KR20090073910A - Apparatus and method for pairing terminals in ul mimo system - Google Patents

Abstract

An apparatus and a method for pairing terminals in up-link MIMO(MULTIPLE INPUT MULTIPLE OUTPUT) system are provided to increase the service quality by paring terminals with low channel correlation by using orthogonal value and unique ratio. A terminal searcher(210) searches terminals capable of C-MIMO in a system environment in which the SISO(Single Input Single Output) and MIMO are mixed. A terminal information gathering unit(220) obtains the information about the terminals which are applicable for C-MIMO. A terminal sorter(230) sorts terminals by using combination of parameter or parameter. The channel correlation calculation unit(240) calculates the channel correlation of terminals by using orthogonal value and unique ratio among terminals.

Description

Apparatus and Method for pairing terminals in UL MIMO System}

The present invention relates to a device and method for pairing (pairing) terminals in a UL MIMO system, and more particularly, MIMO wireless communication that supports OFDM / OFDMA schemes following IEEE 802.16d / e, Wibro, WiMAX, etc. An apparatus and method for efficiently pairing terminals having low channel correlation using orthogonality, eigen ratio, and the like during uplink (UL) burst transmission in a system.

Recently, as the necessity of transmitting a large amount of data at high speed using a wireless channel is rapidly increasing, a wireless / high-speed data transmission system for supporting mobile Internet service in a mobile environment is being actively researched. Multiple input multiple output (MIMO) systems are emerging to support Internet services.

1 is a diagram illustrating an outline of a general SISO system and a MIMO system.

As shown in FIG. 1A, a single input single output (SISO) system uses a single antenna (TxAnt, RxAnt) on a transmitting side and a receiving side, for example, and includes a terminal having one antenna (TxAnt). A signal is transmitted and received through one channel H formed between a base station having one antenna RxAnt.

MIMO (Multiple Input Multiple Output) system is a technology that transmits data through multiple paths by increasing the number of antennas of a terminal and a base station, and can improve transmission efficiency through space-time diversity and spatial multiplexing on the transmitting side, and each path on the receiving side. The interference can be reduced by detecting the received signal. For example, FIG. 1B illustrates a 2x2 MIMO system including a base station having two antennas TxAnt0 and TxAnt1 and a base station having two antennas RxAnt0 and RxAnt1. Four channels, that is, the first channel H00, the second channel H01, and the third channel between the first and second antennas TxAnt0 and TxAnt1 of the base station and the first and second antennas RxAnt0 and RxAnt1 of the base station. H10 and a fourth channel H11 are formed.

Such a MIMO system has a higher data rate by including a plurality of transmit and receive antennas, which is advantageous in that the capacity of the radio link is improved in comparison with the SISO system in the transmit and receive period. That is, in a multipath rich environment, multiple orthogonal channels can be formed between the transceivers, so that data for a single user can be transmitted over the air in parallel over orthogonal channels using the same bandwidth at the same time. As a result, higher spectral efficiency than the SISO system is achieved.

However, in the uplink of the 2 × 2 MIMO system, as described above, two antennas must be present in each terminal, thereby increasing the power loss of the terminal and increasing the complexity of hardware. Therefore, a collaborative MIMO (hereinafter referred to as "C-MIMO") for increasing the transmission rate as a conventional MIMO using a terminal having a simpler hardware structure and one antenna has been studied. More efficient measures are needed to perform MIMO.

The present invention was devised to solve the above-described needs, and an object of the present invention is to provide an apparatus and method for efficiently pairing a plurality of terminals when performing C-MIMO.

Another object of the present invention is to provide an apparatus and method for terminal pairing in a UL MIMO system that can improve service quality by pairing terminals having low channel correlation using orthogonal, eigen ratio, and the like. .

It is still another object of the present invention to provide an apparatus and method for sorting terminals according to power strengths, and calculating the channel correlation according to the sorted order to pair the terminals.

It is still another object of the present invention to provide an apparatus and method for terminal pairing in a UL MIMO system that can improve overall throughput by allocating more resources to a terminal pair having low channel correlation.

For the above purposes, an apparatus for pairing terminals in a UL MIMO system of one embodiment of the present invention includes a channel correlation calculator for calculating a channel correlation for a combination of C-MIMO (Collaborative MIMO) capable terminal pairs; And a terminal pairing unit for pairing in the order of the terminal pair having the lowest channel correlation.

In addition, an apparatus for pairing terminals in a UL MIMO system according to another aspect of the present invention includes: a terminal alignment unit for arranging Collaborative MIMO (C-MIMO) capable terminals based on a predetermined parameter; And a terminal pairing unit configured to pair the terminals based on the sorted order.

On the other hand, in the UL MIMO system of one embodiment of the present invention, a method for pairing a terminal includes: a) calculating a channel correlation for a combination of C-MIMO (Collaborative MIMO) capable terminal pairs; And b) pairing in the order of the terminal pair having the lowest channel correlation.

In addition, a method of pairing terminals in a UL MIMO system according to another aspect of the present invention may include: a) arranging C-MIMO (Collaborative MIMO) capable terminals based on a predetermined parameter; And b) pairing the terminals based on the sorted order.

In addition, a method of pairing UEs in a UL MIMO system according to another aspect of the present invention includes: a) arranging Collaborative MIMO (C-MIMO) capable terminals based on a predetermined parameter; b) calculating channel correlations for the highest priority terminal and the remaining terminals among the aligned terminals, respectively; And c) preferentially pairing the terminal pair having the lowest channel correlation.

Accordingly, the present invention has an effect of increasing the quality of service by pairing terminals having low channel correlation using orthogonal, eigen ratio, and the like.

In addition, the present invention has the effect of efficiently allocating resources to paired terminals on the basis of channel correlation in UL MIMO system to maximize system resources by utilizing limited resources.

Hereinafter, the present invention will be described in detail with reference to the accompanying drawings and preferred embodiments. For reference, detailed descriptions of well-known functions and configurations that may unnecessarily obscure the subject matter of the present invention will be omitted in the following description.

FIG. 2 illustrates an UL (Multilink Multiple Output Multiple Output) system, in particular 2 × 2 C consisting of two mobile stations (Mobile Station / Portable Subscriber Station) and one base station (Base Station / Radio Access Station). -Collaborative MIMO system is illustrated.

In brief, the first terminal transmits through a different pilot pattern through the first transmission antenna TxAnt0 and the second terminal through the second transmission antenna TxAnt1. For reference, FIG. 3 illustrates pilot patterns transmitted by the first transmit antenna TxAnt0 and the second transmit antenna TxAnt1 in the UL PUSC mode of the 2 × 2 C-MIMO system, respectively. Then, the signals of the first channel H00 and the third channel H10 transmitted from the first terminal and the signals of the second channel H01 and the fourth channel H11 transmitted from the second terminal are different from each other. As a result, a base station having spatial multiplexing and transmission through the same subcarrier and having first and second receiving antennas RxAnt0 and RxAnt1 receives both signals transmitted from the first terminal and the second terminal, respectively. .

In this regard, FIG. 4 illustrates an example of a resource allocation scheme for the case of non-MIMO (FIG. 4A) and the case of C-MIMO (FIG. 4B). Referring to FIG. 4, the same subchannel is used for C-MIMO. It can be seen that by transmitting a plurality of bursts (UL Burst # 1 and UL Burst # 2 of FIG. 4B), more data can be transmitted with limited resources.

5 illustrates a configuration of a base station to which a terminal pairing device according to the present invention can be applied.

As shown in FIG. 5, the base station includes an interface 110, a band signal processor 120, a transmitter 130, a receiver 160, a scheduler 150, an antenna 140, and the like. The illustrated base station supports TDD, and also includes 2 antennas to support 2x2 MIMO by transmitting and receiving signals through respective transmission paths and reception paths.

In the reception path, the receiver 160 receives one or more radio signals transmitted by the terminals through the antenna 140 and converts the signal into a baseband signal. For example, the receiver 160 removes and amplifies the noise from the above-described signal for data reception of the base station, down-converts the amplified signal to a baseband signal, and digitizes the down-converted baseband signal. The band signal processor 120 extracts information or data bits from the digitized signal to perform demodulation, decoding, and error correction processes. The received information is transmitted to the adjacent wired / wireless network via the interface 110 or transmitted to other terminals serviced by the base station.

In the transmission path, the interface 110 receives voice, data, or control information from a control station or a wireless network, and the band signal processor 120 encodes the voice, data, or control information and transmits the same to the transmitter 130. do. The transmitter 130 modulates the encoded voice, data, or control information into a carrier signal having a desired transmission frequency or frequencies, amplifies the modulated carrier signal to a level suitable for transmission, and propagates to the air through the antenna 140. do.

Meanwhile, the scheduler 150 performs scheduling by allocating DL_MAP, UL_MAP, uplink burst, downlink burst, and the like to a frame formed of a symbol and a subchannel for transmission and reception with a terminal. At this time, when the C-MIMO is applied, the C-MIMO capable terminals are paired. Therefore, the terminal pairing device according to the present invention may be implemented as a scheduler itself or as a part or as a separate device.

6 illustrates a process of performing a UL MIMO by a base station and a terminal in accordance with the present invention.

In brief, the UE informs the base station of its capability through the SS Basic Capability Request (SBC-REQ) message, and the base station determines the basic capability between the terminals and the base station in consideration of the SBC-REQ message. The response is made through an SS Basic Capability Response (RSP) message (S610). Subsequently, the terminal transmits a REG-REQ (Registration Request) message to the base station, and the base station responds with a REG-RSP message, thereby performing registration (S620). In addition, the base station performs scheduling for data transmission and reception with the terminals (S630). At this time, the scheduler selects and pairs C-MIMO capable terminals and allocates resources by designating a burst area of the paired terminals in an uplink frame. When the UL_MAP is created by allocating the resources, the base station includes the UL_MAP in the downlink frame and transmits the UL_MAP to the terminal (S640). After decoding the terminal (S650), the base station transmits data to the base station in a designated uplink burst (S650). S660). Here, the paired terminal pairs transmit data in different pilot patterns in the same burst.

Hereinafter, an apparatus and method for pairing a terminal in a UL MIMO system according to the present invention will be described with reference to FIGS. 7 to 8. For convenience of description, this embodiment assumes an OFDM / OFDMA based 2x2 C-MIMO system as an example, and assumes that each terminal is capable of C-MIMO.

7 is a block diagram of a terminal pairing device according to an embodiment of the present invention. As shown, the terminal pairing apparatus 200 according to the present invention is the terminal search unit 210, the terminal information collection unit 220, the terminal alignment unit 230, the channel correlation calculator 240, the terminal pairing unit 250 and the like.

The terminal search unit 210 refers to basic capacity such as physical parameter information and bandwidth allocation information provided from the terminals and refers to C- in a system environment in which SISO and MIMO are mixed. Search for terminals capable of MIMO.

The terminal information collecting unit 220 obtains information about C-MIMO applicable terminals found by the terminal searching unit 151. The terminal information may include location information of the terminal and channel quality information of the terminal, and the channel quality information of the terminal may include ranging channel information, uplink sounding channel information, and CQI (Channel). Examples include parameters such as carrier to interference and noise ratio (CINR) information, power allocation information, and modulation coding scheme (MCS) level information through a Quality Indicator (CN) channel.

The terminal sorter 230 sorts the terminals using the parameter or a combination of parameters. For example, the location information of the terminal is used to arrange the terminal close to the base station or the CINR information of the terminal is used to order the terminal in good channel condition. In particular, in connection with the present invention, the terminal aligning unit 230 sorts the terminals in order of power strength or in order of channel gain. At this time, it is also possible to align the terminal in consideration of two or more parameters.

The channel correlation calculator 240 calculates channel correlations of the terminals using orthogonal values, eigen ratios, and the like. The channel correlation indicates the degree to which the channel is affected due to interference between terminals, etc. It is preferable that the channel correlation is low or low in order to guarantee the performance of the C-MIMO system.

The terminal pairing unit 250 pairs the terminals based on information provided by the terminal searching unit 210, the terminal government collecting unit 220, the terminal arranging unit 230, and the channel correlation calculator 240. do. In the simplest case, it is possible to consider pairing or randomly pairing terminals according to the sorting order. In the present invention, the terminals are paired in the following manner (algorithm) using channel correlation.

First, an orthogonal value between terminals is paired in the order of the lowest pair of terminals.

In detail, first, orthogonality is calculated for all user terminals. Then, the terminal pair having the lowest orthogonality is selected and paired. Thereafter, the pair of terminals having the lowest orthogonality is selected and paired with respect to the remaining terminals, and this is repeated to pair all the terminals.

For reference, the orthogonality may be calculated using Equations 1 and 2 below.

[Equation 1]

[Equation 2]

In the above equation, 'user #' represents an index of the #th user terminal, 'sc' represents a subcarrier index, 'H' represents a channel response, and 'H ^* ' represents H Represents a conjugate of. In addition, 'channel_gain' represents channel gain, 'power' represents power of the user terminal, and 'Orth_value' represents an orthogonal value between two user terminals in the sc-th subcarrier. 'Orth' represents an orthogonality between two user terminals for all subcarriers, and 'Nsc' represents the number of subcarriers.

Second, a pairing method is performed in order of the terminal pairs having the lowest eigen ratios between the terminals.

In detail, first, an intrinsic ratio is calculated for all user terminals. The terminal pair having the lowest intrinsic ratio is selected and paired. Thereafter, the pair of terminals having the lowest intrinsic ratio is similarly selected and paired with respect to the remaining terminals, and this is repeated to pair all the terminals.

For reference, the intrinsic ratio may be calculated using Equation 3 below.

[Equation 3]

Eigen_Ratio = Max (eigen value) / Min (eigen value)

Third, a method of pairing terminals in order of a large power offset between terminals.

In detail, first, a product of power and channel gains (Power * Channel_Gain) is obtained for all terminals, and the terminals are sorted based on this. Then, the terminal pair having the largest difference between the product of power and channel gain is selected and paired. Thereafter, similarly, a pair of terminals having the largest difference between the product of power and channel gain is selected and paired with the remaining terminals, and all of the terminals are repeatedly paired.

Example 1 below shows the result of pairing 10 terminals in order of power and channel gain, followed by pairs of terminals having the largest difference in power and channel gain.

Example 1

User index: [1 2 3 4 5 6 7 8 9 10]

Sorted index: [2 4 5 9 10 3 7 6 8 1]

Paired index: {[2 1] [4 8] [5 6] [9 7] [10 3]}

Fourth, the terminals are arranged in order of signal strength and then paired in order.

In detail, first, a product of power and channel gains (Power * Channel_Gain) is obtained for all terminals, and the terminals are sorted based on this. Then, a pair of terminals having the largest product of power and channel gain and the second largest terminal is selected and paired. Thereafter, similarly, a pair of a terminal having the largest power and channel gain and a second largest terminal are paired with the remaining terminals, and the pairs are repeatedly paired with each other.

Example 2 below shows the result of pairing the 10 terminals in the order of the product of power and channel gain (Power * Channel_Gain).

Example 2

User index: [1 2 3 4 5 6 7 8 9 10]

Sorted index: [2 4 5 9 10 3 7 6 8 1]

Paired index: {[2 4] [5 9] [10 3] [7 6] [8 1]}

On the other hand, the case of combining the above-described schemes can be considered. In this case, orthogonal values, eigen ratios, and the like for all combinations of terminals are selected, for example, by selecting one terminal randomly or by selecting the terminal having the largest or smallest signal strength. This can reduce the amount of computation required. This will be described in detail below.

Fifth, the priority is given to the terminal having the largest signal strength, and the orthogonality with the other terminal is calculated to pair with the lowest terminal.

In detail, first, a product of power and channel gains (Power * Channel_Gain) is obtained for all terminals, and the terminals are sorted based thereon. A terminal having the largest product of power and channel gain is selected, and an orthogonality is calculated for each of the selected terminal and the remaining terminals. The terminal having the lowest orthogonality is paired with the selected terminal. Thereafter, similarly, the terminal with the highest power and channel gain is selected for the remaining terminals, and the terminal with the lowest orthogonality is paired.

In the following Example 3, the power and channel gains are arranged in order of power and channel gains for 10 terminals, and then the user terminal (user index: 2) having the largest signal strength and the remaining 9 terminals (user index: 1,3). After calculating the orthogonality for each of 4,5,6,7,8,9,10), it shows pairing with the terminal having the lowest orthogonality (user index: 8). And, by repeating this process, all the terminals are paired.

Example 3

User index: [1 2 3 4 5 6 7 8 9 10]

Sorted index: [2 4 5 9 10 3 7 6 8 1]

2 ^nd Sorted index: [4 5 9 10 3 7 6 1] ← 1 ^st Paired index: [2 8]

Sixth, a method of giving priority to a terminal having the smallest signal strength, calculating an orthogonality with the remaining terminals, and pairing with the lowest terminal.

In detail, first, a product of power and channel gains (Power * Channel_Gain) is obtained for all terminals, and the terminals are sorted based thereon. Then, the terminal with the smallest product of power and channel gain is selected, and orthogonality is calculated for each of the selected terminal and the remaining terminals. The terminal having the lowest orthogonality is paired with the selected terminal. Thereafter, similarly, a terminal having the smallest product of power and channel gain is selected for the remaining terminals, and the terminal having the lowest orthogonality is paired.

Seventh, it gives priority to the terminal with the largest signal strength, calculates the intrinsic ratio with the other terminal and pairs with the lowest terminal.

In detail, first, a product of power and channel gains (Power * Channel_Gain) is obtained for all terminals, and the terminals are sorted based thereon. Then, a terminal having the largest product of power and channel gain is selected, and an inherent ratio for each of the selected terminal and the remaining terminals is calculated. Then, the selected terminal and the terminal with the lowest unique ratio are paired. Thereafter, similarly, the terminal with the largest power and channel gain is selected for the remaining terminals, and the selected terminal and the terminal with the lowest intrinsic ratio are paired.

Eighth, a method of giving priority to a terminal having the smallest signal strength, calculating an inherent ratio with the remaining terminals, and pairing with the lowest terminal.

In detail, first, a product of power and channel gains (Power * Channel_Gain) is obtained for all terminals, and the terminals are sorted based thereon. Then, a terminal having the smallest product of power and channel gain is selected, and an inherent ratio for each of the selected terminal and the remaining terminals is calculated. Then, the selected terminal and the terminal with the lowest unique ratio are paired. Thereafter, similarly, a terminal with the smallest product of power and channel gain is selected for the remaining terminals, and the selected terminal and the terminal with the lowest intrinsic ratio are paired.

Ninth, a method of randomly selecting one terminal and pairing the selected terminal and the terminal having the smallest power size difference. The remaining terminals are paired in the same manner.

Meanwhile, after pairing terminals according to the above-described scheme, system performance may be improved by allocating more resources to a pair of terminals having low orthogonality, inherent ratio, and the like.

Hereinafter, a terminal pairing method in a UL MIMO system according to a preferred embodiment of the present invention will be described with reference to FIG. 8. For reference, a detailed process or operation principle of the terminal paying method according to the present invention may refer to the description of the terminal pairing apparatus described above, and thus, redundant description thereof will be omitted and will be briefly described here, focusing on steps occurring in time series. do.

First, C-MIMO-enabled terminals are searched for in a mixed environment of SISO and MIMO with reference to basic capacity exchanged between the terminal and the base station (S810). Subsequently, parameters for the discovered terminals are obtained (S820). Such parameters may include, for example, the location of the terminal, the CINR of the terminal, the allocated power of the terminal, and the like.

Next, the terminal is aligned using the parameter or a combination of parameters (S830). In this case, the terminal pairing apparatus according to the present invention described above was arranged in order of the signal strength of the terminal.

Thereafter, the aligned terminals are paired (S840). For this, reference may be made to the above-described pairing method using an orthogonal value, an eigen ratio, and the like. Finally, resources are allocated to the paired terminals (S850).

Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art to which the present invention pertains can implement the present invention in other specific forms without changing the technical spirit or essential features, The examples are to be understood in all respects as illustrative and not restrictive.

In addition, the scope of the present invention is specified by the appended claims rather than the detailed description, and all changes or modifications derived from the meaning and scope of the claims and their equivalent concepts are included in the scope of the present invention. Should be interpreted as

1 is a view for explaining the outline of a general SISO system and MIMO system.

2 is a diagram illustrating a C-MIMO system performed between two terminals and one base station in a MIMO system according to an embodiment of the present invention.

3 is a diagram illustrating a pilot pattern applied to an uplink PUSC mode in a 2x2 MIMO system according to an embodiment of the present invention.

4 is a diagram illustrating a resource allocation method for the case of non-MIMO and case of C-MIMO.

5 is a diagram illustrating a configuration of a base station to which a terminal pairing device according to the present invention can be applied.

6 is a diagram illustrating a process of performing a UL MIMO by a base station and a terminal in accordance with the present invention.

7 is a block diagram of a terminal pairing device according to an embodiment of the present invention.

8 is a flowchart of a terminal pairing method according to an embodiment of the present invention.

Claims (19)

An apparatus for pairing terminals in an UL Uplink Multiple Input Multiple Output (MIMO) system, A channel correlation calculator configured to calculate a channel correlation for a combination of C-MIMO capable terminal pairs; And And a terminal pairing unit configured to pair in the order of the terminal pair having the lowest channel correlation. The method of claim 1, The channel correlation may include at least one of an orthogonal value and an eigen ratio. The method of claim 2, The orthogonal value (orthogonal value) is a terminal pairing device, characterized in that calculated by the following equations (1, 2). [Equation 1]

[Equation 2]

In the above equation, 'user #' represents the index of the #th user terminal, 'sc' represents a subcarrier index, 'H' represents a channel response, and 'H ^* ' represents Represents a conjugate of H, 'channel_gain' represents channel gain, 'power' represents power of the user terminal, and 'Orth_value' represents between two user terminals in the scth subcarrier. Orthogonal value, and 'Orth' represents orthogonality between two user terminals for all subcarriers, and 'Nsc' is the number of subcarriers.) The method according to any one of claims 1 to 3, The apparatus may further include a terminal alignment unit for arranging the C-MIMO capable terminals based on a predetermined parameter. And the channel correlation calculator calculates channel correlation between terminals based on the sorted order. The method of claim 4, wherein The terminal alignment unit aligns the terminals according to the signal strength of the terminal, And the terminal pairing unit preferentially pairs the terminal having the largest signal strength or the smallest terminal. An apparatus for pairing terminals in an UL Uplink Multiple Input Multiple Output (MIMO) system, A terminal alignment unit for arranging C-MIMO (Collaborative MIMO) capable terminals based on a predetermined parameter; And And a terminal pairing unit for pairing terminals based on the sorted order. The method of claim 6, Further comprising a channel correlation calculator for calculating a channel correlation for the combination of the C-MIMO (Collaborative MIMO) capable terminal pair, And the terminal pairing unit preferentially pairs the terminal pair having the lowest channel correlation among the terminal pair combinations consisting of the terminal arranged in the highest priority or the lowest priority and the remaining terminals. The method of claim 6, The channel correlation may include at least one of an orthogonal value and an eigen ratio. The method of claim 6, And the terminal pairing unit preferentially pairs the highest priority terminal and the lowest priority terminal. The method according to any one of claims 6 to 9, The parameter is a terminal pairing device, characterized in that the signal strength of the terminal. A method of pairing terminals in an UL Uplink Multiple Input Multiple Output (MIMO) system, a) calculating a channel correlation for a combination of C-MIMO capable terminal pairs; And b) pairing the terminal in the order of the terminal pairs having the lowest channel correlation. The method of claim 11, Prior to step a) Selecting the terminal having the largest signal strength or the smallest terminal among the C-MIMO capable terminals; In the step a), the terminal pairing method characterized in that for calculating the channel correlation for the terminal with the largest or smallest signal strength and the remaining terminals. The method of claim 11, The channel correlation may include at least one of an orthogonal value and an eigen ratio. A method for pairing terminals in an UL Uplink Multiple Input Multiple Output (MIMO) system, a) arranging Collaborative MIMO (C-MIMO) capable terminals based on a predetermined parameter; And b) pairing terminals based on the sorted order. The method of claim 14, Step b) is a terminal pairing method characterized in that the first priority terminal and the lowest priority terminal pairing. A method for pairing terminals in an UL Uplink Multiple Input Multiple Output (MIMO) system, a) arranging Collaborative MIMO (C-MIMO) capable terminals based on a predetermined parameter; b) calculating channel correlations for the highest priority terminal and the remaining terminals among the aligned terminals, respectively; And c) first pairing the terminal pair having the lowest channel correlation. The method of claim 16, The predetermined parameter is a terminal pairing method, characterized in that the signal strength of the terminal. The method of claim 16, The channel correlation may include at least one of an orthogonal value and an eigen ratio. A method for pairing terminals in an UL Uplink Multiple Input Multiple Output (MIMO) system, a) randomly selecting one terminal among C-MIMO (Collaborative MIMO) capable terminals; And b) pairing the terminal with the smallest power size difference; Step a) and step b) is the terminal pairing method characterized in that it is performed repeatedly.

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