the PDCP SDU segmentation function of the PDCP layer according to the request of the base station. ul-DataSplitThresholdinSCMBB is a setting value that allows data to be transmitted to a conventional serving base station when the amount of data to be transmitted in a handover based on simultaneous connection is greater than this threshold. ul-DataSplitDRB-ViaOldServingCell is a value indicating whether a terminal can transmit data to a conventional serving base station after handover to a target base station in a handover based on simultaneous connection. When ul-DataSplitThresholdinSCMBB and ul-DataSplitDRB-ViaOldServingCell are set by the base station, the UE RRC layer sets the PDCP SDU split function of the PDCP layer according to the request of the base station.

How to determine the validity of a conditional handover command

In the conditional handover, the terminal and the base station must know each other how long the conditional handover command (ie, conditional handover command) received by the terminal is valid for. The method of determining the validity of the conditional handover command is as follows.

-Permanently validating conditional handover commands

-Conditional Handover Command validity timer method using timer T3xy

The value of the validity timer T3xy may be set in step 1 or 7 of TS36.300. When the terminal receives the conditional handover command, it may start or restart the T3xy timer for the conditional handover command. When T3xy expires, the terminal deletes the stored conditional handover command.

When the terminal actually performs handover, if present, all stored conditional handover commands are deleted and the T3xy timer is stopped. When deleting the conditional handover command, the terminal may transmit a measurement report to the source base station.

The source base station may set an event that triggers the measurement result report to delete the stored conditional command in reportConfig in step 1 or 7 of TS36.300.

The event may be an A3 event (eg, -9dB) with a negative offset. When the event is satisfied, the terminal transmits a measurement result report and deletes the stored conditional handover command.

The source base station can determine whether to remove the conditional handover command. When deleting the conditional handover command, the source base station may send a handover cancel message to the target base station.

When deleting the conditional handover command, the source base station may transmit an RRCConnectionReconfiguration message with removeconditionalmobilityControlInfo set to the terminal, or if implicitRemoveAfterMR is set, the terminal may implicitly delete the conditional handover command after transmitting the measurement result report. implicitRemoveAfterMR can be set in steps 1 or 7 of TS36.300.

RRC connection reset method including conditional handover command

In the conditional handover, after the source base station transmits the conditional handover command to the terminal, when the RRCConnectionReconfiguration affecting the setting of the target base station is transmitted to the terminal, the target base station setting information of the already stored conditional handover command matches the target base station This may cause problems. The method to solve this is as follows.

-A method for a source base station to transmit an updated conditional handover command

-A method for a source base station to transmit a handover request update to a target base station

-The method for the target base station to include the updated information in the handover request acknowledgment message (or handover preparation acknowledgment message), and to transmit the updated RRCConnectionReconfiguration to the source base station

The source base station transmits the RRCConnectionReconfiguration to the terminal, and includes an updated conditional handover command in this message or a conditional handover command updated in a separate message to the terminal.

Upon receiving the updated conditional handover command, the terminal replaces the stored conditional handover command with the updated conditional handover command, and restarts the T3xy timer as necessary.

-How the target base station transmits the updated RRCConnectionReconfiguration

The source base station sends a UE context update to the target base station, and informs the updated RRCConnectionReconfiguration configuration information.

The target base station sends a UE context update check (UE context ppdate ACK) to the source base station, and informs the user that the updated RRCConnectionReconfiguration can be transmitted to the UE when the UE performs handover to the UE.

When the target base station connects to the terminal, the target base station can immediately transmit the updated RRCConnectionReconfiguration to the terminal to inform the updated configuration information.

8 is a block diagram illustrating a structure of a device to which embodiments of the present invention can be applied.

8 illustrates a general structure applicable to a terminal of a mobile communication system, but the same or similar structure may be applied to a base station of a mobile communication system.

Referring to FIG. 8, the terminal 800 may include at least one processor 810, a memory 820, and a transceiver 830 connected to a network to perform communication. In addition, the terminal 800 may further include an input interface device 840, an output interface device 850, a storage device 860, and the like. Each component included in the terminal 800 may be connected by a bus 870 to communicate with each other. However, each of the components included in the terminal 100 may be connected through a separate interface or a separate bus centered on the processor 810, not the common bus 870. For example, the processor 810 may be connected to at least one of the memory 820, the transceiver 830, the input interface device 840, the output interface device 850, and the storage device 860 through a dedicated interface. .

The processor 810 may execute at least one instruction stored in at least one of the memory 820 and the storage device 860. The processor 810 may refer to a central processing unit (CPU), a graphics processing unit (GPU), or a dedicated processor in which methods according to embodiments of the present invention are performed. Each of the memory 820 and the storage device 860 may be configured as at least one of a volatile storage medium and a non-volatile storage medium. For example, the memory 820 may be configured with at least one of read only memory (ROM) and random access memory (RAM).

The at least one command may be configured such that the processor 810 performs each of the steps constituting the handover methods according to the above-described embodiments of the present invention, and all messages exchanged between the base station and the terminal are the processor ( It may be transmitted or received through the transceiver 830 under the control of 810).

The methods according to the present invention may be implemented in the form of program instructions that can be executed through various computer means and recorded in a computer readable medium. Computer-readable media may include program instructions, data files, data structures, or the like alone or in combination. The program instructions recorded on the computer-readable medium may be specially designed and configured for the present invention or may be known and usable by those skilled in computer software.

Examples of computer-readable media include hardware devices specially configured to store and execute program instructions, such as ROM, RAM, flash memory, and the like. Examples of program instructions include high-level language codes that can be executed by a computer using an interpreter, etc., as well as machine language codes such as those produced by a compiler. The above-described hardware device may be configured to operate with at least one software module to perform the operation of the present invention, and vice versa.

Although described with reference to the above embodiments, those skilled in the art understand that various modifications and changes can be made to the present invention without departing from the spirit and scope of the present invention as set forth in the claims below. Will be able to.

Claims (1)

A method of operating a terminal performing handover between a source base station and a target base station,
Receiving a handover command from the source base station;
Determining whether to perform a single connectivity handover or a simultaneous connectivity handover with the source base station and the target base station based on the handover command;
When the single connection-based handover is performed, performing connection release with the source base station and performing connection establishment with the target base station; And
When the simultaneous connection-based handover is performed, characterized in that it comprises the step of performing a connection release with the source base station after completion of the connection establishment with the target base station without disconnecting from the source base station,
How the terminal operates.

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