Patent Description:
There is conventionally known a communication system capable of cooperatively transmitting data from each of a plurality of base stations to one communication terminal by making one of the plurality of base stations function as a cooperative source base station, receiving data from a core node and distributing the data to another cooperative target base station, which is an anchor-type communication system of controlling the inter-base-station cooperative communication in an n autonomous and distributed manner by an inter-base station communication interface (reefer to Patent Literature <NUM> and Non-Patent Literature <NUM>). According to this anchor-type communication system, it is said that it is capable of realizing an improvement of throughput, an enhancement of communication quality, an effective utilization of communication band, etc. in a cell border area where a plurality of radio communication areas (cells) of base stations overlaps, surely performing a management of cooperative resources in the plurality of base stations, and enhancing an efficiency of cooperative transmission from the plurality of base stations.

In the anchor-type communication system, there is a problem that it is desired to suppressing an interference in a communication terminal located in a cell border area and to cooperatively transmit a data from a plurality of base stations to a plurality of communication terminals including the communication terminal by using a same radio resource.

The present invention has been made in view of the above described problem and an objective is to provide a communication system, and a communication control method which are capable of suppressing an interference in a communication terminal located in a cell border area, and capable of cooperatively transmitting a data from a plurality of base stations to a plurality of communication terminals including the foregoing communication terminal by using a same radio resource.

According to the present invention, it is capable of cooperatively transmitting a data from a plurality of base stations to a plurality of communication terminals by using a same radio resource.

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

<FIG> is an illustration showing one example of a configuration of a communication system according to embodiment of the present invention. The communication system <NUM> of the present embodiment includes a first base station <NUM> and a second base station <NUM> being as a plurality of radio base station apparatus, which are mutually synchronized in time by GPS, etc. These base stations <NUM> and <NUM> are capable of transmitting with each other via a wired or wireless communication link being as inter-base station communication interface, and capable of cooperatively transmitting a desired data for cooperative transmission object respectively to a communication terminal (hereinafter called as "terminal") <NUM> located in a cell border area (hereinafter also called as "cell") where radio communication areas of the base stations <NUM> and <NUM> are overlapped and to a terminal <NUM> located in a cell of the base station <NUM>, by cooperating with each other. A MIMO (Multi Input Multi Output) transmission system with a plurality of antennas is used for the cooperative transmission to each of the terminals <NUM> and <NUM>, and it can be realized to improve at a communication time by each terminal <NUM> and <NUM>, improve a communication quality and effectively use a frequency band.

It is noted that, although the present embodiments will be described by assuming the LTE (Long Term Evolution) as a radio technology, the embodiments can be applied even in case of assuming another radio technology such as the LTE-Advanced or <NUM>, etc..

It is also noted that, although the present embodiments will be described in case of the number of the plurality of base stations capable of cooperatively transmitting with each other is two, the number of the plurality of base stations may be three or more. Furthermore, although the present embodiments will be described in case of two terminals receive a plurality of data cooperatively transmitted from the plurality of base stations, the number of terminals receiving the plurality data cooperatively transmitted may be a plurality of three or more.

In <FIG>, the first base station <NUM>, which is one of the plurality of the base stations <NUM> and <NUM>, is a cooperative source base station having a function of controlling by determining a cooperative transmission start of data from each of the base stations. The other base station <NUM> is a cooperative target base station in which the cooperative transmission start of data is controlled by the cooperative source base station <NUM>. These base stations <NUM> and <NUM> may be called as a "Node B" in the specifications of 3GPP (Third Generation Partnership Project), and also sometimes called as an "eNode B (evolved Node B)" being as an evolved Node B in the specifications of LTE. The cooperative source base station may be sometimes called as an anchor base station or master base station, and the cooperative target base station may be sometimes called as a slave base station.

The communication system <NUM> may include a core node apparatus (hereinafter called as "core node") <NUM> that distributes data as cooperative transmission target data to the plurality of base stations <NUM> and <NUM>. The core node <NUM> is, for example, an EPC (Evolved Packet Core) in the LTE, and capable of communicating with an external network via a data communication network <NUM>. The core node <NUM> and the plurality of base stations <NUM> and <NUM> are connected <NUM> via a data communication network <NUM> such as a packet network. The communication system <NUM> may include the data communication network <NUM>. The communication system <NUM> may also include the terminals <NUM> and <NUM> capable of receiving a plurality of data cooperatively transmitted from the plurality of base stations <NUM> and <NUM>. The terminals <NUM> and <NUM> may be called as user equipment (UE: User Equipment) as they are used by users of communication service, sometimes called as a mobile apparatuses since they are movable, and further sometimes called as radio devices.

The terminals <NUM> and <NUM> may be mobile communication terminals such as portable telephones, etc. and the network configured with the base stations <NUM> and <NUM> and the data communication network <NUM>, etc. may be a cellular network of a mobile communication network. The data communication network <NUM> may be an external network such as the Internet or an IMS (IP multimedia subsystem). Each of radio communication areas of the base stations <NUM> and <NUM> may be any one of various cells that are different in size from each other, such as a macro cell, a micro cell, a femto cell, a pico cell or the like.

One of the plurality of base stations <NUM> and <NUM> capable of radio communicating with the terminals <NUM> and <NUM> is a cooperative source base station having a function of performing a determination and control to start and stop the cooperative transmission operation for cooperatively transmitting data from the plurality of base stations <NUM> and <NUM>. Another base station other than the cooperative source base station is a cooperative target base station being as a cooperated base station that is controlled so as to being cooperatively operated by the cooperative source base station. The base stations <NUM> and <NUM> may be capable of having both of the functions as the cooperative source base station and the cooperative target base station, and may optionally become the cooperative source base station or the cooperative target base station for each of the terminals depending on the state of each terminal.

In the present embodiment, the cases in which the OFDMA (Orthogonal Frequency Division Multiple Access) system is used as a radio communication system between the base stations <NUM> and <NUM> and the terminals <NUM> and <NUM> will be described.

<FIG> is an illustration showing an example of a concept of a radio resource in the OFDMA system. As illustrated in the figure, when adopting the OFDMA system, radio resources are allocated by designating frequencies (F1, F2, F3, • • •) with a predetermined frequency band as one unit, and time of transmission timings (T1, T2, T3,. • •) with a subframe of a predetermined time width as one unit. For example, in the example illustrated in the figure, radio resources with <NUM> blocks designated by frequencies F2 and F3 and a time T2 are allocated for the terminal of user U1, and a radio resource with <NUM> block designated by a frequency F5 and a time T2 is allocated for the terminal of user U2. Moreover, radio resources with <NUM> blocks designated by frequencies F4 and F5 and times T5 and T6 are allocated for the terminal of user U3, and a radio resource with <NUM> block designated by a frequency F2 and a time T5 is allocated for the terminal of user U4. It is noted that, in the present embodiment, although the communication system using the OFDMA system is described, the present invention can be similarly applied to a communication system using a radio communication system other than the OFDMA system.

In the present embodiment, following the general LTE implementation, each of base-station apparatuses manages radio resources in a cell that is a radio management area. In the example of <FIG> described below, the first base station <NUM> manages radio resources of a cell 110a and the second base station <NUM> manages radio resources of a cell 120a. Accordingly, in case that the first base station <NUM> communicates with the terminal <NUM> using a radio resource F1/T1 in <FIG>, when the second base station <NUM> communicates with the terminal <NUM> using the same radio resource F1/T1, if the terminal <NUM> moves into a cell border area A, a communication quality of downlink signal of the terminal <NUM> is deteriorated by an occurrence of interference (refer to <FIG> as described below). Therefore, in case that the first base station <NUM> communicates with the terminal <NUM> using the F1/T1, although the interference can be suppressed by the second base station <NUM> communicating with the terminal <NUM> using the different radio resource F2/T2, a utilization efficiency of the radio resources is degraded since <NUM> blocks of radio resource F1/T1 and radio resource F2/T2 are required. Accordingly, a cooperative transmission system in which the base station <NUM> and the base station <NUM> uses the same radio resource F1/T1 and communicate simultaneously without deteriorating communication quality is required.

Hereinafter, in the present embodiment, the cases in which an inter-base station cooperative MU-MIMO is used as such a cooperative transmission system will be described. It is noted that, in the present embodiment, although a communication system using an inter-base station cooperative MU-MIMO will be described, the present invention can be similarly applied to a communication system using a cooperative transmission system other than the inter-base station cooperative MU-MIMO.

<FIG> are respectively an illustration showing a communication before starting the cooperative transmission, during the cooperative transmission and after stopping the cooperative transmission in the communication system <NUM> according to the present embodiment. Before starting the cooperative transmission shown in <FIG>, the terminal <NUM> as a first communication terminal locates in the cell 110a that is a radio management area of the first base station <NUM>. When the first base station <NUM> receives a transmission target data from the core node <NUM>, the first base station <NUM> transmits a radio-wave transmission signal generated with respect to the data to the terminal <NUM> in the first cell 110a using the radio resource of the first cell <NUM>10a. The terminal <NUM> as a second communication terminal locates in the cell 120a that is a radio management area of the second base station <NUM>. When the second base station <NUM> receives a transmission target data from the core node <NUM>, the second base station <NUM> transmits a radio-wave transmission signal generated with respect to the data to the terminal <NUM> in the second cell 120a using the radio resource of the second cell 120a.

As shown in <FIG>, if the terminal <NUM> and the terminal <NUM> are sufficiently far from the cell border area A between the cell 110a and the cell 120a, even when the base station <NUM> and the base station <NUM> transmit the signals to the terminal <NUM> and the terminal <NUM> using the same radio resource (for example, in the above-mentioned case of OFDMA, in the same frequency and at the same time), the communication can be independently performed for each of the cells, since the signal (interference wave) received by the terminal <NUM> from the base station <NUM> and the signal (interference wave) received by the terminal <NUM> from the base station <NUM> are sufficiently small and can be neglected.

However, as shown in <FIG>, when the terminal <NUM> moves and enters into the cell border area A, the signal (interference wave) received by the terminal <NUM> from the base station <NUM> becomes non-negligible and deteriorates communication property of the terminal <NUM>. Therefore, the base station <NUM> and the base station <NUM> start a cooperative communication using the inter-base station cooperative MU-MIMO, the communication property of the terminal <NUM> is improved by suppressing the signal (interference wave) received by the terminal <NUM> from the base station <NUM>, the base station <NUM> continues a communication with the terminal <NUM> using the same radio resource as the cooperative transmission (for example, in the above-mentioned case of OFDMA, in the same frequency and at the same time).

Furthermore, as shown in <FIG>, when the terminal <NUM> determines to move into the cell 120a that is a radio management area of the base station <NUM>, the terminal <NUM> performs a handover from the base station <NUM> to the base station <NUM> and terminates the cooperative transmission.

Hereinafter, the details with respect to processing procedures when starting the cooperative transmission, processing procedures during the cooperative transmission and processing procedures when terminating the cooperative transmission will be described using the present examples.

<FIG> is a sequence diagram showing an example of processing procedures when staring the cooperative transmission at the transition timing from the state of <FIG> to the state of <FIG>. It is noted that, the numbers shown in parentheses in the figure are indicating the order of control steps for convenience. Moreover, although the example of <FIG> indicates an example of one terminal <NUM> located in the cell 110a of the first base station <NUM> and three terminals <NUM>, <NUM> and <NUM> located in the cell 120a of the second base station <NUM>, the number of terminals located in each of the cells is not limited to those shown in the figure.

In <FIG>, when a normal transmission of transmitting data to the terminal <NUM> located in the cell 110a of the base station <NUM> is performed via the first base station <NUM> from the core node <NUM>, the terminal <NUM> measures a received signal strength of radio wave from neighboring base stations including the first base station <NUM> and the second base station <NUM>, and transmits the received signal strength to the first base station <NUM> as quality information on the radio communication (hereinafter appropriately called as "radio communication quality information", also called "Measurement Report" in the LTE) (STEP <NUM>).

Similarly, when a normal transmission of transmitting data to the plurality of terminals <NUM>, <NUM> and <NUM> located in the cell 120a of the base station <NUM> is performed via the second base station <NUM> from the core node <NUM>, each of the terminals <NUM>, <NUM> and <NUM> measures a received signal strength of radio wave from neighboring base stations including the first base station <NUM> and the second base station <NUM>, and transmits the received signal strength to the second base station <NUM> as radio communication quality information (STEP <NUM>).

When a difference of the received signal strength of the radio wave between the first base station <NUM> and the neighboring second base station <NUM> based on the radio communication quality information received from the terminal <NUM> is equal to or lower than a predetermined threshold value (cooperative transmission start threshold), the base station <NUM> determines the terminal <NUM> locates in the cell border area A in which both of the cells 110a and 120a of base stations <NUM> and <NUM> are overlapped, determines a start of the cooperative transmission using the inter-base station cooperative MU-MIMO to prevent the interference wave (STEP <NUM>), and transmits the cooperative communication start request to the second base station <NUM> (STEP <NUM>). At this time, the first base station <NUM> functions as the cooperative source base station for the terminal <NUM> and the second base station <NUM> functions as the cooperative target base station.

It is noted that, the functions of the cooperative source base station and the cooperative target base station are not predetermined for each of the base stations. For example, when the terminal <NUM> located in the cell of the second base station <NUM> moves into the cell border area A and starts the cooperative communication, the second base station <NUM> becomes the cooperative source base station that determines the start of the cooperative transmission for the terminal <NUM>, and the first base station <NUM> becomes the cooperative target base station. In this manner, each of the base stations <NUM> and <NUM> is capable of having both functions of the cooperative source base station and the cooperative target base station for each of the terminals.

When receiving the cooperative communication start request from the first base station <NUM>, the second base station <NUM> selects the terminal <NUM> and another terminal that performs the cooperative transmission using the inter-base station cooperative MU-MIMO, among the terminals <NUM>, <NUM> and <NUM>, based on the radio communication quality information received from each of the terminals <NUM>, <NUM> and <NUM> located in the cell 120a of the second base station <NUM> (STEP <NUM>). As a cooperative data transmission target terminal that performs a cooperative transmission using the inter-base station cooperative MU-MIMO, for example, a terminal having the weakest interference signal that is transmitted from the second base station <NUM> to the terminals <NUM>, <NUM> and <NUM> and reaches the terminal <NUM> in the cell boundary area A is selected. By thus selecting the terminal with the weakest interference signal, it is capable of surely reducing the inter-base station interference in the terminal <NUM> located in the cell border area A at the time of the cooperative data transmission. It is noted that, location information on the terminal <NUM>, <NUM> and <NUM> acquired by the GPS, etc. may be used for the selection of the terminal. In the present example, the terminal <NUM> is selected as a terminal that performs the cooperative transmission with the terminal <NUM> by the inter-base station cooperative MU-MIMO.

Next, the second base station <NUM> checks the radio resource (vacant resource) available for the data cooperative transmission, determines the vacant resource as a candidate of a cooperative resource, and performs a resource control for using the vacant resource at the time of the cooperative data transmission (STEP <NUM>). The second base station <NUM> sends back a cooperative transmission start response for responding to the cooperative transmission start request to the first base station <NUM>, together with the resource information on the vacant resource (STEP <NUM>).

The first base station <NUM> determines the cooperative resource used for the cooperative transmission of the inter-base station cooperative MU-MIMO based on the vacant resource information received from the second base station <NUM>, and performs a resource control for lending the cooperative resource to the second base station <NUM> so as to remotely controlling the cooperative resource from the second base station <NUM> (STEP <NUM>). After that, the first base station <NUM> determines the start of the cooperative transmission of the inter-base station cooperative MU-MIMO in which the own base station functions as the cooperative source base station and the second base station <NUM> functions as the cooperative target base station (STEP <NUM>). Then, in order to inform the second base station <NUM> of the start of the cooperative transmission of the inter-base station cooperative MU-MIMO, the first base station <NUM> transmits a cooperative transmission start notification to the second base station <NUM> together with the information on the determined cooperative resource used for the cooperative transmission (STEP <NUM>). The second base station <NUM>, which receives the cooperative transmission start notification, performs a resource control setting so as to use the cooperative resource of the own base station for the cooperative transmission of the inter-base station cooperative MU-MIMO to the terminal <NUM>, and performs a resource control setting to use the cooperative resource borrowed from the first base station <NUM> by remotely controlling for the cooperative transmission of the inter-base station cooperative MU-MIMO to the terminal <NUM>.

<FIG> is an illustration showing an example of a concept of lending and borrowing of a cooperative resource during the data cooperative transmission. In <FIG>, the cooperative source base station (the first base station) <NUM> allocates frequency resources F5a and F6a of the own base station as the cooperative resource, and lends the frequency resources F5a and F6a to the cooperative target base station <NUM>. The cooperative resources (frequency resources F5a and F6a) in the cooperative source base station <NUM> are used for transmitting a desired data (first data) from the own base station to the terminal <NUM> at the timing of the cooperative transmission of the inter-base station cooperative MU-MIMO, and used for transmitting a data transferred to the base station <NUM> by the inter-base station communication interface, which is a desired data (second data) from the cooperative target base station <NUM> to the terminal <NUM>. However, the cooperative resources (frequency resources F5a and F6a) in the cooperative source base station <NUM> are not controlled by a scheduler <NUM> of the cooperative source base station <NUM>, and remotely controlled by a scheduler <NUM> of the cooperative target base station <NUM>. Therefore, while the cooperative transmission of the inter-base station cooperative MU-MIMO is performed, the scheduler of the cooperative source base station <NUM> cannot use the cooperative resource for data transmission from the cooperative source base station <NUM> to other terminals than the terminal <NUM>.

The cooperative target base station (the second base station) <NUM> borrows the frequency resources F5a and F6a from the cooperative source base station <NUM> as the cooperative resource, and allocates frequency resources F5b and F6b of the own base station, which are the same frequency resources as the frequency resources F5a and F6a. As described above, the frequency resources F5a and F6a borrowed from the cooperative source base station <NUM> are used for the transmission of the first data from the cooperative source base station <NUM> to the terminal <NUM> at the time of cooperative transmission of the inter-base station cooperative MU-MIMO and the transmission of the second data transferred from the own base station. The cooperative resources (frequency resources F5b and F6b) in the cooperative target base station <NUM> are used for the transmission of the second data as the desired data from the own base station to the terminal <NUM> at the time of cooperative transmission of the inter-base station cooperative MU-MIMO. The cooperative resources (frequency resources F5b and F6b) in the cooperative target base station <NUM> are controlled by the scheduler <NUM> of the cooperative target base station <NUM>, while the cooperative transmission of the inter-base station cooperative MU-MIMO is performed, the cooperative resources cannot be used for a data transmission from the cooperative target base station <NUM> to other terminals than the terminal <NUM>. The cooperative source base station <NUM> and the cooperative target base station <NUM> perform a cooperative transmission of the inter-base station cooperative MU-MIMO at the same time (data cooperative transmission timing, for example, T1), by using the same frequencies F5 and F6 determined by the above-mentioned resource control. Therefore, the cooperative source base station <NUM> and the cooperative target base station <NUM> are capable of performing a cooperative transmission of the inter-base station cooperative MU-MIMO to the terminals <NUM> and <NUM> without negotiating resources with each other during the cooperative transmission.

<FIG> is a sequence diagram showing an example of a processing procedure during the cooperative transmission showing in <FIG>. In <FIG>, after transmitting and receiving the cooperative transmission start notification from the cooperative source base station <NUM> to the cooperative target base station <NUM>, the cooperative source base station <NUM> transmits a cooperative start command to the terminal <NUM> in the cell border area A (STEP <NUM>), and the cooperative target base station <NUM> transmits the cooperative start command to the selected terminal <NUM> located in an own cell (STEP <NUM>).

Upon receiving the cooperative transmission start command from the cooperative source base station <NUM>, the terminal <NUM> becomes a state capable of processing the cooperative transmission data by activating the cooperative transmission processing by starting a predetermined cooperative transmission program for processing a data cooperatively transmitted from the base station <NUM>, and transmits feedback information (also called "CSI feedback" in the LTE) including a propagation channel state (CSI : Channel State Information) of the downlink from the cooperative source base station <NUM> and the cooperative target base station <NUM> to the base station <NUM> (STEP <NUM>). Moreover, upon receiving the cooperative transmission start command from the cooperative target base station <NUM>, the terminal <NUM> becomes a state capable of processing the cooperative transmission data by activating a predetermined cooperative transmission program for processing data transmitted from the base station <NUM> and starting a cooperative transmission process, and transmits feedback information including a propagation channel state (CSI) of the downlink from the cooperative source base station <NUM> and the cooperative target base station <NUM> to the base station <NUM> (STEP <NUM>).

Upon receiving the feedback information from the terminal <NUM>, the cooperative source base station <NUM> calculates a value of "transmission weight" (also called "Precoding Matrix" in the 3GPP specification) as an interference suppression parameter that is applied to the data to be transmitted to the terminal <NUM> by the MIMO transmission system at the time of data cooperative transmission (STEP <NUM>). The transmission weight is a weighting factor consisting of a complex number that is multiplied by a transmission signal transmitted from each of the plurality of transmission antennas in the MIMO transmission system. For example, the value of the transmission weight is calculated so that the transmission signal transmitted from the cooperative source base station <NUM> to the terminal <NUM> of the cell border area A becomes opposite in phase to the transmission signal (interference wave) of the second data transmitted from the cooperative target base station <NUM> in the terminal <NUM> of the cell border area A. By using this transmission weight, the interference wave is suppressed in the terminal <NUM> of the cell border area A, and the communication quality of the downlink signal (first data transmitted from the base station <NUM> to the terminal <NUM>) can be improved. In the example of <FIG>, although it is described that the base station <NUM> performs a calculation of the transmission weight based on the feedback information on the propagation channel state from the terminal <NUM>, the terminal <NUM> may calculate the transmission weight based on the propagation channel state, and the calculated transmission weight may be included in the feedback information transmitted to the base station <NUM>.

Upon receiving the desired data (first data) to the cooperative transmission target terminal <NUM> from the core node <NUM> at the same time when calculating the value of the transmission weight (STEP <NUM>). Upon receiving the desired data (second data) to the cooperative transmission target terminal <NUM> from the core node <NUM> (STEP <NUM>), the cooperative target base station <NUM> duplicates the second data and transmits it to the cooperative source base station <NUM> together with the predetermined control information on the cooperative transmission (cooperative transmission control information) (STEP <NUM>). For the transmission of the second data from the cooperative target base station <NUM> to the cooperative source base station <NUM>, a tunneling protocol such as a GTPv2 (GPRS Tunneling Protocol Version <NUM>), etc. can be used via an inter-base station communication interface such as an X2 interface, etc. which is a standard interface of an inter-base station connection of the LTE. The cooperative transmission control information may include a data cooperative transmission timing and MCS information for remotely controlling the cooperative resource of the cooperative source base station <NUM>. The MCS information is information that defines a modulation system and an encoding system, and is calculated by the scheduler <NUM> of the cooperative target base station <NUM> based on the feedback information (refer to <FIG>).

Then, the cooperative source base station <NUM> multiplies the first data and the second data received from the cooperative target base station <NUM> by the transmission weight to generate a transmission signal (STEP <NUM>), and transmits the transmission signal to the terminal <NUM> of the cell border area A at the data cooperative transmission timing by the predetermined cooperative resource that is set in advance, by the cooperative resource controlled remotely by the cooperative target base station <NUM> (STEP <NUM>). On the other hand, the cooperative target base station <NUM> transmits a transmission signal of the second data to the terminal <NUM> in the cell border area A and the terminal <NUM> in the own cell at the data cooperative transmission timing by the predetermined cooperative resource that is set in advance (STEP <NUM>).

In the cooperative transmission in <FIG>, when the cooperative source base station <NUM> and the cooperative target base station <NUM> are synchronized in time by the GPS, etc. it is capable of performing a cooperative transmission from the cooperative source base station <NUM> and the cooperative target base station <NUM> at the data cooperative transmission timing included in the cooperative transmission control information. Moreover, since the transmission signal of the second data delivered as an interference signal from the cooperative target base station <NUM> to the terminal <NUM> of the cell border area A can be cancelled by the transmission signal with an opposite phase to the second data, which is multiplied by the predetermined transmission weight and transmitted from the cooperative source base station <NUM>, the inter-base station interference in the terminal <NUM> of the cell border area A can be suppressed.

In the terminal <NUM>, information on state of propagation channel from the cooperative source base station <NUM> and the cooperative target base station <NUM> is information that changes every moment. Therefore, the feedback information from the terminal <NUM> is transmitted in a relatively short cycle. For example, in the LTE, the CSI feedback is transmitted from the terminal to the base station every <NUM> millisecond. Therefore, the processing procedures during the cooperative transmission shown in <FIG> are repeatedly executed whenever the feedback information is received from the terminal <NUM>, while the cooperative transmission of the inter-base station cooperative MU-MIMO is performed. As described above, the cooperative transmission of the inter-base station cooperative MU-MIMO can be executed, by calculating the optimal MCS information and the transmission weight in accordance with the state of the propagation channel that changes every moment.

<FIG> is a functional block diagram showing an example of a communication layer structure of the cooperative source base station <NUM> and the cooperative target base station <NUM> during the cooperative transmission. Each of the base stations <NUM> and <NUM> has a multilayered communication layer structure consisting of data convergence protocol layers (PDCP layer) <NUM> and <NUM>, radio link control layers (RLC layer) <NUM> and <NUM>, media access control layers (MAC layer) <NUM> and <NUM>, and physical layer (PHY layer) <NUM> and <NUM>.

The PDCP layers <NUM> and <NUM> perform a process such as a data compression, encryption, extension and decoding. The RLC layers <NUM> and <NUM> perform a process such as a data division, combination, order control and retransmission (ARQ : Automatic Repeat-Request). The MAC layer <NUM> and <NUM> perform a process such as a scheduling, multiplexing, retransmission (HARQ: Hybrid Automatic Repeat Request) of data transmission.

The PHY layers <NUM> and <NUM> perform a process such as a modulation, demodulation, encoding of a high-frequency transceived signal that is transceived with the terminals <NUM> and <NUM>. In case of the cooperative transmission source base station, in the PHY layer <NUM>, a transmission signal is generated by applying the transmission weight calculated in advance based on the feedback information to the desired data (first data) for the terminal located in the cell border area A and the desired data (second data) transferred from the cooperative target base station for the terminal located in the cooperative target cell. The PHY layers <NUM> and <NUM> also function as a radio communication section that performs a radio communication with the terminals <NUM> and <NUM>.

At the start of cooperation, the control section <NUM> of the cooperative source base station <NUM> determines cooperative resources (for example, frequencies) used for a data cooperative transmission in each of the cooperative source base station <NUM> and cooperative target base station <NUM>, and lends the cooperative resources to the cooperative target base station <NUM>.

At the start of cooperation, the control section <NUM> of the cooperative target base station <NUM> borrows the cooperative resources determined by the cooperative source base station <NUM> from the cooperative source base station <NUM>. The control section <NUM> remotely controls the cooperative resources borrowed from the cooperative source base station <NUM>. Furthermore, in order to remotely control the cooperative resources of the cooperative source base station <NUM>, the control section <NUM> transmits the second data of cooperative transmission target for the terminal <NUM> in the own cell and cooperative transmission control information to the cooperative source base station <NUM>.

The plurality of base stations <NUM> and <NUM> is respectively provided with inter-base station communication sections <NUM> and <NUM>, schedulers <NUM> and <NUM>, and cooperative resource control sections <NUM> and <NUM>. The inter-base station communication sections <NUM> and <NUM> communicate with other base stations other than the own base station via the inter-base station communication interface such as the X2 interface using a wired link or radio communication link.

The schedulers <NUM> and <NUM> respectively constitute a part of the control sections <NUM> and <NUM> that control a process and operation in the base stations <NUM> and <NUM>, determine which radio resource is to be used for transmission, which modulation system is to be used for transmission, which encoding system is to be used for transmission, and the like, and perform a data transmission by controlling the above-mentioned RLC layers <NUM> and <NUM>, the MAC layers <NUM> and <NUM> and the PHY layers <NUM> and <NUM>. For example, the scheduler <NUM> of the cooperative source base station <NUM> determines which cooperative resource is to be used for transmission, and controls the RLC layer <NUM>, the MAC layer <NUM> and the PHY layer <NUM> of the cooperative source base station <NUM> during the cooperative transmission. The scheduler <NUM> of the cooperative target base station <NUM> controls the RLC layer <NUM>, the MAC layer <NUM> and the PHY layer <NUM> of the cooperative target base station <NUM> during the cooperative transmission, controls the PHY layer <NUM> of the cooperative source base station <NUM>, and performs a cooperative transmission of the inter-base station cooperative MU-MIMO via the cooperative resource control section <NUM> of the cooperative source base station <NUM>.

The cooperative source base station <NUM> is provided with the cooperative resource control section <NUM> for remotely controlling the cooperative resource of the cooperative transmission target data based on the cooperative transmission control information transmitted from the cooperative target base station <NUM>. The cooperative resource in the cooperative source base station <NUM> is lent to the cooperative target base station <NUM> via the cooperative resource control section <NUM>, and can be remotely controlled by the scheduler <NUM> of the cooperative target base station <NUM>.

It is noted that, the scheduler <NUM> of the cooperative target base station <NUM> controls the PHY layer <NUM> of the cooperative target base station <NUM>, and performs a cooperative transmission of the inter-base station cooperative MU-MIMO via the cooperative resource control section <NUM>.

In <FIG>, the cooperative target base station <NUM> receives the second data from the core node and performs a process in processes the PDCP layer <NUM>, the RLC layer <NUM> and the MAC layer <NUM> to generate a data that is called a an MAC-PDU. (Hereinafter, the MAC-PDU generated from the second data is called as an "MAC-PDU2") The scheduler <NUM> of the cooperative target base station <NUM> calculates information (MCS information) on a modulation system and encoding system to be applied to the MAC-PDU2 from the feedback information from the terminal <NUM>. Then, the cooperative resource control section <NUM> of the cooperative source base station <NUM> receives the MAC-PDU2 of cooperative transmission target from the MAC layer <NUM> of the cooperative target base station <NUM> via the communication link, and transfers the MAC-PDU2 to the PHY layer <NUM> of the cooperative source base station <NUM>. The cooperative resource control section <NUM> receives the cooperative transmission control information (information on the data cooperative transmission timing, MCS information, etc.) from the scheduler <NUM> of the cooperative target base station <NUM> via the communication link (inter-base station communication interface). At this time, the cooperative transmission control information may be included in header information of the MAC-PDU2 or the like. The cooperative resource control section <NUM> processes the MAC-PDU2 in the PHY layer <NUM> in the cooperative source base station <NUM> based on the cooperative transmission control information received from the cooperative target base station <NUM>. Specifically, the modulation processing and encoding processing with the modulation system and the encoding system designated in the MCS information is performed for the MAC-PDU2 in the PHY layer <NUM>. The cooperative source base station <NUM> receives the first data from the core node and performs a process in the PDCP layer <NUM>, the RLC layer 112and the MAC layer <NUM> to generate a data that is called a an MAC-PDU. (Hereinafter, the MAC-PDU generated from the first data is called as an "MAC-PDU1 ") The scheduler <NUM> of the cooperative source base station <NUM> calculates information (MCS information) on a modulation system and encoding system to be applied to the MAC-PDU1 from the feedback information from the terminal <NUM>. Then, the modulation process and encoding process by the modulation system and the encoding system designated in the MCS information is performed for the MAC-PDU1 in the PHY layer <NUM>. Furthermore, the PHY layer <NUM> of the cooperative source base station <NUM> multiplies the MAC-PDU1 and the MAC-PDU2, to which the modulation process and the encoding process are performed, by the calculated transmission weights, and performs a transmission process from the PHY layer <NUM> to the terminal <NUM> in the cell border area A at the time designated by the data cooperative transmission timing information.

In <FIG>, the cooperative resource control section <NUM> of the cooperative target base station <NUM> performs the modulation process and the encoding process in the PHY layer <NUM> by the modulation system and the encoding system designated in the MCS information calculated by the above-mentioned scheduler <NUM>, and also performs a transmission process from the PHY layer <NUM> to the terminal <NUM> in the own cell and the terminal <NUM> in the cell border area A at the time designated in the data cooperative transmission timing information.

It is noted that, the function of the cooperative resource control section <NUM> may be incorporated into the control section <NUM> in the cooperative target base station <NUM> without independently providing the cooperative resource control section <NUM>. Similarly, the function of the cooperative resource control section <NUM> may be incorporated into the control section <NUM> in the cooperative source base station <NUM> without independently providing the cooperative resource control section <NUM>.

The hardware of the base stations <NUM> and <NUM> used for the communication system with the aforementioned configuration, for example, is configured with a transmitting amplifier, a receiving amplifier, a radio signal processing section, a baseband signal processing section, a wire transmission link interface section, a computer apparatus or the like, in addition to an antenna. Among these hardware configuration components, the antenna, the transmitting amplifier and the receiving amplifier correspond to the aforementioned radio communication section, and the wired transmission link interface section corresponds to the aforementioned inter-base station communication section. The computer apparatus is configured with, for example, a micro computer, functions as the aforementioned control section <NUM> and <NUM> and the cooperative resource control section <NUM> and <NUM>, and controls each of the sections based on a predetermined control program that is incorporated in advance. Especially, by controlling the radio signal processing section and the baseband signal processing section based on the predetermined control program, the computer apparatus processes the transceived data and signals for example, via the plurality of communication layer structures such as the aforementioned PDCP layer, RLC layer, MAC layer and PHY layer.

<FIG> is a sequence diagram showing an example of a processing procedure when stopping the cooperative transmission at the time of transition from the state of <FIG> to the state of <FIG>. In <FIG>, the cooperative source base station <NUM> receives radio communication quality information from the terminal <NUM> (STEP <NUM>), determines that the terminal <NUM> moves out the cell border area A and locates in the area of the cell 120a of the cooperative target base station <NUM>, for example, when an electric field strength in the base station <NUM> is larger than an electric field strength in the base station <NUM> by a predetermined threshold value (cooperative stop threshold) based on the radio communication quality information, determines the stop of the cooperative transmission (cooperative transmission of the inter-base station cooperative MU-MIMO) (STEP <NUM>). And the cooperative source base station <NUM> transmits a cooperative transmission stop request to the cooperative target base station <NUM> (STEP <NUM>). Upon receiving the cooperative transmission request from the cooperative source base station <NUM>, the cooperative target base station <NUM> returns the frequency resources borrowed from the cooperative source base station <NUM>, cancels the selection of the terminal <NUM>, and transmits a cooperative transmission stop response to the cooperative source base station <NUM> (STEP <NUM>), and transmits a cooperative transmission stop command to the terminal <NUM> (STEP <NUM>). When receiving the cooperative transmission stop response from the cooperative target base station <NUM> (STEP <NUM>), the cooperative source base station <NUM> transmits the cooperative transmission stop command to the terminal <NUM> (STEP <NUM>).

Upon receiving the cooperative transmission stop command, each of the terminals <NUM> and <NUM> stops the foregoing activated cooperative transmission program and performs a cooperative transmission termination processing (STEP <NUM>). When a normal data communication is enabled, each of the terminals <NUM> and <NUM> transmits a success response of the cooperative transmission stop command to the base stations <NUM> and <NUM> (STEP <NUM>).

Upon receiving the success response of the cooperative transmission stop command from the terminal <NUM>, the cooperative target base station <NUM> transmits a path switching request for performing a handover by switching the data communication path with respect to the terminal <NUM>, to the core node <NUM>(STEP <NUM>). Upon receiving the success response of path switching request from the core node <NUM>, the cooperative target base station <NUM> establishes a link for data transmission and reception via the base station <NUM> (STEP <NUM>). Then, a context release is transmitted from the base station <NUM> to the base station <NUM>, and the handover process of the terminal <NUM> from the base station <NUM> to the base station <NUM> is completed. As described above, the termination process of cooperative transmission is completed, and afterwards the normal data transmission for transmitting a data from the core node <NUM> to the terminal <NUM> via the base station <NUM> is performed.

It is noted that, in the embodiments of <FIG>, although the cases in which only the terminal <NUM> among the terminals <NUM> and <NUM> is located in the cell border area A is described, both of the terminals <NUM> and <NUM> may locate in the cell border area A as shown in the following embodiments.

<FIG> is an illustration showing one example of a configuration of a communication system according to another embodiment of the present invention. In the present embodiment, both of the terminals <NUM> and <NUM> are located in the cell border area A, the base station <NUM> and the base station <NUM> cooperate with each other, and the cooperative transmission using the inter-base station cooperative MU-MIMO is performed for both terminals. It is noted that, in <FIG>, the same reference sign will be used for parts in common with <FIG> described above, and the description thereof will be omitted.

It is noted that, in the present embodiment, it is assumed that the terminal <NUM> located in the cell of the base station <NUM> moves to the cell border area A and starts the cooperative transmission at first, then, the terminal <NUM> located in the cell of the base station <NUM> moves to the cell border area A and starts the cooperative transmission during the cooperative transmission.

As shown in <FIG>, when the terminal <NUM> and the terminal <NUM> move and enter into the cell border area A, the signal (interference wave <NUM>) received by the terminal <NUM> from the base station <NUM> and the signal (interference wave <NUM>) received by the terminal <NUM> from the base station <NUM> become non-negligible, and the signals deteriorate the communication characteristics of the terminal <NUM> and the terminal <NUM>. Therefore, the base station <NUM> and the base station <NUM> start a cooperative transmission of the signals to the terminal <NUM> and the terminal <NUM>, suppress the signal (interference wave <NUM>) received by the terminal <NUM> from the base station <NUM> and the signal (interference wave <NUM>) received by the terminal <NUM> from the base station <NUM>, improve the communication characteristics in the terminal <NUM> and the terminal <NUM>, and also continue each of the communication from the base station <NUM> to the terminal <NUM> and the communication from the base station <NUM> to the terminal <NUM> by using the same radio resources.

<FIG> is a sequence diagram showing an example of a processing procedure during the cooperative transmission shown in <FIG>. In <FIG>, after a cooperative transmission start notification is transmitted and received from the cooperative source base station <NUM> to the cooperative target base station <NUM>, the cooperative source base station <NUM> transmits the cooperative start command to the terminal <NUM> in the cell border area A, and the cooperative target base station <NUM> transmits the cooperative start command to the selected terminal <NUM> located in its own cell (STEP <NUM>).

Upon receiving the cooperative transmission start command from the cooperative source base station <NUM> (STEP <NUM>), the terminal <NUM> activates the cooperative transmission processing by starting the predetermined cooperative transmission program for processing a data that is cooperatively transmitted from the base station <NUM> and becomes a state capable of processing the cooperative transmission data, and transmits feedback information including the propagation channel state of the downlink from the cooperative source base station <NUM> and the cooperative target base station <NUM> to the base station <NUM> (STEP <NUM>). Upon receiving the cooperative transmission start command from the cooperative target base station <NUM> (STEP <NUM>), the terminal <NUM> activates the cooperative transmission processing by starting the predetermined cooperative transmission program for processing a data that is cooperatively transmitted from the base station <NUM> and becomes a state capable of processing the cooperative transmission data, and transmits feedback information including the propagation channel state of the downlink from the cooperative source base station <NUM> and the cooperative target base station <NUM> to the base station <NUM> (STEP <NUM>). The cooperative target base station <NUM> transmits the feedback information received from the terminal <NUM> to the cooperative source base station <NUM> via the inter-base station communication interface (STEP <NUM>).

Upon receiving the feedback information on the each terminals <NUM> and <NUM>, the cooperative source base station <NUM> calculates a value of the transmission weight that is applied to the data to be transmitted to the terminals <NUM> and <NUM> by the MIMO transmission system at the time of data cooperative transmission (STEP <NUM>), and transmits the value of the transmission weight to the cooperative target base station <NUM> (STEP <NUM>). The value of the transmission weight, for example, is calculated so that the transmission signal transmitted from the cooperative source base station <NUM> to the terminal <NUM> of the cell border area A becomes opposite in phase to the transmission signal (interference wave <NUM>) transmitted from the cooperative target base station <NUM> in the terminal <NUM> of the cell border area A, and the transmission signal transmitted from the cooperative target base station <NUM> to the terminal <NUM> of the cell border area A becomes opposite in phase to the transmission signal (interference wave <NUM>) transmitted from the cooperative source base station <NUM> in the terminal <NUM> of the cell border area A. By using this transmission weight, the interference wave <NUM> is suppressed in the terminal <NUM> and the communication quality of the downlink signal (first data transmitted from the base station <NUM> to the terminal <NUM>) can be improved, and the interference wave <NUM> is suppressed in the terminal <NUM> and the communication quality of the downlink signal (second data transmitted from the base station <NUM> to the terminal <NUM>) can be improved.

When the cooperative source base station <NUM> receives the first data of cooperative transmission target as a desired data for the terminal <NUM> from the core node <NUM> (STEP <NUM>), the cooperative source base station <NUM> duplicates the first data and transmits it to the cooperative target base station <NUM> together with the predetermined control information on the cooperative transmission (cooperative transmission control information <NUM>) (STEP <NUM>). When the cooperative target base station <NUM> receives the second data of cooperative transmission as a desired data for the terminal <NUM> from the core node <NUM> (STEP <NUM>), the cooperative target base station <NUM> duplicates the second data and transmits it to the cooperative source base station <NUM> together with the predetermined control information on the cooperative transmission (cooperative transmission control information <NUM>) (STEP <NUM>).

It is noted that, for the transmission of the second data from the cooperative target base station <NUM> to the cooperative source base station <NUM> and the transmission of the first data from the cooperative source base station <NUM> to the cooperative target base station <NUM>, the tunneling protocol such as the GTPv2 can be used via the inter-base station communication interface such as the X2 interface which is a standard interface of the inter-base station connection of the LTE. The cooperative transmission control information <NUM> may be included in the data cooperative transmission timing and the MCS information for remotely controlling the cooperative resource of the cooperative target base station <NUM>. The cooperative transmission control information <NUM> may be included in the data cooperative transmission timing and the MCS information for remotely controlling the cooperative resource of the cooperative source base station <NUM>.

Then, the cooperative source base station <NUM> multiplies the first data and the second data received from the cooperative target base station <NUM> by the transmission weight, generates the transmission signal (STEP <NUM>), and transmits the transmission signal to the terminal <NUM> and the terminal <NUM> in the cell border area A at the data cooperative transmission timing by the predetermined cooperative resource that is set in advance, by remotely controlling the cooperative resource by the cooperative target base station <NUM> (STEP <NUM>). On the other hand, the cooperative target base station <NUM> multiplies the first data received from the cooperative source base station <NUM> and the second data by the transmission weight received from the cooperative target base station <NUM>, generates the transmission signal (STEP <NUM>), and transmits the transmission signal to the terminal <NUM> and the terminal <NUM> in the cell border area A at the data cooperative transmission timing by the predetermined cooperative resource that is set in advance, by remotely controlling the cooperative resource by the cooperative source base station <NUM> (STEP <NUM>).

It is noted that, in the present embodiment, since the cooperative transmission to the terminal <NUM> is determined at first, although the base station <NUM> functions as the cooperative source base station and the base station <NUM> functions as the cooperative target base station, the functions of the cooperative source base station and the cooperative target base station are not predetermined for each of the base stations. For example, in cases in which the terminal <NUM> located in the cell of the base station <NUM> moves to the cell border area A and starts the cooperative communication and the terminal <NUM> located in the cell of the base station <NUM> moves to the cell border area A and starts the cooperative communication during cooperative transmission, the second base station <NUM> becomes the cooperative source base station that determines the start of the cooperative transmission for the terminal <NUM>, and the first base station <NUM> becomes the cooperative target base station. As described above, each of the base stations <NUM> and <NUM> is capable of having both functions of the cooperative source base station and the cooperative target base station for each of the terminals.

In the cooperative transmission in <FIG>, when the cooperative source base station <NUM> and the cooperative target base station <NUM> are synchronized in time by the GPS, etc. it is capable of performing a cooperative transmission at the data cooperative transmission timing included in the cooperative transmission control information from the cooperative source base station <NUM> and the cooperative target base station <NUM>. Since the transmission signal of the second data delivered as the interference wave <NUM> from the cooperative target base station <NUM> to the terminal <NUM> of the cell border area A can be cancelled by the transmission signal with an opposite phase to the second data, which is multiplied by the predetermined transmission weight and transmitted from the cooperative source base station <NUM>, and the transmission signal of the first data delivered as the interference wave <NUM> from the cooperative source base station <NUM> to the terminal <NUM> of the cell border area A can be cancelled by the transmission signal with an opposite phase to the first data, which is multiplied by the predetermined transmission weight and transmitted from the cooperative target base station <NUM>, and the inter-base station interference in the terminal <NUM> and the terminal <NUM> of the cell border area A can be suppressed.

In the terminal <NUM> and the terminal <NUM>, information on state of propagation channel from the cooperative source base station <NUM> and the cooperative target base station <NUM> is information that changes every moment. Therefore, the feedback information from the terminal <NUM> and the terminal <NUM> are transmitted at a relatively short cycle. For example, in the LTE, the CSI feedback is transmitted from the terminal to the base station every <NUM> millisecond. Therefore, the processing procedures during the cooperative transmission shown in <FIG> are repeatedly executed whenever the feedback information are received from the terminal <NUM> and the terminal <NUM>, while the cooperative transmission of the inter-base station cooperative MU-MIMO is performed. As described above, the cooperative transmission of the inter-base station cooperative MU-MIMO can be executed, by calculating the optimal MCS information and the transmission weight with the state of the propagation channel that changes every moment.

<FIG> is a functional block diagram showing an example of a communication layer structure of the cooperative source base station <NUM> and the cooperative target base station <NUM> during the cooperative transmission. It is noted that, with respect to parts in common with <FIG>, the description thereof will be omitted. In <FIG>, the cooperative source base station <NUM> receives the first data from the core node and performs the processes on the PDCP layer <NUM>, the RLC layer <NUM> and the MAC layer <NUM> to generate the MAC-PDU <NUM>. The generated MAC-PDU <NUM> is duplicated and the duplicated MAC-PDU1 is transmitted to the cooperative target base station <NUM> via the communication link (inter-base station communication interface). The cooperative target base station <NUM> receives the second data from the core node and performs the processes on the PDCP layer <NUM>, the RLC layer <NUM> and the MAC layer <NUM> to generate the MAC-PDU <NUM>. The generated MAC-PDU <NUM> is duplicated and the duplicated MAC-PDU2 is transmitted to the cooperative source base station <NUM> via the communication link (inter-base station communication interface).

The cooperative resource control section <NUM> of the cooperative source base station <NUM> receives the MAC-PDU2 of cooperative transmission target from the MAC layer <NUM> of the cooperative target base station <NUM> via the communication link (inter-base station communication interface), and transfers the MAC-PDU2 to the PHY layer <NUM> in the cooperative source base station <NUM>. The cooperative resource control section <NUM> receives the cooperative transmission control information <NUM> (information on the data cooperative transmission timing, MCS information, etc.) from the scheduler <NUM> of the cooperative target base station <NUM> via the communication link (inter-base station communication interface). At this time, the cooperative transmission control information <NUM> may be included in the header information of the MAC-PDU2 or the like. The cooperative resource control section <NUM> processes the MAC-PDU2 in the PHY layer <NUM> in the cooperative source base station <NUM> based on the cooperative transmission control information <NUM> received from the cooperative target base station <NUM>. Specifically, the modulation processing and the encoding processing in the PHY layer <NUM> are performed on the MAC-PDU2 with the modulation system and the encoding system designated in the MCS information. The scheduler <NUM> of the cooperative source base station <NUM> calculates the MCS information and the like to be applied to the MAC-PDU1, by the feedback information from the terminal <NUM>. Then, the modulation processing and the encoding processing in the PHY layer <NUM> are performed on the MAC-PDU1 with the modulation system and encoding system designated in the MCS information. Furthermore, the MAC-PDU1 and the MAC-PDU2, on which the modulation process and the encoding process are performed, are multiplied by the calculated transmission weight, and a transmission process to the terminal <NUM> and the terminal <NUM> in the cell border area A from the PHY layer <NUM> is performed at the data cooperative transmission timing.

The cooperative resource control section <NUM> of the cooperative target base station <NUM> receives the MAC-PDU1 of cooperative transmission target from the MAC layer <NUM> of the cooperative source base station <NUM> via the communication link (inter-base station communication interface), and transfers the MAC-PDU1 to the PHY layer <NUM> in the cooperative target base station <NUM>. The cooperative resource control section <NUM> receives the cooperative transmission control information <NUM> (MCS information, etc.) from the scheduler <NUM> of the cooperative source base station <NUM> via the communication link (inter-base station communication interface). At this time, the cooperative transmission control information <NUM> may be included in the header information of the MAC-PDU1 or the like. The cooperative resource control section <NUM> processes the MAC-PDU1 in the PHY layer <NUM> in the cooperative target base station <NUM> based on the cooperative transmission control information <NUM> received from the cooperative source base station <NUM>. Specifically, the modulation processing and the encoding processing in the PHY layer <NUM> are performed on the MAC-PDU1 with the modulation system and the encoding system designated in the MCS information. The scheduler <NUM> of the cooperative target base station <NUM> calculates the MCS information and the like to be applied to the MAC-PDU2, by the feedback information from the terminal <NUM>. Then, the modulation processing and the encoding processing in the PHY layer <NUM> are performed on the MAC-PDU2 with the modulation system and the encoding system designated in the MCS information. Furthermore, the MAC-PDU1 and the MAC-PDU2, on which the modulation process and the encoding process are performed, are multiplied by the transmission weight received from the base station <NUM>, and a transmission process to the terminal <NUM> and the terminal <NUM> in the cell border area A from the PHY layer <NUM> is performed at the data cooperative transmission timing.

In the example of <FIG>, although the cooperative source base station calculates the transmission weight in the STEP <NUM>, the cooperative target base station may calculate the transmission weight as shown in <FIG>. In the example of <FIG>, by transmitting the feedback information received from the terminal <NUM> to the cooperative target base station <NUM> by the cooperative source base station <NUM> in the STEP <NUM>, calculating the transmission weight by the cooperative target base station <NUM> in the STEP <NUM>, and transmitting the calculated transmission weight from the cooperative target base station <NUM> to the cooperative source base station <NUM> in the STEP <NUM>, the same effect can be realized.

Furthermore, in the aforementioned embodiments, although the cooperative source base station <NUM> transmits the cooperative transmission start request to the cooperative target base station <NUM>, receives the cooperative transmission start response including the information on a vacant resource as a candidate of the cooperative resource from the cooperative target base station <NUM> that received the cooperative transmission start request, and determines the cooperative resource based on the cooperative transmission start response, the cooperative resource may be determined by other methods.

For example, the cooperative source base station <NUM> may transmit the cooperative transmission start request including the information on a vacant resource information as a candidate of the cooperative resource to the cooperative target base station <NUM>, receive the cooperative transmission start response including a result of determining whether the candidate of the cooperative resource are available or not, from the cooperative target base station <NUM> that received the cooperative transmission start request, and determine the cooperative resource based on the cooperative transmission start response.

Moreover, in the aforementioned embodiments, although the determining, lending and borrowing of the cooperative resources are performed at the time of the cooperative transmission start, the determining, lending and borrowing may be performed periodically at predetermined timing set in advance. For example, the determining, lending and borrowing of the cooperative resource may be performed periodically (for example, every one second, five seconds or ten seconds) in a cyclic timing sufficiently longer than the time of inter-base station communication (several milliseconds).

Further, the determining , lending and borrowing of the cooperative resource may be performed based on the channel quality indicator (CQI : channel quality indicator) indicating a quality of radio communication in the terminal <NUM>. This CQI indicates the quality of radio communication channel in the base station that the terminal <NUM> can perform a radio communication therewith, for each of a plurality of radio communication channels (a plurality of frequencies), and the CQI is included in the feedback information from the terminal <NUM>. For example, it is determined whether the CQI value of each radio communication channel (frequency) is changed by more than a predetermined range or not, based on the CQI received from the terminal <NUM>. Then, at the timing when the CQI value changes by more than the predetermined range, the determining, lending and borrowing of the cooperative resource are performed so as to switch to a radio communication channel (frequency) resource that is optimum for the cooperative transmission.

As described above, according to the aforementioned embodiment, while suppressing the interference in the terminal <NUM> located in the cell border area A, it is capable of efficiently performing the cooperative transmission of data from the plurality of base stations <NUM> and <NUM> to the plurality of terminals <NUM> and <NUM> including the terminal <NUM> by using the same radio resources. Furthermore, in cases that the plurality of terminals <NUM> and <NUM> are located in the cell border area A, while suppressing the interference in the terminals <NUM> and <NUM> located in the cell border area A, it is capable of efficiently performing the cooperative transmission of data from the plurality of base stations <NUM> and <NUM> to the plurality of terminals including the terminals <NUM> and <NUM> by using the same radio resources.

Moreover, according to the aforementioned embodiment, by selecting the terminal <NUM> as a terminal for the cooperative transmission target, to which the transmission signal with the weakest interference for the terminal <NUM> in the cell border area A is transmitted from the cooperative target base station <NUM>, among the plurality of terminals <NUM>, <NUM> and <NUM> located in the cell of the cooperative target base station <NUM>, it is capable of securely reducing the inter-base station interference in the terminal <NUM> of the cell border area A at the time of the data cooperative transmission. Further, since the cooperative target base station <NUM> selects the cooperative transmission target terminal <NUM> based on the location information such as the GPS of the plurality of terminals located in the cell of the base station <NUM>, the terminal selection process can be simplified and it is capable of promptly starting the data cooperative transmission from the plurality of base stations <NUM> and <NUM>.

According to the aforementioned embodiment, each of the cooperative source base station <NUM> and the cooperative target base station <NUM> performs the cooperative transmission by the MIMO transmission system that uses a plurality of antennas, and uses the transmission weight that is a weight coefficient multiplied by the transmission signal transmitted from the plurality of transmission antennas, as an interference suppression parameter. By applying this transmission weight to the transmission signals of the first data and the second data to be transmitted to the terminal <NUM> located in the cell border area A, it is capable of easily and surely generating the transmission signal having a phase opposite to the transmission signal of the second data transmitted from the cooperative target base station <NUM>, in the terminal <NUM> in the cell border area A.

Furthermore, according to the aforementioned embodiment, since the cooperative source base station <NUM> can unitarily control the cooperative resource in each of the base stations <NUM> and <NUM>, it is capable of more surely performing the management of cooperative resources used for the cooperative transmission from each of the base stations <NUM> and <NUM>, as compared to the case that each of the base stations <NUM> and <NUM> individually controls the cooperative resources. Moreover, in a state in which the cooperative source base station <NUM> can unitarily control the cooperative resource, it is capable of repeatedly performing the data cooperative transmission from each of the base stations <NUM> and <NUM> to the plurality of terminals <NUM> and <NUM>. Accordingly, it is less susceptible to the communication delay between the base stations <NUM> and <NUM> and the efficiency of cooperative transmission is improved as compared to a case that the cooperative resource is adjusted by a negotiation between the base stations <NUM> and <NUM> every timing of data cooperative transmission.

Moreover, according to the aforementioned embodiment, by performing the determining, lending and borrowing of the cooperative resource at the time of the cooperative transmission start to enable the cooperative transmission, since the optimal cooperative resources can be used based on the vacant state of the radio communication resources at the time of the cooperative transmission start, it is capable of surely performing the data cooperative transmission.

Further, according to the aforementioned embodiment, by periodically performing the determining, lending and borrowing of the cooperative resource at the time of the predetermined timing that is set in advance, since the cooperative resource can be updated to more suitable cooperative resources for the cooperative transmission, it is capable of surely performing the data cooperative transmission.

Furthermore, according to the aforementioned embodiment, by performing the determining, lending and borrowing of the cooperative resource based on the quality information (CQI) of the radio communication between the base stations <NUM> and <NUM> and the terminals <NUM> and <NUM>, the cooperative resource can be updated to more suitable radio communication resources for the cooperative transmission in accordance with the change of situation in the radio communication between the base stations and the terminals.

Moreover, according to the aforementioned embodiment, the cooperative source base station <NUM> transmits the cooperative transmission start request to the cooperative target base station <NUM>, receives the cooperative transmission start response including the information on the vacant radio communication resource as a candidate of the cooperative resource from the cooperative target base station <NUM> that receives the cooperative transmission start request, and determines the cooperative resource based on the cooperative transmission start response. In this way, by determining the cooperative resource based on the cooperative transmission start response including the information on the vacant radio communication resource as a candidate of the cooperative resource in the cooperative target base station <NUM>, the cooperative source base station <NUM> does not need to transmit the determined cooperative resource information to the cooperative target base station <NUM>. Therefore, it is capable of improving the efficiency of initialization process of the cooperative transmission. Especially, in this case, since the vacant radio communication resource as a candidate of the cooperative resource in the cooperative target base station <NUM> is presented to the cooperative source base station <NUM>, the vacant radio communication resource can be set preferentially as a cooperative resource.

Further, according to the aforementioned embodiment, the cooperative source base station <NUM> may transmit the cooperative transmission start request including the candidate of the cooperative resource to the cooperative target base station <NUM>, receive the cooperative transmission start response including the result of determining whether the candidate of the cooperative resource are available or not from the cooperative target base station <NUM> that receives the cooperative transmission start request, and determine the cooperative resource based on the cooperative transmission start response. Also in this case, the cooperative source base station <NUM> does not need to transmit the determined cooperative resource information to the cooperative target base station <NUM>. Therefore, it is capable of improving the efficiency of initialization process of the cooperative transmission. Especially, in this case, since the vacant radio communication resource of the cooperative source base station <NUM> can be presented to the cooperative target base station <NUM> as a candidate of the cooperative resource, the vacant radio communication resource in the cooperative source base station <NUM> can be set preferentially as a cooperative resource.

The process steps and configuration elements in the mobile communication system, the base station and the communication terminal apparatus (terminal, user terminal equipment, mobile station) described in the present specification can be implemented with various means. For example, these process steps and configuration elements may be implemented with hardware, firmware, software, or a combination thereof.

With respect to hardware implementation, means such as processing units or the like used for establishing the foregoing steps and configuration elements in entities (for example, various wireless communication apparatuses, Node B, communication terminal apparatus, hard disk drive apparatus, or optical disk drive apparatus) may be implemented in one or more of an application-specific IC (ASIC), a digital signal processor (DSP), a digital signal processing apparatus (DSPD), a programmable logic device (PLD), a field programmable gate array (FPGA), a processor, a controller, a microcontroller, a microprocessor, a electronic device, other electronic unit, computer, or a combination thereof, which are designed so as to perform a function described in the present specification.

With respect to the firmware and/or software implementation, means such as processing units or the like for establishing the foregoing configuration elements may be implemented with a program (for example, code such as procedure, function, module, instruction, etc.) for performing a function described in the present specification. In general, any computer/processor readable medium of materializing the code of firmware and/or software may be used for implementation of means such as processing units and so on for establishing the foregoing steps and configuration elements described in the present specification. For example, in a control apparatus, the firmware and/or software code may be stored in a memory and executed by a computer or processor. The memory may be implemented within the computer or processor, or outside the processor. Further, the firmware and/or software code may be stored in, for example, a medium capable being read by a computer or processor, such as a random-access memory (RAM), a read-only memory (ROM), a non-volatility random-access memory (NVRAM), a programmable read-only memory (PROM), an electrically erasable PROM (EEPROM), a FLASH memory, a floppy (registered trademark) disk, a compact disk (CD), a digital versatile disk (DVD), a magnetic or optical data storage unit, or the like. The code may be executed by one or more of computers and processors, and a certain aspect of functionalities described in the present specification may by executed by a computer or processor.

Claim 1:
A communication system (<NUM>) capable of cooperatively transmitting a data from a first base station (<NUM>) and a second base station (<NUM>) by radio communication,
wherein the first base station (<NUM>) acquires a value of a transmission weight that is multiplied by a transmission signal from the first base station (<NUM>) so as to suppress an interference from the second base station (<NUM>) in a first communication terminal (<NUM>), by calculating the value of the transmission weight based on quality information on a radio signal received from the first communication terminal (<NUM>), when the first communication terminal (<NUM>) having the first base station (<NUM>) as a desired station locates in a cell border area between a cell (110a) of the first base station (<NUM>) and a cell (120a) of the second base station (<NUM>),
wherein the second base station (<NUM>) replicates a second desired data for a second communication terminal (<NUM>) having the second base station (<NUM>) as a desired station located in a cell (120a) of the second base station (<NUM>), and transmits the replicated second desired data to the first base station (<NUM>) via an inter-base station communication interface,
wherein the first base station (<NUM>) generates a transmission signal by multiplying the value of the transmission weight by a first desired data for the first communication terminal (<NUM>) and the second desired data for the second communication terminal (<NUM>) received from the second base station (<NUM>), in such a way that the second desired data multiplied by the transmission weight becomes opposite in phase to an interference signal from the second base station (<NUM>),
wherein the second base station (<NUM>) generates a transmission signal of the second desired data for the second communication terminal (<NUM>), and
wherein the first base station (<NUM>) cooperatively transmits the generated transmission signal of the first desired data and the second desired data after multiplying by the value of the transmission weight, at a predetermined data cooperative transmission timing, and the second base station (<NUM>) cooperatively transmits the generated transmission signal of the second desired data, at the predetermined data cooperative transmission timing.