KR20210100072A - Method and Device for Performing Handover in Stand Alone Mode - Google Patents

Abstract

Disclosed are a method and apparatus for a handover in an SA mode. In accordance with one embodiment of the present invention, the method for a handover in an SA mode includes the following steps of: receiving an RRC connection resetting message including a measurement report condition for a handover from a serving cell; measuring both the signal quality of adjacent LTE cells and the signal quality of adjacent NR cells; transmitting a measurement report message including information of the NR cells and information of the LTE cells satisfying the measurement report condition to the serving cell; receiving a handover command message for the NR cells from the serving cell after the serving cell performs handover preparation with the NR cells and performs handover preparation with the LTE cells; and when sensing a fail in a wireless link with the NR cells, transmitting an RRE request message to the LTE cells for an RRC connection with the LTE cells. Therefore, the present invention is capable of minimizing time that a service for a terminal is interrupted.

Description

Method and Device for Performing Handover in Stand Alone Mode

Embodiments of the present invention relate to a wireless communication system, and more particularly, to an efficient handover method and apparatus when a radio link failure or a handover failure of a terminal is detected in an SA mode.

The content described in this section merely provides background information on the present invention and does not constitute the prior art.

Recently, wireless communication technology has made rapid development, and communication system technology has also evolved accordingly. Among them, the 5G NR (new radio) system is currently in the spotlight as a 5G mobile communication technology. Such a mobile communication terminal is frequently used while the user is moving, and accordingly, it is required that a seamless service be provided through the terminal even while the user is moving.

In order to provide a seamless communication service to users, the 5G system supports handover scenarios in various scenarios as well as handover support, which is an inter-cell mobility technology supported in the previous generation communication technology. Here, the handover refers to a process in which the terminal moves from a serving cell connected to the terminal to another cell. Specifically, when the terminal moves from the service space of the current serving cell to the service space of another adjacent cell, the handover refers to a function for allowing the terminal to continue receiving the service currently being received from the serving cell from the other cell. . By such handover, the user can always receive an optimal service.

In general, a target cell to which the UE is moving and a serving cell to which the UE is connected use a homogeneous radio access network, and when the UE fails handover with the target cell, another cell is searched for and handover is attempted. do. When the terminal fails to handover to another cell, it falls back to a heterogeneous network from the core network connected to the terminal. For example, when the UE fails handover with the NR target cell while connected to the 5G core network using the NR serving cell, it falls back to the enhanced packet core (EPC). This delays the time it takes for the terminal to receive the service from the network.

In particular, in a stand-alone (SA) mode in which the terminal is connected to one core network using one radio access network, when the terminal fails handover, it frequently falls back from the connected core network to another core network. , there is a problem in that the time required to provide a service to the user is greatly increased.

Accordingly, there is a need to shorten the service interruption time by performing handover between the serving cell's radio access network and a cell using a different type of radio access network, in addition to the cell using the same radio access network as the serving cell's radio access network.

In the embodiments of the present invention, in the SA mode of the 5G system, the terminal measures both the signal of the NR cell and the signal of the LTE cell, and the serving cell performs handover preparation with the NR cell as the target cell as well as the LTE cell. A main object of the present invention is to provide a handover method and apparatus capable of minimizing service interruption time by quickly connecting to an LTE cell despite a handover failure or radio link failure with the NR cell.

According to an aspect of the present invention, there is provided a handover method in SA mode, the method comprising: receiving an RRC connection reconfiguration message including a measurement report condition for handover from a serving cell; The process of measuring both the signal quality of the neighboring LTE cells and the signal quality of the neighboring NR cells; transmitting, to the serving cell, a measurement report message including both LTE cell information and NR cell information that satisfy the measurement report condition; receiving, by the serving cell, a handover command message for the NR cell from the serving cell after performing handover preparation with the NR cell and the LTE cell; Provided is a handover method comprising the step of transmitting an RRE request message to the LTE cell for RRC connection with the LTE cell when a radio link failure with the NR cell is detected.

According to another aspect of this embodiment, in a terminal for handover in SA mode, a transceiver; at least one memory storing instructions; and by executing the instructions, receive an RRC connection reconfiguration message including a measurement report condition for handover from a serving cell, measure both the signal quality of adjacent LTE cells and the signal quality of adjacent NR cells, and the measurement report condition transmits to the serving cell a measurement report message including both LTE cell information and NR cell information that satisfies At least one processor configured to receive a handover command message for the NR cell and, when detecting a radio link failure with the NR cell, send an RRE request message to the LTE cell for RRC connection with the LTE cell It provides a terminal comprising.

As described above, according to an embodiment of the present invention, in the SA mode of the 5G system, the UE measures both the signal of the NR cell and the signal of the LTE cell, and the serving cell performs a hand with the NR cell as the target cell as well as the LTE cell. Over-preparation is performed, and the terminal is quickly connected to the LTE cell despite handover failure or radio link failure with the NR cell, thereby minimizing service interruption time of the terminal.

1 is a diagram for explaining components used for handover in the SA mode of a 5G system.
2 is a flowchart illustrating a handover method using a cell of the same type.
3 is a flowchart illustrating a handover method using heterogeneous cells according to an embodiment of the present invention.
4 is a flowchart illustrating a handover method using heterogeneous cells according to an embodiment of the present invention.
5 is a configuration diagram for explaining the configuration of a terminal according to an embodiment of the present invention.

Hereinafter, some embodiments of the present invention will be described in detail with reference to exemplary drawings. In adding reference numerals to the components of each drawing, it should be noted that the same components are given the same reference numerals as much as possible even though they are indicated on different drawings. In addition, in describing the present invention, if it is determined that a detailed description of a related known configuration or function may obscure the gist of the present invention, the detailed description thereof will be omitted.

In addition, in describing the components of the present invention, terms such as first, second, A, B, (a), (b), etc. may be used. These terms are only for distinguishing the components from other components, and the essence, order, or order of the components are not limited by the terms. Throughout the specification, when a part 'includes' or 'includes' a certain component, it means that other components may be further included, rather than excluding other components, unless otherwise stated. . In addition, terms such as '~ unit' and 'module' described in the specification mean a unit that processes at least one function or operation, which may be implemented as hardware or software or a combination of hardware and software.

Meanwhile, embodiments of the present invention may be supported by standard documents disclosed in at least one of IEEE 802, 3GPP, and 3GPP2, which are wireless access systems. That is, steps or parts not described in order to clearly reveal the technical spirit of the present invention among the embodiments of the present invention may be supported by the above documents. In addition, all terms disclosed in this document may be explained by the standard document. In particular, key terms may be explained by the document of 3GPP TS 38.331.

1 is a diagram for explaining components used for handover in the SA mode of a 5G system.

Referring to FIG. 1 , a 5G system may include a terminal 100 , a serving cell 110 , a new radio cell 120 , and a long-term-evolution cell 130 . .

The terminal 100 is a mobile component, and is connected to a base station (gNodeB) and a 5G core network through the serving cell 110 , and is provided with various services from the base station and the 5G core network.

The terminal 100 may be connected to the RRC layer of the serving cell 110 by transmitting a radio resource control (RRC) connection request message to the serving cell 110 and receiving the RRC establishment message from the serving cell 110 . RRC is a protocol layer configured to perform functions such as broadcasting, paging, RB control, mobility function, and terminal measurement report/control between the terminal and the base station. The terminal 100 may modify radio resources through the RRC reconnection process even after RRC connection with the serving cell 110 , and the terminal 100 may also perform RRC connection with the NR cell 120 or the LTE cell 130 . there is.

The terminal 100 may receive a radio signal from each base station that manages the serving cell 110 , the NR cell 120 and the LTE cell 130 according to the RRC setting, and measure the quality of the signal. Here, the signal quality mainly refers to reference signal received power (RSRP), but may also be reference signal received quality (RSRQ), received signal strength indicator (RSSI), or signal to inference noise ratio (SINR).

When the terminal 100 goes out of coverage of the serving cell 110 , that is, when the quality of a signal received from and from the serving cell 110 is lowered, the terminal 100 is different from the serving cell 110 . A handover with the cell may be attempted. The terminal 100 first attempts handover with the NR cell 120 as a target cell, and if it detects a handover failure or radio link failure, the terminal performs handover with the LTE cell 130 . can try

Hereinafter, the terminal 100 is UE (User Equipment), MS (Mobile Station), UT (user terminal), MSS (Mobile Subscriber Station), SS (Subscriber Station), AMS (Advanced Mobile Station), WT (Wireless terminal) ), a Machine-Type Communication (MTC) device, a Machine-to-Machine (M2M) device, and a Device-to-Device (D2D) device.

The serving cell 110 is a component that initially establishes an RRC connection with the terminal 100 , provides a service to the terminal 100 , and helps the terminal 100 perform handover with other cells.

The serving cell 110 receives a measurement report message for the signal quality of each cell from the terminal 100 (measurement report message). The serving cell 110 transmits an overhand command message to the terminal 100 after comparing the signal quality of each cell using the measurement report message.

Also, the serving cell 110 performs handover preparation with the handover target cell. Handover preparation refers to a process in which a target cell receives a user equipment (UE) context for the terminal 100 from the serving cell 110 . When the serving cell 110 and the target cell perform handover preparation, the terminal 100 may perform handover with the target cell.

The NR cell 120 means a service area range of a base station (gNodeB) using a 5G access network, and is a target cell for the terminal 100 to perform handover first.

The LTE cell 130 means a service area range of a base station (eNodeB) using a 4G access network, and is a target cell for the terminal 100 to perform handover in the second order.

Hereinafter, it will be mainly described that the serving cell 110 and the NR cell 120 are components of the 5G system, and the LTE cell 130 is a component of the 4G system, but the present invention is not limited thereto. Reference numeral 120 may be a component of the 4G system, and the LTE cell 130 may be a component of the 5G system.

Hereinafter, each cell not only means a service area of a base station, but also has a meaning as a terminal node of a network that directly communicates with a terminal. A specific operation described as being performed by the base station in this document may be performed by an upper node of the base station in some cases. That is, it is obvious that various operations performed for communication with the terminal in a network including a plurality of network nodes including the base station may be performed by the base station or other network nodes other than the base station. 'Base station (BS: Base Station)' is a term such as gNB (g-NodeB), fixed station (fixed station), NodeB, eNB (evolved-NodeB), BTS (base transceiver system), access point (AP: Access Point), etc. can be replaced by

2 is a flowchart illustrating a handover method using a cell of the same type.

Referring to FIG. 2 , the terminal 100 performs RRC connection or RRC connection reconfiguration with the serving cell 110 ( S200 ). The UE 100 may set measurement report conditions through RRC connection reconfiguration with the serving cell 110 . The measurement report condition is a condition for the terminal 100 to trigger a handover, and may be set using signal quality of adjacent cells.

After the RRC connection re-establishment, the terminal 100 measures the signal quality of adjacent cells (S202). The terminal 100 measures the signal quality of the serving cell 110 and adjacent NR cells.

When the measurement report condition is satisfied, the terminal 100 transmits a measurement report message to the serving cell 110 (S204). The measurement report condition may mean a case in which the signal quality of the serving cell 110 is low or an NR cell 120 having a high signal quality is found among adjacent NR cells.

The serving cell 110 receiving the measurement report message from the terminal 100 performs handover preparation with the NR cell satisfying the measurement report condition (S206). The serving cell 110 may transmit the UE context of the terminal 100 to the NR cell 120 through handover preparation. Specifically, the serving cell 110 transmits a handover request message to the NR cell 120 , and the NR cell 120 transmits a handover request acknowledgment message to the serving cell 110 , thereby preparing for handover. .

The serving cell 110 transmits a handover command message to the terminal 100 after performing handover preparation (S208). Specifically, the serving cell 110 receives the handover command message from the NR cell 120 and delivers it to the terminal 100 .

The terminal 100 may detect a radio link failure during handover with the NR cell 120 according to the handover command message (S210).

When the terminal 100 detects radio link failure, it transmits an RRE request message to the serving cell 110 to reestablish RRC and the serving cell 110, or an RRE request message to the NR cell 120 to send When the serving cell 110 receives the RRE request message, the serving cell 110 may perform RRC connection using the UE context of the terminal 100 . When the NR cell 120 receives the RRE request message, the NR cell 120 communicates with the terminal 100 using the UE context of the terminal 100 received through handover preparation with the serving cell 110 . RRC connection may be performed.

In this case, even if the serving cell 110 or the NR cell 120 that has failed handover with the terminal 100 attempts handover with the terminal 100 again, there is a high probability that the handover will fail. As a result, the terminal 100 has a problem in that the service interruption time of the 5G system becomes long by falling back to a heterogeneous network.

3 is a flowchart illustrating a handover method using heterogeneous cells according to an embodiment of the present invention.

Referring to FIG. 3 , the terminal 300 according to an embodiment of the present invention may re-establish an RRC connection with the serving cell 310 ( S300 ). The terminal 300 may set the measurement report condition through the serving cell 310 and RRC connection reconfiguration. This may be performed by the UE 300 receiving an RRC connection reconfiguration message including a measurement report condition from the serving cell 310 . In addition, the terminal 300 may be configured to measure the signal quality of the NR cells that are the same as the serving cell 310 as well as the signal quality of the LTE cells that are heterogeneous through the RRC connection reconfiguration.

The terminal 300 may measure the signal quality of the serving cell 310, the signal quality of the adjacent NR cells, and the signal quality of the adjacent LTE cells according to the RRC connection re-establishment (S302). Here, the signal quality may be any one of RSRP, RSRQ, SINR, or RSSI of a signal received by the terminal 300 from each cell. The terminal 300 according to an embodiment of the present invention may measure LTE frequency band signals of LTE cells through an automatic gap operation without setting a measurement gap.

The terminal 300 transmits a measurement report message to the serving cell 310 when the measurement report condition according to the RRC connection reconfiguration is satisfied (S304).

The measurement report condition according to an embodiment of the present invention is that the signal quality of the NR cell 320 is higher than the signal quality of the serving cell 310 by an offset, and the signal quality of the LTE cell 330 is higher than the threshold value. it could be Offset and threshold can be arbitrarily set according to wireless environment or network connection timeout.

The measurement report condition according to another embodiment of the present invention is that the signal quality of the NR cell 320 is higher than the signal quality of the serving cell 310 by an offset, the signal quality of the serving cell 310 is lower than the first threshold, , the signal quality of the LTE cell 330 may be higher than the second threshold. Similarly, the offset, the first threshold, and the second threshold may be arbitrarily set according to a wireless environment or a network connection time limit.

In addition, the measurement report condition according to another embodiment of the present invention is 1) the signal quality of the NR cell 320 is higher than the signal quality of the serving cell 310, and the signal quality of the LTE cell 330 is higher than the threshold value. Case 2) the signal quality of the NR cell 320 is higher than the signal quality of the serving cell 310, the signal quality of the serving cell 310 is lower than the first threshold, and the signal quality of the LTE cell 330 is It may be any one of cases higher than the second threshold value.

The measurement report message transmitted by the terminal 300 to the serving cell 310 includes information on the NR cell 320 that satisfies the measurement report condition and information on the LTE cell 330 . In addition, the measurement report message includes the signal quality of the NR cell 320 and the signal quality of the LTE cell 330 . Here, the signal quality may be any one or more of RSRP, RSRQ, RSSI, and SINR of each cell.

The serving cell 310 performs handover preparation with the NR cell 320 (S306). The NR cell 320 may receive the UE context of the terminal 300 from the serving cell 310 through handover preparation. The serving cell 310 may perform handover preparation based on the NR cell 320 and Xn I/F.

According to an embodiment of the present invention, the serving cell 310 performs handover preparation with the LTE cell 330 (S308). The LTE cell 330 may receive the UE context of the terminal 300 from the serving cell 310 through handover preparation. The serving cell 310 may perform handover preparation based on the LTE cell 330 and N2 I/F.

The serving cell 310 transmits a handover command message for the NR cell 320 to the terminal 300 after performing handover preparation (S310). The serving cell 310 receives the handover command message from the NR cell 320 and delivers it to the terminal 300 .

The terminal 300 may detect a radio link failure (RLF) while performing handover with the NR cell 320 ( S312 ). RLF means that the UE 300 fails to complete handover to the NR cell 320 within the time according to the T304 timer, and may be mixed with handover failure. Here, the T304 timer means a timer for returning the setting of the terminal 300 to the one before the handover when the terminal 300 does not succeed in handover to the NR cell 320 for a predetermined time. That is, if the terminal 300 does not succeed in handover with the NR cell 320 within the time according to the T304 timer, it is determined that the RLF is detected.

When the UE 300 detects the RLF, it transmits an RRC re-establishment (RRE) request message to the LTE cell 330 for RRC connection with the LTE cell 330 (S314). Here, the RRE refers to a process of reconnecting an RRC that has been disconnected due to a handover failure of the terminal 300 . The LTE cell 330 that has received the RRE request message from the terminal 300 may establish an RRC connection with the terminal 300 using the UE context stored in advance through handover preparation, and the terminal 300 is the LTE cell ( 330) can be provided.

Therefore, the terminal 300 measures the signal quality of the LTE cell 330 in advance, and reports the signal quality of the LTE cell 330 to the serving cell 310 for overhand preparation of the LTE cell 330, thereby NR Even if the handover with the cell 320 fails, the service provision interruption time can be minimized.

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

Referring to FIGS. 3 and 4 , steps S400 to S410 are the same as steps S300 to S310, and thus a detailed description thereof will be omitted.

The terminal 300 re-measures the signal quality of adjacent NR cells after detecting the RLF, and compares them (S414).

The terminal 300 according to an embodiment of the present invention may transmit an RRE request message to the LTE cell 330 when the re-measured signal quality of adjacent NR cells is lower than a threshold value.

The threshold value may be an arbitrarily set value, and the threshold value according to another embodiment of the present invention may be the signal quality of the LTE cell 330 . The threshold value according to another embodiment of the present invention may be RSRP-thresholdSSB that the NR cell 320 broadcasts as SIB1 or a value obtained by adding an offset to RSRP-thresholdSSB.

5 is a configuration diagram for explaining the configuration of a terminal according to an embodiment of the present invention.

Referring to FIG. 5 , the terminal according to an embodiment of the present invention includes a processor 510 , a memory 520 , and a communication unit 530 . The processor 510 is composed of at least one or more, and implements the functions, processes and/or methods proposed in FIGS. 1 to 4 above. The layers of the air interface protocol may be implemented by the processor 510 . The memory 520 is connected to the processor 510 and stores various information for driving the processor 510 . The communication unit 530 is connected to the processor 510 to transmit and/or receive a wireless signal.

The memory 520 is composed of at least one or more, and may be inside or outside the processor 510 , and may be connected to the processor 510 by various well-known means. In addition, the terminal 500 may have a single antenna or multiple antennas.

Embodiments according to the present invention may be implemented by various means, for example, hardware, firmware, software, or a combination thereof. In the case of implementation by hardware, an embodiment of the present invention provides one or more application specific integrated circuits (ASICs), digital signal processors (DSPs), digital signal processing devices (DSPDs), programmable logic devices (PLDs), FPGAs ( field programmable gate arrays), a processor, a controller, a microcontroller, a microprocessor, and the like.

In the case of implementation by firmware or software, an embodiment of the present invention may be implemented in the form of a module, procedure, function, etc. that perform the functions or operations described above. The software code may be stored in the memory and driven by the processor. The memory may be located inside or outside the processor, and may transmit and receive data to and from the processor by various known means.

2 to 4, it is described that the steps S200 to S416 are sequentially executed, but this is merely illustrative of the technical idea of an embodiment of the present invention. In other words, those of ordinary skill in the art to which an embodiment of the present invention pertain change the order described in FIGS. Since it will be possible to apply various modifications and variations to executing one or more processes in S416 in parallel, FIGS. 2 to 4 are not limited to a time-series order.

Meanwhile, the processes shown in FIGS. 2 to 4 can be implemented as computer-readable codes on a computer-readable recording medium. The computer-readable recording medium includes all kinds of recording devices in which data readable by a computer system is stored. That is, the computer-readable recording medium may be a non-transitory medium such as ROM, RAM, CD-ROM, magnetic tape, floppy disk, and optical data storage device, and also carrier wave (for example, , transmission over the Internet) and may further include a transitory medium such as a data transmission medium. In addition, the computer-readable recording medium is distributed in a network-connected computer system so that the computer-readable code can be stored and executed in a distributed manner.

The above description is merely illustrative of the technical idea of this embodiment, and various modifications and variations will be possible by those skilled in the art to which this embodiment belongs without departing from the essential characteristics of the present embodiment. Accordingly, the present embodiments are intended to explain rather than limit the technical spirit of the present embodiment, and the scope of the technical spirit of the present embodiment is not limited by these embodiments. The protection scope of this embodiment should be interpreted by the following claims, and all technical ideas within the equivalent range should be interpreted as being included in the scope of the present embodiment.

300: terminal 310: serving cell
320: NR cell 330: LTE cell

Claims (6)

In the handover (handover) method in the SA (Stand-Alone) mode,
Receiving an RRC connection reconfiguration (radio resource control connection reconfiguration) message including a measurement report condition for handover from a serving cell; The measurement report condition is that the signal quality of the adjacent NR cell is higher than the signal quality of the serving cell by an offset, and the signal quality of the adjacent LTE cell is higher than the threshold,
The process of measuring both the signal quality of the neighboring LTE cells and the signal quality of the neighboring NR cells;
transmitting, to the serving cell, a measurement report message including both information on an LTE cell and information on an NR cell satisfying the measurement report condition;
receiving, by the serving cell, a handover command message for the NR cell from the serving cell after performing handover preparation with the NR cell and the LTE cell;
transmitting an RRC reestablishment (RRE) request message to the LTE cell for RRC connection with the LTE cell when detecting radio link failure with the NR cell;
A handover method comprising a. According to claim 1,
The method further comprises the step of re-measuring the signal quality of the adjacent NR cells when detecting a radio link failure with the NR cell,
A handover method for transmitting an RRE request message to the LTE cell when the remeasured signal quality of the adjacent NR cells is lower than the signal quality of the LTE cell. According to claim 1,
The measurement report condition is,
The handover method, wherein the signal quality of the NR cell is higher than the signal quality of the serving cell by an offset, the signal quality of the serving cell is lower than a first threshold, and the signal quality of the LTE cell is higher than a second threshold. . In the terminal for handover in SA mode,
transceivers;
at least one memory storing instructions; and
By executing the above instructions,
Receiving an RRC connection reconfiguration message including a measurement report condition for handover from the serving cell, wherein the measurement report condition is that the signal quality of the adjacent NR cell is higher than the signal quality of the serving cell by an offset, and the adjacent LTE cell that the signal quality of is higher than the threshold,
Measure both the signal quality of adjacent LTE cells and the signal quality of adjacent NR cells,
Transmitting a measurement report message including both LTE cell information and NR cell information that satisfies the measurement report condition to the serving cell,
After the serving cell performs handover preparation with the NR cell and the LTE cell, receiving a handover command message for the NR cell from the serving cell,
A terminal comprising at least one processor configured to transmit an RRE request message to the LTE cell for RRC connection with the LTE cell when detecting a radio link failure with the NR cell. 5. The method of claim 4,
The processor is
When detecting a radio link failure with the NR cell, re-measure the signal quality of the adjacent NR cells,
A terminal configured to transmit an RRE request message to the LTE cell when the remeasured signal quality of the adjacent NR cells is lower than the signal quality of the LTE cell. 5. The method of claim 4,
The measurement report condition is,
The signal quality of the NR cell is higher than the signal quality of the serving cell by an offset, the signal quality of the serving cell is lower than a first threshold, and the signal quality of the LTE cell is higher than a second threshold.

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