SECONDARY BASE STATION, MOBILE STATION, COMMUNICATION CONTROL METHOD, AND MASTER BASE STATION

A secondary base station according to an embodiment is configured to connect to a master base station via an X2 interface and for being used for a dual connectivity communication, which is to transmit downlink data from the master base station through a split bearer to a user equipment. The secondary base station comprises: a receiver configured to receive the downlink data transferred from the master base station; a transmitter configured to transmit the downlink data received by the receiver to the user equipment; and a controller configured to determine a release of the split bearer. The transmitter configured to transmit a release request message for requesting the release of the split bearer to the master base station in response to determining the release of the split bearer.

TECHNICAL FIELD

The present disclosure relates to a secondary base station, a mobile station, a communication control method, and a master base station used in a mobile communication system.

BACKGROUND ART

In 3GPP (3rd Generation Partnership Project), which is a project aiming to standardize a mobile communication system, introduction of a dual connectivity scheme (Dual connectivity) in Release 12 and onward is expected (see Non Patent Document 1). In the dual connectivity scheme, a mobile station simultaneously establishes a connection with a plurality of base stations. The mobile station is allocated with a radio resource from each base station, and thus, it is possible to expect an improvement in throughput.

In the dual connectivity scheme, of the plurality of base stations that establish a connection with the mobile station, only one base station (hereinafter, called the “master base station”) establishes an RRC connection with the mobile station. On the other hand, of the plurality of base stations, another base station (hereinafter, called the “secondary base station”) provides an additional radio resource to the mobile station without establishing an RRC connection with the mobile station. It is noted that the dual connectivity scheme may also be called an inter-base station carrier aggregation (inter-eNB CA).

PRIOR ART DOCUMENT

SUMMARY

A secondary base station according to an embodiment is configured to connect to a master base station via an X2 interface and for being used for a dual connectivity communication, which is to transmit downlink data from the master base station through a split bearer to a user equipment. The secondary base station comprises: a receiver configured to receive the downlink data transferred from the master base station; a transmitter configured to transmit the downlink data received by the receiver to the user equipment; and a controller configured to determine a release of the split bearer. The transmitter configured to transmit a release request message for requesting the release of the split bearer to the master base station in response to determining the release of the split bearer.

A mobile communication system according to an embodiment in which a master base station and a secondary base station are configured to connect each other via an X2 interface and performing a dual connectivity communication, which is to transmit downlink data from the master base station through a split bearer to a user equipment, comprises: the secondary base station. The secondary base station includes: a receiver configured to receive the downlink data transferred from the master base station; a transmitter configured to transmit the downlink data received by the receiver to the user equipment; and controller configured to determine a release of the split bearer. The transmitter configured to transmit a release request message for requesting the release of the split bearer to the master base station in response to determining the release of the split bearer.

A mobile communication method according to an embodiment in which a master base station and a secondary base station are configured to connect each other via an X2 interface and performing a dual connectivity communication, which is to transmit downlink data from the master base station through a split bearer to a user equipment, compries steps of: receiving, at the secondary base station, the downlink data transferred from the master base station; transmitting, from the secondary base station, the downlink data received by the receiver to the user equipment; determining, at the secondary base station, a release of the split bearer; transmitting, from the secondary base station, a release request message for requesting the release of the split bearer to the master base station in response to determining the release of the split bearer.

DESCRIPTION OF THE EMBODIMENT

FIG. 1is a configuration diagram of an LTE system according to an embodiment.

As illustrated inFIG. 1, the LTE system according to the embodiment comprises UEs (User Equipments)100, E-UTRAN (Evolved-UMTS Universal Terrestrial Radio Access Network)10, and EPC (Evolved Packet Core)20.

The UE100corresponds to a mobile station. The UE100is a mobile communication device and performs radio communication with a cell (a serving cell). Configuration of the UE100will be described later.

The E-UTRAN10corresponds to a radio access network. The E-UTRAN10includes eNBs200(evolved Node-Bs). The eNB200corresponds to a radio base station. The eNBs200are connected mutually via an X2 interface. Configuration of the eNB200will be described later.

The eNB200controls a cell or a plurality of cells, and performs radio communication with the UE100that establishes a connection with the cell of the eNB200. The eNB200, for example, has a radio resource management (RRM) function, a routing function of user data, and a measurement control function for mobility control and scheduling. The “cell” is used as a term indicating a minimum unit of a radio communication area, and is also used as a term indicating a function of performing radio communication with the UE100.

The EPC20corresponds to a core network. The EPC20includes MMEs (Mobility Management Entities)/S-GWs (Serving-Gateways)300. The MME performs various mobility controls, etc., for the UE100. The SGW performs transfer control of user data. The MME/S-GW300connected to the eNBs200cia an S1 interface.

FIG. 2is a block diagram of the UE100. As illustrated inFIG. 2, the UE100comprises a plurality of antennas101, a radio transceiver110, a battery140, a memory150, and a processor160. The memory150and the processor160constitute a control unit. The memory150may be integrally formed with the processor160, and this set (that is, a chipset) may be called a processor160′.

The antennas101and the radio transceiver110are used to transmit and receive a radio signal. The radio transceiver110converts a baseband signal (a transmission signal) output from the processor160into the radio signal, and transmits the radio signal from the antennas101. Furthermore, the radio transceiver110converts the radio signal received by the antenna101into a baseband signal (a reception signal), and outputs the baseband signal to the processor160.

The battery140accumulates a power to be supplied to each block of the UE100.

The memory150stores a program to be executed by the processor160and information to be used for a process by the processor160. The processor160includes a baseband processor that performs modulation and demodulation, encoding and decoding and the like of the baseband signal, and a CPU (Central Processing Unit) that performs various processes by executing the program stored in the memory150. The processor160may further include a codec that performs encoding and decoding of sound and video signals. The processor160implements various processes and various communication protocols described later.

FIG. 3is a block diagram of the eNB200. As illustrated inFIG. 3, the eNB200comprises a plurality of antennas201, a radio transceiver210, a network interface220, a memory230, and a processor240. The memory230and the processor240constitute a control unit. The memory230may be integrally formed with the processor240, and this set (that is, a chipset) may be called a processor.

The antennas201and the radio transceiver210are used to transmit and receive a radio signal. The radio transceiver210converts a baseband signal output (a transmission signal) from the processor240into the radio signal, and transmits the radio signal from the antenna201. Furthermore, the radio transceiver210converts the radio signal received by the antenna201into a baseband signal (a reception signal), and outputs the baseband signal to the processor240.

The network interface220is connected to the neighboring eNB200via the X2 interface and is connected to the MME/S-GW300via the S1 interface. The network interface220is used in communication performed on the X2 interface and communication performed on the Si interface.

The memory230stores a program to be executed by the processor240and information to be used for a process by the processor240. The processor240includes the baseband processor that performs modulation and demodulation, and encoding and decoding of the baseband signal and a CPU that performs various processes by executing the program stored in the memory230. The processor240implements various processes and various communication protocols described later.

An LTE system according to an embodiment supports a dual connectivity scheme in an uplink.

In the dual connectivity scheme, of the plurality of eNBs that establish a connection with a UE100, only a master eNB (MeNB)200-1establishes an RRC connection with the UE100. On the other hand, a secondary eNB (SeNB)200-2of the plurality of the eNBs establishes a connection with the UE100in a layer that is lower than the RRC layer, and does not establish an RRC connection with the UE100.

Further, in a user data control of the dual connectivity scheme, the MeNB200-1provides functions of a physical layer, a MAC layer, an RLC layer, and a PDCP layer. On the other hand, the SeNB200-2provides functions of a physical layer, a MAC layer, and an RLC layer, and does not provide a function of a PDCP.

An Xn interface is set between the MeNB200-1and the SeNB200-2. The Xn interface is either an X2 interface or a new interface. The Xn interface is used for exchanging a control signal, and exchanging the user data between the MeNB200-1and the SeNB200-2.

A path of the user data of a bearer of the dual connectivity scheme (a Split Bearer) will be described by usingFIG. 4.

The Split Bearer is set between the UE100and the MeNB200-1(path #A), and from the UE100via the SeNB200-2to the MeNB200-1(path #B). The Split Bearer may be applied to the exchange of both uplink and downlink data, or may be applied to the exchange of either the uplink data or the downlink data.

(Downlink Data Control: Release Request from the MeNB200-1)

When the Split Bearer is set, the MeNB200-1transmits downlink data to the UE100via the SeNB200-2by using the path #B, while directly transmitting the downlink data to the UE100by using the path #A.

Next, an operation (an operation example1) in which the MeNB200-1releases the path #B of the Split Bearer, and transits to a normal connectivity scheme will be described by usingFIG. 5.

The MeNB200-1uses the Xn interface to transfer the downlink data of the Split Bearer to the SeNB200-2(S401).

The MeNB200-1stops the transmission of the downlink data via the SeNB200-2for the Split Bearer, and decides to release the SeNB200-2(S402). Specifically, the MeNB200-1decides to release the path #B. This decision may be made by an RRM (Radio Resource Management) function of the MeNB200-1.

Upon deciding to release the path #B, the MeNB200-1stops the transmission of the downlink data to the SeNB200-2(S403).

The MeNB200-1notifies the SeNB200-2of the release request of the path #B (S404). As a result of the notification, the MeNB200-1requests the release of the additional radio resource provided by the SeNB200-2for the Split Bearer.

The release request may be transmitted by using a SeNB Release Request message. Further, the SeNB Release Request message may include information on the Split Bearer to be released.

The release request may include information for identifying the last downlink data that the MeNB200-1transfers to the SeNB200-2. For example, the information for identifying the last downlink data may be an End Marker that includes the sequence number of the last PDCP packet (PDCP PDU) transmitted by the MeNB200-1.

The SeNB200-2transmits the downlink data received from the MeNB200-1to the UE100(S405).

Further, the SeNB200-2receives an Ack with an indication of a successful data reception, as a delivery acknowledgment signal of the downlink data (a PDCP packet), from the UE100(S406).

The SeNB200-2confirms that an Ack is received for the last downlink data (S407).

After confirming that an Ack is received for the last downlink data, the SeNB200-2transmits a response signal to the release request, to the MeNB200-1(S408). The response signal may use a SeNB Release Response message.

Upon receiving the response signal to the release request that is transmitted from the SeNB200-2, the MeNB200-1performs resetting of the RRC connection (RRC Connection Reconfiguration) with the UE100(S409). As a result of resetting the RRC connection, the setting of the radio interval changes from the dual connectivity scheme to a normal connectivity scheme.

As a result of using this method, the SeNB200-2is capable of releasing the path of the Split Bearer after confirming that the last downlink data has been received by the UE100.

Next, another operation (an operation example 2) in which the MeNB200-1releases the path #B of the Split Bearer, and transits to a normal connectivity scheme will be described by usingFIG. 6.

The MeNB200-1uses the Xn interface to transfer the downlink data of the Split Bearer to the SeNB200-2(S501, data transfer for split bearer).

The MeNB200-1notifies the SeNB200-2of the release request of the path #B (S502). As a result of the notification, the MeNB200-1requests the release of the additional radio resource provided by the SeNB200-2for the Split Bearer.

The release request may be transmitted by using a SeNB Release Request including the information on the Split Bearer to be released.

The SeNB200-2transmits the downlink data (a PDCP packet) to the UE100(S503).

The SeNB200-2receives a Nack signal with an indication of an unsuccessful reception of the downlink data (S504), as a delivery acknowledgment signal, or the SeNB200-2identifies the downlink data (the PDCP packet) for which it is not possible to receive the delivery acknowledgement signal.

The SeNB200-2generates a list of unsuccessfully transmitted PDCP packets (S505).

The SeNB200-2transmits a response to the release request, to the MeNB200-1(S506). The response signal may include a list of unsuccessfully transmitted PDCP packets. Further, the response to the release request may use a SeNB Release Response message.

By using the present method, the MeNB200-1is capable of identifying the PDCP packet that the UE100has failed to receive.

Next, an operation when the UE100detects an anomaly in the path #B of the Split Bearer will be described by usingFIG. 7.

The UE100performs exchange of data (the PDCP packet) with the SeNB200-2by using the path #B of the Split Bearer.

The UE100detects an anomaly in a radio link that is set between the UE100and the SeNB200-2(S601). Specifically, the UE100detects a problem in a radio link, an abnormal release, and a radio link failure (RLF).

Upon detecting an anomaly in the radio link, the UE100confirms the sequence number (PDU SN) of the unreceived PDCP packet (S602).

The UE100transmits the sequence number of the unreceived PDCP packet to the MeNB200-1(S603). For example, the UE100may transmit the sequence number of an untransmitted PDCP packet, which is included in an Undelivered Sequence Number Report message. Alternatively, the UE100may transmit a PDCP Status Report to the MeNB200-1.

As a result, even when the UE100detects an anomaly in a radio link between the UE100and the SeNB200-2, the UE100is capable of notifying the MeNB200-1of information that allows for recognition of an untransmitted PDCP packet. The MeNB200-1is capable of retransmitting an untransmitted PDCP packet.

(Downlink Data Control: Release Request from the SeNB200-2)

Next, an operation (an operation example 3) in which the SeNB200-2releases the path #B of the Split Bearer will be described by usingFIG. 8.

The MeNB200-1transfers the downlink data to the SeNB200-2(S701). The SeNB200-2transmits the received downlink data to the UE100.

The SeNB200-2decides to release the path #B of the Split Bearer (S702). For example, the RRM (Radio Resource Management) of the SeNB200-2decides to release the path #B of the Split Bearer.

The SeNB200-2transmits a release request for the path #B of the Split Bearer to the MeNB200-1(S703). The release request may use a SeNB Release Request message. Further, information for identifying the downlink data transmitted from the SeNB200-1to the UE100, specifically, the sequence number of the PDCP SDU, may be included in the release request and transmitted. The information for identifying the downlink data is information on the downlink data that has already been transmitted as of the release of a connection with the UE100by the SeNB200-1.

Upon receiving the release request from the SeNB200-2, the MeNB200-1stops the transfer of data to the SeNB200-2(S704). More specifically, when the MeNB200-1receives the release request from the SeNB200-2, and determines acceptance (RRM decision), the MeNB200-1stops the transfer of data to the SeNB200-2.

The MeNB200-1confirms the last data transmitted by the SeNB200-2to the UE100(S705).

The SeNB200-2transmits the data transferred from the MeNB200-1to the UE100(S706). The SeNB200-2receives a confirmation signal (Ack) from the UE100(S707).

The MeNB200-1transmits a response to the release request, to the SeNB200-2(S708). The response to the release request may use a SeNB Release Response message.

The MeNB200-1performs reconfiguration of the RRC connection (RRC Connection Reconfiguration) with the UE100(S709). As a result of reconfiguration of the RRC connection, the setting of the radio interval changes from the dual connectivity scheme to a normal connectivity scheme.

(Uplink Data Control: Release Request from the MeNB200-1)

Next, an operation (an operation example 4) in which the MeNB200-1releases the path #B of the Split Bearer will be described by usingFIG. 9.

When transmitting the uplink data by the dual connectivity scheme, the UE transmits a scheduling request (SR) and a buffer status report (BSR) to the SeNB200-2(S801). The buffer status report may be a report indicating a status of a transmission buffer region of the uplink data to the SeNB200-2.

The SeNB200-2allocates, by a PDCCH, a resource for the UE to transmit the uplink data (S802).

Upon receiving the allocation of the resource, the UE100transmits the uplink data (S803).

The MeNB200-1decides to release the path #B of the Split Bearer (S804). For example, the RRM (Radio Resource Management) of the MeNB200-1decides to release the path #B of the Split Bearer.

The MeNB200-1transmits a release request for the path #B of the Split Bearer to the SeNB200-2(S805). The release request may use a SeNB Release Request message.

The UE100transmits the SR and the BSR to the SeNB200-2(S806), but the SeNB200-2stops the allocation of the resource for the uplink data transmission to the path #B of the Split Bearer (S808).

Upon transmitting the release request of the5805, the MeNB200-1stops the order control timer (S807).

The SeNB200-2transmits a response to the release request (S809). The response to the release request may use a SeNB Release Response message. Further, the SeNB200-2may include information on the untransmitted uplink data amount remaining in the uplink transmission buffer of the UE in the response to the release request. Further, the BSR received from the UE100may be included as is in the response to the release request.

The MeNB200-1performs reconfiguration of the RRC connection (RRC Connection Reconfiguration) with the UE100(S810). As a result of resetting the RRC connection, the reconfiguration of the radio interval changes from the dual connectivity scheme to a normal connectivity scheme. The order control timer may be resumed after the completion of the process of5810(S813).

The MeNB200-1allocates, by PDCCH, a resource for the UE to transmit the uplink data (S811).

From the untransmitted uplink data remaining in the uplink transmission buffer, the UE transmits the uplink data to the MeNB200-1(S812).

As a result, even when the path #B of the Split Bearer is released, the UE100is capable of transmitting the uplink data by changing the transmission destination of the uplink data to be transmitted to the SeNB200-2to the MeNB200-1. Further, after releasing the path #B of the Split Bearer, the MeNB200-1is capable of performing appropriate order control from the amount of the untransmitted uplink data to the SeNB200-2.

Next, an operation (an operation example5) in which the MeNB200-1releases the path #B of the Split Bearer will be described by usingFIG. 10. In the embodiment shown inFIG. 10, after releasing the path #B of the Split Bearer, the transmission destination of the untransmitted uplink data to the SeNB200-2is not changed to the MeNB200-1. Next, the portions different from the embodiment shown inFIG. 9will be described.

Since the procedures of S901to S907are the same as the embodiment shown inFIG. 9, a description thereof will be omitted.

The SeNB200-2transmits a response to the release request (S908). The response to the release request may use a SeNB Release Response message.

The MeNB200-1performs reconfiguration of the RRC connection (RRC Connection Reconfiguration) with the UE100(S909). Here, the procedure of S909is similar to that of S810.

The MeNB200-1discards the PDCP packet (the RLC data) awaiting the generation of the PDCP SDU (IP packet) stored in the buffer of the MeNB200-1(S910).

The UE100discards the untransmitted PDCP packet (S911).

The MeNB200-1may notify the sequence number of the PDCP SDU that is to be retransmitted as a result of discard of the PDCP packet (the RLC data) awaiting, by the MeNB200-1, generation (S912).

The MeNB200-1allocates, by PDCCH, a resource for the UE to transmit the uplink data (S913).

The UE100retransmits the uplink data to the MeNB200-1(S914). When notified of the sequence number of the PDCP SDU to be retransmitted, the UE performs retransmission.

When releasing the path #B of the Split Bearer, the MeNB200-1notifies the UE100of the information for identifying the data that is to be retransmitted. As a result, after releasing the path #B of the Split Bearer, the UE100is capable of transmitting the appropriate uplink data to the MeNB200-1.

Other Embodiments

In the above-described embodiment (the operation example 3), the SeNB200-1may notify the MeNB200-1of the information for identifying the downlink data transmitted to the UE100since a release request is transmitted to the MeNB200-1until a response signal is received for the release request from the MeNB200-1.

In the above-described embodiment, description proceeds with a mode where the UE100performs dual connectivity by using the MeNB200-1and one SeNB200-2; however, the UE100may use a similar procedure to perform dual connectivity also by using the MeNB200-1and a plurality of SeNBs200-2.

Further, there is no particular limitation on the types of cell of the MeNB200-1and the SeNB200-2that perform dual connectivity. For example, dual connectivity communication may be performed with a combination of a macrocell and a small cell, a combination of a macro cell and a pico cell, a combination of a pico cell and a femto cell, and a combination of a macro cell and a macro cell.

In addition, the MeNB200-1may be configured to provide an additional resource by using a base station of a radio system other than the LTE, for example a WLAN AP, as the SeNB200-2.

Furthermore, in the embodiment described above, although an LTE system is described as an example of a mobile communication system, the present disclosure is not limited to the LTE system, and may be applied to a system other than the LTE system.