KR102207843B1 - Method and apparatus for handover based on cooperation between base stations - Google Patents

Abstract

A handover method and apparatus through cooperation between base stations are provided. When a message indicating handover completion is received from the terminal that has performed handover to the target base station by determining whether to perform the handover, the target base station transmits a message requesting transmission of information about the terminal to the serving base station of the terminal. In addition, information about the terminal is received from the serving base station and data is transmitted to the terminal based on this.

Description

Method and apparatus for handover through cooperation between base stations {Method and apparatus for handover based on cooperation between base stations}

The present invention relates to handover, and more particularly, to a method and apparatus for performing handover through cooperation between base stations.

In general, in a mobile communication system, a handover is performed when a terminal moves and changes an access base station. The terminal measures the signal strength of the neighboring base station and reports it to the source base station. The source base station prepares a target base station to be handed over based on the reported signal strengths of the neighboring base stations, and commands the terminal to perform handover. At this time, the source base station transmitting the handover command message transmits the data sequence number information and data of the terminal to the target base station to perform a seamless handover. Upon receiving the handover command message, the terminal immediately performs handover and attempts to access the target base station.

However, it is difficult to transmit a signal strength report from the terminal to the base station in real time due to poor radio link conditions in the handover area, and it is difficult to transmit a handover command transmitted from the base station to the terminal to the terminal in real time. Accordingly, the time during which data is disconnected during handover increases, and thus communication quality is inevitable during handover.

The technical problem to be solved by the present invention is to provide a method and apparatus for performing handover through cooperation between base stations.

In addition, the technical problem to be solved by the present invention is to provide a method and apparatus for performing handover without interruption.

A handover method according to a feature of the present invention includes the steps of: receiving, by a target base station, a message indicating completion of handover from a terminal-the terminal determines whether to perform handover and performs handover to the target base station; After receiving the message indicating completion of the handover, transmitting, by the target base station, a message requesting transmission of information about the terminal to a serving base station of the terminal; Receiving, by the target base station, information on the terminal from the serving base station; And transmitting, by the target base station, data to the terminal.

In the step of transmitting the message requesting information transfer, when there is no information about the terminal, it is determined that an abnormal situation in which the handover has been performed without notifying the serving base station of the handover execution has occurred, and requesting the information transfer. Message can be sent.

In the abnormal situation, the terminal determining the handover execution time according to the handover command from the serving base station transmits a handover notification message notifying the handover execution to the serving base station, but the serving base station sends the handover notification message. It may indicate a case where the reception was not performed.

In the transmitting of the message requesting transmission of the information, a message requesting data and data sequence number information, which is information about the terminal, may be transmitted to the serving base station.

In the handover method, after the step of transmitting the message requesting information transfer, the serving base station includes a sequence number of a packet data convergence protocol (PDCP) service data unit (SDU) requiring retransmission, and a packet in a buffer. And transmitting packets received from the gateway to the target base station.

In addition, the handover method, before the step of receiving a message indicating completion of the handover,

Receiving, by the target base station, a handover request message including information including the context of the terminal from the serving base station; And determining whether to accept handover based on the context information of the terminal, and transmitting a handover request response message to the serving base station when the handover is allowed.

In this case, after the serving base station transmits the information on the terminal to the target base station according to the message requesting the information transfer, radio resources prepared for the terminal to neighboring base stations of the terminal prepared for handover excluding the target base station It may further include the step of requesting to release.

Meanwhile, the message indicating the completion of the handover may be received after the terminal determines whether to perform handover based on the occurrence of a handover execution event and performs handover to the target base station. In this case, the handover execution event is a situation in which the signal strength of the target base station is greater than the signal strength of the serving base station by a first offset or more, and the signal strength of the target base station is greater than the signal strength of the serving base station by a second offset or more. When maintained for the set time TTE (Time to Execute), when the signal strength of the target base station is greater than the signal strength of the serving base station, the signal strength of the serving base station is less than a preset first threshold, and the target The signal strength of the base station may be greater than that of the serving base station, and at least one of the case where the signal strength of the target base station is greater than a preset second threshold.

A handover apparatus according to another aspect of the present invention, in a handover apparatus of a target base station, indicates completion of handover from a terminal--the terminal determines whether to perform handover and performs handover to the target base station. A handover completion receiver for receiving a message; A data transmission request unit for transmitting a message requesting information transmission to the terminal to a serving base station of the terminal when a message indicating completion of the handover is received; And a data transmission unit receiving information on the terminal from the serving base station and transmitting data to the terminal.

When there is no information on the terminal, the data transfer requesting unit determines that an abnormal situation in which the handover has been performed without notifying the serving base station of the handover execution has occurred, and sends a message requesting the information transfer to the serving base station. Can be transmitted.

The message for requesting information delivery may be a message for requesting data sequence number information and data, which are information about the terminal.

On the other hand, the handover apparatus, the handover request receiving unit for receiving a handover request message including information including the context of the terminal from the serving base station; And a handover acceptance determination unit that determines whether to accept handover based on the context information of the terminal and transmits a handover request response message to the serving base station when the handover is allowed.

The message indicating the completion of the handover is received from the terminal after the terminal determines whether to perform handover based on the occurrence of a handover execution event, determines a target base station, and then performs handover to the determined target base station. Can be.

According to an embodiment of the present invention, the target base station notifies the serving base station that the terminal has handed over to itself, and directly requests the data sequence number information and data of the terminal to be transmitted to the target base station, thereby achieving a seamless handover.

In addition, as the terminal determines the handover timing, a handover is performed in which the result of measuring the signal strength of the base station of the terminal is reflected in real time. As a result, the handover success rate is improved and the time for data disconnection during handover can be significantly reduced. In addition, in most cases, handover timing synchronization between the terminal and the base station can be synchronized.

1 is a diagram illustrating a network environment in which handover is performed according to an embodiment of the present invention.
2 is a flowchart of a handover method according to an embodiment of the present invention.
3 is an exemplary diagram illustrating occurrence of an event according to signal strength in a handover method according to an embodiment of the present invention.
4 is a diagram showing the structure of a handover device according to an embodiment of the present invention.

Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings so that those of ordinary skill in the art may easily implement the present invention. However, the present invention may be implemented in various forms and is not limited to the embodiments described herein.

In the drawings, parts irrelevant to the description are omitted in order to clearly describe the present invention, and similar reference numerals are assigned to similar parts throughout the specification.

Throughout the specification and claims, when a certain part'includes' a certain element, it means that other elements may be further included rather than excluding other elements unless otherwise stated.

Throughout the specification, a terminal is a mobile terminal (MT), a mobile station (MS), an advanced mobile station (AMS), and a high reliability mobile station (HR-MS). , Subscriber station (SS), portable subscriber station (PSS), access terminal (AT), user equipment (UE), etc., MT, MS, AMS , HR-MS, SS, PSS, AT, may include all or part of the functions such as UE.

In addition, the base station (BS) is an advanced base station (ABS), a high reliability base station (HR-BS), a node B (node B), an advanced node B (evolved node B), eNodeB), access point (AP), radio access station (RAS), base transceiver station (BTS), mobile multihop relay (MMR)-BS, relay serving as a base station station, RS), a relay node (RN) that acts as a base station, an advanced relay station (ARS) that acts as a base station, and a high reliability relay station (HR) that acts as a base station. -RS), small base station (femoto BS), home node B (HNB), home eNodeB (HeNB), pico base station (pico BS), metro base station (metro BS), micro base station (micro BS) ), etc.], etc., and may include all or part of functions such as ABS, NodeB, eNodeB, AP, RAS, BTS, MMR-BS, RS, RN, ARS, HR-RS, small base stations, etc. have.

Hereinafter, a method and apparatus for handover through cooperation between base stations according to an embodiment of the present invention will be described.

1 is a diagram illustrating a network environment in which handover is performed according to an embodiment of the present invention.

As shown in FIG. 1, in a network environment in which a plurality of small cells are mixed, the terminal 1 accesses a corresponding cell through a base station in charge of each cell. The terminal 1 moves from the serving base station or primary cell (2) (or also referred to as the source base station) of the currently accessed cell to the target base station 3 (target station or secondary cell). Can perform a handover.

In such a radio channel environment, in general, a terminal measures the strength of a signal received from a serving base station and a neighboring base station, and transmits channel state information of a serving cell and a neighboring cell to the serving base station through a measurement report message. Then, the serving base station determines whether to handover to the terminal and transmits a handover command message to the terminal. Thereafter, the terminal immediately performs handover, disconnects the wireless connection with the serving base station, and attempts to access the target base station. After the target base station transmits a random access response to the terminal according to the random access of the terminal to the target base station, the target base station determines that the handover was successful and starts transmitting downlink data.

The terminal disconnects the radio connection with the serving base station and attempts to access the target base station until the time when the RACH (random access channel) procedure to the target base station is successful and downlink data is first received, that is, the handover interrupt time. time), the terminal does not receive data. As described above, the handover interrupt time in which the terminal does not receive data during handover is defined as about 10.5 ms according to the standard standard.

However, since downlink data is not actually transmitted from the time when the serving base station transmits a handover command message to the terminal, the handover interrupt time is much longer than the time defined in the standard standard. In the handover area, the signal strength report of the terminal is not delivered to the base station in real time due to poor radio link conditions, and the handover command transmitted from the base station to the terminal is not transmitted to the terminal. In response, handover is not performed or handover itself fails in many cases. As a result, communication quality deterioration during handover is inevitable due to a longer period of time when data is disconnected during handover or a time when data is disconnected during a recovery procedure due to a handover failure.

Accordingly, in an embodiment of the present invention, the terminal finally determines the handover timing so that the handover is performed in real time according to the radio link state and the handover success probability can be increased.

When the terminal determines the handover time, the terminal reports the signal strength measurement result of the neighboring base station, receives a handover command message from the base station to perform handover, and then determines the handover time (e.g., for a certain period of time). When the signal strength of the target base station is greater than the signal strength of the serving base station, a handover is determined) and this is notified to the serving base station through a handover determination message. As the handover is performed in which the result of measuring the base station signal strength of the terminal is reflected in real time, the handover success rate increases and the time for data disconnection during handover can be significantly reduced.

Meanwhile, when the radio link state between the terminal and the serving base station is not good, a transmission error may occur, and thus the serving base station may not normally receive a handover decision message transmitted by the terminal to the serving base station.

In an embodiment of the present invention, when the terminal completes the handover to the target base station, the target base station informs the serving base station that the terminal has handed over to itself, and requests that the terminal data sequence number information and data be transmitted to the target base station. . Accordingly, handover without disconnection can be performed.

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

The terminal 1 measures the signal strength of the serving base station 2 and neighboring base stations, and reports the radio signal measurement result through a measurement report message. For example, the terminal 1 may transmit a measurement report message to the serving base station when the difference in signal strength between the serving base station and the neighbor base station is greater than or equal to HOM (Handover Margin) (ex. 2dB).

The serving base station 2 selects a neighbor base station to which the terminal 1 can handover based on the radio signal measurement result of the neighboring base station received from the terminal 1.

In this case, the serving base station 2 may transmit a handover request message to selected neighbor base stations. The handover request message may include information including the context of the terminal. Nearby base stations that have received the handover request message, that is, potential target base stations, determine whether to accept handover based on the context information of the terminal, and if handover is allowed, it is necessary to access the target base station. A Handover Request ACK message including information may be transmitted to the serving base station. The serving base station 2 transmits a handover command message (e.g., an RRC connection reconfiguration message for setting the neighbor base station as the target base station) to the terminal 1 according to the handover acceptance of the neighbor base stations to perform handover. Can be ordered.

When the handover preparation process is completed (S100), the terminal 1 receives a handover command message instructing to perform a handover from the serving base station 2 (S110), the serving base station 2 ) And the signal strength of neighboring base stations are continuously measured, and a time point to perform handover is determined based on the measurement result (S120). The terminal 1 measures the signal strength of neighboring base stations and, when the measurement result satisfies a preset handover execution event, determines an optimal target base station to perform handover and an optimal handover time point.

In an embodiment of the present invention, the handover execution event means a situation where it is better for the terminal to handover to the target base station than to maintain a wireless connection with the serving base station.

3 is an exemplary diagram showing occurrence of an event according to signal strength in a handover method according to an embodiment of the present invention.

1) When the signal strength of the neighboring base station is greater than that of the serving base station, and the signal strength of the neighboring base station is greater than the signal strength of the serving base station by the first offset (eg, 3dB) or more (A3 in FIG. 3), 2) A situation in which the signal strength of the neighboring base station is greater than the signal strength of the serving base station by more than the second offset is maintained for a set time (for example, TTE (Time to Execute)), 3) the signal strength of the neighboring base station is the signal of the serving base station When the signal strength of the serving base station is greater than the strength and the signal strength of the serving base station is less than the preset first threshold value (Threshold1) (A2 in FIG. 3), and 4) the signal strength of the neighboring base station is greater than that of the serving base station, When the strength is greater than the preset second threshold (Threshold2) (A4 in FIG. 3), the terminal 1 may determine that it is a time point to perform a handover due to the occurrence of a handover execution event. The handover execution event according to an embodiment of the present invention is not limited to those described above.

Meanwhile, the terminal 1 may determine the target base station based on the measured signal strength of neighbor base stations. For example, among neighboring base stations having a signal strength greater than the signal strength of the serving base station, a neighbor base station whose signal strength is greater than a first offset than the signal strength of the serving base station may be selected as a target base station for handover. The terminal 1 may receive one or more handover command messages, and in this case, the target base stations of each handover command message are different from each other. In this case, the terminal 1 may determine an optimal target base station for handover while determining an optimal time point for handover.

In the attached FIG. 2, when the terminal 1 determines that the current state is a state in which handover should be performed, a handover indication message notifying the handover is generated and transmitted to the serving base station 2 Do (S130). The handover notification message may include information on a target base station determined by the terminal. The handover notification message includes various notification methods (e.g., a method of notifying a handover performance using a MAC control element, a method of notifying a handover performance using an RRC message, a physical) channel. It may be transmitted to the serving base station by using a method of notifying the handover performance, etc.). Here, when the terminal 1 notifies the serving base station 2 that the handover is to be performed, the handover notification message may be repeatedly transmitted for reliability of the transmitted message.

The serving base station 2 may transmit a response message on whether to receive a handover notification message to the terminal 1. For example, an ACK (ACKnowledgment) response message informing that the handover notification message has been normally received is transmitted to the terminal 1 or a negative ACKnowledgment (NACK) response message informing that the handover notification message has not been normally received is sent to the terminal 1 ).

In this case, the handover notification message transmitted from the terminal 1 may not be normally received by the serving base station 2 according to the radio link state (in the case of “X” in FIG. 2). After transmitting the handover notification message, the terminal 1 generates, for example, an acknowledgment (ACK) error (eg, PHICH NACK to ACK Error), or a preset maximum number of transmissions (eg, maximum If an ACK indicating normal reception is not received from the serving base station even when a message is transmitted for as many times as HARQ (Hybrid automatic repeat request) transmission number), it may be determined that a handover notification message transmission failure has occurred.

The terminal 1 transmits a handover notification message and then performs handover to the target base station. At this time, when it is determined that transmission failure has occurred, or after receiving a response message (ACK) indicating that reception of the handover notification message has been normally performed from the serving base station 2, or when transmission failure occurs or ACK Regardless of reception, handover to the target base station may be performed immediately after transmitting the handover notification message.

The terminal 1 performs handover to the target base station and transmits a handover completion message to the target base station 3 (S140).

Upon receiving the handover completion message from the terminal 1, the target base station 3 determines whether the data sequence number information and the data of the terminal have been received from the serving base station 2. When the data sequence number information and data for the terminal are not received from the serving base station 2, the terminal fails to notify the serving base station of the handover execution and performs handover, that is, a handover notification message is normally sent to the serving base station. It is determined that there is an abnormal situation that has not been received. In this case, the target base station 3 sends an information transfer request message (e.g., SN (sequence number) Status Transfer Request message) to the serving base station 2 to request data sequence number information and data of the terminal. ) To transmit (S150).

Receiving the information transfer request message, the serving base station 2 transfers the data sequence number information and data of the terminal 1 to the target base station 3 (S160). At this time, the serving base station 2 selects, for example, a sequence number (data sequence number) of a packet data convergence protocol (PDCP) service data unit (SDU) that needs retransmission, a packet in a buffer, and packets received from the gateway. You can forward to (3).

The target base station 3 transmits data according to the current order information to the terminal 1 based on the data order information and data received from the serving base station 2.

On the other hand, when handover preparation (S100) is performed for several neighboring base stations, the serving base station 2 determines that the terminal 1 has normally handed over to the target base station 3, except for the target base station 3 Prepared for the terminal 1 by transmitting a resource release message to neighboring base stations (neighboring base stations that received a handover request message from the serving base station and transmitted a handover request response message to the serving base station) ready for handover. A request to release radio resources may be made (S170).

According to this handover, even if a transmission error occurs due to a poor radio link state between the terminal and the serving base station, and the serving base station does not normally receive the handover notification message transmitted from the terminal, the terminal returns to the target base station. When the handover is performed immediately, and the handover is completed, the target base station notifies the serving base station that the terminal has handed over to itself, and requests that the terminal's data sequence number information and data be transmitted to the target base station, thereby enabling seamless handover. Done.

In addition, the handover interrupt time in which the terminal does not receive data during handover is the downlink data from the target base station from the time the serving base station receives the handover notification message, which is the time when the terminal disconnects the wireless connection with the serving base station. Since it is up to the time point at which is first received, it is significantly reduced compared to the existing handover interrupt time starting from the time when the serving base station transmits the RRC connection reconfiguration message to the terminal.

A handover device for performing such handover is as follows.

4 is a diagram showing the structure of a handover device according to an embodiment of the present invention.

The handover apparatus 100 according to an embodiment of the present invention includes a handover request receiving unit 110, a handover acceptance decision unit 120, a handover completion receiving unit 130, and a data transfer requesting unit, as shown in FIG. 4. 140), and a data transmission unit 150.

The handover apparatus 100 is implemented in a form included in the target base station 3 and performs handover based on cooperation between base stations according to an embodiment of the present invention.

The handover request receiving unit 110 receives a handover request message from a serving base station. The handover request message may include, for example, information including the context of the terminal.

The handover acceptance determination unit 120 determines whether to accept the handover for the corresponding terminal according to the handover request from the serving base station. The handover acceptance determination unit 120 determines whether to accept handover based on the context information of the terminal, and when the handover is allowed, transmits a handover request response message including information necessary to access the target base station to the serving base station. do.

The handover completion receiving unit 130 receives a message indicating handover completion from the terminal. In particular, a handover completion message is received from a terminal that has decided to accept handover according to a request from a serving base station.

The data transfer requesting unit 140 requests information on the corresponding terminal from the serving base station according to whether the data sequence number information and data of the terminal have been received from the serving base station. If the data sequence number information and data for the corresponding terminal are not delivered, it is determined that the terminal fails to notify the serving base station of the handover execution and an abnormal situation has occurred. It transmits an information transfer request message requesting data (eg, SN Status Transfer Request message) to the serving base station.

The data transmission unit 150 receives data sequence number information and data of the terminal from the serving base station, and transmits data according to the current sequence information to the terminal.

The embodiments of the present invention are not implemented only through the apparatus and/or method described above, but may be implemented through a program for realizing a function corresponding to the configuration of the embodiment of the present invention, a recording medium on which the program is recorded. Also, such an implementation can be easily implemented by an expert in the technical field to which the present invention belongs from the description of the above-described embodiment.

Although the embodiments of the present invention have been described in detail above, the scope of the present invention is not limited thereto, and various modifications and improvements of the business operator using the basic concept of the present invention defined in the following claims are also provided. It belongs to the scope of rights.

Claims (13)

Obtaining, by the target base station, context information of the terminal through a handover preparation process with the serving base station;
The target base station determines a time point to perform handover after receiving a handover command from the serving base station, and performs handover to the target base station at the determined handover time point. Receiving a message indicating completion;
When the target base station receives a message indicating completion of the handover, determining whether there is data sequence number information on data transmitted to the terminal;
If the target base station does not have data sequence number information of the terminal, informing the serving base station of the completion of handover of the terminal to request data sequence number information and data transfer of the terminal;
Receiving, by the target base station, data sequence number information and data for the terminal from the serving base station; And
Transmitting, by the target base station, data to the terminal based on the received information and data
Including,
Receiving the message indicating the completion of the handover, the target base station obtained the context information of the terminal through the handover preparation process, but the sequence number information about the data transmitted from the serving base station to the terminal or data In a state in which the message is not received, receiving a message indicating completion of the handover. According to claim 1
In the step of informing the serving base station of the completion of the handover of the terminal, when there is no information on the terminal, it is determined that an abnormal situation in which the handover has been performed without notifying the serving base station of the handover execution has occurred. A handover method for transmitting a message requesting data sequence number information and data delivery. According to claim 2
In the abnormal situation, the terminal determining the handover execution time according to the handover command from the serving base station transmits a handover notification message notifying the handover execution to the serving base station, but the serving base station sends the handover notification message. A handover method indicating a case in which reception is not performed. delete According to claim 1
After the step of notifying the completion of handover of the terminal to the serving base station,
Delivering, by the serving base station, a sequence number of a packet data convergence protocol (PDCP) service data unit (SDU) requiring retransmission, a packet in a buffer, and packets received from a gateway to the target base station
The handover method further comprising. According to claim 1
Obtaining the context information,
Receiving, by the target base station, a handover request message including information including the context of the terminal from the serving base station; And
Determining whether to accept handover based on the context information of the terminal, and transmitting a handover request response message to the serving base station when handover is allowed
Including, handover method. According to claim 6
After the serving base station transmits the information on the terminal to the target base station according to the message requesting the information transfer, release the radio resources prepared for the terminal to the neighboring base stations of the terminal prepared for handover excluding the target base station. Steps to request
The handover method further comprising. According to claim 1
The message indicating completion of the handover is received after the terminal determines whether to perform handover based on the occurrence of a handover execution event and performs handover to the target base station,
The handover execution event is
When the signal strength of the target base station is greater than the signal strength of the serving base station by a first offset or more,
When a situation in which the signal strength of the target base station is greater than the signal strength of the serving base station by a second offset or more is maintained for a set time TTE (Time to Execute),
When the signal strength of the target base station is greater than the signal strength of the serving base station, and the signal strength of the serving base station is less than a preset first threshold, and
When the signal strength of the target base station is greater than that of the serving base station, and the signal strength of the target base station is greater than a preset second threshold
Handover method comprising at least one of. In the handover device of the target base station,
A handover completion receiver configured to receive a message indicating completion of a handover from a terminal-the terminal determines whether to perform a handover and performs a handover to the target base station;
A data transmission request unit for transmitting a message requesting information transmission to the terminal to a serving base station of the terminal when a message indicating completion of the handover is received; And
A data transmission unit that receives information about the terminal from the serving base station and transmits data to the terminal
Including,
The handover completion receiving unit is in a state in which the target base station acquires context information of the terminal through the handover preparation process, but does not receive sequence number information or data regarding data transmitted from the serving base station to the terminal, Receiving a message indicating the completion of the handover,
The data transfer request unit determines whether there is data sequence number information of the terminal when a message indicating completion of the handover is received, and if there is no data sequence number information of the terminal, the data sequence number of the terminal is sent to the serving base station. Request information and data transfer from the terminal,
The data transfer unit transmits data to the terminal based on the received information and data. The method of claim 9
When there is no information on the terminal, the data transfer requesting unit determines that an abnormal situation in which the handover has been performed without notifying the serving base station of the handover execution has occurred, and requests data sequence number information and data transfer of the terminal To do, handover device. delete The method of claim 9
The handover request receiving unit receives a handover request message including information including the context of the terminal from the serving base station,
The handover device,
Handover acceptance determination unit for determining whether to accept handover based on the context information of the terminal and transmitting a handover request response message to the serving base station when handover is allowed
The handover device further comprising. The method of claim 9
The message indicating the completion of the handover is received from the terminal after the terminal determines whether to perform handover based on the occurrence of a handover execution event, determines a target base station, and then performs handover to the determined target base station. Which is a handover device.

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