System and method for aligning interference in uplink

Provided is a method of generating a transmit beamforming vector and a receive beamforming vector to substantially eliminate the effect of interference transmitted from macro terminals to a pico base station in a hierarchical cell environment. Also, provided is a method of selecting, from a plurality of macro terminals, a macro terminal for transmitting data to a macro base station.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application claims the benefit under 35 U.S.C. §119(a) of Korean Patent Application No. 10-2010-0076981, filed on Aug. 10, 2010, in the Korean Intellectual Property Office, the entire disclosure of which is incorporated herein by reference for all purposes.

BACKGROUND

The following description relates to a method of determining a transmit beamforming vector and a receive beamforming vector to substantially eliminate the effect of interference transmitted from macro terminals to a pico base station in a hierarchical cell environment.

2. Description of Related Art

To secure sufficient frequency resources, it has been suggested that a next generation mobile communication system may use a high frequency compared to a carrier frequency used in a current mobile communication system. Accordingly, the use of a higher frequency may result in a path loss increase and a cell coverage decrease.

Research has been conducted to decrease an inter-cell distance in preparation for the decrease in the cell coverage of next generation mobile communication systems. However, one consequence of this research has been an increase in the effect of interference from a plurality of adjacent cells to a user positioned at a cell edge. Accordingly, a data transmission rate of a user positioned at a cell edge may decrease, which may present difficulties in guaranteeing a quality of service (QoS).

SUMMARY

In one general aspect, a method of transmitting and receiving at a macro terminal includes receiving, by a receiver of the macro terminal, information associated with a transmit beamforming vector from a pico base station, wherein the pico base station receives a transmit beamformed first data stream from a pico terminal, transmit beamforming of a second data stream according to the transmit beamforming vector, and transmitting, by a transmitter of the macro terminal, the transmit beamformed second data to a macro base station, wherein the transmit beamforming vector is calculated based on a channel matrix of a channel formed between the pico terminal and the pico base station.

The method may further include transmit beamforming of a third data steam by a second macro terminal, and transmitting, by the second macro terminal, the transmit beamformed third data stream to the macro base station, wherein the third data stream and the second data stream are received by the pico base station with substantially the same phase.

A received phase of the first data stream may be substantially orthogonal to a received phase of the second data stream.

The second macro terminal may generate a transmit beamforming vector that substantially minimizes a phase difference between the third data stream and the second data stream.

In another general aspect, a pico base station includes a channel matrix generator to generate a first channel matrix by estimating a channel formed between the pico base station and a pico terminal, a beamforming vector generator to generate a transmit beamforming vector of the pico terminal and a transmit beamforming vector of a macro terminal based on the first channel matrix, a transmitter to transmit, to the pico terminal, the transmit beamforming vector of the pico terminal, and to transmit, to the macro terminal, the transmit beamforming vector of the macro terminal, and a receiver to receive, from the pico terminal, a first data stream that is transmit beamformed according to the transmit beamforming vector of the pico terminal, and to receive, from the macro terminal, a second data stream that is transmit beamformed according to the transmit beamforming vector of the macro terminal.

In the pico base station, the beamforming vector generator may be configured to generate a transmit beamforming vector of a second macro terminal, the transmitter may be configured to transmit, to the second macro terminal, the transmit beamforming vector of the second macro terminal, and the receiver may be configured receive, a third data that is transmit beamformed according to the transmit beamforming vector of the second macro terminal, wherein the third data stream is transmitted with substantially the same phase as the second data stream.

In the pico base station, the beamforming vector generator may be configured to generate a receive beamforming vector based on the first channel matrix, and the receiver may be configured to apply receiving beamforming of to the first data stream according to the receive beamforming vector.

The beamforming vector generator may be configured to generate the transmit beamforming vector of the macro terminal such that the first data stream received by the receiver is substantially orthogonal to the second data stream received by the pico base station.

The pico base station may include a singular value decomposition (SVD) performing unit to perform SVD of the channel matrix.

The channel matrix generator may be configured to generate a second channel matrix by estimating a channel between the pico base station and the macro terminal, and the beamforming vector generator may be configured to calculate the transmit beamforming vector of the macro terminal according to one of Equation 1 or Equation 2:

v=μ·[-(h~(2))H(h~(1))H][Equation⁢⁢1]v=μ·[(h~(2))H-(h~(1))H],[Equation⁢⁢2]wherein v corresponds to the transmit beamforming vector of the macro terminal, μ corresponds to a predetermined constant, and {tilde over (h)}(1)and {tilde over (h)}(2)are determined according to Equation 3:

h~=α·uH·H=[h~(1)h~(2)],[Equation⁢⁢3]wherein u1corresponds to a receive beamforming vector of the pico base station, H corresponds to the second channel matrix, and a corresponds to a predetermined constant.

The second macro terminal may be configured to apply beamforming to the third data stream and to transmit the beamformed third data stream to the macro base station; and the beamforming vector generator may be configured to generate the transmit beamforming vector by calculating a vector that minimizes a phase difference between the second data stream received by the receiver and the third data stream received by the receiver.

The channel matrix generator may be configured to generate a second channel matrix by estimating a channel between the pico base station and the macro terminal, and the beamforming vector generator may be configured to generate an autocorrelation matrix according to Equation 4, and to calculate an eigenvector corresponding to a minimum eigenvalue of the autocorrelation matrix as the transmit beamforming vector of the macro terminal:
R=HH·H[Equation 4]where R corresponds to the autocorrelation matrix, and H corresponds to the second channel matrix.

In still another general aspect, a macro base station includes a signal quality information generator to generate signal quality information associated with each channel formed between the macro base station and each of a plurality of macro terminals, a terminal grouping unit to determine a plurality of terminal groups from the plurality of macro terminals, based on the signal quality information, a data rate predictor to predict a data rate with respect to each of the terminal groups by scheduling each of the terminals groups, and a terminal selector to select, from the plurality of terminal groups, a terminal group for receiving data from the macro base station, wherein the terminal selector selects the terminal group based on the predicted data rate.

The signal quality information may include a signal to noise ratio (SNR) or a signal to interference and noise ratio (SINR).

The macro base station may further include a threshold value setting unit to set a signal quality threshold value, the signal quality threshold value including a value greater than a predetermined reference value when a number of the macro terminals is less than a predetermined threshold value. The terminal grouping unit may determine the plurality of terminal groups according to the macro terminals that have a signal quality information value greater than the signal quality threshold value.

The macro base station may further include a threshold value setting unit to set a signal quality threshold value, the signal quality threshold value comprising a value less than a predetermined reference value when a number of the macro terminals is greater than or equal to a predetermined threshold value. The terminal grouping unit may determine the plurality of terminal groups according to the macro terminals that have a signal quality information value greater than the signal quality threshold value.

According to certain examples herein, inter-cell interference transmitted from terminals included in a macro cell to a pico cell may be aligned in a hierarchical cell environment.

Further according to certain examples herein, a communication quality may be enhanced by decreasing inter-cell interference in a hierarchical cell.

DETAILED DESCRIPTION

FIGS. 1A and 1Billustrate an example of inter-cell interference in a hierarchical cell.

FIG. 1Aillustrates a signal received at each of a pico base station130and a macro base station110in a hierarchical cell.

The pico base station130is a base station associated with the macro base station110. The pico base station130may be assigned with a portion of radio resources, for example, a frequency domain and a time domain allocated to the macro base station110, and may use the assigned radio resource for data transmission. Accordingly, the radio resource used by the pico base station130may be the same as the radio resource used by the macro base station110, or may be a subset of the radio resource used by the macro base station110.

A macro terminal120may transmit first data to the macro base station110. In general, a coverage area of the macro base station110is wider than a coverage area of the pico base station130. Accordingly, the macro terminal120may transmit the first data using a relatively large transmission power as compared to a transmission power of the pico terminal140. The first data transmitted from the macro terminal120may be transmitted to the pico base station130. In this case, the first data may act as an interference signal to the pico base station130.

A pico terminal140may transmit second data to the pico base station130. In general, the coverage area of the pico base station130is narrower than the coverage area of the macro base station110. Accordingly, the pico terminal140may transmit the second data using a relatively small transmission power as compared to a transmission power of the macro terminal120. The second data transmitted from the pico terminal140may be transmitted to only the pico base station130positioned to be close from the pico terminal140; that is, the data may not be received by the macro base station110, for example, due to the macro base station being positioned away from the pico terminal140. Referring toFIG. 1A, it can be assumed that an interference signal transmitted from the pico terminal140to the macro base station110does exist, but that a strength of the interference signal is comparatively small.

FIG. 1Billustrates an example of modeling a channel environment ofFIG. 1A.

Second data transmitted from a pico terminal150may be transmitted to a pico base station160using a channel191formed between the pico terminal150and the pico base station160. The second data transmitted from the pico terminal150may not be received by a macro base station180. For example, it can be assumed that a strength of a channel193formed between the pico terminal150and the macro base station180is comparatively small.

First data transmitted from a macro terminal170may be transmitted to the pico base station160using a channel192formed between the macro terminal170and the pico base station160. In this case, the first data may act as an interference signal in the pico base station160. The first data transmitted from the macro terminal170may be transmitted to the macro base station180using a channel194formed between the macro terminal170and the macro terminal180.

InFIG. 1B, it can be assumed that an interference signal transmitted from the pico terminal150to the macro base station180does not exist, or that a strength of the interference signal is comparatively small. Accordingly, in this example, the channel193formed between the pico terminal150and the macro base station180may be ignored. When considering only channels191,192, and194inFIG. 1B, the channels191,192, and194may form a shape similar to the letter “Z”. Accordingly, the channel environment illustrated inFIG. 1Bmay be referred to as a Z channel model.

FIG. 2illustrates an example of cancelling interference transmitted to a pico base station.

Referring to the Z channel model ofFIG. 1B, it can be assumed that an interference signal transmitted from a pico terminal210to a macro base station260does not exist, or has a comparatively small strength. Accordingly, if it is possible to control the effect of interference signals transmitted from macro terminals230,240, and250to a pico base station220, a data receive performance between the pico base station220and the macro base station260may be enhanced.

In the example illustrated inFIG. 2, the pico base station220includes two receive antennas, the macro base station260includes three receive antennas, and each of the pico terminal210and the macro terminals230,240, and250includes two transmit antennas. In addition, for this example it can be assumed that the macro base station260and the pico base station220receive data using the same radio resource.

InFIG. 2, a number of data streams (degree of freedom (DOF)) that can be transmitted by the entire system may be increased by aligning a receive phase of an interference signal transmitted from each of the macro terminals230,240, and250to the pico base station220.

Each of the macro terminals230,240, and250may transmit a data stream using a plurality of transmit antennas by controlling a phase of the data stream. Each of the macro terminals230,240, and250may control a phase of each of corresponding data streams231,241, and251transmitted by the macro terminals230,240, and250so that the data streams231,241, and251are aligned and thereby received at the pico base station220using the same phase222,223, and224.

A data stream transmitted from each terminal may be expressed by Equation 1:
xi=√{square root over (pi)}visi.  [Equation 1]

In Equation 1, i corresponds to a terminal index. Thus, i=1 with respect to the pico terminal210and i=2, 3, 4 with respect to the macro terminals230,240, and250. sicorresponds to a data stream to be transmitted. vicorresponds to a transmit beamforming vector used for controlling a phase of a data stream by each of the pico terminal210and the macro terminals230,240, and250. picorresponds to a transmission power of the data stream.

In this example, a received signal ypicoreceived by the pico base station220may be expressed by Equation 2, and a received signal ymacroreceived by the macro base station260may be expressed by Equation 3:

In Equation 2 and Equation 3, Hj,icorresponds to a channel matrix of a radio channel formed between a terminal i and a base station j. InFIG. 2, the pico base station220may correspond to a base station1and the macro base station260may correspond to a base station2. In addition, njcorresponds to a thermal noise component added to the base station j.

The data stream211transmitted from the pico terminal210may be received using a phase different from phases of the data streams222,223, and224transmitted from the macro terminals230,240, and250. The pico base station220may cancel an interference signal aligned and received using the same phase by employing a plurality of receive antennas and a receive beamforming vector.

According to one example, a data transmission system ofFIG. 2may determine a substantially optimal transmit beamforming vector and a substantially optimal receive beamforming vector by performing singular value decomposition (SVD) with respect to a channel matrix H11of a channel formed between the pico terminal210and the pico base station220as expressed by Equation 4:

The pico terminal210may perform transmit beamforming using vector V1, and the pico base station220may perform receive beamforming using vector U1. Accordingly, it may be possible to achieve a substantially optimal data transmission efficiency.

In this example, each of the macro terminals230,240, and250may control a phase of each of data streams transmitted by each of the macro terminals230,240, and250so that the data streams may be aligned into a direction substantially orthogonal to U1.

If a data stream is substantially orthogonal to U1, the pico base station220can cancel an interference signal from each of the macro terminals230,240, and250. When a vector orthogonal to U1is U2, U2may be expressed by Equation 5:

In Equation 5, Vicorresponds to a transmit beamforming vector used by a macro terminal i.

Equation 5 may be arranged to Equation 6:
u2=α1H12v2=α2H13v3=α3H14v4.  [Equation 6]

If U1His multiplied by Equation 6, it may be expressed as Equation 7:

In Equation 7, {tilde over (h)}1i=αi-1u1HH1i, and viis present in a null space of {tilde over (h)}1i. To satisfy this criterion, vimay be expressed by Equation 8:

h~1⁢i=[h~1⁢i(1)h~1⁢i(2)],
and μ corresponds to a constant for maintaining a magnitude of vk.

If each of the macro terminals230,240, and250perform transmit beamforming of a data stream using vkof Equation 8, the data stream may be substantially orthogonal to the data stream221transmitted from the pico terminal210to the pico base station220. The pico base station210may cancel an interference signal using a variety of receiving schemes, for example, a zero-forcing (ZF) scheme, a minimum mean square error (MMSE) scheme, and the like, and may receive a data stream from the pico terminal210.

When a data stream is transmitted according to the aforementioned scheme, each terminal, for example, each of the pico terminal210and the macro terminals230,240, and250may transmit the corresponding data stream without causing inter-cell interference.

When a transmit beamforming vector and a receive beamforming vector are generated according to the scheme described above with reference toFIG. 2, DOF may also linearly increase according to an increase in a number of receive antennas of the macro base station260, as expressed by Equation 9:
DOF=M1+M2−1.  [Equation 9]

In Equation 9, M1corresponds to a number of receive antennas installed in the pico base station220, and M2corresponds to a number of receive antennas installed in the macro base station260.

When an additional macro terminal uses the transmit beamforming vector determined according to Equation 8, a data stream transmitted from the additional macro terminal may also use the same phase as the data streams231,241, and251transmitted from the macro terminals230,240, and250. Accordingly, the pico base station220may cancel an interference signal from a macro terminal regardless of a number of macro terminals.

FIG. 3illustrates an example of decreasing interference transmitted to a pico base station. Accordingly,FIG. 3illustrates an example where a pico base station320includes at least three receive antennas, and a macro base station370includes four receive antennas.

As described above with reference toFIG. 2, when data streams331,341,351, and361transmitted from macro terminals330,340,350, and360are aligned using the same phase at the pico base station320, a pico terminal310may transmit two data streams311and312. When the data streams331,341,351, and361are not aligned in phase, the pico terminal310may transmit only a single data stream.

Accordingly, compared toFIG. 2, a number of transmittable data streams may not increase and thus, a DOF gain may not occur.

As shown inFIG. 3, the pico terminal310may determine a substantially optimal transmit beamforming vector and a substantially optimal receive beamforming vector by performing SVD of a channel matrix H11as expressed by Equation 10:

The pico terminal310may perform transmit beamforming using V1and V2, and may transmit two data streams311and312to the pico base station320. The pico base station320may perform receive beamforming using U1and U2.

When the data streams331,341,351, and361transmitted from the macro terminals330,340,350, and360are aligned into a direction of U3substantially orthogonal to U1and U2, the effect of the data streams331,341,351, and361against the pico base station320is substantially minimized.

A criterion of aligning the data streams331,341,351, and361transmitted from the macro terminals330,340,350, and360may be expressed by Equation 11:

Equation 11 may be arranged to Equation 12:
u3=α1H12v2=α2H13v3=α3H14v4=α4H15v5.  [Equation 12]

If [U1·U2]His multiplied by Equation 12, it may be expressed as Equation 13:

In Equation 13, {tilde over (H)}1i=αi-1[u1u2]HH1i. Vimay be determined from vectors present in a null space of {tilde over (H)}1i.

If Viis not found from the null space of {tilde over (H)}1i, it may not be possible to completely eliminate the effect of interference signals transmitted from the macro terminals330,340,350, and360to the pico base station320. In this case, Visubstantially minimizing the effect of the interference signals may be found.

Vifor minimizing the effect of interference signals transmitted from the macro terminals330,340,350, and360to the pico base station320may be expressed by Equation 14:

A result of Equation 14 may be obtained according to Equation 15:

The range of

vkH⁢R1⁢k⁢vkvkH⁢vk
may be determined according to Equation 16:

In Equation 16, A corresponds to a symmetrical matrix. In Equation 15, since R1kcorresponds to a symmetrical matrix,

vkH⁢R1⁢k⁢vkvkH⁢vk
may have eigenvalues of R1k. Accordingly, a minimum value of

When each of the macro terminals330,340,350, and360determines a transmit beamforming vector according to Equation 14, data streams transmitted from the macro terminals330,340,350, and360may be set within a predetermined range. A distribution range of the data streams may be reduced by selecting a suitable macro terminal according to an increase in a number of macro terminals, and by enabling only the selected macro terminal to transmit a data stream. When the distribution range of data streams is reduced, an interference alignment may be more accurately performed. Accordingly, if a sufficient number of macro terminals are present, data streams may be aligned without completely cancelling the interference from the macro terminals330,340,350, and360, and DOF may be calculated according to Equation 17:
DOF˜M1+M2−1  [Equation 17]

In Equation 17, M1corresponds to a number of receive antennas installed in the pico base station220, and M2corresponds to a number of receive antennas installed in the macro base station370.

Accordingly, when a number of receive antennas installed in the macro base station370is greater than a number of macro terminals, a data transmission system ofFIG. 3may effectively transmit data.

When the number of macro terminals increases, difficulties may arise in selecting, from the macro terminals, a macro terminal for transmitting data. According to aspects that a gain is high in a signal to noise ratio (SNR) area using a relatively high interference alignment technology and that difficulty arises in accurately aligning interference as a number of macro terminals decreases, it may be possible to select, from the macro terminals, the macro terminal for transmitting data.

According to one example, when a channel has a high SNR or when a number of macro terminals is comparatively small, a relatively small number of candidate terminal groups may be selected from a plurality of macro terminals, and a terminal group maximizing a data transmission capacity of a macro base station may also be selected from the selected candidate terminal groups.

According to another example, when a channel has a low SNR or when a number of macro terminals is comparatively large, a relatively large number of candidate terminal groups may be selected from a plurality of macro terminals.

FIGS. 4A through 4Cillustrate an example of transmitting transmit beamforming vector information.

FIG. 4Aillustrates an example of transmitting transmit beamforming vector information of a macro terminal411using a backhaul link between base stations. InFIG. 4A, a pico base station412measures a macro channel formed between the macro terminal411and the pico base station412. The pico base station412also measures a pico channel formed between a pico terminal413and the pico base station412.

The pico base station412may determine a transmit beamforming vector of the macro terminal411according to the examples described above with reference toFIG. 2orFIG. 3. Referring back toFIG. 4A, the pico base station412transmits, using the backhaul link, information associated with the transmit beamforming vector of the macro terminal411to a macro base station410. The macro base station410transmits, to the macro terminal411, information associated with the transmit beamforming vector to be used by the macro terminal411.

FIG. 4Billustrates an example of transmitting transmit beamforming vector information of a macro terminal431using an over-the-air (OTA) channel. InFIG. 4B, a pico base station432measures a macro channel formed between the macro terminal431and the pico base station432. The pico base station432also measures a pico channel formed between a pico terminal433and the pico base station432.

The pico base station432may determine a transmit beamforming vector of the macro terminal431according to the examples described above with reference toFIG. 2orFIG. 3. Referring back toFIG. 4B, the pico base station432transmits, to the macro terminal431, information associated with the transmit beamforming vector of the macro terminal431.

FIG. 4Cillustrates an example of transmitting, to a macro terminal451, information associated with a transmit beamforming vector of the macro terminal451in a data transmission system using a time division duplex (TDD) scheme.

InFIG. 4C, a pico base station452measures a macro channel formed between the macro terminal451and the pico base station452. The pico base station452also measures a pico channel formed between a pico terminal453and the pico base station452.

The pico base station452may transmit a reference signal to the macro terminal451by employing a receive beamforming vector as a transmit beamforming vector. The macro terminal451may determine the transmit beamforming vector so that all the macro terminals may perform nulling of a channel vector using the reference signal.

FIG. 5illustrates an example of a data interference alignment scheme for decreasing interference transmitted to a pico base station511. Referring toFIG. 5, the pico base station511may transmit a transmit beamforming vector of each of macro terminals512and513, and may also transmit the transmit beamforming vector to each of the macro terminals512and513via a macro base station514.

In operation520, a pico terminal510transmits first data to the pico base station511.

In operation521, the macro terminals512and513transmit second data to the macro base station514.

In operation522, the second data transmitted from the macro terminals512and513is received as an interference signal by the pico base station511.

In operation530, the pico base station511compares a strength of the interference signal received from each of the macro terminals512and513with a predetermined threshold value. If the strength of the interference signal is greater than the threshold value, the pico base station511may transmit a sounding request to the macro base station514in operation531.

In operation532, the macro base station514performs sounding coordination in response to the sounding request.

In operation533, the pico base station511receives a sounding response.

In operation540, the macro base station514transmits a sounding trigger to the macro terminals512and513.

In operation541, the macro terminals512and513transmit a sounding signal to the pico base station511.

In operation542, the pico terminal510transmits a sounding signal to the pico base station511.

In operation550, the pico base station511estimates a pico channel formed between the pico terminal510and the pico base station511, and a macro channel formed between the pico base station511and each of the macro terminals512and513, based on the sounding signal received from the pico terminal510and the sounding signal received from each of the macro terminals512and513.

In operation560, the pico base station511generates a transmit beamforming vector of each of the macro terminals512and513, and a transmit beamforming vector of the pico terminal510. It is described above with reference toFIG. 2orFIG. 3.

In operation561, the pico base station511transmits, to the pico terminal510, information associated with the transmit beamforming vector of the pico terminal510.

In operation561, the pico base station511transmits, to the macro base station514, information associated with the transmit beamforming vector of each of the macro terminals512and513. The pico base station511transmits, to the macro base station514, information associated with the transmit beamforming vector of the macro terminals512and513using a backhaul link between the pico base station511and the macro base station514.

In operation562, the macro base station514transmits, to the corresponding macro terminals512and513, information associated with the transmit beamforming vector of each of the macro terminals512and513.

In operation590, the pico terminal510performs transmit beamforming of first data using the transmit beamforming vector of the pico terminal510.

In operation570, the macro terminal513performs transmit beamforming of second data using the transmit beamforming vector of the macro terminal513. In571, the second data transmitted from the macro terminal513may be transmitted to the pico base station511as an interference signal.

In operation580, the macro terminal512performs transmit beamforming of third data using the transmit beamforming vector of the macro terminal512. In581, the third data transmitted from the macro terminal512may be transmitted to the pico base station511as an interference signal.

In operations571and581, the interference signals transmitted from the macro terminals512and513are received using the same phase at the pico base station511. In addition, the phase of the interference signals may be orthogonal to a received phase of the first data transmitted from the pico terminal510.

FIG. 6illustrates another example of a data interference alignment scheme decreasing interference transmitted to a pico base station611.

Operations620through630are similar to operations520through530ofFIG. 5and thus, further detailed description will be omitted here.

If a strength of an interference signal is greater than a predetermined threshold value, the pico base station611may perform sounding coordination in operation641.

In operation642, the pico base station611transmits a sounding request to macro terminals612and613. In operation643, the macro terminals612and613transmit a sounding request to a macro base station614.

In operation644, the macro base station614transmits a sounding allowance message to the macro terminals612and613in response to the sounding request.

In operation645, the macro terminals612and613transmit a sounding signal to the pico base station611.

In operation646, a pico terminal610transmits a sounding signal to the pico base station611.

Operations650through690are similar to550through590ofFIG. 5and thus, further detailed description will be omitted here.

FIG. 7illustrates an example of a macro terminal700.

Referring toFIG. 7, the macro terminal700includes a receiver710and a transmitter720.

A pico base station740may receive a transmit beamformed first data stream from a pico terminal760. The macro terminal700may perform transmit beamforming of a second data stream and transmit the transmit beamformed second data stream to a macro base station730. A second macro terminal750may perform transmit beamforming of a third data stream and transmit the transmit beamformed third data stream to the macro base station730. As described above with reference toFIG. 1, it can be assumed that the macro terminal700and a second macro terminal750transmit a significant interference signal to the pico base station740, however, the pico terminal760can be assumed to not transmit an interference signal to the macro base station730or to transmit only a weak interference signal.

The pico base station740may generate a transmit beamforming vector of the macro terminal700. As described above in the examples illustrated inFIG. 2andFIG. 3, the pico base station740may generate the transmit beamforming vector of the macro terminal700based on a channel formed between the macro terminal700and the pico base station740, and a channel formed between the pico base station740and the pico terminal760.

The receiver710may receive, from the pico base station740, information associated with the transmit beamforming vector of the macro terminal700.

The transmitter720may identify the transmit beamforming vector based on information associated with the transmit beamforming vector, and may perform transmit beamforming of the second data stream using the identified transmit beamforming vector. The transmitter720may transmit the transmit beamformed second data stream to the macro base station730.

The second macro terminal750may receive, from the pico base station740, information associated with the transmit beamforming vector of the second macro terminal750. The second macro terminal750may perform transmit beamforming of the third data stream using the transmit beamforming vector of the second macro terminal750.

The second data stream and the third data stream may be transmitted to the pico base station740in addition to the macro base station730. Accordingly, the second data stream and the third data stream may act as interference signals at the pico base station740.

In the above example, the pico base station740may determine the transmit beamforming vector of each of the macro terminal700and the second macro terminal750so that the second data stream and the third data stream may be received using the same phase at the pico base station740.

In addition, the pico base station740may determine the transmit beamforming vector of each of the macro terminal700and the pico terminal760so that a phase of the first data stream received by the pico base station740may be substantially orthogonal to a phase of the second data stream received by the pico base station740.

FIG. 8illustrates an example of a pico base station800.

Referring toFIG. 8, the pico base station800includes a transmitter830and a receiver840, and may also include a channel matrix generator810and a beamforming vector generator820.

The channel matrix generator810generates a channel matrix by estimating a state of a channel formed between the pico base station800and a pico terminal880.

The beamforming vector generator820generates a transmit beamforming vector of the pico terminal880based on the channel matrix formed between the pico base station800and the pico terminal880. Further, the beamforming vector generator820may generate a transmit beamforming vector of each of a first macro terminal860and a second macro terminal870based on the channel matrix between the pico base station800and the pico terminal880.

The transmitter830transmits, to the pico terminal880, information associated with a to transmit beamforming vector of the pico terminal880. Further, the transmitter830may transmit, to the first macro terminal860, information associated with the transmit beamforming vector of the first macro terminal860, and may transmit, to the second macro terminal870, information associated with the transmit beamforming vector of the second macro terminal870.

The pico terminal880identifies the transmit beamforming vector of the pico terminal880based on information associated with the transmit beamforming vector of the pico terminal880, and performs transmit beamforming of a first data stream using the identified transmit beamforming vector.

The receiver840receives the transmit beamformed first data stream. As one example, the beamforming vector generator820may generate a receive beamforming vector based on the channel matrix. Examples of generating the receive beamforming vector are described above with reference toFIG. 2andFIG. 3and thus, further detailed description will be omitted here. The receiver840may perform receive beamforming of the first data stream using the receive beamforming vector.

The first macro terminal860may receive information associated with the transmit beamforming vector of the first macro terminal860. The second macro terminal870may receive information associated with the transmit beamforming vector of the second macro terminal870. The first macro terminal860may perform transmit beamforming of a second data stream using the transmit beamforming vector of the first macro terminal860, and may transmit the transmit beam formed second data stream to a macro base station850. Similarly, the second macro terminal870may perform transmit beamforming of a third data stream and transmit the transmit beam formed third data stream to the macro base station850.

The second data stream and the third data stream may also be transmitted to the pico base station800. Accordingly, the second data stream and the third data stream may act as interference signals at the pico base station800.

The beamforming vector generator820may generate the transmit beamforming vector of the first macro terminal860and the transmit beamforming vector of the second macro terminal870, so that a phase of the second data stream received at the pico base station800may be the same as a phase of the third data stream received at the pico base station800. Accordingly, the receiver840may receive the transmit beam formed third data stream using the same phase as the transmit beam formed second data stream.

The beamforming vector generator820may generate the transmit beamforming vector of the first macro terminal860so that the first data stream received by the receiver840may be substantially orthogonal to the second data stream received by the receiver840. A corresponding example of generating the transmit beamforming vector is described above with reference toFIG. 2and thus, further detailed description will be omitted here.

The beamforming vector generator820may generate the transmit beamforming vector of the first macro terminal860and the transmit beamforming vector of the second macro terminal870, so that a phase difference between the second data stream and the third data stream received by the receiver840may be minimized. A corresponding example of generating the transmit beamforming vector is described above with reference toFIG. 3and thus, further detailed description will be omitted here.

FIG. 9illustrates an example of a macro base station900.

Referring toFIG. 9, the macro base station900may include a signal quality information generator910, a terminal grouping unit920, a data rate predictor930, and a terminal selector940.

The signal quality information generator910generates signal quality information associated with channel(s) formed between the macro base station900and macro terminal(s), such as illustrated macro terminals951,952,953, and954. The signal quality information may be an SNR or a signal to interference and noise ratio (SINR).

The terminal grouping unit920groups macro terminals into terminal groups. As illustrated inFIG. 9, the terminal grouping unit920may group the plurality of macro terminals951,952,953, and954into a plurality of terminal groups961,962, and963. The terminal grouping unit920may group macro terminals based on the signal quality information.

The terminal grouping unit920compares signal quality information of each macro terminal with a predetermined threshold value, and may group the plurality of macro terminals951,952,953, and954based on a comparison result.

The macro base station900may select a single terminal group from the terminal groups961,962, and963, and receive data from terminals included in the selected terminal group.

If the macro base station900receives data using an interference alignment scheme and an SNR of a corresponding terminal is relatively high, a gain of the interference alignment scheme may increase. Further, if the macro base station900receives data using an interference alignment scheme and a number of macro terminals is relatively small, a gain of the interference alignment scheme may increase.

Accordingly, if the number of macro terminals is relatively small, the terminal grouping unit920may set a terminal group from among macro terminals having a relatively high signal quality. For example, if a number of macro terminals is greater than a predetermined threshold value, the terminal grouping unit920may set a signal quality threshold value to be a value less than a predetermined reference value, and may determine a terminal group from terminals having a signal quality information value greater than the signal quality threshold value.

If a number of macro terminals is relatively large, the terminal grouping unit920may set the terminal group from among macro terminals having a relatively high signal strength. For example, if a number of macro terminals is less than a predetermined threshold value, the terminal grouping unit920may set the signal quality threshold value to be a value greater than the predetermined reference value and may determine a terminal group from terminals having the signal quality information value greater than the signal quality threshold value. In this example, a majority of the macro terminals may be included in the terminal group.

The data rate predictor930predicts a data rate of each terminal group by scheduling each terminal group.

To improve performance, a data rate prediction including a beamforming vector calculation may be performed with respect to all probable combinations of macro terminals. However, this example of data rate prediction may use a significant number of calculations. Accordingly, the data rate predictor930may constitute a plurality of terminal groups with respect to macro terminals having a relatively high signal quality information, and may select, from the plurality of terminals groups, a terminal group for transmitting data.

The data rate predictor930may predict a data rate with respect to each macro terminal by virtually applying an interference alignment scheme to macro terminals included in each terminal group, and may predict a data rate with respect to each terminal group by adding up data rates of macro terminals included in a corresponding terminal group.

The terminal selector940selects a terminal group for receiving data from the macro base station900. As illustrated inFIG. 9, the terminal selector940may select, from the plurality of terminal groups961,962, and963, a terminal group for receiving data from the macro base station900based on the data rate with respect to the terminal groups961,962, and963.