Patent Description:
The discussion in this section is only to provide background information of the present embodiment and does not constitute an admission of prior art.

Unlike the past when radio communication provided a service centered on voice traffic, the number of backhaul links for transmitting hundreds of megabytes per second (Mbps) or more are gradually increasing with the rapid development of third generation/fourth generation (<NUM>/<NUM>) mobile communication for providing a multimedia service. Also, with the advent of next-generation mobile communication such as <NUM> mobile communication, a necessity of wireless transmission of gigabytes per second (Gbps) or more is increasing such that frequencies of a millimeter wave band which facilitate ensuring a bandwidth of hundreds of MHz or higher are attracting attention. Further, <NUM> partnership project (3GPP) began a discussion about standardization. In this way, considerable discussions about <NUM> element technology development and <NUM> standard technology are taking place among companies and organizations.

Here, millimeter waves denote electromagnetic waves having a frequency of <NUM> or higher (<NUM> to <NUM>). Currently, <NUM>, <NUM>, <NUM>, <NUM>, and the like are taken into consideration as frequencies to be used in <NUM> mobile communication networks.

Such a millimeter wave band shows a larger transmission loss and a lower diffractive feature than an existing <NUM> frequency band, and thus a beamforming technology for concentrating radio waves toward a desired direction using a plurality of antennas is generally used in wireless transmission.

Meanwhile, <NUM> mobile communication employing such millimeter waves requires a large number of small cells to cover many shadow areas caused by a high transmission rate and a low diffractive feature. Therefore, in consideration of capital expenditure (CAPEX) and operating expenses (OPEX), a necessity of low-priced small cells increases.

However, when a wired transmission network is used to backhaul data from a small cell to a macro cell, a large amount of cost is required to build the wired network separately from the low-priced small cell.

As a solution to this problem, a self-backhauling technology for separating, while using the same frequency/time resources, a wireless backhaul between a macro-cell base station (BS) and a small-cell BS and a wireless link between a BS and a terminal according to a beamforming technique is attracting attention.

In a general wireless backhaul, a backhauling frequency band between BSs and a frequency band used for a terminal are different to prevent interference. In this case, since it is necessary to assign predetermined frequency resources for backhauling, a frequency capacity of a small-cell BS is reduced. On the other hand, the millimeter wave band enables backhauling in which the same frequency/time resources are used due to high directivity based on beamforming.

However, even when a beamforming-based wireless backhaul is implemented between a macro-cell BS and a small-cell BS, there is a problem of interference between each cell and a terminal.

Since a macro-cell BS transmits and receives radio waves in a relatively larger range than a small-cell BS, there is no significant difference in distance and angle between a small-cell BS and terminals connected to the small-cell BS from the viewpoint of the macro-cell BS, and thus interference occurs even when radio waves are transmitted and received based on beamforming. This problem worsens in a downlink in which the macro-cell BS emits a wireless backhauling wave with high transmission power.

In other words, during beamforming-based wireless backhauling, there is an interference factor for a terminal that receives a downlink signal from a small-cell BS.

<CIT> relates to a method for transferring data within a radio telecommunication network comprising a base station, a relay system, a first user equipment and a second user equipment. The base station and the relay system are wirelessly connected to form a backhaul communication. Self-interference measurements and two backhaul-to-access interference measurements are made. The relay system generates a report and transmits it to the base station which then decides on the usage of the radio transmission resources. A backhaul-to-access interference which the user equipment suffers from is avoided to a certain extent.

<CIT> relates to a method of self-backhauling. The method includes the steps of searching for an available backhaul connection, detecting a available backhaul connection, electronically conforming a radiation pattern associated with an antenna to the detected backhaul connection and establishing a backhaul using the conformed radiation pattern.

The present invention is directed to providing a method for connecting a wireless backhaul by generating reception status information and interference information or location information on the basis of a beamforming signal, which can minimize interference with a terminal receiving a downlink signal from a small-cell base station (BS), during a beamforming process for a wireless backhaul between a macro-cell BS and the small-cell BS.

The present invention provides a method of connecting a wireless backhaul, as defined in the appended set of claims.

According to the present invention, when a backhaul is connected using a wireless network, information on interference with a terminal caused by the wireless backhaul is checked, and beamforming is performed in consideration of the interference information. In this way, it is possible to reduce radio wave interference with a terminal connected to a small-cell base station (BS) caused by the wireless backhaul from a macro-cell BS.

In particular, according to the present invention, even when the same frequency/time resources are used in radio communication between a terminal and a BS to implement a wireless backhaul, radio wave interference with the terminal is minimized such that a stable service can be provided.

Effects of the present invention are not limited to those mentioned above, and other effects of the present invention will be apparently understood by those of ordinary skill in the art to which the present invention pertains from the description below.

The accompanying drawings which are included as a portion of the detailed description to help understanding of the present invention provide embodiments of the present invention and illustrate technical features of the present invention along with the detailed description.

Throughout the specification, when a part is referred to as "comprising" or "including" a component, this indicates that the part may further include another component instead of excluding another component unless particularly stated otherwise. The terms such as "unit," "-er (or)," and "module" used in the specification refer to a unit that performs at least one function or operation, and may be implemented in hardware, software or a combination of hardware and software. Also, "a" or "an," "one," "the," and the like may be used to include both the singular form and the plural form unless indicated otherwise in the context of the present invention (particularly in the context of the claims) or clearly denied in the context.

In addition, terminology including ordinal numbers such as first and second may be used to describe a variety of components. The terminology is only used to distinguish one component from other components and is not used to limit the components. For example, a second component may be referred to as a first component without departing from the scope of the present invention, and similarly, the first component may also be referred to as the second component.

Specific terms used in the following description are provided only to help understanding of the present invention, and the use of the specific terms may be modified in a different form. The present invention relates to the formation of a wireless backhaul for a base station (BS) that processes a wireless connection with a user.

Here, backhauling denotes collecting and transferring data between a BS and a backbone network. In the case of a radio communication system, a backhaul may be generally connected between a BS and a backbone network in a wired or wireless manner.

Here, a BS may indicate a node B, an evolved node B (eNodeB), an access point (AP), a radio access station (RAS), a base transceiver station (BTS), a mobile multihop relay (MMR)-BS, and the like, and may include functions of all or some of a nodeB, an eNodeB, an AP, an RAS, a BTS, an MMR-BS, and the like.

In particular, in the present invention, a fifth generation (<NUM>)-based mobile communication system whose small-cell BS is installed in a shadow area or an area in which communication is concentrated within the coverage of a macro-cell BS is taken into consideration. In the present invention, backhauling includes the formation of a backhaul between the macro-cell BS and the small-cell BS.

For reference, the macro-cell BS denotes a BS that is applied to a general mobile communication system and has high transmission power and wide coverage, and the small-cell BS denotes, as a comparative concept of a macro cell, a BS that has lower transmission power and smaller coverage than the macro-cell BS. Such a small-cell BS is a common designation of, for example, BS equipment with a low output of <NUM> W or less per antenna, a pico-cell, a femto-cell, a micro-cell, and the like.

For reference, the small-cell BS is disposed in a shadow area, in a boundary area between BSs, indoors, or the like and used for covering the shadow area, traffic distribution in a high traffic area, or transmission in a heterogeneous network such as mobile communication, wireless fidelity (WiFi), and the like.

A method for connecting a wireless backhaul according to the present invention will be described below based on a radio communication environment including a macro-cell BS and a small-cell BS with reference to <FIG>.

<FIG> is a diagram schematically showing a radio communication system in which a wireless backhaul according to an embodiment of the present invention is implemented.

As shown in <FIG>, a mobile communication system includes at least one terminal <NUM>, a small-cell BS <NUM>, and a macro-cell BS <NUM>.

The terminal <NUM> denotes user equipment (UE) that is connected to a radio communication network and transmits and receives data. Here, "terminal" may be replaced by terms such as "UE," "mobile station (MS)," "mobile subscriber station (MSS)," "subscriber station (SS)," "advanced mobile station (AMS)," "wireless terminal (WT)," "machine-type communication (MTC) device," "machine-to-machine (M2M) device," "device-to-device (D2D) device," "station (STA)," and the like. However, a terminal mentioned in the specification is not limited thereto and can be any device that is connected to a radio communication network provided by the present invention. The small-cell BS <NUM> and the macro-cell BS <NUM> provide a wireless link to the terminal <NUM> in the radio communication system, and a wireless backhaul is connected between the small-cell BS <NUM> and the macro-cell BS <NUM>.

In particular, the small-cell BS <NUM> makes a connection with the at least one terminal <NUM> within the communication coverage thereof and exchanges data with the at least one terminal <NUM>. Here, with regard to the terminal <NUM>, data transmitted to the terminal <NUM> is referred to as a downlink signal, and data transmitted from the terminal <NUM> is referred to as an uplink signal.

An uplink signal of the at least one terminal <NUM> that is received from the at least one terminal <NUM> by the small-cell BS <NUM> is transmitted to the macro-cell BS <NUM> through the wireless backhaul and then transferred to a wired backhaul and a backbone network through the macro-cell BS <NUM>. On the other hand, a downlink signal transferred from the backbone network and the wired backhaul is transferred from the macro-cell BS <NUM> to the small-cell BS <NUM> through the wireless backhaul and then transmitted to the at least one terminal <NUM> through the small-cell BS <NUM>.

These small-cell BS <NUM> and macro-cell BS <NUM> have wireless backhaul connecting apparatuses shown in <FIG> and may connect a wireless backhaul through the wireless backhaul connecting apparatuses.

For reference, the wireless backhaul may be connected for each of a downlink and an uplink. Since the present invention is intended to connect a wireless backhaul while minimizing interference with the terminal <NUM>, the following description will be made on the basis of the formation of a wireless backhaul for a downlink in which relatively severe interference with the terminal <NUM> may occur. However, this is not intended to limit the scope of the present invention, and the present invention can also be applied to an uplink as necessary.

The small-cell BS <NUM> and the macro-cell BS <NUM> having the wireless backhaul connecting apparatuses perform a beamforming process to determine a beam direction for connecting a wireless backhaul. In particular, the macro-cell BS <NUM> performs downlink beamforming for a wireless backhaul for a downlink signal.

Specifically, the macro-cell BS <NUM> emits a plurality of beamforming signals one by one in different directions and receives feedback information of the plurality of beamforming signals from the small-cell BS <NUM>.

At this time, the small-cell BS <NUM> receives a plurality of beamforming signals including training symbols from the macro-cell BS <NUM> and generates reception status information using the received beamforming signals. Also, the small-cell BS <NUM> collects interference information of each of the plurality of beamforming signals from the terminal <NUM> connected thereto. Subsequently, a beamforming signal for a downlink is determined from among the plurality of beamforming signals based on the reception status information and the interference information. According to an embodiment of the present invention, the small-cell BS <NUM> feeds the interference information and the reception status information back to the macro-cell BS <NUM> so that the macro-cell BS <NUM> selects a beamforming signal and connects a wireless backhaul. According to another embodiment of the present invention, the small-cell BS <NUM> selects one of the plurality of beamforming signals based on the interference information and the reception status information and then transmits selection information to the macro-cell BS <NUM> so that the macro-cell BS <NUM> connects a wireless backhaul based on the selection information.

Here, beamforming may be performed using a millimeter wave band.

For reference, millimeter waves have a low penetrability and a very small beam width. Therefore, BSs are connected on a one-to-one basis through precise adjustment of antennas thereof, and it is possible to obtain a high quality when a wireless backhaul is implemented.

Also, the small-cell BS <NUM> makes a wireless connection with the terminal <NUM> in the communication coverage thereof and exchanges a downlink signal and an uplink signal with the terminal <NUM> through connected wireless resources. At this time, the communication coverage of the small-cell BS <NUM> may be an area 200a indicated by an alternate long and short dash line in <FIG>. A wireless link between the small-cell BS <NUM> and the terminal <NUM> and the wireless backhaul between the macro-cell BS <NUM> and the small-cell BS <NUM> may use the same frequency/time resources.

Further, the macro-cell BS <NUM> may connect the wireless backhaul to the small-cell BS <NUM>, exchange a downlink signal/uplink signal through the wireless backhaul, and simultaneously exchange a downlink signal/uplink signal with an arbitrary terminal (not shown) in the communication coverage thereof by connecting a wireless link to the terminal.

The communication coverage of the macro-cell BS <NUM> may be an area 300a indicated by a broken line in <FIG>.

Next, a process of connecting a wireless backhaul in the radio communication system having the above-described structure will be described in detail with reference to <FIG>.

<FIG> is a message sequence diagram showing a process of connecting a wireless backhaul according to a claimed embodiment of the present invention.

Referring to <FIG>, it is possible to see a process in which the macro-cell BS <NUM> transmits beamforming signals and the terminal <NUM> and the small-cell BS <NUM> generate information based on the beamforming signals transmitted by the macro-cell BS <NUM> such that a wireless backhaul is connected.

The macro-cell BS <NUM> radially transmits beamforming signals, and the signals arrive at the small-cell BS <NUM> and the terminal <NUM> within the beamforming signal range thereof (S200a and S200b).

The terminal <NUM> generates interference information based on the beamforming signals arriving from the macro-cell BS <NUM> (S202) and transmits the generated interference information to the small-cell BS <NUM> through a wireless link between the terminal <NUM> and the small-cell BS <NUM> (S206). Specifically, the terminal <NUM> may recognize a plurality of beamforming signals on the basis of training signals included in the plurality of beamforming signals and generate the interference information using a signal-to-noise ratio (SNR) and the like. Here, the plurality of beamforming signals act as noise for a downlink signal received by the terminal <NUM>. The plurality of beamforming signals can be transmitted at the same time or one by one.

The small-cell BS <NUM> generates reception status information based on the beamforming signals arriving from the macro-cell BS <NUM> (S204). Here, the reception status information includes a received signal intensity. The small-cell BS <NUM> transmits the interference information received from the terminal <NUM> and the reception status information generated by itself to the macro-cell BS <NUM> (S208).

The macro-cell BS <NUM> selects a beamforming signal for connecting a wireless backhaul based on the received interference information and reception status information (S210). The macro-cell BS <NUM> connects a wireless backhaul by beamforming with the selected signal toward the small-cell BS <NUM> (S212).

<FIG> is a message sequence diagram showing a process of connecting a wireless backhaul according to another claimed embodiment of the present invention.

Referring to <FIG>, in the other embodiment of the present invention, it is possible to see a process in which the macro-cell BS <NUM> transmits beamforming signals and the terminal <NUM> and the small-cell BS <NUM> generate information based on the beamforming signals transmitted by the macro-cell BS <NUM> such that a wireless backhaul is connected, like in <FIG>.

Specifically, the macro-cell BS <NUM> transmits a plurality of beamforming signals one by one in different directions. At this time, the plurality of beamforming signals are radially transmitted and arrive at the small-cell BS <NUM> and the terminal <NUM> near the small-cell BS <NUM> (S300a and S300b).

The terminal <NUM> generates interference information based on the beamforming signals arriving from the macro-cell BS <NUM> (S302) and transmits the generated interference information to the small-cell BS <NUM> through a wireless link between the terminal <NUM> and the small-cell BS <NUM> (S306).

The small-cell BS <NUM> generates reception status information based on the beamforming signals arriving from the macro-cell BS <NUM> (S304). The small-cell BS <NUM> selects a beamforming signal to be received based on the interference information received from the terminal <NUM> and the reception status information generated by itself (S308). The small-cell BS <NUM> transmits selection information to the macro-cell BS <NUM> (S310).

The macro-cell BS <NUM> connects a wireless backhaul by beamforming toward the small-cell BS <NUM> based on the received selection information (S312).

According to the embodiments shown in <FIG> and <FIG>, in a non-line-of-sight (NLOS) environment in which an obstacle and the like is present, it is possible to check beam reception status and interference in which influence of the obstacle is taken into consideration and perform optimal beamforming.

Also, in a line-of-sight (LOS) environment, it is possible to connect a wireless backhaul which can minimize interference based on location information of the terminal <NUM>.

<FIG> is a message sequence diagram showing a process of connecting a wireless backhaul using location information according to still another embodiment of the present invention.

Referring to <FIG>, the macro-cell BS <NUM> radially transmits beamforming signals, and the signals arrive at the small-cell BS and the terminal within the beamforming signal range thereof (S400a and S400b).

The small-cell BS <NUM> generates reception status information based on the beamforming signals arriving from the macro-cell BS <NUM> (S402). When it is determined based on the reception status information that it is easy to collect location information of the terminal <NUM> (when the small-cell BS <NUM> and the macro-cell BS <NUM> are not in an NLOS environment), the small-cell BS <NUM> collects location information of the terminal (S404). Here, the location information may be received from the terminal <NUM> or collected from anywhere such as upper and lower structures of a communication network in which the location information of the terminal <NUM> exists. The small-cell BS <NUM> transmits the collected location information of the terminal <NUM> and the reception status information generated by itself to the macro-cell BS <NUM> (S406).

The macro-cell BS <NUM> nulls a beamforming signal directed to the terminal <NUM> based on the received reception status information and location information of the terminal <NUM> (S408). The macro-cell BS <NUM> transmits beamforming signals other than the nulled beamforming signal and connects a wireless backhaul to the small-cell BS <NUM> using the transmitted beamforming signals (S410).

Next, a configuration and operation of a wireless backhaul connecting apparatus applied to the above-described small-cell BS <NUM> and macro-cell BS <NUM> will be described with reference to <FIG> and <FIG>.

<FIG> is a diagram showing a structure of a wireless backhaul connecting apparatus. The wireless backhaul connecting apparatus is provided in the small-cell BS <NUM> and the macro-cell BS <NUM>.

As shown in <FIG>, the wireless backhaul connecting apparatus may include a wireless transceiving unit <NUM> and a control unit <NUM>.

Among the components shown in <FIG>, the wireless transceiving unit <NUM> is a component for exchanging wireless signals between BSs. In an example, the wireless transceiving unit <NUM> may transmit and receive a plurality of beamforming signals having different directivities. Also, the wireless transceiving unit <NUM> may receive interference information generated by the terminal <NUM> and transmit and receive reception status information and selection information of a beamforming signal. In another example, the wireless transceiving unit <NUM> may transmit and receive location information of the terminal <NUM>.

Next, the control unit <NUM> is a component for checking information received through the wireless transceiving unit <NUM>, controlling the wireless transceiving unit <NUM>, and connecting a wireless backhaul by generating information and selecting a beamforming signal.

The control unit <NUM> may include a beamforming search module <NUM>, a collection module <NUM>, and a backhaul connecting module <NUM>.

The beamforming search module <NUM> is a component for controlling the wireless transceiving unit <NUM> so that a plurality of beamforming signals having different directivities are transmitted from the macro-cell BS <NUM> and received by the small-cell BS <NUM>, and for checking information on reception status of the plurality of beamforming signals at the small-cell BS <NUM>. The plurality of beamforming signals may include training symbols.

The collection module <NUM> is a component for collecting information on the plurality of beamforming signals' interference with the terminal <NUM> connected to the small-cell BS <NUM> or location information of the terminal <NUM>. The collection module <NUM> may use the wireless transceiving unit <NUM> for information collection and provide collected information to the backhaul connecting module <NUM>.

The backhaul connecting module <NUM> is a component for selecting one of the plurality of beamforming signals based on the reception status information of the plurality of beamforming signals and the interference information or the location information and for connecting a wireless backhaul between the macro-cell BS <NUM> and the small-cell BS <NUM>. The backhaul connecting module <NUM> may null a beamforming signal of a specific direction or transmit a beamforming signal in a selected direction.

<FIG> is a flowchart illustrating a wireless backhaul connecting process performed by the above-described wireless backhaul connecting apparatus, according to a claimed embodiment.

Referring to <FIG>, a wireless backhaul connecting apparatus performs a beam search using a plurality of beamforming signals (S100). During this process, the wireless backhaul connecting apparatus in the macro-cell BS <NUM> transmits a plurality of beamforming signals one by one in different directions. At this time, the plurality of beamforming signals are radially transmitted. The wireless backhaul connecting apparatus in the small-cell BS <NUM> receives the beamforming signals that have been radially transmitted. The beamforming signals may include training symbols.

The wireless backhaul connecting apparatus in the small-cell BS <NUM> generates reception status information based on the beamforming signal (S110).

Also, the wireless backhaul connecting apparatus in the small-cell BS <NUM> collects interference information or location information based on the signal (S120). The wireless backhaul connecting apparatus in the small-cell BS <NUM> transmits the collected information to the wireless backhaul connecting apparatus in the macro-cell BS <NUM>.

A wireless backhaul connecting apparatus selects a beamforming signal using the reception status information and the interference information or the location information (S130). The selection may be made by any of the wireless backhaul connecting apparatus in the small-cell BS <NUM> and the wireless backhaul connecting apparatus in the macro-cell BS <NUM>.

The wireless backhaul connecting apparatus connects a wireless backhaul between the BSs using the selected beamforming signal (S140). This process may be performed by transmitting the at least one beamforming signal selected by the wireless backhaul connecting apparatus or by nulling at least one beamforming signal that interferes with the terminal <NUM>.

<FIG> is a schematic diagram showing a beamforming control status according to an embodiment of the present invention.

When there is no obstacle between the small-cell BS <NUM> and the macro-cell BS <NUM>, operation may be performed as shown in the message sequence diagram of <FIG>, and the macro-cell BS <NUM> nulls a beamforming signal directed to the terminal <NUM>. In other words, the macro-cell BS <NUM> reduces an antenna gain of the beamforming signal directed to the terminal <NUM>, thereby preventing the beamforming signal from being transferred to the terminal <NUM>.

When there is an obstacle between the small-cell BS <NUM> and the macro-cell BS <NUM>, operation may be performed as shown in the message sequence diagram of <FIG> or <FIG>.

Here, some of a plurality of beamforming signals emitted from the macro-cell BS <NUM> may not be transferred to the small-cell BS <NUM> due to an obstacle.

In the above environment, it is assumed as a result of a search that each of a first beamforming signal 300b and a second beamforming signal 300c is reflected by an obstacle and then transferred to the small-cell BS <NUM> along a changed path and reception status of the case is the best.

At this time, the terminal <NUM> connected to the small-cell BS <NUM> is more affected by the second beamforming signal 300c than the first beamforming signal 300b that is transmitted along a farther path. In other words, interference information of the second beamforming signal 300c measured by the terminal <NUM> is greater than interference information of the first beamforming signal 300b.

In this case, the first beamforming signal 300b may be selected to connect a wireless backhaul between the small-cell BS <NUM> and the macro-cell BS <NUM>.

Although the specification and drawings illustrate exemplary configurations of devices, embodiments of the subject matter and the functional operations described in this specification can be provided in another type of digital electronic circuitry, or in computer software, firmware, or hardware, including the structures disclosed in this specification and their structural equivalents, or in combinations of one or more thereof. Embodiments of the subject matter described in this specification can be implemented as one or more computer program products, that is, one or more modules of computer program instructions encoded on a tangible program storage medium for execution by, or to control the operation of, the devices according to the present invention. A computer-readable medium can be a machine-readable storage device, a machine-readable storage substrate, a memory device, a composition of matter having influence on a machine-readable propagated signal, or a combination of one or more thereof.

In particular cases, multitasking and parallel processing may be advantageous. Also, the separation of various system components in the embodiments described above should not be understood as requiring such separation in all embodiments, and it should be understood that the described program components and systems can generally be integrated together in a single software product or packaged into multiple software products.

The present invention can be applied to the field of wireless communication technology and industrially used as a method for implementing a wireless backhaul in various communication systems.

According to the present invention, when a backhaul is connected using a wireless network, information on interference with a terminal caused by the wireless backhaul is checked, and beamforming is performed in consideration of the interference information. Therefore, it is possible to reduce radio wave interference with a terminal connected to a small-cell base station (BS) caused by the wireless backhaul from a macro-cell BS.

Claim 1:
A method of connecting a wireless backhaul, the method comprising:
receiving (S300a), by a small-cell base station, BS, (<NUM>), a plurality of beamforming signals from a macro-cell BS (<NUM>) to connect a wireless backhaul;
collecting (S306), by the small-cell BS (<NUM>), information on the plurality of beamforming signals' interference with a terminal (<NUM>) connected to the small-cell BS (<NUM>) or location information of the terminal (<NUM>);
generating (S304), by the small-cell BS (<NUM>), reception status information of the plurality of beamforming signals from the macro-cell BS (<NUM>), wherein the reception status information of the plurality of beamforming signals comprises a received signal intensity of the plurality of beamforming signals received (S300a) by the small-cell BS (<NUM>) from the macro-cell BS (<NUM>);
selecting (S308), by the small-cell BS (<NUM>), a beamforming signal for connecting the wireless backhaul based on the reception status information of the plurality of beamforming signals and further based on one of the interference information or the location information of the terminal (<NUM>);
transmitting (S310), by the small-cell BS (<NUM>), a selection result to the macro-cell BS (<NUM>); and
connecting (S312), by the macro-cell BS (<NUM>), a wireless backhaul by beamforming with the selected beamforming signal toward the small-cell BS (<NUM>) based on the received selection result.