Mobile communication system, handover control method, base station, and mobile station

A mobile communication system according to an exemplary aspect of the invention is a mobile communication system that includes a mobile station, base stations, and a core network that transmits data destined for the mobile station to at least both a handover source base station and a handover destination base station among the base stations at the time of the handover of the mobile station, the handover source base station transmitting the data to the mobile station using a shared channel, wherein the handover destination base station starts transmission of the data to the mobile station using a shared channel in response to the completion of the handover of the mobile station.

This application is based upon and claims the benefit of priority from Japanese patent application No. 2006-191022, filed on Jul. 12, 2006, the disclosure of which is incorporated herein in its entirety by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a mobile communication system, a handover control method, a base station, and a mobile station. More particularly, the present invention relates to a mobile communication system in which data transmission is performed from a base station to a mobile station using a shared channel.

2. Related Art

In a mobile communication system in which data is transmitted from a radio base station as an Enhanced NodeB (eNodeB) to a mobile terminal as User Equipment (UE) using a shared channel, for making a handover, the mobile terminal measures received signal levels for adjacent cells, and a radio base station which will be the source (or the origin of a handover) determines whether or not to carry out a handover based on the received signal levels of the adjacent cells as measured by the mobile terminal, and decides a radio base station which will be the destination (or the destination of a handover) (see 3GPP TR 25.813 V7.0.0 (2006-06) 3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Evolved Universal Terrestrial Radio Access (E-UTRA) and Evolved Universal Terrestrial Radio Access Network (E-UTRAN); Radio interface protocol aspects (Release7), 9.1.5 Handover).

Then, the source radio base station makes a handover request to the destination radio base station, and instructs the mobile terminal to undergo a handover to the destination radio base station. Also, the source radio base station starts transfer of user data sent from a core network, which is a Mobility Management Entity (MME)/User Plane Entity (UPE), to the destination radio base station.

In response to the instruction for a handover, the mobile terminal establishes synchronization with the destination radio base station and transmits a handover confirmation response to the destination radio base station, thereby completing the handover procedure. The destination radio base station notifies the source radio base station of completion of the handover procedure, and also provides the core network with location information for the mobile terminal so that the core network can directly transmit user data to the destination radio base station.

Instead of the source radio base station transferring user data to the destination radio base station at the time of a handover, it is also possible that the core network transmits user data to both the source and destination radio base stations by arranging the core network to transmit user data to any radio base station that manages a cell a pilot signal from which is received on the mobile terminal with a reception quality equal to or above a certain threshold value.

In a shared channel transmission scheme, a hard handover is conducted because it is difficult to establish temporal synchronization among cells due to the fact that packet scheduling is performed at radio base stations. Accordingly, there can be concerns such as occurrence of data loss and associated reduction in a data transmission rate. In addition, as stated above, since a radio base station decides whether to carry out a handover or not after receiving a measurement report from a mobile terminal as a procedural step of a handover, switching of radio channels takes time.

Services currently under consideration that can be provided using a shared channel include non-real-time services for data transfer, viewing of web information and the like. In such a case, when packet loss has occurred in the course of a handover, packet retransmission is carried out at a higher layer to ensure that the mobile terminal receives data. However, when a real-time service for which occurrence of delay can significantly affect QoS (Quality of Service) is provided, such as VoIP (Voice over IP) and streaming, or when high QoS is required even for a non-real-time service, retransmission control at a higher layer is not effective and delay in received data significantly affects quality of service. To realize the provision of such services on a shared channel, there is a need for a handover scheme that does not cause data loss and significant delay.

BRIEF SUMMARY OF THE INVENTION

An exemplary object of the invention is to provide a mobile communication system, a handover control method, a base station, and a mobile station that can prevent occurrence of data loss during a handover using a shared channel.

A mobile communication system according to an exemplary aspect of the invention is a mobile communication system that includes a mobile station, base stations, and a core network that transmits data destined for the mobile station to at least both a handover source base station and a handover destination base station among the base stations at the time of the handover of the mobile station, the handover source base station transmitting the data to the mobile station using a shared channel, wherein the handover destination base station starts transmission of the data to the mobile station using a shared channel in response to the completion of the handover of the mobile station.

A mobile communication system according to an exemplary aspect of the invention is a mobile communication system that includes a mobile station, base stations, and a core network that transmits data destined for the mobile station to at least both a handover source base station and a handover destination base station among the base stations at the time of the handover of the mobile station, the handover source base station transmitting the data to the mobile station using a shared channel, wherein, during the handover of the mobile station, the handover source base station alternately sets an interruption period in which transmission of the data to the mobile station using a shared channel is interrupted and a period in which the transmission is performed, and the handover destination base station performs transmission of the data to the mobile station using a shared channel in the interruption period during the handover of the mobile station.

A handover control method according to an exemplary aspect of the invention is a handover control method for a mobile communication system that includes a mobile station, base stations, and a core network that transmits data destined for the mobile station to at least both a handover source base station and a handover destination base station among the base stations at the time of the handover of the mobile station, the handover source base station transmitting the data to the mobile station using a shared channel, wherein the method including: starting transmission of the data from the handover destination base station to the mobile station using a shared channel in response to the completion of the handover of the mobile station.

A handover control method according to an exemplary aspect of the invention is a handover control method for a mobile communication system that includes a mobile station, base stations, and a core network that transmits data destined for the mobile station to at least both a handover source base station and a handover destination base station among the base stations at the time of the handover of the mobile station, the handover source base station transmitting the data to the mobile station using a shared channel, wherein the method including: during the handover of the mobile terminal, alternately setting an interruption period in which transmission of the data from the handover source base station to the mobile station using a shared channel is interrupted and a period in which the transmission is performed; and performing transmission of the data from the handover destination base station to the mobile station using a shared channel in the interruption period during the handover of the mobile station.

A base station according to an exemplary aspect of the invention is a base station in a mobile communication system that includes a mobile station, base stations, and a core network that transmits data destined for the mobile station to at least both a handover source base station and a handover destination base station among the base stations at the time of the handover of the mobile station, the handover source base station transmitting the data to the mobile station using a shared channel, wherein the base station starts transmission of the data to the mobile station using a shared channel in response to the completion of the handover of the mobile station to the base station.

A base station according to an exemplary aspect of the invention is a base station in a mobile communication system that includes a mobile station, base stations, and a core network that transmits data destined for the mobile station to at least both a handover source base station and a handover destination base station among the base stations at the time of the handover of the mobile station, the handover source base station transmitting the data to the mobile station using a shared channel, wherein the base station alternately sets an interruption period in which transmission of the data to the mobile station using a shared channel is interrupted and a period in which the transmission is performed during the handover of the mobile station from the base station to another base station, and the base station performs, during the handover of the mobile station to the base station, transmission of the data to the mobile station using a shared channel in the interruption period set in another base station.

A recording medium according to an exemplary aspect of the invention is a recording medium having recorded thereon a program for causing a computer to execute an operation control method for a base station in a mobile communication system that includes a mobile station, base stations, and a core network that transmits data destined for the mobile station to at least both a handover source base station and a handover destination base station among the base stations at the time of the handover of the mobile station, the handover source base station transmitting the data to the mobile station using a shared channel, wherein the program includes processing of starting transmission of the data to the mobile station using a shared channel in response to the completion of the handover of the mobile station to the base station.

A recording medium according to an exemplary aspect of the invention is a recording medium having recorded thereon a program for causing a computer to execute an operation control method for a base station in a mobile communication system that includes a mobile station, base stations, and a core network that transmits data destined for the mobile station to at least both a handover source base station and a handover destination base station among the base stations at the time of the handover of the mobile station, the handover source base station transmitting the data to the mobile station using a shared channel, wherein the program includes processing of, during the handover of the mobile station from the base station to another base station, alternately setting an interruption period in which transmission of the data to the mobile station using a shared channel is interrupted and a period in which the transmission is performed; and processing of, during the handover of the mobile station from another base station to the base station, performing transmission of the data to the mobile station using a shared channel in the interruption period set in the other base station.

A mobile station according to an exemplary aspect of the invention is a mobile station in a mobile communication system that includes a mobile station, base stations, and a core network that transmits data destined for the mobile station to at least both a handover source base station and a handover destination base station among the base stations at the time of the handover of the mobile station, the handover source base station transmitting the data to the mobile station using a shared channel, wherein the mobile station receives the data which starts to be transmitted from the handover destination base station using a shared channel in response to the completion of the handover of the mobile station.

A mobile station according to an exemplary aspect of the invention is a mobile station in a mobile communication system that includes a mobile station, base stations, and a core network that transmits data destined for the mobile station to at least both a handover source base station and a handover destination base station among the base stations at the time of the handover of the mobile station, the handover source base station transmitting the data to the mobile station using a shared channel, wherein the mobile station receives, during the handover thereof, the data from the handover source base station which alternately sets an interruption period in which transmission of the data to the mobile station using a shared channel is interrupted and a transmission period in which the transmission is performed, in the transmission period, and receives, during the handover thereof, in the interruption period the data from the handover destination base station that performs transmission of the data to the mobile station using a shared channel in the interruption period.

A recording medium according to an exemplary aspect of the invention is a recording medium having recorded thereon a program for causing a computer to execute an operation control method for a mobile station in a mobile communication system that includes a mobile station, base stations, and a core network that transmits data destined for the mobile station to at least both a handover source base station and a handover destination base station among the base stations at the time of the handover of the mobile station, the handover source base station transmitting the data to the mobile station using a shared channel, wherein the program includes processing of receiving the data which starts to be transmitted from the handover destination base station using a shared channel in response to the completion of the handover of the mobile station.

A recording medium according to an exemplary aspect of the invention is a recording medium having recorded thereon a program for causing a computer to execute an operation control method for a mobile station in a mobile communication system that includes a mobile station, base stations, and a core network that transmits data destined for the mobile station to at least both a handover source base station and a handover destination base station among the base stations at the time of the handover of the mobile station, the handover source base station transmitting the data to the mobile station using a shared channel, wherein the program includes processing of receiving, during the handover of the mobile station, the data from the handover source base station which alternately sets an interruption period in which transmission of the data to the mobile station using a shared channel is interrupted and a transmission period in which the transmission is performed, in the transmission period; and receiving, during the handover of the mobile station, in the interruption period the data from the handover destination base station that performs transmission of the data to the mobile station using a shared channel in the interruption period.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

Exemplary embodiments of the invention will be described below with reference to drawings.

FIG. 1shows a configuration of a mobile communication system using a shared channel transmission scheme according to a first exemplary embodiment of the invention. InFIG. 1, the mobile communication system according to the first embodiment of the invention includes a mobile terminal1, radio base stations11to13, and a core network20.

The mobile terminal1supports a shared channel transmission scheme and can access the radio base stations11to13. The radio base stations11to13support the shared channel transmission scheme, and are each linked to the core network20by wired connection. The core network20is an exchange that performs call control, and is connected to other exchanges and/or external networks not shown.

FIG. 2shows the configuration of each of the radio base stations11to13shown inFIG. 1. InFIG. 2, each of the radio base stations11to13includes an antenna101and a wireless communication unit102for data transmission and reception to and from the mobile terminal1, a communication unit103for data transmission and reception to and from the core network20, a scheduling unit104which performs scheduling for deciding the order of priority in data transmission using a shared channel, a buffer105for temporarily saving user data destined for the mobile terminal1which is received from the core network20, a memory107in which programs are stored, and a control unit (CPU)106which controls the components described above in accordance with the programs in the memory107.

The configuration of the first exemplary embodiment has been described above. As the configurations of the mobile terminal1and the core network20ofFIG. 1are well-known to those skilled in the art, detailed description of their configuration is omitted.

Next, the operation of the mobile communication system according to the first exemplary embodiment of the invention will be described with reference to drawings.FIG. 3is a sequence diagram showing the operation of the mobile communication system ofFIG. 1at the time of a handover, andFIG. 5Ais a diagram showing the user data transmission operation at the radio base stations ofFIG. 1at the time of a handover.

It is assumed that the mobile terminal1is moving from a cell covered by the radio base station11toward a cell covered by the radio base station12, and is in communication with the radio base station11by a shred channel transmission scheme. The mobile terminal1includes the functions to receive pilot signals transmitted from adjacent cells and measure the quality of their reception, and report the quality to the core network20via the radio base station11together with information identifying the cells. The core network20transmits user data destined for the mobile terminal1to any radio base station that manages a cell the pilot signal from which is received with a reception quality equal to or above a certain threshold value on the mobile terminal1; consequently, data transmission is being performed from the core network20to both the radio base stations11and12. However, transmission of user data within a wireless section is being performed only by the radio base station11, which is the origin of movement.

In this situation, the mobile terminal1receives a pilot signal transmitted in each cell, decides a destination cell based on the magnitude of propagation loss of the signal, and makes a handover implementation request to the source radio base station11(step S1). The mobile terminal1may decide a destination cell based on the reception quality of the pilot signal (e.g., Block Error Rate (BLER)).

At the source radio base station11, in response to the handover implementation request from the mobile terminal1, the scheduling unit104performs scheduling so as to interrupt the transmission of user data to the mobile terminal1using a shared channel (step S2), and the control unit106transmits a handover implementation response to the mobile terminal1via the antenna101and the wireless communication unit102(step S3). As a result, as shown inFIG. 5A, user data transmission using a shared channel from the source radio base station11to the mobile terminal1is interrupted in accordance with the scheduling result from the scheduling unit104.

The mobile terminal1transmits a data transmission request directly to the radio base station12that manages the destination cell (step S4). Here, a cell identification number is included in the data transmission request for specifying the requested destination of data transmission. Upon reception of the data transmission request, the control unit106of the destination radio base station12checks available radio resources and determines whether the base station12can accommodate the mobile terminal1(step S5). If the mobile terminal1can be accommodated, the control unit106starts to accumulate user data destined for the mobile terminal1which is received from the core network20in the buffer105, and transmits to the mobile terminal1a data transmission response indicating that the mobile terminal1can be accommodated (step S6).

After receiving the data transmission response, the mobile terminal1receives a control signal necessary for a handover from the destination radio base station12to complete the handover, and then transmits a handover completion notice to the source and destination radio base stations11and12(step S7). As shown inFIG. 5A, upon receipt of the notice, the source radio base station11terminates user data transmission to the mobile terminal1that uses a shared channel. On the other hand, at the destination radio base station12, the scheduling unit104decides the order of transmitting user data using a shared channel to the mobile terminal1, and the control unit106starts transmission of user data destined for the mobile terminal1which is accumulated in the buffer105in accordance with the scheduling result.

Since user data transmission to the mobile terminal1using a shared channel is performed in accordance with the scheduling by the scheduling unit104, the data transmission may not be performed immediately after the destination radio base station12receives the handover completion notice. As shown inFIG. 5A, during steps S3to S7, data transmission to the mobile terminal1is not performed and thus the transmission rate can decrease. Therefore, the scheduling unit104checks the amount of data accumulated in the buffer105, and if the amount of accumulated user data destined for the mobile terminal1exceeds a certain value, it performs scheduling giving higher priority to data transmission to the mobile terminal1using a shared channel than to that to other mobile terminals, and conducts data transmission to the mobile terminal1by using a number of successive transmission slots on the shared channel. This can prevent decrease in the data transmission rate.

Here, in a case where the modulation scheme is changed in accordance with the reception condition on the mobile terminal1, the scheduling unit104may also decide the number of transmission slots that will be successively used in consideration of the reception condition on the mobile terminal1. The QoS required by the mobile terminal1may be stored in the memory107, and the scheduling unit104may give higher priority to data transmission to the mobile terminal1than that to other mobile terminals when those conditions are met that high QoS is required and the amount of data accumulated in the buffer105exceeds a certain value.

Next, the operation performed when a handover fails will be described with reference to drawings.FIG. 4is a sequence diagram showing the operation for when a handover fails in the mobile communication system ofFIG. 1, andFIG. 5Bshows the user data transmission operation at the radio base stations ofFIG. 1when a handover fails. As processing from step S11to S15ofFIG. 4is similar to those at steps S1to S5ofFIG. 3, description of them is omitted.

InFIG. 4, if the control unit106of the destination radio base station12determines that the base station12cannot accommodate the mobile terminal1(step S15), it transmits to the mobile terminal1a data transmission response indicating that the mobile terminal1cannot be accommodated (step S16). Upon receipt of the data transmission response, the mobile terminal1transmits a handover failure notice to the source radio base station11(step S17). At the source radio base station11, in response to the handover failure notice from the mobile terminal1, the scheduling unit104performs scheduling so that interrupted transmission of user data using a shared channel to the mobile terminal1will be resumed. Then, as shown inFIG. 5B, the control unit106starts transmission of user data destined for the mobile terminal1accumulated in the buffer105in accordance with the scheduling result (step S18), and waits for the next handover implementation request from the mobile terminal1(step S19).

After step S17, the source radio base station11, which is to resume data transmission, may give higher priority to data transmission to the mobile terminal1than that to other mobile terminals based on the amount of data accumulated in the buffer105and/or the QoS required by the mobile terminal1.

As has been thus far described, in the first exemplary embodiment of the invention, since the destination radio base station12starts transmission of user data destined for the mobile terminal1that has been accumulated in its buffer105during the handover of the mobile terminal1in response to the completion of the handover, the mobile terminal1can perform a handover without causing packet loss.

Also, in the first exemplary embodiment of the invention, since the source radio base station11interrupts data transmission to the mobile terminal1during the handover of the mobile terminal1, even if the handover of the mobile terminal1fails, packet loss can be prevented by resuming data transmission from the source radio base station11.

In addition, the first exemplary embodiment of the invention can prevent decrease in the data transmission rate because the destination radio base station12gives higher priority to data transmission to the mobile terminal1than that to other mobile terminals by making reference to the amount of data accumulated in the buffer or QoS.

Further, the first exemplary embodiment of the invention can simplify and speed up the procedure of a handover because it performs a handover which is led by the mobile terminal1by transmitting a data transmission request directly to the destination radio base station12.

In the following, a second exemplary embodiment of the invention will be described with reference to drawings. A mobile communication system according to the second exemplary embodiment of the invention has a similar configuration to that of the mobile communication system according to the first exemplary embodiment of the invention shown inFIG. 1, and the configuration of the radio base stations is similar to the one shown inFIG. 2. However, the user data transmission operation at radio base stations during a handover is different from the operation shown inFIG. 5.FIG. 6shows the operation for user data transmission at the radio base stations ofFIG. 1during a handover according to the second exemplary embodiment of the invention.

The operation of the mobile communication system according to the second exemplary embodiment of the invention will be described below with reference toFIGS. 1 to 3andFIG. 6. InFIG. 3, the mobile terminal1receives a pilot signal transmitted from each cell, decides a destination cell based on the magnitude of propagation loss or the reception quality of the signal, and makes a handover implementation request to the source radio base station11(step S1).

At the source radio base station11, as shown inFIG. 6, in response to the handover implementation request from the mobile terminal1, the scheduling unit104sets an interruption period T1in which transmission of user data using a shared channel to the mobile terminal1is interrupted and a transmission period T2in which the transmission is performed (step S2). As shown inFIG. 6, the interruption period T1and the transmission period T2are alternately set during a handover in the source radio base station11, and user data transmission from the source radio base station11to the mobile terminal1using a shared channel is performed only in the transmission period T2in accordance with a scheduling result from the scheduling unit104. The periods T1and T2are values given to the radio base station11in advance as station data.

Then, the control unit106transmits a handover implementation response to the mobile terminal1via the antenna101and the wireless communication unit102(step S3). Here, the control unit106also notifies the mobile terminal1of information on the interruption period T1and transmission period T2. Upon receipt of the handover implementation response from the source radio base station11, the mobile terminal1transmits a data transmission request to the radio base station12which manages the destination cell (step S4). Here, in order to specify the requested destination of data transmission, the mobile terminal1also notifies the destination radio base station12of a cell identification number as well as information on the interruption period T1and the transmission period T2of the source radio base station11.

Upon receiving the data transmission request, the control unit106of the destination radio base station12checks available radio resources and determines whether the base station12can accommodate the mobile terminal1or not (step S5). If the mobile terminal1can be accommodated, the control unit106starts to accumulate user data destined for the mobile terminal1which is received from the core network20in the buffer105, and transmits to the mobile terminal1a data transmission response indicating that the mobile terminal1can be accommodated (step S6).

At the destination radio base station12, the scheduling unit104performs scheduling so that user data transmission to the mobile terminal1using a shared channel is performed in the interruption period T1of the source radio base station11. As shown inFIG. 6, the control unit106starts the transmission of user data destined for the mobile terminal1accumulated in the buffer105in accordance with the result of the scheduling. Consequently, during its handover, the mobile terminal1can receive user data that is transmitted using a shared channel from the source radio base station11in the transmission period T2of the source radio base station11while receiving user data that is transmitted using a shared channel from the destination radio base station12in the interruption period T1of the source radio base station11.

After receiving the data transmission response from the destination radio base station12, the mobile terminal1receives a control signal from the destination radio base station12to complete the handover, and then transmits a handover completion notice to the source and destination radio base stations11and12(step S7). As shown inFIG. 6, upon receipt of the notice, the source radio base station11terminates the user data transmission to the mobile terminal1using a shared channel. On the other hand, at the destination radio base station12, the scheduling unit104decides the order of transmitting user data to the mobile terminal1using a shared channel in accordance with a normal scheduling, instead of a scheduling that takes into consideration the interruption period T1and the transmission period T2of the source radio base station11, and the control unit106transmits user data destined for the mobile terminal1which is accumulated in the buffer105in accordance with the scheduling result.

In the data transmission from the destination radio base station12after step S7in the second exemplary embodiment of the invention, the scheduling unit104may give higher priority to data transmission to the mobile terminal1than that to other mobile terminals based on the amount of data accumulated in the buffer105and/or the QoS required by the mobile terminal1.

Also, although the interruption period T1and the transmission period T2at the source radio base station11are fixed values that are given in advance as station data in the second exemplary embodiment of the invention, the source radio base station11may monitor the frequency of handover occurrence in a cell which it covers, and the control unit106of the source radio base station11may change the length of interruption period T1based on the frequency of handover occurrence and notify the scheduling unit104of the length. Specifically, the control unit106sets a large interruption period T1when the frequency of handover occurrence is high.

As has been thus far described, in the second exemplary embodiment of the invention, the interruption period T1and the transmission period T2are alternately set during a handover at the source radio base station11, and the destination radio base station12transmits user data during the interruption period T1and the source radio base station11transmits user data during the transmission period T2. Consequently, the mobile terminal1can receive user data from the source radio base station11in the transmission period T2while receiving user data from the destination radio base station12in the interruption period T1during its handover, which thereby can prevent occurrence of data loss at the time of the handover.

Also, as stated above, in the second exemplary embodiment of the invention, the destination radio base station12transmits user data in the interruption period T1and the source radio base station11transmits user data in the transmission period T2during a handover. Therefore, reduction in the data transmission rate can be prevented compared to a case where data transmission to the mobile terminal1is interrupted during a handover.

The processing operations of each of the radio base stations and mobile terminals in accordance with the sequence diagrams shown inFIGS. 3 and 4can be realized by having a computer which serves as a CPU (or a control unit) reading and executing a program prestored in a storage medium such as ROM.

An exemplary advantage according to the invention is that occurrence of data loss can be prevented during a handover using a shared channel.