KR101382420B1 - Method of performing mimo in a radio network and apparatus therefore - Google Patents

Abstract

The present invention relates to a method of performing a multiple input multiple output (MIMO) in a wireless network and an apparatus therefor. Specifically, the present invention relates to the method and the apparatus therefor of which a radio unit (RU) receives data from a digital unit (DU); receives precoding information from the DU; precodes the data using the precoding information; and transmits the precoded data through multiple antennas, wherein the RU and the DU are remotely located from each other.

Description

METHOD OF PERFORMING MIMO IN A RADIO NETWORK AND APPARATUS THEREFORE}

The present invention relates to a wireless network system, and more particularly, to a method and apparatus for performing multiple input multiple output (MIMO) in a cloud wireless access network.

Background of the Invention [0002] Wireless communication systems are widely deployed to provide various types of communication services such as voice and data. In general, a wireless communication system is a multiple access system capable of supporting communication with multiple users by sharing available system resources (bandwidth, transmission power, etc.). Examples of multiple access systems include a code division multiple access (CDMA) system, a frequency division multiple access (FDMA) system, a time division multiple access (TDMA) system, an orthogonal frequency division multiple access (OFDMA) system, a single carrier frequency division multiple access) systems.

An object of the present invention is to provide a method and apparatus for efficiently transmitting and receiving signals using multiple antennas in a wireless network system, specifically, a cloud radio access network (C-RAN) system.

The technical problems to be solved by the present invention are not limited to the technical problems and other technical problems which are not mentioned can be understood by those skilled in the art from the following description.

In one aspect of the present invention, a method for downlink transmission of a radio unit (RU) in a wireless communication system, the method comprising: receiving data from a digital unit (DU); Receiving precoding information from the DU; And precoding the data using the precoding information. And transmitting precoded data via multiple antennas, wherein the RU and the DU are located remotely from each other.

In another aspect of the present invention, a radio unit (RU) for use in a wireless communication system, comprising: a radio frequency (RF) module; And a processor, the processor receiving data from a digital unit (DU), receiving precoding information from the DU, precoding the data using the precoding information, and multiplexing precoded data. And configured to transmit via an antenna, wherein the DU and the RU are located remote from each other.

Advantageously, said data is received in accordance with a first period, said precoding information is received in accordance with one or more second periods, and said first period is shorter than said one or more second periods.

Preferably, the precoding information is received according to a plurality of second periods, each second period corresponding to a precoding update period of the corresponding terminal group.

Preferably, the data includes in-phase quadrature (IQ) -data.

Preferably, the precoding information includes precoding matrix indication information.

In another aspect of the present invention, a method for downlink transmission of a digital unit (DU) in a wireless communication system, the method comprising: transmitting data to a radio unit (RU); Receiving downlink channel state information from the RU; And transmitting, to the RU, precoding information to be applied to the data in consideration of the channel state information, wherein the RU and the DU are located remotely from each other.

According to another aspect of the present invention, a digital unit (DU) used in a wireless communication system, the data signal generating unit; A precoding information generator; And a processor, wherein the processor transmits data to a Radio Unit (RU), receives downlink channel state information from the RU, and takes into account the channel state information, and provides precoding information to be applied to the data. DU is provided, wherein the DU and the RU are located remotely from each other.

Advantageously, said data is transmitted according to a first period, said precoding information is transmitted according to one or more second periods, and said first period is shorter than said one or more second periods.

Preferably, the precoding information is transmitted according to a plurality of second periods, each second period corresponding to a precoding update period of the corresponding terminal group.

Preferably, the data includes in-phase quadrature (IQ) -data.

Preferably, the precoding information includes precoding matrix indication information.

According to the present invention, there is provided a method and an apparatus therefor for efficiently transmitting and receiving signals using multiple antennas in a wireless access network system, specifically a cloud wireless access network system.

The effects obtained by the present invention are not limited to the above-mentioned effects, and other effects not mentioned can be clearly understood by those skilled in the art from the following description will be.

BRIEF DESCRIPTION OF THE DRAWINGS The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of the specification, illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention.
1 illustrates the structure of an E-UMTS network system.
2 illustrates a wireless communication system using multiple-input multiple-output (MIMO).
3 shows an example of performing communication using a closed-loop MIMO technique.
4 illustrates base station equipment in a cloud radio access network (C-RAN).
5 shows an example of MIMO transmission in C-RAN according to the conventional scheme.
6 shows an example of C-RAN MIMO transmission according to an embodiment of the present invention.
7A to 7D show simulation results when a plurality of IQ-data transfer schemes and a plurality of beamforming schemes are adaptively applied.
8 shows an example of performing MIMO transmission in the C-RAN when configuring a terminal group according to the present invention.
9 to 10 show sum-rates according to resource allocation methods.
11 illustrates base station equipment that may be used in a C-RAN, in accordance with the present invention.

Specific terms used in the present specification are provided for the purpose of understanding, and the specific terms may be changed into other forms without departing from the technical spirit of the present invention. Unless defined otherwise, the terms used herein, including technical or scientific terms, have the same meaning as commonly understood by one of ordinary skill in the art to which the present invention belongs. Unless the context clearly indicates otherwise, a singular expression includes a plural expression. In this specification, the terms "comprises" or "having" and the like refer to the presence of stated features, integers, steps, operations, elements, components, or combinations thereof, But do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, or combinations thereof.

The following techniques are various, such as code division multiple access (CDMA), frequency division multiple access (FDMA), time division multiple access (TDMA), orthogonal frequency division multiple access (OFDMA), single carrier frequency division multiple access (SC-FDMA), and the like. It can be used in a wireless access system. 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). 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE) is part of Evolved UMTS (E-UMTS) using E-UTRA and LTE-A (Advanced) is an evolution of 3GPP LTE.

For the sake of convenience, this specification mainly describes a case where it is applied to a 3rd Generation Partnership Project (3GPP) system, but this is exemplified by the present invention. The present invention is a cloud-based network (eg, CCC (Cloud Communication Center), W-SCAN (WCDMA-Smart Cloud). It can be used in any communication system (e.g., IEEE) that supports (Access Network).

1 illustrates the structure of an E-UMTS network system.

Referring to FIG. 1, a network system includes a user equipment (UE), a base station (eNode B), and an access gateway (AG) located at an end of a network and connected to an external network. The terminal receives a signal from the base station through downlink (DL), and transmits information to the base station through uplink (UL). The base station may simultaneously transmit multiple data streams for broadcast services, multicast services, and / or unicast services. The base station controls data transmission and reception for a plurality of terminals. The base station transmits downlink scheduling information to the downlink (DL) data, and informs the corresponding terminal of the time / frequency region in which data is to be transmitted, coding, data size, and information related to HARQ (Hybrid Automatic Repeat and ReQuest). Also, the base station transmits uplink scheduling information to uplink (UL) data, and notifies the time / frequency region, coding, data size, and HARQ related information that the UE can use. An interface for transmitting user traffic or control traffic may be used between base stations. The Core Network (CN) can be composed of AG and a network node for user registration of the terminal. The AG manages the mobility of the UE in units of a tracking area (TA) composed of a plurality of cells.

2 illustrates a wireless communication system using multiple-input multiple-output (MIMO). As shown in FIG. 2, when the number of transmitting antennas is increased to N _{T and the} number of receiving antennas is increased to N _R , the theoretical channel transmission capacity increases in proportion to the number of antennas, unlike when a plurality of antennas are used only in a transmitter or a receiver. do. Therefore, the transmission rate can be improved and the frequency efficiency can be improved.

Specifically, when there are N _T transmit antennas, the maximum information that can be transmitted is N _T. The transmission information can be expressed as follows.

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.

Also,

Is a diagonal matrix of transmit power

Can be expressed as follows.

Transmitted Power Information Vector

Weighting matrix

N _T transmitted signals actually applied by applying

Can be configured. Weighting matrix

Plays a role in properly distributing transmission information to each antenna according to a transmission channel situation.

Vector

Can be expressed as follows.

From here,

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

Is called the precoding matrix.

On the other hand, when there are N _R receive antennas, the received signal at each antenna

Can be expressed as a vector as follows. In the case of MU-MIMO, N _R receiving antennas are replaced with N _R terminals, and y _i may mean a reception signal of an _i -th terminal.

A channel arriving from the N _T transmitting antennas to the receiving antenna i (or the i-th terminal) may be represented as follows.

Accordingly, all channels arriving from N _T transmit antennas to N _R receive antennas (or N _R terminals) may be expressed as follows.

The actual channel includes a channel matrix

After passing through, Additive White Gaussian Noise (AWGN) is added. N _R receive antennas (or N _R of terminals) white noise is added to each

Can be expressed as follows.

Through the mathematical modeling described above, the received signal may be expressed as follows.

Whether MIMO technology is applied or not, MIMO mode (eg, spatial multiplexing, spatial diversity, etc.) is determined based on the downlink channel state. To this end, the terminal feeds back channel state information (eg, CQI (Channel Quality Indicator), RI (Rank Information), Precoding Matrix Indicator (PMI), Precoding Type Indicator (PTI), etc.) to the base station periodically (non-). . MIMO technology is classified as open-loop MIMO when there is no PMI feedback, and as closed-loop MIMO when there is PMI feedback.

3 shows an example of performing communication using a closed-loop MIMO technique.

Referring to FIG. 3, a transmitter Tx (eg, a base station) and a receiver Rx (eg, a terminal) share a codebook for applying MIMO technology. Codebook means a set of precoding matrices. The precoding matrix has a size N _T × N _L. N _T represents the number of antennas (ports) used for signal transmission, and N _L represents the number of layers. The number of layers may be determined according to the rank of the channel matrix. The precoding matrix may be configured in a nested form. For example, when LTE uses two antenna ports, the codebook is defined as shown in Table 1. Reference may be made to codebooks of more various sizes in 3GPP TS36.211.

For convenience, FIG. 3 shows that the transmitter and the receiver are W _i ∈ {W ₀ , W ₁ ,. Suppose you share a codebook that contains, W _{L -1} }. The transmitter selects a precoding matrix from the codebook. The transmitter then transmits the processed signal using the precoding matrix to the receiver via multiple antennas. The receiver inverses the received signal using the precoding matrix in the codebook. In this process, the receiver may select a suitable precoding matrix in consideration of the MIMO channel situation. In this case, the receiver may feed back an indicator (PMI # {0, 1, ..., L-1}) for a specific precoding matrix in the codebook to the transmitter. In Table 2, the codebook index may correspond to PMI. The transmitter may determine a precoding matrix to be used for downlink transmission in consideration of the fed back PMI.

4 illustrates base station equipment in a cloud radio access network (C-RAN).

Referring to FIG. 4, the base station equipment 100 includes a radio unit (RU) 110 and a digital signal processor (Digital Unit, DU) 120. C-RAN technology (eg, CCC, W-SCAN, etc.) can efficiently use resources by remotely separating the RU (110) and the DU (120) in one equipment in the existing base station. Specifically, the DU provides a medium access control (MAC) / physical (PHY) layer function and is concentrated in a DU center in a national office. The RU provides an RF function and is installed in a service area. The DU delivers data (eg, in-phase quadrature (IQ) data) processed by the MAC / PHY layer in the DU to the RU, and the RU converts the data received from the DU into a radio signal and transmits the data to the terminal. One DU 120 may manage a plurality of RUs 110. The RU 110 and the DU 120 may be remotely connected via a wire-link (eg, an optical interface such as a common public radio interface (CPRI)). In the CRPI specification, the DU-RU link rate is 6.x to 9.82 Gbps.

A signal transmission method using multiple antennas in C-RAN will be described. As described with reference to FIG. 4, the transmission signal of the RU in the C-RAN is generated by the DU. The signal transmitted from each antenna of the RU is determined by the symbol for each stream and the beamforming method (eg, Zero Forcing (ZF) method, Maximum Ratio Transmission (MRT) method, etc.) In each user antenna of the RU in multi-user beamforming The transmitted signal is a mixture of signals for different users, and which signal is transmitted from each antenna for each user is determined by the user's data symbol and precoding vector.

For convenience, when the transmission power adjusting portion (Equations 2 to 3) are not considered in Equations 1 to 9, the signal that the RU should transmit wirelessly is as follows.

In addition, the signals received by the terminals are as follows.

Where P is the total transmit power of RU, H is the channel matrix, W is the precoding matrix, wj is the j-th column vector of the precoding matrix W, and s is the data symbol for each stream. Vector represents n represents white noise. y _j represents the j-th terminal. T represents the number of antennas of the RU, J represents the number of terminals.

When the ZF beamforming method and the MRT beamforming method are used as the beamforming method, the relationship between the precoding matrix and the channel matrix is as follows.

5 shows an example of MIMO transmission in C-RAN according to the conventional scheme.

Referring to FIG. 5, the DU completes and sends IQ-data to be transmitted by each antenna of the RU. That is, the DU precodes a data symbol to be transmitted to the UE, and then transmits a transmission signal that is a sum of data symbols to which weights (ie, precoding matrices) to which the antenna is applied is transmitted to the RU. For convenience, the conventional IQ-data delivery scheme is referred to as the "after-precoding" scheme. When delivering IQ-data in a post-precoding scheme, the bit-rate of the required DU-RU link is as follows.

Here, m represents the number of antennas (hereinafter, referred to as active antennas) used for actual transmission among the antennas of the RU, N _sub represents a frequency band (eg, the number of subcarriers in an OFDM system), and f _sym represents a symbol-rate ( symbol rate (= 1 / T _sym , T _sym : symbol interval), and b _IQ represents the number of bits required to represent an IQ sample. CPRI defines the number of bits for IQ-data as 16-40 bits.

Note that in the case of the post-precoding scheme, the bit-rate of the required DU-RU link is proportional to the number of active antennas, but is not related to the number of receiving terminals. That is, in the post-precoding scheme, since the bit-rate of the DU-RU link depends only on the number of antennas, the wireless communication environment (eg, the number of terminals, the rate of change of channel state, etc.) may not be properly reflected. In addition, in a wireless environment where the change in channel state is slow, the weight multiplied by the IQ-data (i.e., the elements of the precoding matrix) may also change slowly. However, according to the conventional post-precoding scheme, the DU always multiplies the IQ-data by weight, and then delivers it to the RU. In this case, it is possible to waste the bit-rate of the DU-RU link unnecessarily. Therefore, these problems may degrade the sum-rate of the C-RAN system.

In order to solve the above problems, the present invention provides an improved IQ-data delivery scheme that can support the C-RAN MIMO scheme that can reflect a wireless communication environment (eg, the number of terminals, the rate of change of channel state, etc.). Suggest.

6 shows an example of C-RAN MIMO transmission according to an embodiment of the present invention.

Referring to FIG. 6, a DU does not complete IQ-data to be transmitted in each antenna, but a data symbol vector s to be transmitted to each terminal and weight information (eg, a weight vector and a precoding matrix) to be applied to the data symbol vector. The vector of vector, precoding matrix, PMI) (W) is separately transmitted to the RU. The RU multiplies the data symbol vector s by the weight information W to generate IQ-data to be transmitted in each antenna (ie, precoding), and then wirelessly transmit the IQ-data through multiple antennas. For convenience, the IQ-data transmission scheme proposed in the present invention is referred to as a "before-precoding" scheme. IQ-data and weight information may be transmitted in the same message or in separate messages. In addition, the IQ-data s is transmitted to the RU every first period (eg, transmission time interval (TTI), subframe interval, slot interval, symbol interval, etc.), and the weight information W is transmitted every second period or It may be delivered to the RU aperiodically. The transmission interval of the weight information W may be set longer than the transmission interval of the IQ-data s.

According to the present invention, when transmitting IQ-data using the pre-precoding scheme, the required DU-RU link bit-rate may be given as follows. In the following equation, it is assumed that the DU transfers a precoding matrix to the RU, and the size of the precoding matrix in MU-MIMO is given by m * k.

Here, m, N _sub , b _IQ and f _sym are as defined in Equation 13. f _W denotes a precoding matrix update frequency, b _DS denotes the number of bits required to represent a modulated signal (e.g., a modulation symbol vector), and k denotes a user (terminal) provided for service in a cell , Service users, service terminals).

In Equation 14, unlike a data symbol transmitted every symbol time, a weight vector (or information related thereto) may be transmitted every weight vector update period. The update period of the weight vector (or related information) may be determined according to the coherence time of the radio channel. The update period of the weight vector (or information related thereto) may be determined based on channel state information (eg, CQI, RI, PMI, PTI, etc.) fed back from the terminal. The DU may set / change an update period of the weight vector (or information related thereto). If there is a change in the update cycle, the DU may inform the RU of the update cycle change information.

According to Equation 14, in the pre-precoding scheme, it can be seen that the DU-RU link bit-rate is determined according to the number of active antennas, the number of service terminals, and the frequency of updating precoding information. Thus, unlike the conventional post-precoding scheme, the pre-precoding scheme of the present proposal can reflect the radio channel environment in which the DU-RU link bit-rate is reflected.

Previously, the conventional beamforming strategy has been the question of which beamforming technique is used to transmit the data simultaneously to all users using a given antenna. However, in a C-RAN environment where the IQ-data rate is limited by the DU-RU link capacity, an antenna that can be used for transmission according to the IQ-data transmission method (ie, post-precoding method or pre-precoding method). There may be a trade-off relationship between the number and the number of sending users.

Accordingly, the present invention further proposes a method of adaptively using a plurality of IQ-data transmission schemes according to radio channel conditions in the C-RAN. For example, in the C-RAN, the post-precoding scheme of FIG. 5 and the pre-precoding scheme of FIG. 6 may be adaptively used according to a radio channel situation. In addition, a scheme of adaptively using a plurality of beamforming schemes may be considered according to a radio channel situation and an IQ-data transfer scheme.

Hereinafter, a method of adaptively using a plurality of IQ-data transmission schemes and a plurality of beamforming schemes to obtain an optimal wireless sum-rate will be described. For illustrative purposes, the plurality of IQ-data transfer schemes includes a "post-precoding" scheme (FIG. 5) and a "pre-precoding" scheme (FIG. 6), and the plurality of beamforming schemes are ZF scheme and MRT scheme. Assume that it contains. For simplicity, it is assumed that the terminal environments are all the same. That is, all terminals may experience the same average Signal to Noise Ratio (SNR) and have the same precoding matrix update period (or interval).

In a single cell, the radio sum-rate according to the beamforming scheme (eg, ZF, MRT), the number of active antennas (m), and the number of service terminals (k) is given as follows.

Here, p represents the average SNR of the terminals.

In C-RAN MIMO, the number of service terminals and the number of active antennas are limited by the IQ-data scheme (e.g., post- / pre-precoding) and DU-RU link bit-rate. Considering this limitation, the sum-maximization problem can be expressed as

Here, S ^D = {"after", "before"} represents an IQ-data transfer method in the DU-RU link. "after" represents a pre-precoding scheme and "before" represents a pre-precoding scheme. S ^B = {"ZF", "MRT"} indicates a beamforming method. K represents the total number of terminals in the cell, and M represents the total number of antennas in the RU. θ represents the maximum bit-rate of the DU-RU link. N _sub , f _sym , b _IQ , f _w and b _DS are as defined above.

The computational complexity for finding the optimal S ^D , S ^B , k and m is O (4NK) for M> K and O (4M ² ) for M <= N.

According to Equation 15, the radio sum-rate C monotonically increases with the number of active antennas (m), which means that a large m value is always good if other variables are constant. In this sense, for the "post-precoding" scheme where the IQ-data transfer bit-rate depends only on m and does not depend on S ^D , S ^B and k, it is best to select the maximum possible m value for the radio sum-rate. Maximize. Therefore, the optimal number of active antennas (m ^* ) in the "post-precoding" scheme may be given as follows.

Where θ ₁ = θ / (N _sub * f _sym * b _IQ ), where M is the total number of antennas in the RU.

For the "pre-precoding" scheme, m and k are in a trade-off relationship for a given DU-RU link capacity. As with the "post-precoding" scheme, for the "pre-precoding" scheme, selecting the maximum possible m value for a given k and θ can maximize the radio sum-rate. Therefore, the optimal number of active antennas m ^* in the “pre-precoding” scheme may be given as follows.

Where θ ₂ = θ / (N _sub * f _W * b _IQ ), θ ₃ = (f _sym * b _DS ) / (f _W * b _IQ ), and M is the total number of antennas in the RU.

In consideration of Equations 17 to 18, possible k satisfying 1 ≦ m ≦ M, 1 ≦ k ≦ K, and k ≦ m may be given as follows.

Accordingly, the number of active antennas m can be simplified in Equation 15 for obtaining the maximum sum-rate by substituting the variables in Equation 15 with the contents of Equations 17 to 19.

Therefore, the remaining process for obtaining the maximum sum-rate is to find the optimal number of service terminals (k ^* ) representing the maximum sum-rate for each combination of S ^D and S ^B (i.e.,

). k _max represents the maximum possible k value in equation (19).

7A to 7D show simulation results when a plurality of IQ-data transfer schemes and a plurality of beamforming schemes are adaptively applied. Specifically, FIGS. 7A-7D illustrate the change in radio sum-rate (C) according to the number of users (k) for various average SNRs, precoding matrix update intervals (or periods) (= T _W = 1 / f _W ). Indicates. Simulation results were obtained using Equation 20, and the figures are shown within the range of possible k values.

Table 2 shows the values of the parameters used in the simulation.

Referring to Figures 7A-7D, for the "after-precoding" scheme, the sum-rate curve is independent of the precoding matrix update interval T _W , since the required IQ-data transfer rate is precoded. This is because it does not depend on the matrix update interval. On the other hand, in the "pre-precoding" manner, the sum-rate of Figs. 7B / 7D is lower than the sum-rate of Figs. 7A / 7C, as the precoding matrix update interval f _W is shortened. This is because the optimal number of active antennas (m ^* ) decreases. In Equation 15, since the sum-ratio is monotonically proportional to the number m of active antennas, the sum-ratio also decreases when the optimal number of active antennas m ^* decreases.

On the other hand, in the "post-precoding" scheme, the maximum sum-rate of MRT at low SNR is higher than the maximum sum-rate of ZF, while the maximum sum-rate of ZF is higher than the maximum sum-rate of MRT at high SNR. Is observed. However, for the "pre-precoding" scheme, the maximum sum-rate of ZF is always observed to be higher than the maximum sum-rate of MRT, regardless of the SNR value. This difference is because the number m of active antennas is fixed in the "post-precoding" scheme, but it is possible to flexibly select m and k in the "pre-precoding" scheme.

Table 3 shows m ^? , k ^? , k _max . Referring to Table 3, in comparison to the "post-precoding" scheme, the smaller k ^? And larger m ^? It can be noted that this is selected.

In the present invention, adaptively using a plurality of IQ-data delivery schemes (and a plurality of beamforming schemes) may be implemented in various ways. For example, referring to FIG. 7D, the following scheme may be implemented.

If the maximum sum-rate is important: The maximum sum-rate of [MRT, after], [ZF, after], [MRT, before] and [MRT, before] is obtained from [MRT, before]. Accordingly, according to [MRT, before], the DU separates the data symbols and the precoding matrix (information) and delivers them to the RU. In this case, the number of service terminals may be set to 8 ± A (integer including A: 0) and the number of active antennas is 67 ± B (integer including B: 0).

When the number of service terminals is important: For example, when the number of terminals requiring service provision in a cell is 30, the maximum sum-ratio is obtained at [MRT, after]. Thus, according to [MRT, after], the DU combines the data symbols and the precoding matrix and delivers them to the RU. In this case, the number of service terminals may be set to 30 ± C (an integer including C: 0), and the number of active antennas may be fixed to 39.

In order to facilitate understanding of the present invention, it is assumed that all users (or terminals) experience the same wireless environment. In practice, however, users (or terminals) may have different SNRs, different precoding matrix update intervals (or periods) due to different locations, mobility, and the like. Accordingly, in the present invention, after grouping terminals that experience the same / similar wireless environment (eg, SNR, precoding matrix update interval, etc.), the C-RAN MIMO scheme and the beamforming scheme are adaptively applied. Suggest further application.

8 shows an example of performing MIMO transmission in the C-RAN when configuring a terminal group according to the present invention.

Referring to FIG. 8, G terminal groups (Groups 1 to G) may be configured according to channel state information (eg, SNR) and a precoding matrix update interval (or period). For example, the IQ-data transmission scheme and / or the beamforming scheme may be independently applied to each UE group as described with reference to Equation 20 and FIGS. 7A to 7D. Downlink physical resources (eg, time intervals, frequency intervals, time-frequency intervals) (group resources for convenience) may be allocated for each UE group. The figure illustrates an amount of frequency resources for each UE group and a transmission strategy for each UE group in an OFDMA system. w_i is the ratio of resources allocated to group-i among all frequency resources. Resources allocated to each terminal group are determined according to a resource allocation policy, the number of terminals belonging to the terminal group, and a transmission strategy for each terminal group.

When terminals in a terminal group share resources equally, the average throughput of the terminals in group-i may be expressed as follows.

Here, v _i is the normalized resource ratio allocated to group- _i , K _i is the total number of terminals belonging to group- _i , k _i ^* is the number of terminals receiving service simultaneously in group-i, r _i (k _i ^* ) is an achievable rate of one UE in group-i.

A case where a throughput fair resource allocation method and a proportional fair resource allocation method are used will be described.

In the case of the throughput fair resource allocation method, resource allocation for each terminal group is performed so that the expected throughput of all terminals is equal. According to the following equation, terminals in each group can obtain the same expected throughput.

Here, G represents the number of terminal groups.

In this case, the average sum-rate of all terminals is given as follows.

Meanwhile, in the proportional fair resource allocation scheme, resource allocation for each terminal group is performed in proportion to the number of terminals in the terminal group as shown in the following equation.

In this case, the average sum-rate of all terminals is given as follows.

The proportional fair resource allocation method may be performed in other ways. For example, instead of the number of terminals in the terminal group, resource allocation may be performed based on the access opportunity of the terminals in the terminal group in consideration of the multiplexing order of the terminal group.

9 to 10 show sum-rates according to resource allocation methods. Four types of terminals are assumed in the simulation: {cell-center, high speed}, {cell-center, low speed}, {cell-boundary, high speed}, {cell-boundary, low speed}. Each type of terminal is grouped, and terminals in each group share resources allocated for the group. Table 4 shows the average SNR and precoding vector update interval for each group.

9-10, the gain of adaptive "post- / pre-precoding" compared to the existing "post-precoding" is 31.3% (throughput fair resource allocation) and 26.1% (proportional fair resource allocation). In addition, it can be seen that the gain is high when both ZF and MRT are used as compared with the case where only MRT is used as the beamforming method.

Although the above description has been mainly focused on downlink MIMO transmission, this is exemplarily applicable to uplink MIMO transmission in the present invention. Specifically, in the case of uplink MIMO transmission, the base station receives an uplink signal from one or more terminals through multiple antennas, and then performs inverse operations (for convenience and postcoding) corresponding to precoding. Applying the present invention, the RU transmits the signal received from each antenna to the DU as it is (for convenience, referred to as the "pre-postcoding" scheme), or "pre-precoding" scheme, similar to the "post-precoding" scheme. Similar to the scheme, post-coding is performed on signals received from a plurality of antennas to extract data symbols of each terminal (for convenience, referred to as a "post-postcoding" scheme), and then may be transferred to the DU. The precoding matrix (or information about this) used for uplink transmission for "pre-postcoding" may be shared between the DU and the RU in advance. In addition, in the case of uplink MIMO transmission, a "pre-postcoding" scheme and a "post-postcoding" scheme may be adaptively applied based on Equation 20 or the like (adaptive pre / post-). Post coding).

11 illustrates base station equipment that may be used in a C-RAN, in accordance with the present invention.

Referring to FIG. 11, base station equipment includes a DU and a RU. The DU and RU are remotely located and may be connected via a wired link (eg, an optical cable). The DU performs the functions of the MAC layer and the PHY layer, and the RU performs the RF function. In the case of the present invention, the DU may functionally include a processor, a data symbol generation module, a precoding command determination module, and a precoding / postcoding module. The processor of the DU may control the DU to perform the operation proposed in the present invention. For example, in the case of downlink MIMO transmission, the processor of the DU may adaptively apply the IQ-data transmission scheme or the beamforming scheme based on Equation 20. When the "post-precoding" scheme is used, the data symbols and the precoding matrix are combined by the precoding module and then passed to the RU. When the "pre-precoding" scheme is applied, the data symbols and the precoding matrix (or information about them) are separated and passed to the RU. In the case of uplink MIMO transmission, the processor of the DU may adaptively apply the IQ-data reception scheme or the beamforming scheme based on Equation 20. When the "pre-postcoding" scheme is used, the DU receives the signal received from each antenna of the RU as it is, and the DU post-codes the signal received from the RU to extract data symbols of each terminal. have. When the "post-postcoding" scheme is applied, it is received from the data symbol RU of each terminal.

Meanwhile, the RU may functionally include a processor, a precoding / postcoding module, an RF module, and multiple antennas. The processor of the RU may control the RU to perform the operation proposed in the present invention. For example, in the case of downlink MIMO transmission, when the "post-precoding" scheme is used, the RU converts a signal received from the DU into a radio signal through the RF module and then transmits the signal through the multiple antennas as it is. When the "pre-precoding" scheme is applied, the RU combines the data symbols and the precoding matrix in the precoding module, converts the generated signal into a wireless signal, and transmits it through the multiple antennas. In the case of uplink MIMO transmission, the processor of the RU may adaptively apply the IQ-data scheme or the beamforming scheme according to the configuration / instruction of the DU. When the "pre-postcoding" scheme is used, the RU delivers the signal received from each antenna to the DU as it is. When the "post-postcoding" scheme is applied, the RU may perform postcoding on a signal received from multiple antennas, extract a data symbol of each terminal, and deliver the data symbol to the DU.

The embodiments described above are those in which the elements and features of the present invention are combined in a predetermined form. Each component or feature shall 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. It is also possible to construct embodiments of the present invention by combining some of the elements and / or features. 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. It is clear that the claims that are not expressly cited in the claims may be combined to form an embodiment or be included in a new claim by an amendment after the application.

Embodiments in accordance with the present invention may be implemented by various means, for example, hardware, firmware, software, or a combination thereof. In the case of hardware implementation, an embodiment of the present invention may include one or more application specific integrated circuits (ASICs), digital signal processors (DSPs), digital signal processing devices (DSPDs), programmable logic devices (PLDs) field programmable gate arrays, processors, controllers, microcontrollers, microprocessors, and the like.

In the case of implementation by firmware or software, an embodiment of the present invention may be implemented in the form of a module, procedure, function, etc. that performs 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.

It will be apparent to those skilled in the art that the present invention may be embodied in other specific forms without departing from the spirit of the invention. 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.

The present invention can be used in a wireless network system, specifically, a C-RAN and an apparatus therefor (eg, DU, RU, base station, relay, terminal, etc.).

Claims (20)

In a downlink transmission method of a radio unit (RU) in a wireless communication system,
Receiving data from a digital unit (DU) according to a first period;
Receiving precoding information from the DU according to one or more second periods; And
Precoding the data using the precoding information; And
Transmitting the precoded data via multiple antennas,
The RU and the DU are remote from each other,
And the first period is shorter than the one or more second periods. delete The method of claim 1,
Wherein the precoding information is received according to a plurality of second periods, each second period corresponding to a precoding update period of the corresponding terminal group. The method of claim 1,
Wherein the data comprises in-phase quadrature (IQ) -data. The method of claim 1,
Wherein the precoding information comprises precoding matrix indication information. In a radio unit (RU) used in a wireless communication system,
An RF (Radio Frequency) module; And a processor,
The processor receives data from a digital unit (DU) according to a first period, receives precoding information from the DU according to one or more second periods, precodes the data using the precoding information, Is configured to transmit precoded data through multiple antennas,
The DU and the RU are remote from each other,
The first period is shorter than the one or more second periods. delete The method according to claim 6,
The precoding information is received according to a plurality of second periods, each second period corresponding to a precoding update period of the terminal group. The method according to claim 6,
The data comprises in-phase quadrature (IQ) -data. The method according to claim 6,
The precoding information comprises precoding matrix indication information. In a downlink transmission method of a digital unit (DU) in a wireless communication system,
Transmitting data to a radio unit (RU) according to a first period;
Receiving downlink channel state information from the RU; And
Considering the channel state information, transmitting precoding information to be applied to the data to the RU according to one or more second periods,
The RU and the DU are remote from each other,
And the first period is shorter than the one or more second periods. delete 12. The method of claim 11,
The precoding information is transmitted according to a plurality of second periods, each second period corresponding to a precoding update period of the corresponding terminal group. 12. The method of claim 11,
Wherein the data comprises in-phase quadrature (IQ) -data. 12. The method of claim 11,
Wherein the precoding information comprises precoding matrix indication information. In the DU (Digital Unit) used in the wireless communication system,
A data signal generator; A precoding information generator; And a processor,
The processor transmits data to a Radio Unit (RU) according to a first period, receives downlink channel state information from the RU, and takes into account one or more precoding information to be applied to the data in consideration of the channel state information. Configured to transmit to the RU according to two cycles,
The DU and the RU are remote from each other,
The first period is shorter than the one or more second periods. delete 17. The method of claim 16,
The precoding information is transmitted according to a plurality of second periods, each second period corresponding to a precoding update period of the corresponding terminal group. 17. The method of claim 16,
The data comprises in-phase quadrature (IQ) -data. 17. The method of claim 16,
Wherein the precoding information includes precoding matrix indication information.

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