JP2013012819A - Handover control method, base station device, and radio communication system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve the through-put of a system by controlling useless consumption of radio resources related to retransmission of a handover command.SOLUTION: A handover control method includes the steps of: detecting, by a radio base station (10), whether there is a link fault during radio communication with a mobile station (1); transmitting a handover command to the mobile station (1) by the base station (10) if an execution condition of handover has been satisfied; retransmitting the handover command if feedback information from the mobile station (1) has not been received after transmission of the handover command; changing a detection condition to determine that a radio link fault exists toward the direction where a radio link fault will be detected sooner after transmission of the handover command; and stopping retransmission of the handover command if it has been detected that a radio link fault exists after transmission of the handover command.

Description

  The present invention relates to a handover control method, a base station apparatus, and a radio communication system in a communication system such as LTE.

  Currently, standardization of the LTE (Long Term Evolution) method is being advanced as an access method for mobile communication systems. In the LTE system, the following regulations are made in a handover (H / O) process of a mobile station (for example, UE: User Equipment). That is, in order to reduce the handover delay, the mobile station that has received the handover command from the base station (eNB) does not need to return a layer 2 acknowledgment (ACK) in response to this command. And it is a regulation that allows the handover process to proceed such as resetting of internal resources as it is (see Non-Patent Document 1).

  A general handover procedure of the LTE system is shown in FIGS. As shown in FIG. 4, when handover execution determination and handover preparation processing are performed in steps S1 to S6, a handover command is subsequently transmitted from the handover source source base station (source eNB) 10 to UE1. (Step S7). Receiving this command, UE1 performs internal processing in step J3 to start synchronization with the targate base station 30 in order to change the connection destination of the radio link from the source base station 10 to the targate base station 30 (step S9). ). When the synchronization is successful, uplink resource allocation and timing advance information is transmitted from the target base station 10a to the UE1 (step S10). UE1 switches the setting about an uplink based on these information, Then, the notification of the completion of RRC connection and resetting which notifies completion of a handover is transmitted to the target base station 10a (step S11).

  In response to the handover completion notification, the targate base station 30 transmits the user data of UE1 to the upper station (for example, a service gateway: Serving Gateway) 52 in the subsequent handover final step J4 (see FIG. 5). Thus, processing for switching the data transfer path is executed (steps S12 to S16). Then, after this switching is completed, the targate base station 30 notifies the source base station 10 of the release of the UE context (step S17). Based on this notification, the source base station 10 releases resources allocated to the UE 1 (step S18).

  On the other hand, if the source base station 10 does not receive the UE context release notification in step S17 even after a predetermined time has elapsed after transmitting the handover command in step S7, it is regarded as a handover failure, and the process of step S18 is voluntarily performed. Perform resource release.

3GPP TS36.331 V10.1.0 "Reception of an RRC Connection Reconfiguration including the mobility Control Info by the UE (handover)"

  In the handover process of the LTE system, as described above, the UE 1 does not need to return a layer 2 acknowledgment in response to the handover command in step S7. Therefore, the source base station 10 that has transmitted the handover command may not be able to immediately know whether the handover command has been normally received by the UE 1. In this case, the layer 2 automatic retransmission process causes the handover command to be retransmitted to UE1 wastefully, causing a problem that radio frequency resources are wasted. Further, in the LTE system, radio frequency resources are allocated to a plurality of UEs 1 in common, and therefore, the consumption of frequency resources leads to a decrease in the throughput of the entire system.

  An object of the present invention is to provide a handover control method, a base station apparatus, and a wireless communication system capable of suppressing the wasteful consumption of radio resources required for retransmission of a handover command and improving the system throughput.

  The handover control method according to the present invention includes a step in which a base station detects whether there is a radio link failure with the mobile station during radio communication with the mobile station, and the base station when a handover execution condition is met. Transmitting a handover command to the mobile station, retransmitting the handover command when feedback information is not received from the mobile station after transmitting the handover command, and transmitting the handover command after transmitting the handover command. A step of changing the detection condition to be present in a direction in which the radio link failure is detected earlier, and the retransmission of the handover command is stopped when the presence of the radio link failure is detected after the handover command is transmitted. A configuration including steps is adopted.

  According to the present invention, even if there is no acknowledgment for the handover command, the retransmission of the handover command is quickly stopped. Therefore, the consumption of radio resources for this retransmission can be quickly eliminated, and the system throughput can be improved.

The block diagram which shows the structure of the radio | wireless communications system containing the base station of 1st Embodiment of this invention. The block diagram which shows the structure of the radio | wireless communications system containing the base station of 2nd Embodiment of this invention. FIG. 9 is a sequence diagram illustrating a handover procedure of the wireless communication system according to the second embodiment. Sequence diagram showing a handover procedure of a conventional wireless communication system FIG. 4 is a sequence diagram showing a detailed procedure of the final handover step J4 in FIG.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings.

[First Embodiment]
FIG. 1 is a block diagram showing a configuration of a radio communication system including a base station according to the first embodiment of the present invention.

  The wireless communication system of this embodiment includes a plurality of mobile stations (for example, UE1), a plurality of base stations 10, and a plurality of upper stations (51, 52) connected to the plurality of base stations 10 through a network. . The host station includes an MME (Mobility Management Entity) 51 that performs user authentication and network connection management of the mobile station, and a service gateway (S-GW, see FIG. 3) 52 that routes user data. For the base station 10, the handover source base station 10 is called a source base station, and the handover destination base station 10 is called a target base station.

  The base station 10 is connected to a host station (MME 51, service gateway 52, etc.) and transmits / receives data, a wireless unit 16 that transmits / receives radio signals to / from UE1, and a signal from UE1 A receiving unit 12 that performs layer 2 uplink signal processing, a transmitting unit 13 that performs layer 2 downlink signal processing and transmits a signal to UE1, and determines that transmission data is not normally received by UE1 A retransmission control unit 14 that retransmits transmission data when the transmission is performed, and an RLF detection unit (failure detection unit) 15 that detects that a radio link failure has occurred between the UE 1 and the like. Yes. The transmission unit 13 also functions as a transmission control unit that transmits a handover command.

  The retransmission control unit 14 monitors whether feedback information “ACK” is received from the UE 1 for the data transmission of the transmission unit 13. When a negative response “NACK” is received or when no positive response “ACK” is received, the retransmission control information is output to the transmitter 13, and the MAC layer (media access control layer) or the RLC layer ( Data retransmission of the radio link control layer) is performed. This data retransmission control is similarly performed when a handover command is transmitted from the transmission unit 13.

  In parallel with the uplink signal processing, the reception unit 12 detects the synchronization shift of the radio signal received from the UE 1 and outputs this synchronization information to the RLF detection unit 15. When a response to the transmission data is received, feedback information indicating the response content is output to the retransmission control unit 14.

  In parallel with downlink signal processing, the transmission unit 13 retransmits the target data when retransmission control information is input from the retransmission control unit 14. Further, when performing data retransmission, a signal 22 indicating the number of retransmissions is output to the RLF detection unit 15. When transmitting a handover command, this signal 22 indicates the number of retransmissions of the handover command. Even when the retransmission control information is input from the retransmission control unit 14, the transmission unit 13 retransmits the target data when the transmission control information 21 for stopping the retransmission is input from the RLF detection unit 15. It is comprised so that it may stop (1st cancellation control part).

  The RLF detection unit 15 includes the above-described synchronization information supplied from the reception unit 12, a signal 22 indicating the number of data retransmissions supplied from the transmission unit 13, and various control timers (RRCs) used by the reception unit 12 and the transmission unit 13. Timer: radio resource control timer, etc.). When the synchronization deviation indicated in the synchronization information exceeds a predetermined threshold, when the number of data retransmissions exceeds the upper limit value, or when the control timer expires, it is detected that there is a radio link failure.

  When detection of the presence of a radio link failure is made, failure information is sent from the RLF detection unit 15 to the reception unit 12 or the transmission unit 13, and the reception unit 12 and the transmission unit 13 are used to repair the radio link with the UE1. A process or a process of releasing a radio link is executed. Further, when the RLF detection unit 15 detects that there is a radio link failure, the RLF detection unit 15 outputs transmission control information 21 for stopping data retransmission for the UE 1 that has failed to the transmission unit 13 to stop data retransmission for the UE 1. It is like that.

The threshold value for the above-described synchronization shift and the upper limit value of the number of retransmissions (upper limit value of the number of retransmissions in the MAC layer and the RLC layer) are set to normal values during normal radio link. For example, the following setting is performed in accordance with the rules of 3GPP_TS36.331.
Threshold for synchronization deviation: Threshold value “N310” indicating “out-of-sync”
MAC layer retransmission count upper limit value ... upper limit value "maxHARDQ-Tx"
RLC layer retransmission count upper limit value ... upper limit value "maxRetxThreshold"

  Further, the threshold for the above-described synchronization shift and the upper limit value of the number of retransmissions are set in a direction in which occurrence of a radio link failure is promptly detected in a period from when a handover command is transmitted to UE1 until resource release of UE1 is performed. Settings are changed. That is, the threshold value for synchronization deviation is small, and the upper limit value of the number of retransmissions is reduced. These setting changes are executed by the control unit of the RLF detection unit 15 or another control unit of the base station 10. These control units function as a condition changing unit.

  In the conventional wireless communication system, the time from the start of the handover process until the UE 1 advances the random access procedure to the handover destination base station and the synchronization is successful is counted. When this time exceeds a predetermined time-up time “T304” (specified in 3GPP_TS36.331), it is determined that the handover process has failed, and a process of releasing the radio link with the UE 1 is performed. It is like that. The threshold value and the upper limit value after the setting change are set such that detection of occurrence of a radio link failure is performed in a time shorter than the time-up time “T304”.

[Description of operation]
In the base station 10 of this embodiment, when the wirelessly connected UE1 satisfies the handover execution condition, a handover command is transmitted from the transmission unit 13 to the UE1.

  When the handover command is transmitted, the threshold value and the upper limit value of the RLF detection unit 15 are changed based on the transmission, so that a radio link failure with the target UE 1 is easily detected more quickly than usual. Become.

  Here, it is assumed that the transmission of the acknowledgment “ACK” is omitted after the UE 1 receives the handover command. Then, the retransmission of the handover command is continuously performed from the transmission unit 13 under the control of the retransmission control unit 14 of the base station 10.

  Here, at an early stage, the synchronization shift of the radio signal received from the UE 1 exceeds the changed threshold value, or the data retransmission count of the handover command exceeds the changed upper limit value, and the RLF detection unit 15, a radio link failure is detected.

  When a radio link failure is detected, transmission control information 21 for stopping data retransmission is sent from the RLF detection unit 15 to the transmission unit 13, and data retransmission of the handover command is stopped.

  As described above, according to the base station 10 and the radio communication system of this embodiment, even when the transmission of an acknowledgment to the handover command is omitted in the UE1, the retransmission of the handover command from the base station 10 is promptly stopped. The Therefore, the consumption of radio resources for this retransmission is quickly eliminated, and the system throughput is improved.

[Second Embodiment]
FIG. 2 is a block diagram showing a configuration of a radio communication system including the base stations 10 and 10a according to the second embodiment of the present invention.

  In the second embodiment, in addition to the configuration of the first embodiment, when the target UE 1 to be handed over starts wireless connection to the target base station 10a, the target base station 10a notifies the source base station 10 of the start of handover. Is added. Even when the data retransmission of the handover command from the source base station 10 to the UE 1 is continued, the data retransmission is stopped by the notification of the handover start.

  In order to realize the above operation, the base station 10a of this embodiment is further provided with a handover start determination unit 18 as a notification transmission unit. Although omitted in FIG. 2, the handover start determination unit 18 is similarly provided in all base stations on the system including the source base station 10. The configurations of the retransmission control unit 14 and the RLF detection unit 15 are similarly provided in all base stations on the system including the target base station 10a.

  The handover start determining unit 18 receives a handover request from another base station 10 and makes a permission response in the handover process for newly connecting the UE 1 by radio (see step S6 in FIG. 3), or thereafter When the packet data to be transmitted from the base station 10 to the handover target UE 1 is transferred (see step S8 in FIG. 3), the fact that the handover process is started is stored. The reception of these handover requests or the transfer of packet data is recognized by the handover start determining unit 18 when handover transmission information representing them is sent from the transmission unit 13.

  In addition, when the receiving unit 12 receives data (for example, a random access channel preamble) for synchronization from the handover target UE 1, message reception information for notifying the handover start determining unit 18 is received by the receiving unit 12. 12 sent from.

  Alternatively, the following configuration may be applied instead of the message reception information. That is, when a handover completion message (for example, notification of completion of RRC connection / reconfiguration) is received from the target UE 1 by the reception unit 12, message reception information notifying this is received from the reception unit 12 to the handover start determination unit 18. You may comprise so that it may be sent.

  The handover start determination unit 18 determines that the target UE 1 has started the handover procedure when the message reception information is sent while the start of the handover process is stored. And based on this determination, it is comprised so that the information of a handover start may be transmitted to the source base station 10 via a network.

  In addition, when the retransmission control unit 14 of the source base station 10 in the second embodiment receives the above handover start information in a situation where the data retransmission of the handover command to the UE 1 is continued, the data of the handover command The retransmission is stopped (second cancellation control unit).

[Description of operation]
FIG. 3 is a sequence diagram showing a handover procedure in the wireless communication system according to the second embodiment.

  The base stations 10 and 10a and the upper stations (MME 51 and service gateway 52) perform processing related to area restriction of each UE 1 (step S0), and the source base station 10 performs signal measurement of neighboring cells on each UE 1 according to the area restriction information. (Step S1).

  As a result of the signal measurement, when a measurement report that requires execution of handover is transmitted from the UE 1 to the source base station 10 (step S2), the source base station 10 determines that the target base station 10a to be handed over is based on this measurement report. Determined (step S3). Subsequently, a handover request is made from the source base station 10 to the target base station 10a (step S4). When the handover request is permitted by the internal processing of the target base station 10a (step S5), an affirmative response is made (step S5). S6).

  With this affirmative response, handover transmission information is sent from the transmission unit 13 of the target base station 10a to the handover start determination unit 18 as shown in FIG. Thereby, the handover start determining unit 18 stores that the handover process has been started for the target UE1.

  If an affirmative response is made, the source base station 10 transmits a handover command to the UE 1 (step S7). Specifically, this handover command is an RRC connection reconfiguration message including mobility control information.

  When receiving the handover command, UE1 starts a process of disconnecting the radio link with source base station 10 and synchronizing with target base station 10a (step J3). Here, the case where UE1 omits the response to the handover command may occur. When the response is omitted, the source base station 10 continuously performs data retransmission of the handover command.

  When the handover command is transmitted, the packet data stored for transmission to the UE 1 is transferred between the source base station 10 and the target base station 10a (steps J1 and J2). At the time of this data transfer, an SN status indicating a packet sequence number (SN) is also transferred in parallel (step S8).

  In the process of synchronizing the UE1 and the target base station 10a, the UE1 first performs a process of transmitting data (random access channel preamble) for synchronization to the target base station 10a (step S9). As a result, if the synchronization is successful, uplink resource allocation information and timing advance information are transmitted from the target base station 10a to the UE 1 (step S10). UE1 performs internal setting based on these pieces of information, and transmits an RRC connection and notification of completion of resetting to the target base station 10a (step S11).

  When the target base station 10a receives the data in step S9 during the above series of synchronization processing, as shown in FIG. Sent to section 18. Based on this information, the handover start determining unit 18 determines that the handover process has been started in UE1, and transmits a handover start notification to the source base station 10 via the network (step J5).

  Alternatively, when the target base station 10 a receives the data in step S <b> 11, message reception information indicating this data reception is sent from the receiving unit 12 to the handover start determining unit 18. Based on this information, the handover start determination unit 18 transmits a handover start notification to the source base station 10 via the network (step J6).

  At the time of transmitting the handover start notification in steps J5 and J6, if the data retransmission of the handover command in step S7 is continued in the source base station 10, this notification is sent to the retransmission control unit 14 and the data retransmission is stopped. The

  After the UE 1 and the target base station 10a are wirelessly connected, as shown in FIG. 5, processing for switching the user data transfer path from the service gateway 52 to the UE 1 (steps S12 to S16) is performed. After that, the source base station 10 releases the UE context and the resources of the target UE 1 (steps S17 and S18), and the handover process is completed.

  As described above, according to the base stations 10 and 10a and the radio communication system of this embodiment, even when the data retransmission of the handover command is continued from the source base station 10 to the UE1, the handover start of step J5 is started. Based on the notification, this data retransmission is stopped. Therefore, consumption of radio resources for data retransmission of the handover command can be quickly eliminated, and the system throughput can be improved.

  Alternatively, the same operation can be obtained even if the data retransmission is stopped based on the handover start notification in step J6 based on the completion notification of UE1 instead of the handover start notification in step J5. While the data retransmission can be stopped more quickly by the control based on the notification at Step J5, it is possible to more accurately determine that UE1 has executed the handover by the control based on the notification at Step J6.

  In the above embodiment, the handover start notification is transmitted from the target base station 10a. However, a similar notification may be transmitted from the upper station of the target base station 10a to the source base station 10. Also, the order of transmission of the handover start notification in step J5 in FIG. 3 and the transmission of uplink resource allocation information in step S10 may be reversed.

  The present invention can be applied to an LTE wireless communication system, a base station, and a handover control method thereof.

1 UE
10, 10a Base station 12 Receiving unit 13 Transmitting unit 14 Retransmission control unit 15 RLF detecting unit 18 Handover start determining unit 51 MME
52 service gateway S7 handover command transmission step S9 synchronization transmission step J5 handover start notification transmission step S11 RRC connection / reconfiguration completion notification transmission step J6 handover start notification transmission step

Claims (6)

The base station detecting whether there is a radio link failure with the mobile station during radio communication with the mobile station;
The base station transmitting a handover command to the mobile station when a handover execution condition is satisfied; and
Retransmitting the handover command when feedback information is not received from the mobile station after transmitting the handover command;
After the handover command is transmitted, the detection condition that the radio link failure is present is changed to a direction in which the radio link failure is detected early;
Stopping the retransmission of the handover command when the radio link failure is detected after the handover command is transmitted;
A handover control method including: A handover start notification to the handover source base station when another handover destination base station or an upper station of the base station starts wireless communication with the mobile station that has received the handover command; ,
When the base station of the handover source receives the start notification after transmission of the handover command, canceling retransmission of the handover command;
The handover control method according to claim 1, further comprising: A failure detection unit that detects presence or absence of a radio link failure with the mobile station during wireless communication with the mobile station;
A transmission control unit for transmitting a handover command to the mobile station when a handover execution condition is satisfied;
A retransmission control unit for retransmitting the handover command when feedback information is not received from the mobile station after transmitting the handover command;
A condition changing unit that changes the detection condition that the failure detection unit has the radio link failure after transmitting the handover command in a direction in which the radio link failure is detected earlier;
A first cancellation control unit that cancels retransmission of the handover command by the retransmission control unit when the failure detection unit detects that there is a radio link failure after transmission of the handover command;
A base station apparatus comprising: After the handover command is transmitted, when the mobile station receives a start notification indicating that the mobile station has started a handover process from another handover destination base station or an upper station of the base station, the retransmission control unit performs the handover A second cancel control unit for canceling the retransmission of the command;
The base station apparatus according to claim 3, further comprising: A notification transmitter that transmits the start notification to another base station that is a handover source when wireless communication with a mobile station to be handed over is started by another handover process;
The base station apparatus according to claim 4, further comprising: A plurality of mobile stations, and a plurality of base stations capable of wireless communication with the plurality of mobile stations,
A wireless communication system, wherein each of the plurality of base stations is a base station apparatus according to any one of claims 3 to 5.

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