KR20160002078A - Method and Apparatus of transmitting or receiving channel information of multi-user in a wireless communication system - Google Patents

Abstract

According to an embodiment of the present invention, a method for a base station supporting multi-user-multi-input multi-output (MU-MIMO) to feed back channel information of multiple users to a network entity comprises the following steps: estimating multiple channels between each antenna and each terminal based on signals of the terminals received through a plurality of antennas; projecting at least a part of channel estimation values of the multiple channels from a first space to a second space; and transmitting, to the network entity, feedback information including at least a part of the channel estimation values projected to the second space.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method and apparatus for transmitting or receiving channel information of multiple users in a wireless communication system,

The present invention relates to a method and apparatus for transmitting or receiving channel information of multiple users in a wireless communication system supporting MU-MIMO.

The multi-input multi-output (MIMO) technique is a technique for improving transmission / reception efficiency of data by using multiple transmit antennas and multiple receive antennas by avoiding the use of one transmit antenna and one receive antenna. With a single antenna, the receiver receives data through a single antenna path, but with multiple antennas, the receiver receives data over multiple paths. Therefore, the MIMO technique can improve the data transmission rate and transmission amount, and can increase the coverage. The single-cell MIMO operation is a single user-MIMO (SU-MIMO) scheme in which one UE receives a downlink signal in one cell and two or more UEs in a downlink MIMO (MU-MIMO) scheme for receiving link signals.

Meanwhile, studies on a Coordinated Multi-Point (CoMP) system for improving the throughput of a user at a cell boundary by applying improved MIMO transmission in a multi-cell environment have been actively conducted. CoMP system can reduce the inter-cell interference and the inter-user interference in a multi-cell environment and improve the overall system performance.

Channel estimation refers to a process of determining channel characteristics to compensate for signal distortion caused by fading or the like, and restoring a received signal using a channel model according to the determined channel characteristics. Here, fading refers to a phenomenon that the characteristics such as the size and phase of a signal vary due to a multi-path-time delay in a wireless communication system environment. A pre-negotiated reference signal (RS) may be used between the transmitter and the receiver for channel estimation. The reference signal may also be referred to as a pilot signal.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a method for efficiently feedbacking channel information of multiple users to a network entity in a multi-cell cooperative communication system based on MU-MIMO.

The technical problem to be solved by the present invention is not limited to the technical problems described above, and other technical problems can be deduced from the embodiments of the present invention.

According to an aspect of the present invention, a method of feeding back a channel information of multiple users to a network entity by a base station supporting MU-MIMO includes the steps of: determining, based on signals from terminals received through a plurality of antennas, Estimating multiple channels between each of the antennas and each of the terminals; Projecting at least some of the channel estimates of the multiple channels from a first space to a second space; And transmitting feedback information to the network entity, the feedback information including the at least some channel estimate values projected into the second space.

According to another aspect of the present invention, a method for receiving channel information of multiple users from a base station supporting a MU-MIMO by a network entity includes the steps of receiving, from a first base station among the base stations, And receiving first feedback information including first channel estimates between terminals; And receiving second feedback information from a second one of the base stations, the second feedback information comprising second channel estimates between the antennas of the second base station and the terminals, wherein the first channel estimates and the second At least some of the channel estimates are projected from the first space to the second space.

According to another aspect of the present invention, a base station for feeding channel information of multiple users to a network entity includes: a plurality of antennas for MU-MIMO; A radio frequency (RF) interface for transmitting and receiving signals to and from the terminals via the plurality of antennas; Estimating multiple channels between each of the antennas and each of the terminals based on signals received from the terminals and transmitting at least some of the channel estimates of the multiple channels from the first space to the second space A processor for projecting; And a backhaul interface for transmitting feedback information including the at least some of the channel estimate values projected to the second space to the network entity under control of the processor.

According to another aspect of the present invention, a network entity for receiving channel information of multiple users from base stations supporting MU-MIMO comprises: a processor; And receiving first feedback information comprising first channel estimates between antennas of the first base station and terminals from a first one of the base stations in accordance with the control of the processor, And a backhaul interface for receiving second feedback information comprising second channel estimates between the antennas of the second base station and the terminals, wherein at least one of the first channel estimates and the second channel estimates And some are projected from the first space to the second space.

According to an embodiment of the present invention, by projecting channel information estimated by the base station to multiple users, it is possible to reduce the size of the feedback information transmitted to the network entity and reduce the backhaul overhead.

The effects of the present invention are not limited to the effects described above, and other effects can be deduced from the embodiments of the present invention.

1 is a diagram illustrating a structure of a downlink radio frame.
2 is an exemplary diagram illustrating an example of a resource grid for one downlink slot.
3 is a diagram showing a structure of a downlink sub-frame.
4 is a diagram illustrating the structure of an uplink subframe.
5 is a configuration diagram of a wireless communication system having multiple antennas.
6 is a diagram illustrating a cooperative communication (CoMP) system of multiple cells based on Massive MIMO according to an embodiment of the present invention.
7 is a flowchart illustrating a method of transmitting or receiving channel information according to an embodiment of the present invention.
8 is a flowchart illustrating a method of feedback of channel information by a base station according to an embodiment of the present invention.
9 is a flowchart illustrating a method for a network entity to receive channel information from a base station according to an embodiment of the present invention.
10 is a diagram illustrating a structure of feedback information according to an embodiment of the present invention.
11 is a diagram illustrating a terminal according to an embodiment of the present invention.
12 is a diagram illustrating a base station according to an embodiment of the present invention.
13 is a diagram illustrating a network entity according to an embodiment of the present invention.
FIG. 14 is a diagram illustrating a terminal and a base station according to another embodiment of the present invention.

The following embodiments are a combination of elements and features of the present invention in a predetermined form. Each component or characteristic may be considered optional unless otherwise expressly stated. Each component or feature may be implemented in a form that is not combined with other components or features. In addition, some of the elements and / or features may be combined to form an embodiment of the present invention. The order of the operations described in the embodiments of the present invention may be changed. Some configurations or features of certain embodiments may be included in other embodiments, or may be replaced with corresponding configurations or features of other embodiments.

A base station has a meaning as a terminal node of a network that directly communicates with a terminal. The specific operation described herein as performed by the base station may be performed by an upper node of the base station, as the case may be.

That is, it is apparent that various operations performed for communication with a terminal in a network composed of a plurality of network nodes including a base station can be performed by a network node other than the base station or the base station. A 'base station (BS)' may be replaced by a term such as a fixed station, a Node B, an eNode B (eNB), an access point (AP) Repeaters can be replaced by terms such as Relay Node (RN), Relay Station (RS), and so on. The term 'terminal' may be replaced with terms such as User Equipment (UE), Mobile Station (MS), Mobile Subscriber Station (MSS), and Subscriber Station (SS).

The specific terminology used in the following description is provided to aid understanding of the present invention, and the use of such specific terminology may be changed into other forms without departing from the technical idea of the present invention.

In some instances, well-known structures and devices may be omitted or may be shown in block diagram form, centering on the core functionality of each structure and device, to avoid obscuring the concepts of the present invention. In the following description, the same components are denoted by the same reference numerals throughout the specification.

Embodiments of the present invention may be supported by standard documents disclosed in at least one of the IEEE 802 systems, 3GPP systems, 3GPP LTE and LTE-Advanced (LTE-Advanced) systems, and 3GPP2 systems, which are wireless access systems. That is, the steps or portions of the embodiments of the present invention that are not described in order to clearly illustrate the technical idea of the present invention can be supported by the documents. In addition, all terms disclosed in this document may be described by the standard document.

The following description will be made on the assumption that the present invention is applicable to a CDMA system such as Code Division Multiple Access (CDMA), Frequency Division Multiple Access (FDMA), Time Division Multiple Access (TDMA), Orthogonal Frequency Division Multiple Access (OFDMA), and Single Carrier Frequency Division Multiple Access And can be used in various wireless access systems. CDMA may be implemented in radio technology such as Universal Terrestrial Radio Access (UTRA) or CDMA2000. The TDMA may be implemented in a wireless technology such as Global System for Mobile communications (GSM) / General Packet Radio Service (GPRS) / Enhanced Data Rates for GSM Evolution (EDGE). OFDMA may be implemented in wireless technologies such as IEEE 802.11 (Wi-Fi), IEEE 802.16 (WiMAX), IEEE 802-20, and Evolved UTRA (E-UTRA). UTRA is part of the Universal Mobile Telecommunications System (UMTS). 3GPP (3rd Generation Partnership Project) LTE (Long Term Evolution) is a part of E-UMTS (Evolved UMTS) using E-UTRA, adopting OFDMA in downlink and SC-FDMA in uplink. LTE-A (Advanced) is the evolution of 3GPP LTE. WiMAX can be described by the IEEE 802.16e standard (WirelessMAN-OFDMA Reference System) and the advanced IEEE 802.16m standard (WirelessMAN-OFDMA Advanced system). For clarity, the following description will be made with reference to 3GPP LTE and LTE-A standards, but the technical idea of the present invention is not limited thereto.

The structure of the radio frame will be described with reference to Fig.

In a cellular OFDM wireless packet communication system, uplink / downlink data packet transmission is performed on a subframe basis, and one subframe is defined as a predetermined time interval including a plurality of OFDM symbols. The 3GPP LTE standard supports a Type 1 radio frame structure applicable to Frequency Division Duplex (FDD) and a Type 2 radio frame structure applicable to TDD (Time Division Duplex).

Fig. 1 (a) shows the structure of a type 1 radio frame. A downlink radio frame is composed of 10 subframes, and one subframe is composed of two slots. The time taken for one subframe to be transmitted is referred to as a transmission time interval (TTI). For example, the length of one subframe may be 1 ms and the length of one slot may be 0.5 ms. One slot includes a plurality of OFDM symbols in a time domain and a plurality of resource blocks (RBs) in a frequency domain.

The number of OFDM symbols included in one slot may vary according to the configuration of the CP. The CP has an extended CP and a normal CP. For example, when an OFDM symbol is configured by a general CP, the number of OFDM symbols included in one slot may be seven. When the OFDM symbol is configured by an extended CP, since the length of one OFDM symbol is increased, the number of OFDM symbols included in one slot is smaller than that of a normal CP. In the case of the extended CP, for example, the number of OFDM symbols included in one slot may be six. If the channel condition is unstable, such as when the UE moves at a high speed, an extended CP may be used to further reduce inter-symbol interference.

When a normal CP is used, one slot includes 7 OFDM symbols, and therefore one subframe includes 14 OFDM symbols. At this time, the first two or three OFDM symbols of each subframe may be allocated to a physical downlink control channel (PDCCH), and the remaining OFDM symbols may be allocated to a physical downlink shared channel (PDSCH).

Fig. 1 (b) shows the structure of a Type 2 radio frame. The Type 2 radio frame is composed of two half frames, and each half frame is composed of five subframes. The subframes can be classified into a general subframe and a special subframe. The special subframe is a subframe including three fields of a Downlink Pilot Time Slot (DwPTS), a Gap Period (GP), and an Uplink Pilot Time Slot (UpPTS). The lengths of these three fields can be set individually, but the total length of the three fields should be 1 ms. One subframe consists of two slots. That is, one subframe consists of two slots regardless of the type of radio frame.

The structure of the radio frame is merely an example, and the number of subframes included in a radio frame, the number of slots included in a subframe, and the number of symbols included in a slot can be variously changed.

2 is an exemplary diagram illustrating an example of a resource grid for one downlink slot. This is the case where the OFDM symbol is composed of a general CP. Referring to FIG. 2, a downlink slot includes a plurality of OFDM symbols in a time domain and a plurality of resource blocks in a frequency domain. Herein, one downlink slot includes 7 OFDM symbols, and one resource block includes 12 subcarriers by way of example, but the present invention is not limited thereto. Each element on the resource grid is called a resource element (RE). For example, the resource element a (k, l) is a resource element located in the kth subcarrier and the lth OFDM symbol. In the case of a generic CP, one resource block contains 12 x 7 resource elements. In the case of an extended CP, 12 × 6 resource elements are included. Since the interval of each subcarrier is 15 kHz, one resource block includes about 180 kHz in the frequency domain. The NDL is the number of resource blocks included in the downlink slot. The value of the NDL may be determined according to the downlink transmission bandwidth set by the scheduling of the base station.

3 is a diagram showing a structure of a downlink sub-frame. In a subframe, a maximum of three OFDM symbols in the first part of the first slot corresponds to a control area to which a control channel is allocated. The remaining OFDM symbols correspond to a data area to which a Physical Downlink Shared Chanel (PDSCH) is allocated. The basic unit of transmission is one subframe. That is, PDCCH and PDSCH are allocated over two slots. The downlink control channels used in the 3GPP LTE system include, for example, a physical control format indicator channel (PCFICH), a physical downlink control channel (PDCCH), a physical HARQ indicator channel (Physical Hybrid Automatic Repeat Request Indicator Channel (PHICH)). The PCFICH includes information on the number of OFDM symbols transmitted in the first OFDM symbol of the subframe and used for control channel transmission in the subframe. The PHICH includes an HARQ ACK / NACK signal as a response to the uplink transmission. The control information transmitted through the PDCCH is referred to as downlink control information (DCI). The DCI includes uplink or downlink scheduling information or includes an uplink transmission power control command for an arbitrary terminal group. The PDCCH includes a resource allocation and transmission format of a downlink shared channel (DL-SCH), resource allocation information of an uplink shared channel (UL-SCH), paging information of a paging channel (PCH), system information on a DL- A set of transmission power control commands for individual terminals in an arbitrary terminal group, transmission power control information, activation of VoIP (Voice over IP), resource allocation of upper layer control messages such as random access response And the like. A plurality of PDCCHs may be transmitted within the control domain. The UE can monitor a plurality of PDCCHs. The PDCCH is transmitted in a combination of one or more contiguous Control Channel Elements (CCEs). The CCE is a logical allocation unit used to provide the PDCCH with a coding rate based on the state of the wireless channel. The CCE corresponds to a plurality of resource element groups. The format of the PDCCH and the number of available bits are determined according to the correlation between the number of CCEs and the coding rate provided by the CCE. The base station determines the PDCCH format according to the DCI transmitted to the UE and adds a cyclic redundancy check (CRC) to the control information. The CRC is masked with an identifier called a Radio Network Temporary Identifier (RNTI) according to the owner or use of the PDCCH. If the PDCCH is for a particular UE, the cell-RNTI (C-RNTI) identifier of the UE may be masked in the CRC. Alternatively, if the PDCCH is for a paging message, a Paging Indicator Identifier (P-RNTI) may be masked in the CRC. If the PDCCH is for system information (more specifically, the System Information Block (SIB)), the system information identifier and the system information RNTI (SI-RNTI) may be masked to the CRC. A random access-RNTI (RA-RNTI) may be masked to the CRC to indicate a random access response that is a response to the transmission of the UE's random access preamble.

4 is a diagram illustrating the structure of an uplink subframe. The UL subframe may be divided into a control region and a data region in the frequency domain. A physical uplink control channel (PUCCH) including uplink control information is allocated to the control region. A physical uplink shared channel (PUSCH) including user data is allocated to the data area. In order to maintain a single carrier characteristic, one UE can selectively transmit a PUCCH and a PUSCH. A PUCCH for one terminal is allocated to a resource block pair (RB pair) in a subframe. Resource blocks belonging to a resource block pair occupy different subcarriers for two slots. It is assumed that the resource block pair allocated to the PUCCH is frequency-hopped at the slot boundary.

Coordinated Multi-Point (CoMP)

For the 3GPP LTE-A system, CoMP is supported through transmission mode 10. According to transmission mode 10, multiple CSI (Channel State Information) processes can be set in the UE. CoMP techniques may be expressed as co-MIMO, collaborative MIMO, or network MIMO. The CoMP technique can increase the performance of the UE located at the cell-edge and increase the average sector throughput.

Generally, in a multi-cell environment with a frequency reuse factor of 1, the performance of a UE located at a cell boundary due to inter-cell interference (ICI) and average sector yield may be reduced have. In order to reduce such ICI, in a conventional LTE system, a simple passive technique such as fractional frequency reuse (FFR) through UE-specific power control is used, A method of allowing the terminal to have a proper yield performance has been applied. However, rather than lowering the frequency resource usage per cell, it may be more desirable to reduce the ICI or reuse the ICI as the desired signal. In order to achieve the above object, the CoMP technique can be applied.

CoMP techniques that can be applied in the case of downlink can be roughly classified into a joint-processing (JP) technique and a coordinated scheduling / beamforming (CS / CB) technique.

The JP technique can utilize the data at each point (base station) of the CoMP cooperating unit. The CoMP cooperative unit refers to a set of base stations used in the cooperative transmission scheme. The JP method can be classified into Joint Transmission method and Dynamic cell selection method.

The joint transmission scheme refers to a scheme in which the PDSCH is transmitted from a plurality of points (a part or all of CoMP cooperation units) at one time. That is, data transmitted to a single terminal can be simultaneously transmitted from a plurality of transmission points. According to the joint transmission scheme, the quality of a coherently or non-coherently received signal may be improved and also actively cancel interference to other terminals.

The dynamic cell selection scheme refers to a scheme in which the PDSCH is transmitted from one point (at CoMP cooperation unit) at a time. That is, data transmitted to a single terminal at a specific time point is transmitted from one point, and other points in the cooperating unit at that point do not transmit data to the terminal, and points for transmitting data to the terminal are dynamically selected .

Meanwhile, according to the CS / CB scheme, CoMP cooperation units can cooperatively perform beamforming of data transmission to a single terminal. Here, the data is transmitted only in the serving cell, but the user scheduling / beamforming can be determined by adjusting the cells of the CoMP cooperation unit.

On the other hand, in the case of an uplink, coordinated multi-point reception means receiving a signal transmitted by coordination of a plurality of points that are geographically separated. The CoMP scheme that can be applied in the case of uplink can be classified into Joint Reception (JR) and coordinated scheduling / beamforming (CS / CB).

The JR scheme means that a signal transmitted through the PUSCH is received at a plurality of reception points. In the CS / CB scheme, the PUSCH is received at only one point, but the user scheduling / beamforming is determined by adjustment of cells in CoMP cooperation unit .

Modeling of Multi-antenna (MIMO) Systems

MIMO (Multiple Input Multiple Output) system is a system that improves data transmission / reception efficiency by using multiple transmit antennas and multiple receive antennas. The MIMO technique can receive the entire data by combining a plurality of pieces of data received through a plurality of antennas without depending on a single antenna path to receive the entire message.

The MIMO technique includes a spatial diversity technique and a spatial multiplexing technique. The spatial diversity scheme can increase transmission reliability or diversity gain through diversity gain and is suitable for data transmission to a terminal moving at a high speed. Spatial multiplexing can increase the data rate without increasing the bandwidth of the system by transmitting different data at the same time.

5 is a configuration diagram of a wireless communication system having multiple antennas. As shown in FIG. 5A, when the number of transmission antennas is increased to NT and the number of reception antennas is increased to NR, unlike the case where a plurality of antennas are used only in a transmitter and a receiver, a theoretical channel transmission Capacity increases. Thus, MIMO can improve the transmission rate and improve the frequency efficiency. As the channel transmission capacity increases, the transmission rate can theoretically increase by the rate of rate increase Ri multiplied by the maximum transmission rate Ro in single antenna use.

[ Equation 1 ]

For example, in a MIMO communication system using four transmit antennas and four receive antennas, it is possible to obtain a transmission rate four times higher than the single antenna system.

A communication method in a multi-antenna system will be described in more detail using mathematical modeling. It is assumed that there are NT transmit antennas and NR receive antennas in the system.

Looking at the transmitted signal, if there are NT transmit antennas, the maximum transmittable information is NT. The transmission information can be expressed as follows.

& Quot; (2 ) & quot ;

는 전송 전력이 다를 수 있다. 각각의 전송 전력을

라고 하면, 전송 전력이 조정된 전송 정보는 다음과 같이 표현될 수 있다.Each transmission information

The transmission power may be different. Each transmission power

, The transmission information whose transmission power is adjusted can be expressed as follows.

& Quot; (3 ) & quot ;

는 전송 전력의 대각행렬

를 이용해 다음과 같이 표현될 수 있다.Also,

Is a diagonal matrix of transmit power

Can be expressed as follows.

& Quot; (4 ) & quot ;

에 가중치 행렬

가 적용되어 실제 전송되는 NT개의 송신신호

가 구성되는 경우를 고려해 보자. 가중치 행렬

는 전송 정보를 전송 채널 상황 등에 따라 각 안테나에 적절히 분배해 주는 역할을 한다.

는 벡터

를 이용하여 다음과 같이 표현될 수 있다.An information vector whose transmission power is adjusted,

A weighting matrix

Lt; RTI ID = 0.0 > NT < / RTI > transmitted signals

. Weighting matrix

Which distributes the transmission information to each antenna according to the transmission channel condition and the like.

Vector

Can be expressed as follows.

& Quot; (5 ) & quot ;

는 i번째 송신 안테나와 j번째 정보간의 가중치를 의미한다.

는 프리코딩 행렬이라고도 불린다.From here,

Denotes a weight between the i-th transmit antenna and the j-th information.

Is also referred to as a precoding matrix.

On the other hand, the transmission signal x can be considered in different ways depending on two cases (for example, spatial diversity and spatial multiplexing). In the case of spatial multiplexing, different signals are multiplexed and the multiplexed signal is transmitted to the receiving side, so that the elements of the information vector (s) have different values. On the other hand, in the case of spatial diversity, the same signal is repeatedly transmitted through a plurality of channel paths, so that the elements of the information vector (s) have the same value. Of course, a combination of spatial multiplexing and spatial diversity techniques may also be considered. That is, the same signal may be transmitted according to a spatial diversity scheme, for example, through three transmission antennas, and the remaining signals may be spatially multiplexed and transmitted to the receiving side.

은 벡터로 다음과 같이 표현될 수 있다.If NR receive antennas are present,

Can be expressed as a vector as follows.

& Quot; (6 ) & quot ;

로 표시하기로 한다.

에서, 인덱스의 순서가 수신 안테나 인덱스가 먼저, 송신 안테나의 인덱스가 나중임에 유의한다.When a channel is modeled in a multi-antenna wireless communication system, the channel may be classified according to the transmission / reception antenna index. The channel passing through the receiving antenna i from the transmitting antenna j

.

, It is noted that the order of the index is the reception antenna index, and the index of the transmission antenna is the order of the index.

FIG. 5 (b) shows a channel from NT transmit antennas to receive antennas i. The channels can be grouped and displayed in vector and matrix form. In FIG. 5 (b), a channel arriving from a total of NT transmit antennas to receive antennas i may be expressed as follows.

& Quot; (7 ) & quot ;

Therefore, all channels arriving from the NT transmit antennas to the NR receive antennas can be expressed as:

& Quot; (8 ) & quot ;

를 거친 후에 백색잡음(AWGN; Additive White Gaussian Noise)이 더해진다. NR 개의 수신 안테나 각각에 더해지는 백색잡음

은 다음과 같이 표현될 수 있다.The actual channel includes a channel matrix

And additive white Gaussian noise (AWGN) is added. White noise added to each NR receive antenna

Can be expressed as follows.

& Quot; (9 ) & quot ;

Through the above-described equation modeling, the received signal can be expressed as follows.

& Quot; (10 ) & quot ;

의 행과 열의 수는 송수신 안테나의 수에 의해 결정된다. 채널 행렬

에서 행의 수는 수신 안테나의 수 NR과 같고, 열의 수는 송신 안테나의 수 NT와 같다. 즉, 채널 행렬

는 행렬이 NR×NT된다.A channel matrix representing the channel state

The number of rows and columns of the antenna is determined by the number of transmitting and receiving antennas. Channel matrix

The number of rows is equal to the number NR of receiving antennas, and the number of columns is equal to the number of transmitting antennas NT. That is,

The matrix is NR x NT.

의 랭크(

)는 다음과 같이 제한된다.The rank of a matrix is defined as the minimum number of independent rows or columns. Thus, the rank of the matrix can not be greater than the number of rows or columns. Channel matrix

Rank of

) Is limited as follows.

& Quot; (11 ) & quot ;

In MIMO transmission, 'rank' represents the number of paths that can independently transmit signals, and 'number of layers' represents the number of signal streams transmitted through each path. In general, since the transmitting end transmits a number of layers corresponding to the number of ranks used for signal transmission, the rank has the same meaning as the number of layers unless otherwise specified.

The massive MIMO

Massive MIMO is a method of integrating more antennas in a conventional antenna array, placing antennas in a space for obtaining a directional radiation pattern, pencil beamforming by mounting several hundred antennas at a base station, It is the same performance as a single large antenna with many small antennas in one array. Due to the integration of several (e.g., hundreds) of antennas, the mechanical problem of a single large antenna can be solved by the electrical problem of feeding a small antenna.

The massive MIMO can transmit a narrow beam to a large number of users using spatial freedom of multiple antennas, thereby providing high received signal power and low interference power. However, in order to transmit with a narrow beam, channel information between a user and a base station must be completely known. If the channel information is not accurate, the user can not provide a beam with a narrow width, so that the received signal power is lowered and intercell interference is caused to other users.

Therefore, it is essential for a base station to properly control inter-cell interference in order for a cellular system using massive MIMO to provide high frequency efficiency.

According to an embodiment of the present invention, a CoMP technique is used to handle such interference. According to CoMP techniques applied to a massive MIMO system, a plurality of neighboring base stations transmit channel information to a network entity. Here, the network entity may be any network node in the cellular network. For example, the network entity may be one of the base stations operating according to the CoMP technique, or may be a node that controls base stations in the core network, but is not limited thereto. The network entity may create a coordinated transmit and receive precoder and remove the inter-cell interference and the inter-user interference using the cooperated transmit and receive precoders.

Meanwhile, each base station must feedback channel information of each user to a network entity. Since the amount of channel information to be fed back is proportional to the number of antennas and the number of users, the channel information to be fed back through the backhaul is very large in a system using a massive MU-MIMO.

Massive MIMO is used for transmission and reception using the following favorable propagation.

& Quot; (11 ) & quot ;

Where M is the number of antennas of the base station and K is the number of terminals (users). For convenience of explanation, the number of antennas of each terminal is assumed to be 1, but is not limited thereto.

은 K×M 행렬로 ㅣ번째 기지국의 각 안테나들과 각 단말들간의 채널을 표현한다. 만약 단말의 안테나 개수가 N개라면,

은 K×MN 행렬이 됨을 당업자라면 이해할 수 있다.

은 K×K 대각행렬로 (k,k)번째 엘리먼트가 β _lk 이다. β _lk 는 l번째 기지국과 k번째 단말 간의 롱텀(long-term) 페이딩 성분 또는 평균 채널 파워를 의미한다. 참고로,

은 기지국의 안테나들과 단말들 간의 숏텀(short-term) 페이딩 성분과 롱텀(long-term) 페이딩 성분을 모두 포함한다.

Represents a channel between each of the antennas of each base station and each of the terminals using a K × M matrix. If the number of antennas of the UE is N,

Lt; RTI ID = 0.0 > KxNM < / RTI > matrix.

Is a K × K diagonal matrix with the (k, k) th element being β _lk . β _lk denotes a long-term fading component or average channel power between the l- th base station and the k-th UE. Note that,

Includes both a short-term fading component and a long-term fading component between the antennas and the terminals of the base station.

Since the long-term fading component is determined by path attenuation, shadowing, and antenna characteristics due to the distance between the base station and the terminal, the long-term fading component changes over a relatively long time. On the other hand, the short-time fading component changes over a relatively short period of time to a portion affected by the cancellation or constructive interference of the signal. In other words, the coherence time of the long-term fading component is longer than the coherence time of the short-term fading component.

를 전달하여야 한다. 코드북 기반의 제한된 피드백 (limited feedback) 또는 아날로그 피드백 (analog feedback) 등의 방식이 사용될 수 있다. 제한된 피드백 방식을 사용하면, 채널 행렬

의 크기는 단말들의 개수와 기지국의 안테나들의 개수에 비례하므로 필요한 피드백 정보의 비트 수도 증가시켜야 한다. 따라서, 매시브 MIMO를 사용하는 시스템의 경우 제한된 채널 상관 시간 안에 채널 행렬

을 피드백 하기 어렵거나, 적은 비트로 채널 행렬

을 표현해야 하므로 추정된 채널 행렬에 대한 열화가 발생한다. 이와 같은 문제점은 아날로그 피드백을 사용해도 동일하게 나타난다.For cooperative transmission and reception by the CoMP technique, each base station transmits to the network entity a channel matrix

. Limited feedback based on a codebook or analog feedback may be used. Using a limited feedback scheme, the channel matrix

Is proportional to the number of UEs and the number of antennas of the Node B, it is necessary to increase the number of bits of the necessary feedback information. Therefore, in the system using the massive MIMO, the channel matrix

Is difficult to be fed back, or the channel matrix

So that deterioration of the estimated channel matrix occurs. The same problem can be avoided by using analog feedback.

를 소정의 벡터 공간으로 프로젝션 (projection) 시키면, K개의 기지국들과 단말들 간의 롱텀 페이딩 또는 평균 채널 파워 성분으로 구성된 대각행렬

이 획득된다. 따라서 KM개의 복소수 엘리먼트들로 구성된 채널 행렬

를 피드백하는 것 보다, K개의 실수 엘리먼트들로 구성된 대각행렬

를 피드백하면, 피드백에 의한 백홀의 오버헤드가 감소될 수 있다. 특히, 대각행렬

의 크기는 기지국의 안테나들의 개수 M과는 독립적이므로, 매시브 MIMO를 사용하는 협력 송수신에 적합하다.However, when the antenna of the base station is very large, by using the Favorable propagation characteristic of Equation (11), the estimated channel information can be fed back more effectively. In Equation (11), the channel matrix

Is projected to a predetermined vector space, a diagonal matrix composed of long-term fading or average channel power components between K base stations and terminals

Is obtained. Therefore, a channel matrix composed of KM complex elements

Than a diagonal matrix of K real elements

The overhead of the backhaul due to the feedback can be reduced. In particular,

Is independent of the number M of antennas of the base station, and thus is suitable for cooperative transmission and reception using a massive MIMO.

Massive MIMO-based cooperative communication system

Figure 6 shows a cooperative communication (CoMP) system of multiple cells based on Massive MIMO. The system of FIG. 6 includes a plurality of base stations BS1 to BS4, a plurality of terminals UE1 to UE5, and a network entity. According to FIG. 6, the network entity is shown as a separate node for centralizing the feedback information gathered from the base stations. According to other embodiments of the present invention, the network entity may be implemented as a base station or any node of the core network.

Each base station has at least one serving cell. If carrier aggregation is set, multiple serving cells may be set in one base station. For example, one primary cell and one or more secondary cells may be set in one base station.

The number of antennas of the base stations and the number of antennas of the terminals may be different from each other. For convenience, it is assumed that the base station has M (M is a natural number equal to or greater than 1) antennas, and the user has one antenna. However, since this assumption is only one embodiment of the present invention, the number of antennas of the terminals can be changed.

로 표현될 수 있다.The number of base stations participating in cooperative communication is not limited, and it is assumed that L (L is a natural number equal to or greater than 1) base stations participates in the present embodiment. The number of terminals participating in the cooperative communication should be smaller than the LM. In this embodiment, it is assumed that K (K is a natural number equal to or greater than 1) terminals participate in cooperative communication. The downlink channel between the 1 < th > base station and the k < th >

. ≪ / RTI >

& Quot; (12 ) & quot ;

은 l번째 기지국과 k번째 단말 간의 숏텀 페이딩(short-term fading) 성분이다. 본 발명에서는 설명의 편의를 위해서 숏텀 페이딩 성분이 T개의 타임 슬롯 동안 변화하지 않는다고 가정한다. 롱텀 페이딩 성분은 숏텀 페이딩 성분보다 수십 내지 수백 배 더 긴 시간 동안 변화하지 않는다고 가정한다. 타임 슬롯은 3GPP LTE-A 표준의 리소스 블록 (resource block)일 수 있으며 이에 한정되지 않는다.Where beta _lk is the average received power or long-term fading component between the l < th > base station and the k <

Is a short-term fading component between the l- th base station and the k-th terminal. For convenience of explanation, it is assumed that the short-time fading component does not change during T time slots in the present invention. It is assumed that the longtop fading component does not change over a period of several tens to hundreds of times longer than the short term fading component. The time slot may be a resource block of the 3GPP LTE-A standard and is not limited thereto.

Each base station is connected to a network entity via a backhaul network. The backhaul network may be a network substantially free of transmission capacity since it has a considerably larger transmission capacity than a wireless channel, or conversely, there may be a network with a limited transmission capacity. If the transmission capacity is not limited, it is possible to transmit feedback information between the base station and the network entity without data compression or data loss. If the transmission capacity is limited, the feedback information transmitted between the base station and the network entity may deteriorate due to data compression or the like. The compression and loss of this data is an important part of the network design because it has a significant impact on the transmission capacity. The following embodiments will be described mainly on the case where the transmission capacity of the backhaul network is limited.

A reference signal (RS)

When a packet is transmitted in a wireless communication system, since the transmitted packet is transmitted through a wireless channel, signal distortion may occur in the transmission process. In order to properly receive the distorted signal at the receiving side, the distortion should be corrected in the received signal using the channel information. To know the channel information, a signal known to both the transmitting side and the receiving side is transmitted, and the channel information is estimated using the degree of distortion when the signal is received through the channel. Such a signal is referred to as a pilot signal or a reference signal.

When transmitting and receiving data using multiple antennas, it is necessary to know the channel condition between each transmitting antenna and the receiving antenna so that a correct signal can be received. Therefore, there is a separate reference signal for each transmission antenna.

In the mobile communication system, the reference signal RS can be roughly classified into two types according to its purpose. One is an RS used for channel information acquisition and the other is an RS used for data demodulation. The RS for acquiring channel information needs to be transmitted in a wide band, and periodically transmitted even if there is no data transmission / reception. The RS for acquiring channel information is also used for measurement for handover and the like. An RS for data demodulation is an RS that transmits data through a data area when transmitting data, and a receiving side can demodulate data by receiving a corresponding RS.

라고 하면, 단말 k에 의해 전송된 신호는 다음과 같다.For example, the sequence of the RS corresponding to the terminal k is

, The signal transmitted by the terminal k is as follows.

& Quot; (13 ) & quot ;

는 상향링크 송신 전력, T ^UL은 RS에 전송에 할당된 시간 자원의 길이(예컨대, 3GPP LTE의 경우 OFDMA 심볼의 개수), π _k 는 단말 k에 대응하는 RS의 인덱스이다. 인덱스 π _k 가 서로 상이한 RS들은 서로 직교하는 특성을 갖을 수 있다.here

T ^UL is the length of time resource allocated to transmission to the RS (e.g., the number of OFDMA symbols in case of 3GPP LTE), and ? _K is the index of the RS corresponding to the terminal k. The RSs whose indexes [ pi] _k differ from each other can have mutually orthogonal characteristics.

For example, when the RS has transmitted the terminal k has been received by the base station l, the RS receives from the base station l can be expressed by equation (14).

& Quot; (14 ) & quot ;

V _l is an M × T ^UL- sized matrix, which represents uplink noise.

Estimation of channel information

The base station receiving the RS signal can estimate the channel information based on the received RS. For example, the channel information obtained using a least square (LS) channel estimation scheme is represented by Equation (15).

& Quot; (15 ) & quot ;

는 M개의 채널 추정값들을 포함하는 M차원의 벡터로서, l번째 기지국과 k번째 단말 간의 실제 하향링크 채널

에 대한 채널 추정 벡터이다. J _j 는 인덱스가 j인 RS를 전송한 단말들의 집합,

는 잡음 성분이다.

Is an M-dimensional vector including M channel estimates, and is a vector of the actual downlink channel between the l < th > base station and the k <

/ RTI > J _j is a set of terminals transmitting an RS having an index j,

Is a noise component.

Other channel estimation schemes other than the least square based channel estimation scheme described above can be used and the scope of the present invention is not limited to the type of channel estimation scheme used.

The projection of the estimated channel information

The estimated channel information may be projected into another vector space using a projection matrix. The term "projection" is used to encompass, for example, converting, substituting, compressing, coding, or mapping values of a first space (or a first domain) to values of a second space (second domain) May be replaced by other terms having meaning.

Since the estimated channel information is present in the vector space of the complex, the channel estimation values of the non-projected channel information have complex values. The estimated channel information can be projected into a real vector space. When projected into a real vector space, complex channel estimates are mapped to real values.

라고 할 때, 프로젝션된 채널 정보

는 수학식 16과 같이 정의 될 수 있다.The projection matrix used by the base station l

, The projected channel information

Can be defined as shown in Equation (16).

& Quot; (16 ) & quot ;

는 기지국 l과 K개의 단말들간의 채널 정보(또는 채널 행렬)이다. 기지국이 사용할 수 있는 다수의 프로젝션 방식들이 미리 정의되어 있을 수 있다. 사용가능한 프로젝션 방식들을 모은 집합은 수학식 17과 같이 표현 될 수 있다.here

(Or channel matrix) between the base station 1 and the K terminals. A number of projection schemes available to the base station may be predefined. A set of available projection schemes can be expressed as Equation (17).

& Quot; (17 ) & quot ;

For example, a normalized matched filter may be used as a projection filter in one embodiment of the projection system. The projection filter of the base station 1 when the normalized matched filter is used is shown in Equation (18).

& Quot; (18 ) & quot ;

은 추정된 채널 정보의 에르미트 행렬이다.In Equation 18,

Is an Hermitian matrix of estimated channel information.

According to Equation (16) and Equation (18), the result of projecting one channel estimation vector among the channel information of the terminal k and the base station 1 is expressed by Equation (19).

& Quot; (19 ) & quot ;

는 프로젝션된 채널 추정 벡터로서, k와 i는 각각 단말의 인덱스이고, l은 기지국의 인덱스이다. 만약, 단말들의 총 개수가 K 라면, 프로젝션된 채널 추정 값들은 총 K X K 행렬의 엘리먼트들에 대응한다. 따라서, 프로젝션된 채널 추정값들의 개수는 단말들의 개수 K에만 의존적이며, 기지국의 안테나 개수 M에 독립적이다.

Where k and i are the index of the terminal, and l is the index of the base station. If the total number of terminals is K, then the projected channel estimate values correspond to the elements of the total KXK matrix. Thus, the number of projected channel estimates depends only on the number K of terminals and is independent of the number M of antennas of the base station.

는 수학식 20과 같이 간주될 수 있다.On the other hand, when M is sufficiently large in the Massive MIMO,

Can be regarded as Equation (20).

& Quot; (20 ) & quot ;

According to Equation (20), the projected channel estimation vector is expressed by the uplink transmission power and the long-term fading component of the terminal i and the terminal k. For example, it can be understood that the short-fading component is removed.

개의 0이 아닌 실수 값과

개의 0을 포함한다. 참고로 프로젝션되지 않은 하나의 채널 추정 벡터는, M개 복소수의 채널 추정값들을 포함한다.Further, according to Equation (20), the projected one channel estimation vector is composed of K real values,

Non-zero real numbers and

0 < / RTI > For reference, one channel estimation vector that is not projected includes channel estimates of M complex numbers.

For example, in another embodiment of the projection system, a zero-forcing filter can be used as a projection filter. The projection filter of the base station 1 in the case where the zero-forcing filter is used is represented by the following expression (21).

& Quot; (21 ) & quot ;

If M in Massive MIMO is sufficiently large, the result of projecting one channel estimation vector among channel information (or channel matrix) of terminal k and base station 1 on the basis of Equation (16) and Equation (21) is expressed by Equation (22).

& Quot; (22 ) & quot ;

개의 값이 1,

는 값이 0이다.According to Equation (22), the values of the projected channel estimation vector are total K,

If the value of 1 is 1,

Has a value of zero.

Since the above-mentioned projection filters are examples of projection methods applicable to the present invention, other projection filters can be used.

As explained, since the projected channel estimation vector is composed of a total of K real values, the data size decreases as compared to the complex values of the non-projected channel estimation vector M (M > K).

In particular, when different RS indices are assigned to terminals, RSs are orthogonal to each other, so that the projected channel information (or channel matrix) can be composed of a diagonal matrix. Thus, the projected channel information can be represented by only K non-zero elements. Thus, the base station may feed back K real elements to the network instead of feeding back the KM complex elements to the network.

Operation and structure of base station (BS), network entity (NE) or terminal (UE)

In the following description of the operation and structure of the base station, the network entity, and the terminal, descriptions overlapping with the above description are omitted.

7 is a flowchart illustrating a method of transmitting or receiving channel information according to an embodiment of the present invention. 7 includes a plurality of terminals and a plurality of base stations and one network entity. However, this is for convenience of description, and the number of terminals, base stations, and network entities may be changed to one or more.

The UEs measure the average power of the signal received in the downlink (705), and feed back the measured average power to the base stations (710). For example, BSs can transmit downlink RSs to UEs, and UEs can measure RSRP (Reference Signal Received Received Strength) of downlink RSs received from BSs. The terminals can feed back the measured RSRP to the base station. In this case, the BSs can estimate the downlink channel based on the average power received from the UEs. Meanwhile, in the case of the TDD system, the base station can estimate the uplink channel based on the downlink channel estimation. Meanwhile, in order to estimate the uplink channel of the base station, the terminals can transmit the uplink RSs to the base stations (715).

The BS estimates an uplink channel based on the uplink RS (720). If there are K terminals and there are M antennas in one base station, each base station estimates KM channels. If the number of antennas of the UE is N, each base station estimates NKM channels.

Meanwhile, in the case of the TDD system, the base station may estimate the downlink channel based on the uplink channel estimation.

705 through 720 illustrate various methods of estimating uplink or downlink channels in the base stations. For example, it is possible to perform the uplink channel estimation of the FDD system and the uplink / downlink channel estimation of the TDD system with only 715 and 720. Therefore, channel estimation may be performed with 705, 710, and 720 only in the same sense.

The base stations project at least a portion of the estimated channel information (725). The projection technique described above may be used for projection. The base stations may project the entire channel information, but may only project some channel information of the channel information. For example, channel estimation values or channel estimation vectors corresponding to some antennas in the channel information may be selected and projected, or only channel estimation values corresponding to some terminals may be selected and projected.

(7-i) transmission of the projected channel information

The base stations transmit feedback information including at least a portion of the projected channel information to the network entity (730). The feedback information may include channel information of the remaining unprojected channel, information on the projection method, and identification information (or index) of the terminal.

For example, when full channel information is projected, the projected channel information transmitted by the base station l to the network entity may be expressed as Equation (23).

& Quot; (23 ) & quot ;

이다. 프로젝션된 채널 정보는 총 K ²개의 실수 값으로 이중

개는 0이 아닌 값이고,

개는 0인 값이다.here

to be. The projected channel information is a total of K ² real numbers,

A dog is a non-zero value,

The value of 0 is zero.

For reference, when the channel information estimated by the base station 1 is not projected but transmitted to a network entity, it can be expressed as: < EMI ID = 24.0 >

& Quot; (24 ) & quot ;

Where the channel information is a total of MK complex values. Thus, the projected channel information may be less than the estimated channel information.

The base station 1 may transmit the channel information projected to the network entity and the channel information not projected together. In this case, the channel information to be transmitted may be expressed by Equation (25).

& Quot; (25 ) & quot ;

Where K ⁽¹⁾ and K ⁽²⁾ may be a subset of K.

(7-ii) Determination of the first precoding matrix

Returning to FIG. 7, the base stations determine 735 a first precoding matrix. For example, the base station may determine a first precoding matrix based on the estimated channel information for beamforming for each of the terminals. However, since the first precoding matrix is determined to be local by each base station, the channel state of the other base station and the terminal is determined in a state that is not considered. That is, the first precoding matrix is not for CoMP transmission / reception with other base stations, but refers to a beamforming weight determined locally.

은 수학식 26 과 같다.The projection matrix used to project the channel information as the first precoding matrix determined by the base station can be used. For example, if a matched filter is used, the first precoding matrix determined at base station l

Is expressed by Equation (26).

& Quot; (26 ) & quot ;

은 수학식 27 과 같다.If a zero-forcing filter is used, the first precoding matrix determined at base station l

Is expressed by Equation (27).

& Quot; (27 ) & quot ;

Other precoding schemes may be used for the matched filter and the zero-forcing filter.

(7-iii) Determination of the second precoding matrix

The network entity determines a second precoding matrix and transmit power based on the feedback information received from the base stations (740). The second precoding matrix is for CoMP transmission / reception of a plurality of base stations, and is determined based on feedback information by a plurality of base stations.

를 다음과 같이 결정할 수 있다. 네트워크 엔터티가 각 기지국들로부터 수학식 23과 같이 전체가 프로젝션된 채널 정보를 수신할 때, 하향링크 송수신은 수학식 28과 같이 표현될 수 있다. 참고로, 설명의 편의를 위해 수학식 28에서는 제1 프리코딩 행렬은 고려되지 않았다.For example, the network entity may transmit a second precoding matrix

Can be determined as follows. When the network entity receives the entirely projected channel information from each base station as shown in Equation 23, the downlink transmission / reception can be expressed as Equation (28). For reference, the first precoding matrix is not considered in Expression (28) for convenience of explanation.

& Quot; (28 ) & quot ;

은 프로젝션 채널이고,

는 단말의 수신 신호,

는 네트워크 엔터티에서 결정한 제2 프리코딩 행렬이다.In equation (28)

Is a projection channel,

The reception signal of the terminal,

Is a second precoding matrix determined by the network entity.

The network entity may determine the second precoding matrix according to various manners. In one embodiment, a second precoding matrix based on a matched filter is given by: < EMI ID = 29.0 > In another embodiment, the second precoding matrix based on a zero forcing filter is as shown in equation (30).

& Quot; (29 ) & quot ;

& Quot; (30 ) & quot ;

Meanwhile, when the channel information received by the network entity from the base station is channel information that is not projected as in Equation 24, the downlink transmission and reception can be expressed as Equation (31). In this case, the second precoding matrix based on the matched filter and the second precoding matrix based on the zero-forcing filter can be determined as shown in Equation 32 and Equation 33, respectively.

& Quot; (31 ) & quot ;

[Equation 32]

& Quot; (33 ) & quot ;

On the other hand, when the channel entity received by the network entity at the base station is projected with only a part of channel information as shown in Equation 25 and the remaining part is not projected, Equation (28) and Equation (31) A transmission / reception relationship can be obtained. In contrast, the second precoding matrix can be determined in various manners using an effective channel matrix.

Returning to FIG. 7, the network entity transmits at least one of the determined second precoding matrix and transmit power information to each base station (745).

For example, the information of the second precoding matrix transmitted from the central processing unit to the base station 1 can be expressed as Equation (34).

& Quot; (34 ) & quot ;

Then, each base station transmits and receives uplink or downlink signals to / from the terminals through the information of the received second precoding matrix and information of the transmit power (750).

와 전송 전력

과 기지국 l에서 결정한 제1 프리코딩 행렬

를 사용하여 단말들과 신호를 송수신할 수 있다. 이때, 기지국 l에서 하향링크로 송신하는 신호는 수학식 35와 같이 나타낼 수 있다.For example, the base station 1 may include a second precoding matrix

And transmission power

And first precoding matrix determined by the base station l

And can transmit and receive signals to / from the terminals. At this time, the signal transmitted from the base station 1 to the downlink can be expressed by Equation (35).

& Quot; (35 ) & quot ;

는 K×1 벡터로 K명의 단말들에게 전송되는 정보를 나타낸다.In Equation (35)

Denotes a K × 1 vector indicating information transmitted to K terminals.

8 is a flowchart illustrating a method of feedback of channel information by a base station according to an embodiment of the present invention. The description overlapping with the above embodiments is omitted.

The base station estimates multiple channels (805) between each of the antennas and each of the terminals based on signals from one or more terminals received via multiple antennas. At this time, the signal received by the base station may be RS.

The base station selects at least one terminal among the terminals (810). The base station may select at least one terminal to project the channel estimate corresponding to the selected terminal from the first space to the second space. The first space may correspond to the complex space, and the second space may correspond to the real space.

The BS may consider the average received power or the panel estimation error of the signal received from the MSs to select the MS.

(8-i) Selection of terminal based on average received power

According to one embodiment, the base station may select a terminal among the terminals whose average received power is below a first threshold.

The BS may measure the average received power of the MS and may select the MS that has transmitted the average received power below the first threshold. That is, the channel estimation values corresponding to the terminals whose average reception power is equal to or less than the first threshold value can be projected, and the channel estimation values corresponding to the terminals whose average reception power exceeds the first threshold value can be transmitted to the network entity without projection. This can be expressed as Equation (36).

& Quot; (36 ) & quot ;

는 제1 임계치 이다.here

Is a first threshold.

(8-ii) selection of a terminal based on channel estimation error

The base station can select the terminal using the error of the estimated channel information. According to one embodiment, the base station can select a terminal among the terminals whose channel estimation error is equal to or greater than the second threshold.

The channel estimation error can be expressed by Equation (37).

& Quot; (37 ) & quot ;

는 잡음 성분에 대한 기대값이다. 기지국은 제2 임계치 이상의 채널 추정 오차를 갖는 채널 추정값들에 대응하는 단말을 선택할 수 있다. 즉, 제2 임계치 이상의 채널 추정 오차를 갖는 채널 추정값들을 프로젝션하고, 제2 임계치 미만의 채널 추정 오차를 갖는 채널 추정값들을 프로젝션하지 않을 수 있다. 이는 수학식 38과 같이 표현될 수 있다.In equation (37)

Is the expected value for the noise component. The base station may select a terminal corresponding to channel estimation values having a channel estimation error equal to or greater than a second threshold value. That is, it is possible to project channel estimation values having a channel estimation error equal to or larger than the second threshold value, and not to project channel estimation values having channel estimation errors below the second threshold value. This can be expressed as Equation (38).

& Quot; (38 ) & quot ;

는 제2 임계치이다. 서술된 방식 이외의 다양한 방식으로 단말이 선택 될 수 있다.here

Is the second threshold. The terminal can be selected in various ways other than the described manner.

Returning to Fig. 8, the base station determines a projection scheme (815). The projection scheme may be, for example, a normalized matched filter-based scheme or a zero-forcing filter-based scheme, but is not limited thereto.

The base station projects the channel estimation values corresponding to the selected terminal according to the determined projection method (820).

For example, if the number of selected terminals is K1, the base station selects K1 × M complex channel estimation values selected from the K × M complex channel estimation values. The base station obtains K1 x K1 real channel estimates from the selected K1 x M complex channel estimates.

The base station projects using a normalized matched filter or a zero-forcing filter according to the determined projection method. The projection result using the normalized matched filter corresponds to the expression (20), and the projection result using the zero-forcing filter corresponds to the expression (22).

On the other hand, it can be understood that the projection of the base station removes a fading component having a small coherence time from the first fading component and the second fading component included in the channel estimation values. That is, the short-term fading component of the long-term fading component and the short-term fading component included in the channel estimation values can be removed by projection. Thus, the data size of the projected channel estimates is less than before projection.

The base station transmits feedback information including the projected channel estimate to the network entity (825). The feedback information includes projection scheme information indicating a projection scheme used to project at least some of the channel estimation values of the plurality of projection schemes, terminal information indicating a terminal corresponding to at least some of the projected channel estimates, And may further include at least one of the remaining channel estimation values.

10 shows a structure of feedback information according to an embodiment of the present invention. Referring to FIG. 10, the feedback information includes N1-bit projection-type index, N2-bit terminal information, N3-bit projected channel information, and N4-bit estimated channel information. The terminal information of N2 bits is information for distinguishing which terminal's channel information is projected and which terminal's channel information is not projected.

The base station determines information of the first precoding matrix from the projection matrix used to project the channel estimate (830).

The base station receives (835) at least one of information of the second precoding matrix and information of transmit power from the network entity. The information of the second precoding matrix may be based on at least some of the projected channel estimates transmitted by the base station and channel estimates sent to the network entity by other base stations performing Coordinated Multi-Point (CoMP) It has already been explained that it has been decided by.

The base station precodes the downlink signals to be transmitted to the UEs based on the information of the first precoding matrix and the information of the second precoding matrix (840).

The base station transmits 845 precoded downlink signals based on information on downlink transmission power for terminals received from the network entity.

9 is a flowchart illustrating a method for a network entity to receive channel information from a base station according to an embodiment of the present invention. The description overlapping with the above description is omitted.

The network entity receives feedback information including channel estimates by each base station from one or more base stations (905). For example, the network entity receives first feedback information including first channel estimates between first base station antennas and terminals from a first base station. The network entity receives second feedback information including second channel estimates between the antennas of the second base station and the terminals from the second base station. At this time, at least some of the first channel estimation values and the second channel estimation values are projected from the first space to the second space.

The network entity determines 910 how the channel estimates are projected. The network entity may use the information on the projection method included in the feedback information.

The network entity determines a second precoding matrix according to the projection scheme (915). In order to determine the second precoding matrix, feedback information may be used. The network entity determines the transmit power based on the feedback information (920). The network entity transmits information of the second precoding matrix and information of the transmission power to the base stations (925).

11 is a diagram illustrating a terminal according to an embodiment of the present invention. The terminal shown in FIG. 11 can perform the operation of the terminal in the above-described embodiments.

11, the terminal includes a wireless interface 1101, a processor 1104, and a memory 1103.

The wireless interface 1101 may include a transceiver for transmitting and receiving RF signals.

Memory 1103 may store the average signal power and RS. Memory 1103 may store signal power from the neighboring cells measured by processor 1104 to the base station. The stored average signal power may be fed back to the base station over the wireless channel upon request of the base station. Memory 1103 may store RS sequences used in the network. The stored RS sequences may be used for uplink channel estimation.

The processor 1104 may measure the average signal power. The processor 1104 may accumulate and measure the average signal power from other terminals of the cell adjacent to the serving base station or from other terminals of the serving cell. The processor 1104 stores the measured average signal power in the memory 1103. The processor 1104 transmits the RS for estimation of the uplink channel to the base station. The processor 1104 may utilize the RS sequence stored in the memory 1103. The processor 1104 controls transmission / reception of user data between the terminal and the base station.

12 is a diagram illustrating a base station according to an embodiment of the present invention. The base station shown in Fig. 12 can perform the operation of the base station in the above-described embodiments.

The base station includes a plurality of antennas for MU-MIMO, a wireless interface 1201, a backhaul interface 1202, a memory 1203 and a processor 1204.

The wireless interface 1201 transmits and receives signals to and from the terminals through a plurality of antennas.

The processor 1204 estimates multiple channels between each of the antennas and each of the terminals based on signals received from the terminals. The processor 1204 projects at least some of the channel estimates of the multiple channels from the first space to the second space.

The backhaul interface 1202 sends feedback information to the network entity, including at least some of the channel estimates projected into the second space, under the control of the processor 1204.

The memory 1203 may store the received signal power of the terminal measured by the processor 1204 or fed back from the terminal. The memory 1203 may store RS sequences used in the network. The stored RS sequences are used for uplink channel estimation. The memory 1203 may store channel information estimated by the processor 1204. The stored channel information may be transmitted to the base station via the backhaul interface 1203 or used to determine a first precoding matrix. The memory 1203 may store the feedback information or information received from the network entity. The memory 1203 may store the first precoding matrix determined by the processor 1204.

The processor 1204 may measure the power of the signal transmitted by each terminal. The processor 1204 stores the measured average received power in the memory 1203. The processor 1204 estimates channel information through the RS from the terminal. The processor 1204 stores the estimated channel information in the memory 1203. The processor 1204 can project the estimated channel information. The processor 1204 may determine a first precoding matrix. The processor 1204 may send feedback information to a network entity or may receive information from a network entity. The processor 1204 may be responsible for transmitting and receiving user data to and from the terminal.

13 is a diagram illustrating a network entity according to an embodiment of the present invention. The network entity shown in FIG. 13 may perform the operation of the network entity in the above-described embodiments.

The network entity includes a backhaul interface 1302, a memory 1303, and a processor 1304.

The backhaul interface 1302 receives first feedback information from the first base station, including first channel estimate values between the antennas of the first base station and the terminals, under the control of the processor 1304. The backhaul interface 1302 receives second feedback information including second channel estimates between the antennas of the second base station and the terminals from the second base station. At least some of the first channel estimates and the second channel estimates may be projected from the first space to the second space.

The memory 1303 can store feedback information received from the base station. The memory 1303 may store a second precoding matrix determined by the processor 1304. The memory 1303 may store information on the transmission power determined by the processor 1304. [

The processor 1304 may receive feedback information from the base station and may control the backhaul interface 1302 to transmit information to the base station. The processor 1304 may determine a second precoding matrix and store the second precoding matrix in the memory 1303. The processor 1304 can determine the information of the transmission power using the information stored in the memory 1303. [

FIG. 14 is a diagram illustrating a terminal and a base station according to another embodiment of the present invention. The terminal and the base station in FIG. 14 can perform operations of the terminal and the base station in the above-described embodiments, respectively.

The base station 1410 may include a receiving module 1411, a transmitting module 1412, a processor 1413, a memory 1414 and a plurality of antennas 1415. A plurality of antennas 1415 refers to a base station supporting MIMO transmission / reception. The receiving module 1411 can receive various signals, data, and information on the uplink from the terminal. The transmission module 1412 can transmit various signals, data, and information on the downlink to the terminal. The processor 1413 may control the operation of the entire base station 1410.

The processor 1413 of the base station 1410 further functions to process information received by the base station 1410 and information to be transmitted to the outside and the memory 1414 stores the processed information and the like for a predetermined time And may be replaced by a component such as a buffer (not shown).

The terminal 1420 may include a receiving module 1421, a transmitting module 1422, a processor 1423, a memory 1424 and a plurality of antennas 1425. A plurality of antennas 1425 refers to a terminal supporting MIMO transmission / reception. The receiving module 1421 can receive various signals, data, and information on the downlink from the base station. The transmission module 1422 can transmit various signals, data, and information on the uplink to the base station. The processor 1423 can control the operation of the entire terminal 1420.

The processor 1423 of the terminal 1420 also performs a function of calculating information received by the terminal 1420 and information to be transmitted to the outside and the memory 1424 stores the processed information and the like for a predetermined time And may be replaced by a component such as a buffer (not shown).

The specific configurations of the base station and the terminal as described above may be implemented such that the elements described in the various embodiments of the present invention described above are applied independently or two or more embodiments are applied at the same time and the duplicated description is omitted for the sake of clarity .

14, the description of the base station 1410 may be similarly applied to a repeater device as a downlink transmission entity or an uplink reception entity. The description of the terminal 1420 may be applied to a downlink receiving entity The present invention can be similarly applied to a repeater device as an uplink transmission entity.

The above-described embodiments of the present invention can be implemented by various means. For example, embodiments of the present invention may be implemented by hardware, firmware, software, or a combination thereof.

In the case of hardware implementation, the method according to embodiments of the present invention may be implemented in one or more Application Specific Integrated Circuits (ASICs), Digital Signal Processors (DSPs), Digital Signal Processing Devices (DSPDs), Programmable Logic Devices (PLDs) , FPGAs (Field Programmable Gate Arrays), processors, controllers, microcontrollers, microprocessors, and the like.

In the case of an implementation by firmware or software, the method according to embodiments of the present invention may be implemented in the form of a module, a procedure or a function for performing the functions or operations described above. The software code can be stored in a memory unit and driven by the processor. The memory unit may be located inside or outside the processor, and may exchange data with the processor by various well-known means.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The foregoing description of the preferred embodiments of the invention disclosed herein has been presented to enable any person skilled in the art to make and use the present invention. While the present invention has been particularly shown and described with reference to preferred embodiments thereof, it will be understood by those skilled in the art that various changes and modifications may be made therein without departing from the spirit and scope of the invention as defined by the appended claims. For example, those skilled in the art can utilize each of the configurations described in the above-described embodiments in a manner of mutually combining them. Accordingly, the present invention is not intended to be limited to the embodiments shown herein but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

The present invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. Accordingly, the above description should not be construed in a limiting sense in all respects and should be considered illustrative. The scope of the present invention should be determined by rational interpretation of the appended claims, and all changes within the scope of equivalents of the present invention are included in the scope of the present invention. In addition, claims that do not have an explicit citation in the claims may be combined to form an embodiment or be included in a new claim by amendment after the filing.

Claims (17)

A method for a base station supporting MU-MIMO feedback channel information of multiple users to a network entity,
Estimating multiple channels between the respective antennas and the respective terminals based on signals from the terminals received through the multiple antennas;
Projecting at least some of the channel estimates of the multiple channels from a first space to a second space; And
And transmitting feedback information to the network entity including the at least some of the channel estimates projected into the second space. The method according to claim 1,
Further comprising selecting at least one terminal among the terminals,
Wherein the projecting step projects a channel estimate corresponding to the selected at least one terminal from the first space to the second space. 3. The method of claim 2,
Selecting a terminal having an average received power of the signals less than or equal to a first threshold or selecting a terminal having a channel estimation error equal to or greater than a second threshold. 3. The method of claim 2,
Obtaining K1 x K1 real channel estimation values from the K1xM complex channel estimation values selected from the KxM complex channel estimation values,
Wherein K is the number of terminals transmitting the signals, K1 is the number of the selected at least one terminal, and M is the number of antennas. 2. The method of claim 1,
And removing a fading component having a small coherence time from the first fading component and the second fading component for the multiple channels included in the at least some channel estimation values in the first space. The method according to claim 1,
Wherein the first space corresponds to a complex space, the second space corresponds to a real space,
Wherein the data size of the at least some of the channel estimates projected into the second space is reduced prior to being projected. 2. The method of claim 1,
Projection scheme information indicating a projection scheme used to project the at least some of the plurality of projection schemes;
Terminal information indicating a terminal corresponding to the projected at least a part of the channel estimation value; And
And at least one of the remaining channel estimates not projected to the second space among the channel estimates. 2. The method of claim 1,
And projecting a channel estimate corresponding to a first one of the terminals using a normalized matched filter or a zero-forcing filter. 9. The method of claim 8,
The result of projection using the normalized matched filter is:

Respectively,
The result of the projection using the zero forcing filter,

Respectively,
Wherein l is the index k is the index of the received signal, from the π _k is the first terminal of the second terminal of the index, the i of the first terminal is the terminal, an index of the BS, the

A set of terminals that transmitted the signal [ pi] _k ,

The power of the signal received from the first terminal,

The power of the signal received from the second terminal,

Is a first fading component of the channel estimated for the second terminal. The method according to claim 1,
Obtaining information of a first precoding matrix from a projection matrix used to project the at least one channel estimate;
Receiving information of a second precoding matrix from the network entity; And
Precoding the downlink signals to transmit to the terminals based on the information of the first precoding matrix and the information of the second precoding matrix. 11. The method of claim 10,
Further comprising transmitting the precoded downlink signals based on information on downlink transmission power for the terminals received from the network entity. 11. The method of claim 10, wherein the information of the second precoding matrix includes:
Wherein the at least some of the projected channel estimates transmitted by the base station and the channel estimates sent to the network entity by another base station performing Coordinated Multi-Point (CoMP) Way. A method for receiving channel information of multiple users from a base station supporting a MU-MIMO by a network entity,
Receiving first feedback information including first channel estimation values between antennas of the first base station and terminals from a first one of the base stations; And
Receiving second feedback information including second channel estimates between the antennas of the second base station and the terminals from a second one of the base stations,
Wherein at least some of the first channel estimates and the second channel estimates are projected from a first space to a second space. 14. The method of claim 13,
Further comprising determining information of a downlink transmission power and a precoding matrix of each of the first base station and the second base station based on the first feedback information and the second feedback information. 15. The method of claim 14, wherein determining the precoding matrix further comprises:
Based on a projection scheme used to project at least some of the first and second channel estimate values. A base station for feeding back channel information of multiple users to a network entity,
Multiple antennas for MU-MIMO;
A radio frequency (RF) interface for transmitting and receiving signals to and from the terminals via the plurality of antennas;
Estimating multiple channels between each of the antennas and each of the terminals based on signals received from the terminals and transmitting at least some of the channel estimates of the multiple channels from the first space to the second space A processor for projecting; And
And a backhaul interface for transferring feedback information to the network entity, the feedback information comprising the at least some of the channel estimates projected into the second space under control of the processor. A network entity for receiving channel information of multiple users from base stations supporting MU-MIMO,
A processor; And
Receiving first feedback information including first channel estimation values between antennas of the first base station and the UEs from a first one of the base stations under control of the processor, And a backhaul interface for receiving second feedback information comprising antennas of a second base station and second channel estimates between the terminals,
Wherein at least some of the first channel estimates and the second channel estimates are projected from a first space to a second space.

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