KR101608777B1 - Method of dl transmitting reference signal in a wireless communication having multiple antennas - Google Patents

Abstract

The present invention relates to a method of performing communication in a mobile communication system having multiple antennas. More particularly, the present invention relates to a method for downlink transmission of a reference signal in a wireless communication system having multiple antennas, the method comprising: allocating a reference signal for a first antenna group to a subframe including a specific area, The method comprising the steps of: pairing a reference signal for a second antenna group with the first antenna group in the specific region and allocating the reference signal in a code division multiplexing manner to the first antenna group; A method for downlink transmission of a signal is provided.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention [0001] The present invention relates to a method for downlink transmission of a reference signal in a wireless communication system having multiple antennas,

The present invention relates to a method for performing communication in a wireless communication system. More particularly, the present invention relates to a method for downlink transmission of a reference signal in a wireless communication system having multiple antennas.

A 3rd Generation Partnership Project (3GPP) wireless communication system based on Wideband Code Division Multiple Access (WCDMA) radio access technology is widely deployed all over the world. HSDPA (High Speed Downlink Packet Access), which can be defined as the first evolutionary phase of WCDMA, provides 3GPP with highly competitive wireless access technology in the mid-term future.

There is E-UMTS to provide high competitiveness in the long term future. E-UMTS is a system that evolved from existing WCDMA UMTS and is being standardized in 3GPP. E-UMTS is also called Long Term Evolution (LTE) system. For details of the technical specifications of UMTS and E-UMTS, refer to Release 7 and Release 8 of "3rd Generation Partnership Project (Technical Specification Group Radio Access Network)" respectively.

The E-UMTS is largely composed of an Access Gateway (AG) located at the end of a User Equipment (UE), a base station and a network (E-UTRAN) and connected to an external network. Typically, a base station may simultaneously transmit multiple data streams for broadcast services, multicast services, and / or unicast services. In the LTE system, Orthogonal Frequency Divisional Multiplexing (OFDM) and Multi-input Multi-out (MIMO) are used to downlink various services.

OFDM represents a high-speed data downlink access system. The advantage of OFDM is the high spectral efficiency that the entire spectrum allocated can be used by all base stations. In OFDM modulation, a transmission band is divided into a plurality of orthogonal subcarriers in the frequency domain and a plurality of symbols in the time domain. Since OFDM divides the transmission band into a plurality of subcarriers, the bandwidth per subcarrier decreases and the modulation time per carrier increases. Since the plurality of subcarriers are transmitted in parallel, the digital data or symbol transmission rate of a specific subcarrier is lower than that of a single carrier.

A multiple input multiple output (MIMO) system is a communication system using a plurality of transmit and receive antennas. The MIMO system can linearly increase the channel capacity without increasing the additional frequency bandwidth as the number of transmit and receive antennas increases. The MIMO technique uses a spatial diversity scheme that can improve transmission reliability using symbols that have passed through various channel paths and a spatial diversity scheme in which each antenna transmits a separate data stream at the same time using a plurality of transmit antennas, And a spatial multiplexing method for increasing the number of received signals.

MIMO technology can be roughly divided into open-loop MIMO technology and closed-loop MIMO technology depending on whether channel information is known at a transmitter. In the open-loop MIMO technique, the transmitter does not know the channel information. Examples of the open-loop MIMO technique include per antenna rate control (PARC), per common basis rate control (PCBRC), BLAST, STTC, and random beamforming. On the other hand, in the closed-loop MIMO technique, the transmitter knows the channel information. The performance of the closed-loop MIMO system depends on how accurately the channel information is known. Examples of the closed-loop MIMO technique include per-stream rate control (PSRC), TxAA, and the like.

Channel information means radio channel information (e.g., attenuation, phase shift, or time delay) between a plurality of transmission antennas and a plurality of reception antennas. In the MIMO system, there are various stream paths by a plurality of transmission / reception antenna combinations, and the channel state has a fading characteristic that varies irregularly in the time / frequency domain due to multipath time delay. Therefore, the transmitting end calculates channel information through channel estimation. The channel estimation is to estimate channel information necessary for restoring a distorted transmission signal. For example, channel estimation refers to estimating the size and reference phase of a carrier wave. That is, the channel estimation is to estimate the frequency response of the radio section or the radio channel.

As a channel estimation method, there is a method of estimating a reference value based on a reference signal (RS) of several base stations using a two-dimensional channel estimator. In this case, RS is a symbol having a high output although it does not have data in order to facilitate carrier phase synchronization and acquisition of base station information. The transmitting side and the receiving side can perform channel estimation using the RS. The channel estimation by the RS estimates a channel through a symbol commonly known to the transmitting and receiving sides, and restores the data using the estimated values. RS is also referred to as pilot.

MIMO systems support Time Division Duplex (TDD) systems and Frequency Division Duplex (FDD) systems. In a time division duplex (TDD) system, since the forward link transmission and the reverse link transmission are in the same frequency domain, estimation can be made on the forward link channel from the reverse link channel by the reciprocity principle.

Wireless communication technologies have been developed to LTE based on WCDMA, but the demands and expectations of users and operators are continuously increasing. In addition, since other wireless access technologies are continuously being developed, new technology evolution is required to be competitive in the future. Cost reduction per bit, increased service availability, use of flexible frequency band, simple structure and open interface, and proper power consumption of terminal.

In this regard, 3GPP is preparing work to standardize the follow-on technology for LTE. This technique is referred to herein as "LTE-Advanced" or "LTE-A ". One of the major differences between LTE systems and LTE-A systems is the number of multiple antennas that they support. Currently, LTE systems are designed to support up to four multiple antennas. On the other hand, the LTE-A system aims to support up to eight multiple antennas. Therefore, the LTE-A system should support downlink transmission of reference signals for up to eight antennas. In particular, in the LTE-A system, an LTE terminal capable of recognizing up to four antennas and an LTE-A terminal capable of recognizing up to eight antennas coexist.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a method for effectively downlinking a reference signal in a wireless communication system having multiple antennas.

Another object of the present invention is to provide a method of efficiently downlinking a reference signal when the number of multiple antennas is increased.

It is another object of the present invention to provide a method of downlinking a reference signal with backward compatibility when the number of multiple antennas is extended.

It is another object of the present invention to provide a method for efficiently transmitting a reference signal in an environment where terminals having different capabilities coexist.

It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are not restrictive of the invention, unless further departing from the spirit and scope of the invention as defined by the appended claims. It will be possible.

According to one aspect of the present invention, there is provided a method for downlink transmission of a reference signal in a wireless communication system having multiple antennas, the method comprising: allocating a reference signal for a first antenna group to a subframe including a specific area, The method comprising the steps of: pairing a reference signal for a second antenna group with the first antenna group in the specific region and allocating the reference signal in a code division multiplexing manner to the first antenna group; A method for downlink transmission of a signal is provided.

According to another aspect of the present invention, there is provided a method of downlinking a reference signal in a wireless communication system having multiple antennas, the method comprising: receiving a reference signal for a first antenna group in a subframe including a specific area having a different accessibility according to a terminal capability; Assigning a reference signal for a second antenna group to the first antenna group in a code division multiplexing manner by paired with the first antenna group when the rank is equal to or greater than a predetermined value; Downlink transmission of the reference signal is provided.

According to another aspect of the present invention, there is provided a method of estimating a channel in a wireless communication system having multiple antennas, comprising: receiving a subframe allocated with a reference signal for first and second antenna groups; Extracting a reference signal for a second antenna group and performing channel estimation with the extracted reference signal, wherein the subframe includes a specific region defined such that a terminal of a predetermined capability can not access, And a reference signal for the second antenna group is paired with the first antenna group in the specific region and allocated in a code division multiplexing manner.

According to another aspect of the present invention, there is provided a method of estimating a channel in a wireless communication system having multiple antennas, comprising: receiving a subframe allocated with a reference signal for first and second antenna groups; Extracting a reference signal for a second antenna group and performing channel estimation using the extracted reference signal, wherein the subframe includes a specific area having a different accessibility according to a terminal capability, And a reference signal for the antenna group is assigned to the first antenna group in the specific region by a code division multiplexing scheme when the rank is equal to or greater than a predetermined value.

According to another aspect of the present invention, there is provided a method for downlink transmission of a reference signal in a wireless communication system having multiple antennas, the method comprising: allocating a reference signal for a first antenna group to a subframe; And allocating a reference signal for a second antenna group in a code division multiplexing manner to a specific position defined to be allocated a Dedicated Reference Signal (DRS), wherein the specific position is a frequency within a subframe And a downlink transmission method of a reference signal dispersed in the time domain.

According to another aspect of the present invention, there is provided a method of estimating a channel in a wireless communication system having multiple antennas, comprising: receiving a subframe allocated with a reference signal for first and second antenna groups; The method comprising: extracting a reference signal for a second antenna group; and performing channel estimation using the extracted reference signal, wherein the reference signal for the second antenna group is a dedicated reference signal (DRS) Is allocated to a specific location defined by a code division multiplexing scheme, and the specific location is distributed in frequency and time domain in the subframe so that the frequency interval is constant.

According to the embodiments of the present invention, the following effects are obtained.

First, a reference signal can be efficiently transmitted downlink in a wireless communication system having multiple antennas.

Second, when the number of multiple antennas is extended, the reference signal can be efficiently transmitted downlink.

Third, when the number of multiple antennas is extended, the reference signal can be downlinked while having backwardness.

Fourth, the reference signal can be efficiently transmitted in an environment where terminals having different capabilities coexist.

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.

The structure, operation and other features of the present invention can be easily understood by the preferred embodiments of the present invention described with reference to the accompanying drawings. The embodiments described below are examples in which the technical features of the present invention are applied to an OFDM system having multiple antennas.

Modeling of Multi-antenna (MIMO) Systems

1 is a configuration diagram of a wireless communication system having a general multiple antenna. As shown in FIG. 1, if the number of transmit antennas is increased to N _{T and the} number of receive antennas is increased to N _R , unlike the case where multiple antennas are used only in a transmitter and a receiver, a theoretical channel transmission capacity . Therefore, the transmission rate can be improved and the frequency efficiency can be remarkably improved. As the channel transmission capacity increases, the transmission rate may theoretically increase by the rate of increase R _i multiplied by the maximum transmission rate R _{o at the} time of single antenna use.

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. After the theoretical capacity increase of the multi-antenna system has been proved in the mid-90s, various techniques have been actively researched to bring it up to practical data rate improvement. In addition, several technologies have already been reflected in various wireless communication standards such as 3G mobile communication and next generation wireless LAN.

The research trends related to multi-antenna up to now include information theory study related to calculation of multi-antenna communication capacity in various channel environment and multiple access environment, study of wireless channel measurement and modeling of multi-antenna system, improvement of transmission reliability and improvement of transmission rate And research on space-time signal processing technology.

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

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

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.

는 전송 전력의 대각행렬

를 이용해 다음과 같이 표현될 수 있다.Also,

Is a diagonal matrix of transmit power

Can be expressed as follows.

에 가중치 행렬

가 적용되어 실제 전송되는 N _T 개의 송신신호

가 구성되는 경우를 고려해 보자. 가중치 행렬

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

는 벡터

를 이용하여 다음과 같이 표현될 수 있다.Transmission power adjusted information vector

A weighting matrix

_Lt ; RTI ID = 0.0 _& gt; N _& lt; / RTI >

. Weighting matrix

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

Vector

Can be expressed as follows.

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.

은 벡터로 다음과 같이 표현될 수 있다.If there are N _R reception antennas,

Can be expressed as a vector as follows.

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. 2 shows channels from N _T transmit antennas to receive antenna i . The channels can be grouped and displayed in vector and matrix form. In FIG. 2, a channel arriving from a total of N _T transmit antennas to receive antenna i may be expressed as follows.

Thus, all channels arriving from N _T transmit antennas to N _R receive antennas may be expressed as:

에서 행의 수는 수신 안테나의 수 N _R 과 같고, 열의 수는 송신 안테나의 수 N _T 와 같다. 즉, 채널 행렬

는 행렬이 N _R ㅧN _T 된다.Channel matrix

The number of rows is equal to the number N _R of receive antennas and the number of columns is equal to the number N _T of transmit antennas. That is,

Is the matrix N _R ㅧ N _T.

를 거친 후에 백색잡음(AWGN; Additive White Gaussian Noise)이 더해진다. N _R 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 of the N _R receive antennas

Can be expressed as follows.

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

의 랭크(

)는 다음과 같이 제한된다.On the other hand, 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.

Another definition of the rank is defined as the number of eigenvalues that are not zero when the matrix is eigenvalue decomposition. Similarly, another definition of a rank is defined as the number of non-zero singular values when singular value decomposition is performed. Therefore, the physical meaning of a rank in a channel matrix is the maximum number that can transmit different information in a given channel.

A transmitter and a receiver of a multi-antenna (MIMO) system

3 shows a block diagram of a base station that may be applied to an embodiment of the present invention.

3, a base station generally includes a control system 302, a baseband processor 304, a transmit circuit 306, a receive circuit 308, multiple antennas 310, and a network interface 312 . The receiving circuit 308 receives the radio signal transmitted from the terminal through the multiple antennas 310. Preferably, a low noise amplifier and filter (not shown) amplify the signal and eliminate broadband interference. A downconversion and digitization circuit (not shown) downconverts the filtered received signal to an intermediate or baseband frequency signal and digitizes it into one or more digital streams.

The baseband processor 304 processes the digitized received signal to extract information or data bits from the received signal. The processing includes demodulation, decoding, error correction, and the like. The baseband processor 304 is typically implemented with one or more digital signal processors (DSPs). The received information is then transmitted over the wireless network via the network interface or to another terminal served by the base station. The network interface 312 interacts with the central network controller and the circuit-switched network forming part of the wireless network that can be connected to the public switched telephone network (PSTN).

On the transmission side, the baseband processor 304 receives digitized data, which may represent voice, data or control information, from the network interface 312 under the control of the control system 302 and encodes the data for transmission. The encoded data is input to a transmission circuit 306. In transmission circuitry 306, the encoded data is modulated by a carrier having a desired transmission frequency or frequencies. A power amplifier (not shown) amplifies the modulated carrier signal to an appropriate level for transmission. The amplified signal is transmitted to multiple antennas 310.

FIG. 4 shows a block diagram of a terminal that may be applied to an embodiment of the present invention.

4, a terminal may include a control system 402, a baseband processor 404, a transmit circuit 406, a receive circuit 408, multiple antennas 410 and a user interface circuit 412 . The receiving circuit 408 receives a radio signal containing information from the one or more base stations via the multiple antennas 410. Preferably, a low noise amplifier and filter (not shown) amplify the signal and eliminate broadband interference. A downconversion and digitization circuit (not shown) then downconverts the received signal filtered with the intermediate or baseband frequency signal. The signal is then digitized into one or more digital streams. The baseband processor 404 processes the digitized received signal to extract information or data bits from the received signal. The processing includes demodulation, decoding, and error correction operations. The baseband processor 404 is typically implemented with one or more digital signal processors (DSPs) and an application specific integrated circuit (ASIC).

At the transmitting end, the baseband processor 404 receives digitized data, which may represent voice, data or control information, from the user interface 312 under the control of the control system 402 and encodes the data for transmission. The encoded data is input to a transmission circuit 406. In transmission circuitry 406, the encoded data is modulated by a carrier having a desired transmission frequency or frequencies. A power amplifier (not shown) amplifies the modulated carrier signal to an appropriate level for transmission. The amplified signal is transmitted to multiple antennas 410.

5 shows a block diagram of a transmitter that may be applied to an embodiment of the present invention.

5, although the transmitter structure is described with reference to a base station, those skilled in the art will appreciate that the structure shown may be used for uplink and downlink transmission. The transmission structure also represents various multiple access structures including, but not limited to, code division multiple access (CDMA), frequency division multiple access (FDMA), time division multiple access (TDMA), orthogonal frequency division multiplexing (OFDM) .

Initially, the network transmits data to be transmitted to the terminal to the base station. Scheduled data, which is a bit stream, is scrambled using data scramble module 504 in a manner that reduces the peak to average power ratio associated with the data. A CRC (Cyclic Redundancy Check) for the scrambled data is determined by the CRC adding module 506 and attached to the scrambled data. To facilitate data recovery and error correction at the terminal, the channel encoder module 508 performs channel coding. Channel coding can effectively add redundancy to data. The channel encoder module 508 may use a turbo encoding technique.

The processed data bits are systematically mapped to the corresponding symbol by the mapping module 514 depending on the selected baseband modulation. Quadrature amplitude modulation (QAM) or quadrature phase shift key (QPSK) modulation forms can be used. The bit group is mapped to a symbol representing the position in the amplitude and phase constellation. The symbol block is then processed by a spatial time code (STC) encoder module 518. The STC encoder module 518 processes the symbols according to the selected STC encoding mode and will provide N outputs corresponding to the number of multiple transmit antennas 310 of the base station. The symbol stream output from the STC encoder module 518 is inverse Fourier transformed by the IFFT processing module 520. Thereafter, the prefix and RS addition module 522 adds a cyclic prefix (CP) and an RS to the inverse Fourier transformed signal. The digital upconversion (DUC) module and digital to analog (D / A) conversion module 524 then upconverts the previously processed signal to an intermediate frequency in the digital domain and converts it to an analog signal. The analog signal is then simultaneously modulated, amplified, and transmitted at the desired RF frequency through the RF module 526 and the multiple antennas 310.

Figure 6 shows a block diagram of a receiver that may be applied to an embodiment of the present invention.

6, although the receiver structure is described with respect to a terminal, those skilled in the art will appreciate that the structure shown may be used for uplink and downlink transmission. When the transmission signal arrives at the multiplex transmission antenna 410, each signal is demodulated and amplified by the corresponding RF module 602. For convenience, only one of the multiple receive paths in the receiver is shown. The analog to digital (A / D) conversion and down conversion module (DCC) 604 digitizes and downconverts the analog signal for digital processing. The digitized signal may be used in the automatic gain control module (AGC) 606 to control the amplifier gain in the RF module 602 based on the received signal level.

The CRC checked data is then recovered by the descrambling module 646 to the original data 648.

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. In order to determine the channel information, a method is used in which a signal known to both the transmitting side and the receiving side is transmitted, and channel information is detected with a degree of distortion when the signal is received through the channel. The 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, a separate reference signal must exist for each transmission antenna.

The structure of the 3GPP LTE downlink RS is shown in FIG. For one resource block, the horizontal axis represents the time axis and the vertical axis represents the frequency axis. One subframe consists of two slots. In the case of a normal CP, each slot is composed of seven symbols [7 (a)]. For extended CPs, each slot consists of six symbols [7 (b)]. Extended CPs are commonly used in environments with long delays. The number of supported base station transmit antennas is 4, and the resources used for transmitting the RS signal to each of the antenna ports 0-3 are shown as '0', '1', '2', and '3'. The RSs in antenna ports 0-3 are common RSs (CRs). l denotes an OFDM symbol index, and sc denotes a subcarrier index.

Time / frequency spacing of RS

Considering a 20 MHz system in 3GPP LTE, one symbol consists of 2048 samples. At this time, the symbol duration (duration) is 66.67us (1us = 10- ⁶ second). In the case of a normal CP, the first symbol among the 7 symbols uses 160 samples as a CP, and the remaining 6 symbols use 144 samples as a CP. Therefore, the first symbol and the remaining symbols using the normal CP can cover a channel delay spread of 5.2 us and 4.69 us, respectively. For the extended CP, the six symbols are CP with 512 samples. Thus, a symbol using an extended CP can cover a channel delay spread of 16.67 us. Referring to FIG. 7 (a) and FIG. 7 (b), RS for a specific antenna has a frequency spacing of 6 within one symbol. However, since the RS for a specific antenna is staggered in a slot or a subframe, the frequency interval between RSs becomes 3 as a result. Here, the frequency interval means a subcarrier interval between adjacent RSs in the frequency axis. In this case, adjacent RSs need not be within the same OFDM symbol but may be distributed over several OFDM symbols, slots, and subframes on the time axis. The frequency interval may vary according to the profile of the OFDM system. As an example, the frequency interval may be 15 kHz.

The relationship between the frequency interval of the RS and the channel estimation capability will be described in more detail using the formulas.

에 IDFT (Inverse Discrete Fourier Transform)를 취하면 다음과 같이 표현될 수 있다.For example,

(Inverse Discrete Fourier Transform) can be expressed by the following equation.

가 된다.

에 IDFT를 취하면 다음과 같이 표현될 수 있다.Assuming that the frequency interval of RS is Z + 1, there are Z data signals between each RS. It is assumed that only the RS is transmitted in order to observe the effect of the frequency spacing of the RS on the channel estimation. In this case, Z 0s are filled as data signals between each RS, and the transmission sequence is

.

The IDFT can be expressed as follows.

과

는

이다.From here,

and

The

to be.

와

의 관계를 살펴보면, 다음과 같이

신호가 0의 개수만큼 반복적으로 나타남을 알 수 있다.Using Equations 11 and 12,

Wow

In the following,

It can be seen that the signal repeatedly appears as many as zero.

Figures 8 and 9 show the RS observed in the time domain when the frequency spacing of RS is 6. Referring to FIG. 13, it can be seen that, according to Equation (13), RS appears in six repeated forms within one OFDM symbol duration. Assuming that the OFDM symbol duration is 66.66us, the RS is repeated in the time domain with a period of 11.11us. Therefore, if the channel delay spread does not exceed 11.11us, restoration of the RS signal is theoretically possible. It assumes a wireless environment with a channel delay of up to 5.2 us or 4.69 us when the general CP is applied (CP length: 5.2 us or 4.69 us). Therefore, even if the frequency interval of the RS is 6 in the environment where the general CP is applied, since there is no interference between the RSs, it is possible to restore the RSs (Fig. 8). On the other hand, when the extended CP is applied (CP length: 16.67us), the wireless environment assumes a maximum channel delay of 16.67us. Therefore, in the environment where the extended CP is applied, interference may exist between RSs when the frequency interval of RS is 6 (FIG. 9).

To overcome this problem, in LTE, the RS of each antenna is staggered and the RS frequency interval is set to 3. FIG. 10 shows the RS observed in the time domain when the frequency interval of RS is 3. Referring to FIG. 10, since the frequency interval of the RS is 3, it can be seen that RS appears in three repeated forms within one OFDM symbol duration. Assuming that the OFDM symbol duration is 66.66us, the RS is repeated in the time domain with a cycle of 22.22us. That is, since the coverable channel delay spread is 22.22 us, it is possible to restore the original state of the RS irrespective of the CP length.

Code division multiplexing (CDM) - Cyclic delay in time domain

와 같은 형태일 수 있다. 주파수 도메인에서 직교 코드를 곱함으로써, RS는 시간 도메인에서 순환 지연(Cyclic dealy)될 수 있다. 시퀀스

에 직교 코드를 곱한 경우에, 시간 도메인에서 순환 지연되는 것을 수학식 15에 나타내었다.Assignment of RSs for different antennas to the same resource area is called multiplexing. The multiplexing scheme may be time division multiplexing, frequency division multiplexing or code division multiplexing. Among them, code division multiplexing means that an orthogonal code (sequence) set differently for each antenna is multiplied by RS in the frequency domain and assigned to the same radio resource (frequency / time). The orthogonal code

Lt; / RTI > By multiplying the orthogonal code in the frequency domain, the RS can be cyclically delayed in the time domain. sequence

Is multiplied by an orthogonal code, a cyclic delay in the time domain is shown in Equation (15).

를 곱하면 시간축 상에서 순환 지연된다.Therefore, in the frequency domain

Multiply it by a cyclic delay on the time axis.

Evolution from LTE to LTE-A: Extension of multi-antenna systems

One embodiment of the present invention relates to a method for downlink transmission of RSs when the number of transmit antennas is extended to M (> N) in a conventional N private system. In LTE, the number of downlink antennas is assumed to be 4 (= N). Therefore, the LTE terminal can recognize up to four antennas. On the other hand, in LTE-A, the number of antennas used for downlink transmission is considered to be extended to 8 (= M). It should be noted that an embodiment of the present invention to be described later is exemplified using LTE-A, but can be applied to the same principle on any MIMO system satisfying the condition of M > N.

In such an environment, LTE UEs and 8 (= M) antennas capable of recognizing only the existing 4 (= N) transmit antennas can be recognized in the cell of the base station supporting LTE-A The LTE-A terminals exist together. In this case, in addition to the reference signal for supporting the existing N antennas, a reference signal for supporting an additional 4 (= M-N) antennas should be transmitted. In this case, it is necessary to efficiently transmit data and reference signals in an environment where an LTE-A UE recognizing eight antennas is added to an LTE UE recognizing only four existing antennas without additional signaling. In LTE, it is defined to use antenna port 5 in the closed loop rank 1 transmission mode. However, in the future, for the sake of convenience, the antenna port for the LTE-A system extended to 8 antennas is changed to 0-7 Define it.

On the other hand, as shown in FIG. 7 (a), RS overhead is 14.3% based on the normal CP. Therefore, in an embodiment of the present invention, an RS transmission scheme using CDM will be proposed so that the overhead does not exceed 15% even if the number of antennas is increased. So far, we have used the term CP and extended CP based on the LTE system. However, an embodiment of the present invention described below may be used in a system that covers multiple channel delays. Therefore, thereafter, the case where the channel delay is small and the case where the channel delay is large will be described.

First Embodiment: Accessibility to  8Tx antenna design using

As described above, the LTE UE and the LTE-A UE coexist in the LTE-A system. In this case, in order to maintain backward compatibility with the LTE UE, the LTE-A system may operate a plurality of subframes having different accessibility according to the terminal capability. For convenience, the subframes to be used in the LTE-A system are classified into two subframes according to the terminal accessibility (the first subframe and the second subframe).

In this specification, the first subframe means a subframe in which all terminals can freely access regardless of their capabilities. On the other hand, the second sub-frame means a sub-frame including a specific area having a different accessibility depending on the terminal capability. As an extreme example, the entire area of the second subframe may be composed only of a specific area having different accessibility depending on the terminal capability. The different accessibility according to the terminal capability means that a terminal having a predetermined capability can access only or a terminal having a specific capability can access. For example, the first subframe can be accessed and read by both the LTE UE and the LTE-A UE. On the other hand, the second subframe consists of (i) an area where both the LTE UE and the LTE-A UE can access and an area where only the LTE-A UE can access, or (ii) Or < / RTI > Therefore, in case (i), the LTE system can use only a part of the second sub-frame. (ii), the LTE system can not use the entire area of the second subframe (e.g. slot 1: l = 0 to slot 2: l = 5 (6)). The second subframe may include a Multimedia Broadcast / Multicast Single Frequency Network (MBSFN) subframe defined in the LTE system.

Therefore, in one embodiment of the present invention, it is proposed to downlink RS for an antenna added to an LTE system using a second subframe. Hereinafter, an embodiment of the present invention will be described in detail with reference to an MBSFN subframe as an example. In the present invention, the LTE, LTE-A, LTE UE, LTE-A UE, and MBSFN subframes are exemplified to facilitate understanding of the present invention, and the scope of the present invention is not limited thereto.

FIG. 11 shows the structure of the MBSFN subframe. The MBSFN is a multimedia broadcast / multicast over a single frequency network, an MBMS single frequency network, a multicast / broadcast over a single frequency network, a multicast broadcast single frequency network ", "multicast broadcast single frequency network ", and the like. Referring to FIG. 11, the MBSFN is composed of two slots. One slot is composed of 7 OFDM symbols for a normal CP and 6 OFDM symbols for an extended CP. A resource block (RB) included in the MBSFN is composed of 12 subcarriers in the frequency domain.

On the other hand, when signaling that the LTE UE is an MBSFN subframe in the LTE system, the LTE UE does not read the data portion of the MBSFN subframe and does not use the channel information. That is, when the LTE UE receives signaling that the received subframe is an MBSFN subframe, it uses only information of l = 0 and l = 1 of slot 1 (denoted by 'LTE UE'). Therefore, the remaining symbols except for the two OFDM symbols are resources that can be freely used by the LTE-A UE regardless of the operation of the LTE UE (shaded portion, denoted by 'LTE-A'). That is, the LTE UE is restricted from reading information by accessing slot 1: l = 2-5 (6) and slot 2: l = 0-5 (6) of the MBSFN subframe. On the other hand, the LTE-A UE can freely access and read information for Slot 1: l = 2-5 (6) and Slot 2: l = 0-5 (6). Therefore, it is possible to transmit RS for LTE-A UE to slot 1: l = 2-5 (6) and slot 2: l = 0-5 (6).

12 is a flowchart illustrating an example of downlink transmission of RSs at antenna ports 0 through 7 using a second subframe according to an embodiment of the present invention.

Referring to FIG. 12, the BS allocates an RS for a first antenna group to a subframe including a specific area having a different accessibility according to the terminal capability (S1210). The specific area may be an area defined to be unreadable by a terminal supporting only the antenna port 0-3. The UE includes an LTE UE. The subframe may be an MBSFN subframe. The specific area may be slot 1: l = 0 to slot 2: l = 5 (6). If the subframe is an MBSFN subframe, the specific area may be slot 1: l = 2 to slot 2: l = 5 (6). The first antenna group includes an antenna used in a conventional multi-antenna system. The conventional multi-antenna system may be a system supporting 1-4 multi-antennas. Preferably, the conventional multi-antenna system may be an LTE system. For example, the first antenna group consists of antenna ports 0-3. The RS may be a common RS (CRS) or a dedicated RS (DRS).

The base station then pairs the reference signal for the second antenna group with the first antenna group in the specific area, and allocates the reference signal in the code division multiplexing manner (S1220, FIG. 12 (a)). As another example, the base station may pair the reference signal for the second antenna group with the first antenna group in the specific area and allocate the reference signal in a code division multiplexing manner only when the rank is equal to or greater than a predetermined value (S1220, FIG. 12 b)). The predetermined value may vary depending on the system. For example, if the system supports eight multiple antennas, the predetermined value may be five. When the code division multiplexing scheme is applied when the rank is smaller than the predetermined value, the RS transmission power is reduced. Therefore, when the rank size is smaller than the predetermined value, it may be more efficient to operate as an LTE system than to reduce the transmission power of the antenna by half. The second antenna group is composed of an antenna newly added to the existing multi-antenna system. The second antenna group may include one to four antennas. The system to which the second antenna group is applied is a system supporting multiple antennas of the antenna port 4-7. The system to which the second antenna group is applied may be an LTE-A system. For example, the second antenna group is composed of antenna ports 4-7.

Thereafter, the base station transmits the subframe downlink (S1230).

FIG. 13 is a flowchart illustrating a method of estimating a channel in a case where an RS of an antenna port 0-7 is transmitted using a second subframe according to another embodiment of the present invention.

Referring to FIG. 13, the UE receives subframes to which reference signals for the first and second antenna groups are allocated (S1310). The subframe includes a specific area having a different accessibility according to the terminal capability, and a reference signal for the second antenna group is paired with the first antenna group in the specific area and is allocated in a code division multiplexing manner 13 (a)). As another example, the reference signal for the second antenna group is paired with the first antenna group in the specific area and allocated in a code division multiplexing manner only when the rank is equal to or greater than a predetermined value (Fig. 13 (b) ). The predetermined value may vary depending on the system. For example, if the system supports eight multiple antennas, the predetermined value may be five. The RS may be a common RS (CRS) or a dedicated RS (DRS). Then, the terminal extracts reference signals for the first and second antenna groups (S1320). The terminal performs channel estimation with the extracted reference signal (S1330). The terminal is a terminal capable of supporting the antenna port 4-7. The UE includes an LTE-A UE. The first antenna group, the second antenna group, the terminal capability, the specific area, and the like are the same as those described in FIG.

 Hereinafter, a method of allocating an RS for an LTE-A UE in an MBSFN sub-frame will be described in detail.

In the LTE system, the number of RSs is defined as shown in Table 1 below in order to keep the maximum overhead below 15% when transmitting RSs. The LTE system supports antenna ports 0-3, and the number of each RS is designed as 8, 8, 4, and 4 per subframe. Since the number of RSs for antenna ports 2 and 3 is 4, the number of RSs for additional antenna ports 4-7 is limited to 4, respectively, when the LTE-A system supports antenna ports 0-7. Lt; / RTI > As the number of RSs increases, accurate channel estimation is possible. However, in the LTE environment in which four antennas are supported, RSs of the antenna ports 2-3 are limited to four, so it is preferable that the same restrictions are also applied to the additional antennas. The limitation on the number of RSs for the added antenna port is shown in the dark portion of Table 1 below.

As can be seen from Table 1, 16 additional RSs are needed for antenna ports 4-7 of the LTE-A UE. In this regard, referring to the MBSFN subframe in FIG. 7, in LTE, the number of RSs transmitted through the remaining symbols except for the two preceding symbols is 16. Therefore, in an embodiment of the present invention, it is proposed that the same position as the LTE UE is used as it is for the RS for the antenna added in the LTE-A UE. If the RS position used by the LTE UE is used as it is, the LTE-A UE can advantageously use the hardware used in the LTE UE as it is.

An antenna pairing is applied to apply the CDM to the RS transmitted to the antenna port 0-3 and the RS transmitted to the antenna port 4-7. The antenna ports 0-3 used in the LTE system and the antenna ports 4-7 added in the LTE-A system can be paired in any combination. As an example, the RS for the antenna ports 0-3 may be allocated as a pattern 1-1 or 1-2 in a specific area set to be unreadable by the LTE UE in the MBSFN subframe. The specific area is l = 2-5 (6) of slot 1 and l = 0-5 (6) of slot 2.

[Pattern 1-1]

[Pattern 1-2]

Here, 1 denotes an OFDM symbol index, numbers in the table indicate subcarrier indices in a resource block, '-' denotes that a reference signal is not allocated,

Here, antenna ports 0-3 may be permutated, and the reference signal may be shifted along the frequency axis or the time axis. The transition may be determined using a cell identifier (ID) or the like.

A) When the channel delay is small

To cover small channel delays such as 5-6us, the frequency spacing of the RS can be 3 or 6. 14 (a) and 14 (b) illustrate an RS pattern for pairing antenna ports 0-3 and antenna ports 4-7. In the figure, only the specific area to which the CDM is applied (i.e., the area defined so that the LTE terminal can not read) is shown. In the figure, the reference signals for antenna ports 0-7 are shown as 0-7, respectively.

The reference signal patterns shown in Figs. 14 (a) and 14 (b) are summarized as follows.

[Pattern 1-3]

[Pattern 1-4]

Here, 1 denotes an OFDM symbol index, numbers in the table indicate subcarrier indexes in a resource block, '-' denotes that a reference signal is not allocated, numbers in parentheses indicate paired antennas,

Here, antenna ports 0-3 and antenna ports 4-7 may each be independently permutated, and the reference signal may be shifted along the frequency axis or the time axis. The transition may be determined using a cell identifier (ID) or the like.

Referring to pattern 1-3 shown in FIG. 14A, antenna pairing is performed in slot 2 with (0,4), (1,5), (2,6), and (3,7). However, since the number of RSs in antenna ports 0 and 1 is greater than the number of RSs in antenna ports 2 and 3, pairing is performed differently in slots 1 (0, 7) and (1, 6).

Referring to pattern 1-4 shown in FIG. 14 (b), antenna pairing is performed in slot 2 with (0,6), (1,7), (2,4), and (3,5). However, since the number of RSs of antenna ports 0 and 1 is larger than the number of RSs of antenna ports 2 and 3, the pairing is different from (0,5) and (1,4) in slot 1.

On the other hand, orthogonality between the RSs must be maintained by using codes / sequences in order to prevent the RSs of the paired antennas from interfering with each other. As an example of the CDM, it is proposed to give an appropriate cyclic delay value on the time axis as shown in Equation (14).

Fig. 15 shows an example in which CDM is applied to a part of the antenna pair shown in Fig. 14 (a).

와 안테나 포트 2

은 안테나 포트 6의 RS에 각각 CDM으로 페어링된다[슬롯 1:(1,6), 슬롯 2:(2,6)]. 즉, 안테나 포트 6은 슬롯 1과 슬롯 2에서 페어링되는 안테나가 달라진다. 따라서, 도 15에서 안테나 포트 6에 대한 RS는 주파수 간격이 3으로 보이지만, 실제 주파수 간격은 6이 된다. 안테나 포트 6에 대한 RS의 주파수 간격이 6이므로, 페어링되는 모든 안테나의 RS 시퀀스에

을 곱해준다. 이와 같이, 모든 안테나 페어에

을 곱함으로써 CDM 방법으로 기존의 안테나와 추가된 안테나를 구분할 수 있다.15 (a), the antenna port 1

And antenna port 2

Are respectively paired to the RS of the antenna port 6 as CDM (Slot 1: (1,6), Slot 2: (2,6)). That is, antenna port 6 is different from antenna to be paired in slot 1 and slot 2. Therefore, in FIG. 15, the RS for antenna port 6 has a frequency interval of 3, but the actual frequency interval is 6. Since the frequency interval of RS to antenna port 6 is 6, the RS sequence of all antennas to be paired

. In this manner,

The CDM method can distinguish an existing antenna from an added antenna.

Referring to FIG. 15 (b), in the time domain, the RS of the antenna port 6 is expressed in the form of a cyclic delay for the RS of the paired antenna port 1 or 2. 15A, since the CDM code multiplied by the paired RS causes a phase difference of? In the frequency domain, the RS of the antenna port 6 in the time domain is shifted by 1/2 the RS of the antenna port 1 or 2 Cyclic delay. Therefore, the RS of the added antenna port 6 can be distinguished from the RS of the existing antenna port 1 or 2. The RS of antenna port 6 is spaced by 11.11 / 2us on the time axis from RS of antenna port 1 or 2, so it can cover a small channel delay such as 5-6us.

Meanwhile, as described above, the same CDM cyclic delay may be applied to all antenna pairs, but a different cyclic delay may be applied to each antenna pair. FIG. 16 shows an example in which CDM is applied in a manner different from FIG.

을 곱해준다. 여기에서, θ는 시간 도메인에서 CDM 코드가 곱해진 RS의 순환 지연 값을 결정한다. 시간 도메인에서의 순환 지연 값은 Pㅧ(θㆇ2π)로 결정된다. P는 CDM 코드가 곱해지는 RS의 주기를 나타낸다. 따라서, 상기 θ는 RS의 순환 지연을 무선 환경에 따라 허용된 채널 딜레이 보다 크거나 같게 하는 값 가운데서 독립적으로 선택될 수 있다. 예를 들어, 상기 θ는 순환 지연 값을 5-6us 보다 크게 하는 임의의 값일 수 있다. 예를 들어, θ는 π/2≤θ≤3π/2 일 수 있다. 이 경우, RS는 시간 도메인에서 페어링된 RS의 1/4 내지 3/4 주기 순환 지연된다. 도 16(b)에 θ의 변화에 따라 안테나 포트 4의 RS가 소정 범위 내에서 순환 지연되는 것을 나타냈다. 이와 같이, 주파수 간격이 3인 안테나 페어에

을 곱함으로써 CDM 방법으로 기존의 안테나와 추가된 안테나를 구분할 수 있다. 그러나, 간단한 필터 디자인을 위해, 페어링되는 안테나의 RS 시퀀스에

을 곱해줄 수 있다. 상기 경우를 도 16(c)에 도시하였다. θ가 π이므로 추가된 안테나의 RS는 1/2 주기 순환 지연된다(11.11us).Referring to slot 2 in FIG. 14 (a), antenna port 4 is paired with antenna port 0 (0, 4). Within slot 2, the antenna port 0 has a frequency spacing of 3 by RS due to staggering. Therefore, the frequency interval of the RS of the antenna port 4 paired with the antenna port 0 becomes 3. Since the frequency interval of RS is 3, the RS sequence of the antenna to be paired

. Here, θ determines the cyclic delay value of the RS multiplied by the CDM code in the time domain. The cyclic delay value in the time domain is determined by P ㅧ ( θ ㆇ 2π). P represents the period of the RS in which the CDM code is multiplied. Therefore, the &thetas; may be independently selected from among the values for making the cyclic delay of the RS equal to or greater than the channel delay allowed according to the radio environment. For example, theta may be any value that makes the cyclic delay value larger than 5-6us. For example, θ may be a π / 2≤ θ ≤3π / 2. In this case, RS is delayed by 1/4 to 3/4 of the RSs paired in the time domain. According to the change of θ in FIG 16 (b) are shown to be the antenna port 4 RS delay a predetermined cycle within a range. Thus, in an antenna pair having a frequency interval of 3

The CDM method can distinguish an existing antenna from an added antenna. However, for a simple filter design, the RS sequence of the paired antenna

. ≪ / RTI > This case is shown in Fig. 16 (c). Since θ is π, the RS of the added antenna is delayed by 1/2 cycle (11.11us).

을 곱해 CDM 방법으로 안테나를 구분하는 예를 나타낸다.FIG. 17 is a diagram illustrating an example of the RS sequence of the antenna to be paired when the frequency interval of RS is 3

And the antenna is divided by the CDM method.

Referring to slot 2 in FIG. 14 (a), antenna port 4 is paired with antenna port 0 (0, 4). In this case, the receiver can think that the frequency interval of RS is 6 based on one OFDM symbol. Also, the frequency interval of the RS is 3 based on one slot.

와

로 구분되고, 각각

및

이 곱해진 형태가 된다. 이 경우,

와

로부터 형성된 RS는 시간 도메인에서 1/2 주기만큼 순환 천이된다. 한 슬롯을 기준으로 RS 처리가 이뤄지면,

에

이 곱해진 형태가 된다.

로부터 형성된 RS는 시간 도메인에서 1/4 주기 순환 천이된다.If an RS process is performed based on an OFDM symbol, the RS of the paired antenna port 0

Wow

, Respectively

And

Is multiplied. in this case,

Wow

Is cyclically shifted by 1/2 period in the time domain. If RS processing is performed based on one slot,

on

Is multiplied.

Is cyclically transited in the 1/4 period in the time domain.

B) Large channel delay

We propose to apply the proposed RS transmission scheme in case of small channel delay when the channel delay is large. As described above, in order to cover the case where the channel delay is as large as 16.67 us, the frequency interval of RS must be 3. However, in the case where the above-mentioned "A) channel delay is small ", the frequency interval of RS is 6 in some cases.

In this regard, referring again to Figs. 14 (a) and (b), antenna pairing in slot 1 and slot 2 did not exactly match. Therefore, some antennas apply the CDM with a frequency interval of 6 for RS. Therefore, in consideration of an environment in which the channel delay spread is long, it is proposed to change the RS position of the antenna port 0-3 so that the frequency interval becomes 3.

Figs. 18 (a) and 18 (b) show an example of changing the position of the antenna so that the frequency interval of RS is 3. In the figure, only the specific area to which the CDM is applied (i.e., the area defined so that the LTE terminal can not read) is shown. In the figure, the reference signals for antenna ports 0-7 are shown as 0-7, respectively.

The reference signal patterns shown in Figs. 18 (a) and 18 (b) are summarized as follows.

[Pattern 1-5]

[Pattern 1-6]

Here, 1 denotes an OFDM symbol index, numbers in the table indicate subcarrier indexes in a resource block, '-' denotes that a reference signal is not allocated, numbers in parentheses indicate paired antennas,

Here, antenna ports 0-3 and antenna ports 4-7 may each be independently permutated, and the reference signal may be shifted along the frequency axis or the time axis. The transition may be determined using a cell identifier (ID) or the like.

If the RS positions of the antenna ports 0-3 are changed as shown in patterns 1-5 and 1-6, the antenna ports 0-3 and the additional antenna ports 4-7 can be correctly paired and staggered to apply the CDM . In Patterns 1-5, all antennas in the MBSFN subframe are paired with (0,4), (1,5), (2,6), and (3,7), and then CDM is applied to each. Similarly, patterns 1-6 can consider CDM in the same way. As described above, by changing the RS position of the antenna port 0-3, the frequency interval of the added antenna port 4-7 can be reduced to 3.

Figures 19 and 20 show RSs observed in the time domain when RSs are assigned as patterns 1-5 or the like. Referring to FIG. 19, it can be seen that the RS of the antenna port 6 is delayed by 11.11 us compared to the RS of the paired antenna port 0. Therefore, if the channel delay spread exceeds 11.11 us, interference may occur between the RSs of the antenna. However, since the signal strength of the RS signal decreases exponentially along the time axis, it is expected that the interference will not be large. On the other hand, when the channel delay is large, it is required to cover the channel delay spread to 16.67us. However, in the case of using the antenna port 0-7, the channel delay spread is rarely 16.67us. However, when the channel delay is large, it is preferable to apply the above method in an environment in which a channel delay spread of 11.11us can cover (Fig. 20).

In the current LTE-A, there are discussions about RS for channel measurement and RS for data demodulation. In order to support LTE-A in the MBSFM subframe, the present invention can transmit RSs of newly increased antennas using CDM. The present invention has the advantage that it can be used for both RS for channel measurement and RS for data demodulation.

Example 2: dedicated RS design for 8Tx antenna

The method of downlink RS using MBSFN subframe has a problem that it can notify the UE only when the MBSFN subframe is transmitted. Therefore, even in the case of transmitting a general subframe, a method of downlinking RSs for up to 8 multiple antennas is required. In the LTE-A UE, it is possible to use the DRS position defined in LTE as an RS for the antenna port 4-7. Therefore, in another embodiment of the present invention, it is proposed to use dedicated RSs (DRs) transmitted in all subframes for supporting terminals with 5-8 antennas.

FIG. 21 shows a flowchart of allocating the RS of the antenna port 4-7 to the DRS position according to the CDM scheme according to an embodiment of the present invention.

Referring to FIG. 21, the BS allocates an RS for a first antenna group to a subframe (S2110). The first antenna group includes an antenna used in a conventional multi-antenna system. The conventional multi-antenna system may be a system supporting multiple antennas of antenna ports 0-3. The conventional multi-antenna system may be an LTE system. For example, the first antenna group consists of antenna ports 0-3. The RS for the first antenna group may be a common RS (CRS).

Then, the BS allocates the RS for the second antenna group to the specific location defined to be allocated DRS in the subframe in a code division multiplexing manner (S2120).

At this time, the BS may allocate the RS for the first antenna group and the RS for the second antenna group in a code division multiplexing manner to a specific location defined to be allocated DRS in the subframe.

The particular location may be distributed in the frequency and time domain within the subframe such that the frequency spacing is constant. The frequency interval may be 3 or less when a normal CP is used and may be 1 when an extended CP is used. In addition, the specific position may be staggered between OFDM symbols. In addition, the specific position may have a frequency interval of 2 or 3 within the same OFDM symbol. The second antenna group is composed of an antenna newly added to the existing multi-antenna system. The second antenna group may include one to four antennas. The system to which the second antenna group is applied is a system supporting 5-8 multiple antennas. The system to which the second antenna group is applied may be an LTE-A system. For example, the second antenna group is composed of antenna ports 4-7.

Thereafter, the base station transmits downlink subframes allocated to the RS to the mobile station (S2130).

The BS may transmit to the MS the subframe identification information indicating that the subframe is for a particular MS supporting 5-8 multiple antennas. The subframe identification information may be transmitted to the UE through system information (SI), RRC message, L1 / L2 control signaling (e.g. PDCCH) or MAC / RLC / PDCP PDU. The RRC signal may be a signal related to RRC disconnection, RRC connection request, RRC connection establishment, radio bearer setup, radio bearer reset, RRC connection reset, and RRC connection reestablishment.

FIG. 22 is a flowchart illustrating a method of estimating a channel in a case where an RS of an antenna port 4-7 is transmitted using a DRS position according to another embodiment of the present invention.

Referring to FIG. 22, the UE receives a subframe to which reference signals for the first and second antenna groups are allocated (S2210). Wherein a reference signal for the second antenna group is allocated in a code division multiplexing manner to a specific position defined to be allocated a dedicated reference signal (DRS), and the specific position is allocated to a frequency within the subframe And the time domain. Alternatively, a dedicated reference signal for the first antenna group and a dedicated reference signal for the second antenna group may be code division multiplexed and allocated to the specific location.

Then, the terminal extracts reference signals for the first and second antenna groups (S2220). The terminal performs channel estimation using the extracted reference signal (S2230). The first antenna group and the second antenna group are the same as those described in Fig. Also, the terminal can receive from the base station the subframe identification information indicating that the subframe is for a specific terminal supporting 5-8 multiple antennas.

23 (a) shows a subframe using a general CP used for downlink transmission in the LTE system. In the figure, RSs for antenna ports 0-3 are shown as 0-4, respectively. The RSs in antenna ports 0-3 are common RSs (CRs). The DRS position is indicated by D in the resource element. l denotes an OFDM symbol index, and sc denotes a subcarrier index.

Referring to FIG. 23 (a), the frequency interval between DRSs is 2. Therefore, the RS allocated to the DRS position appears twice in one symbol in the time domain. Letting the duration of the symbol be 66.67us, the resolvable time resolution of the RS assigned to the DRS location is 33.33us. Therefore, if the RSs for the four antennas added in the LTE-A system are allocated to the DRS positions in the CDM scheme, each RS occupies 33.33 / 4 = 8.33us. Since the expected channel delay in a typical CP is 5-6us, it is possible to transmit the RS for antenna 4-7 without interference in the DRS position. In the case of a subframe using a general CP, it is also possible to arrange the DRS so that the frequency interval between DRSs is 1 or 2.

23 (b) shows a subframe using an extended CP used for downlink transmission in the LTE system. In the figure, RSs for antenna ports 0-3 are shown as 0-3 respectively. The antenna port 0-3 RS is a common RS (CRS). The DRS position is indicated by D in the resource element. l denotes an OFDM symbol index, and sc denotes a subcarrier index.

Referring to FIG. 23 (b), in the extended CP, the frequency interval between DRSs is 3. Therefore, the RS allocated to the location in the DRS is repeated three times in one symbol in the time domain. Letting the duration of the symbol be 66.67us, the resolvable time resolution of the RS assigned to the DRS location is 22.22us. Therefore, if the RSs for the four antennas added in the LTE-A system are allocated to the DRS positions in the CDM scheme, each RS occupies 22.22 / 4 = 5.55us. The expected channel delay at the extended CP is about 15us. Therefore, when the RS of the antenna port 4-7 is transmitted in the CDM scheme to the DRS position, interference may occur between the RSs of the antenna. That is, performance degradation may occur if the RS of the antenna port 4-7 is transmitted using the DRS position of the subframe using the extended CP of the LTE system.

Therefore, in one embodiment of the present invention, we propose 10 DRS patterns that can transmit the RS of the antenna port 4-7 without performance degradation in a long channel delay environment.

24 shows an example of allocating a DRS position to a subframe using an extended CP according to an embodiment of the present invention. The allocation pattern is summarized as follows.

[Pattern 2-1]

Here, 1 denotes an OFDM symbol index, numbers in the table indicate subcarrier indices in a resource block, '-' denotes that a reference signal is not allocated, and the reference signal shifts along a frequency axis or a time axis. .

Referring to RS pattern 2-1, the frequency interval between DRSs is maintained at 3 in one OFDM symbol. However, within one subframe, three OFDM symbols are staggered so that the frequency interval between DRSs is equal to one. That is, the DRS allocated to the OFDM symbol l = 4 of the first slot and the DRS allocated to the OFDM symbols l = 1 and l = 4 of the second slot are not overlapped with each other in the frequency domain. In this case, the frequency interval between the DRSs may ultimately be one. When the RS for the antenna port 4-7 is allocated to the DRS position in the CDM scheme, the resolution at which the RS of each antenna can estimate the channel is 66.66 us / 4 = 16.67 us. The expected maximum channel delay spread for a long-channel radio environment is 15us. Since a time resolution longer than the maximum channel delay spread is guaranteed for each antenna, channel estimation is possible without performance degradation.

25 shows an example of allocating a DRS position to a subframe using an extended CP according to another embodiment of the present invention. The allocation pattern is summarized as follows.

[Pattern 2-2]

Here, 1 denotes an OFDM symbol index, numbers in the table indicate subcarrier indices in a resource block, '-' denotes that a reference signal is not allocated, and the reference signal shifts along a frequency axis or a time axis. .

The method of arranging the DRS in the RS pattern 2-2 is basically the same as that described in the RS pattern 2-1. However, in the RS pattern 2-2, the DRS is not located in the OFDM symbol through which the CRS of the antenna port 0-3 is transmitted. Generally, in order to improve the performance of the RS, the power used for the RS can be boosted higher than the power used for other resource elements. If the power of all the RSs in the resource block is boosted, the power range to be supported by the power amplifier varies depending on the number of RSs allocated in the same OFDM symbol. For example, referring to pattern 2-1, four RSs are allocated to one OFDM symbol, while eight RSs are allocated to l = 1 OFDM symbols of slot 2. Therefore, the power range to be supported by the power amplifier must be designed to be high in order to boost the eight RSs. In addition, if power is boosted to only four RSs in the same manner as other OFDM symbols in order to keep the total transmission power constant, the remaining four RSs may have relatively low power and deteriorate RS performance. In the RS pattern 2-2, since the number of RSs included in the OFDM symbol is uniform, the above power problem is not caused.

26 shows an example of allocating a DRS position to a subframe using an extended CP according to another embodiment of the present invention. The allocation pattern is summarized as follows.

[Pattern 2-3]

Here, 1 denotes an OFDM symbol index, numbers in the table indicate subcarrier indices in a resource block, '-' denotes that a reference signal is not allocated, and the reference signal shifts along a frequency axis or a time axis. .

The method of arranging the DRS in the RS pattern 2-3 is basically the same as that described in the RS pattern 2-1. However, in the RS pattern 2-3, the DRS is arranged as far as possible in the time domain, and the time interpolation effect is maximized at the time of channel estimation. RS pattern 2-3 characteristically does not place the DRS in the last OFDM symbol (l = 5 in slot 2). This is because the RS for channel measurement can be transmitted in the last OFDM symbol.

FIG. 27 shows an example of allocating a DRS position to a subframe using an extended CP according to another embodiment of the present invention. The allocation pattern is summarized as follows.

[Pattern 2-4]

Here, 1 denotes an OFDM symbol index, numbers in the table indicate subcarrier indices in a resource block, '-' denotes that a reference signal is not allocated, and the reference signal shifts along a frequency axis or a time axis. .

If the number of DRSs included in each slot is different, there may be a power problem described in RS pattern 2-2 in boosting RS in the base station. Therefore, in pattern 2-4, one OFDM symbol is DRS arranged in each slot. However, the frequency interval between DRSs in one OFDM symbol is changed from 3 to 2 by changing the total number of OFDM symbols to be allocated DRS to 2. In this case, the frequency interval between DRSs is finally 1. Therefore, the channel estimation can be performed without degradation even in the extended CP. The RS pattern 2-4 is characterized in that channel estimation can be performed using two OFDM symbols located relatively close in time. Since it is not necessary to perform channel estimation by extracting all the RSs distributed to the three OFDM symbols spread widely in the time domain, performance can be expected to be improved when the channel value changes with time in a high-speed environment.

28 shows an example of allocating a DRS position to a subframe using an extended CP according to an embodiment of the present invention. The allocation pattern is summarized as follows.

[Pattern 2-5]

Here, 1 denotes an OFDM symbol index, numbers in the table indicate subcarrier indices in a resource block, '-' denotes that a reference signal is not allocated, and the reference signal shifts along a frequency axis or a time axis. .

The method of arranging the DRS in the RS pattern 2-5 is basically the same as that described in the RS pattern 2-1. However, the pattern for staggering OFDM symbols is different. That is, in the RS pattern 2-1, the DRS allocated to the OFDM symbol l = 4 of the first slot is allocated to the subcarriers 1, 4, 7 and 10, and the DRS allocated to the OFDM symbol l = 4 of the second slot is allocated to the subcarrier 0 In RS pattern 2-5, DRS allocated to OFDM symbol l = 4 of the first slot is allocated to subcarriers 0, 3, 6 and 9, and OFDM symbol l = 4 are allocated to the subcarriers 1, 4, 7, and 10, respectively.

29 shows an example of allocating a DRS position to a subframe using an extended CP according to an embodiment of the present invention. The allocation pattern is summarized as follows.

[Pattern 2-6]

Here, 1 denotes an OFDM symbol index, numbers in the table indicate subcarrier indices in a resource block, '-' denotes that a reference signal is not allocated, and the reference signal shifts along a frequency axis or a time axis. .

The method of arranging the DRS in the RS pattern 2-6 is basically the same as that described in the RS pattern 2-5. However, in the RS pattern 2-6, the DRS is not located in the OFDM symbol through which the CRS of the antenna port 0-3 is transmitted. This is to solve the power boosting problem as described in RS Pattern 2-2.

30 shows an example of assigning an RS position to a subframe using an extended CP according to an embodiment of the present invention. The allocation pattern is summarized as follows.

[Pattern 2-7]

Here, 1 denotes an OFDM symbol index, numbers in the table indicate subcarrier indices in a resource block, '-' denotes that a reference signal is not allocated, and the reference signal shifts along a frequency axis or a time axis. .

The RS pattern 2-7 is a modification of the OFDM symbol to which the DRS is allocated in the RS pattern 2-6.

31 shows an example of assigning RS positions to subframes using an extended CP according to an embodiment of the present invention. The allocation pattern is summarized as follows.

[Pattern 2-8]

Here, 1 denotes an OFDM symbol index, numbers in the table indicate subcarrier indices in a resource block, '-' denotes that a reference signal is not allocated, and the reference signal shifts along a frequency axis or a time axis. .

The method of arranging the DRS in the RS pattern 2-8 is basically the same as that described in the RS pattern 2-1. However, the pattern for staggering OFDM symbols is different. That is, in the RS pattern 2-1, the DRS allocated to the OFDM symbol l = 4 of the first slot is allocated to the subcarriers 1, 4, 7, and 10 and the DRS allocated to the OFDM symbol l = 1 of the second slot is allocated to the subcarrier 2 DRS allocated to the OFDM symbol l = 4 of the second slot is allocated to subcarriers 0, 3, 6 and 9. In the RS pattern 2-8, the OFDM symbol l = The DRS assigned to the OFDM symbol l = 1 of the second slot is allocated to the subcarriers 1, 4, 7 and 10, the OFDM symbol of the second slot is allocated to the subcarriers 1, The DRS allocated to l = 4 is allocated to subcarriers 2, 5, 8,

32 illustrates an example of assigning a DRS position to a subframe using an extended CP according to an embodiment of the present invention. The allocation pattern is summarized as follows.

[Pattern 2-9]

Here, 1 denotes an OFDM symbol index, numbers in the table indicate subcarrier indices in a resource block, '-' denotes that a reference signal is not allocated, and the reference signal shifts along a frequency axis or a time axis. .

The method of arranging the DRS in the RS pattern 2-9 is basically the same as that described in the RS pattern 2-8. However, in the RS pattern 2-9, the DRS is not located in the OFDM symbol through which the CRS of the antenna port 0-3 is transmitted. This is to solve the power boosting problem as described in RS Pattern 2-2.

33 shows an example of allocating a DRS position to a subframe using an extended CP according to an embodiment of the present invention. The allocation pattern is summarized as follows.

[Pattern 2-10]

Here, 1 denotes an OFDM symbol index, numbers in the table indicate subcarrier indices in a resource block, '-' denotes that a reference signal is not allocated, and the reference signal shifts along a frequency axis or a time axis. .

The RS pattern 2-10 is a modification of the OFDM symbol to which the DRS is allocated in the RS pattern 2-9.

In the patterns 2-1 to 2-10, the case where the RS of the antenna 4-7 is allocated in a code division multiplexing manner to the resource element indicated by D in FIGS. 24 to 33 is explained, but in an environment with a small channel delay spread, The RSs of antennas 0-7 may be allocated in a code division multiplexing manner to the resource elements indicated by D in Figs.

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 an implementation by firmware or software, an embodiment of the present invention may be implemented in the form of a module, a procedure, a function, or the like which 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 applied to a method of performing communication in a wireless communication system. More specifically, the present invention can be applied to a method of downlinking a reference signal in a wireless communication system having multiple antennas.

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 shows an antenna configuration diagram of a multi-antenna system.

2 shows a channel from the N _T transmit antenna to the receive antenna i.

3 shows a block diagram of a base station that may be applied to an embodiment of the present invention.

FIG. 4 shows a block diagram of a terminal that may be applied to an embodiment of the present invention.

5 shows a block diagram of a transmitter that may be applied to an embodiment of the present invention.

Figure 6 shows a block diagram of a receiver that may be applied to an embodiment of the present invention.

7 shows a structure in which a reference signal RS is allocated to a subframe using a normal CP and an extended CP in 3GPP LTE.

8 shows channel delay spreading of the RS observed in the time domain when the frequency interval of the RS is 6 in the environment where the general CP is applied.

9 shows channel delay spreading of the RS observed in the time domain when the frequency interval of the RS is 6 in the environment where the extended CP is applied.

10 shows the channel delay spreading of the RS observed in the time domain when the frequency interval of the RS is 3 in the environment where the normal CP and the extended CP are applied.

11 shows an example of assigning RSs to MBSFN subframes in an environment with small channel delay.

12 is a flowchart illustrating an example of transmitting an RS of 5-8 Tx antennas using an MBSFN subframe according to an embodiment of the present invention.

FIG. 13 is a flowchart illustrating a method of estimating a channel in a terminal when an RS of 5-8 Tx antennas is transmitted using an MBSFN subframe according to another embodiment of the present invention.

14A and 14B illustrate an RS pattern in which a CDM scheme is applied to antenna pairing according to an embodiment of the present invention, and channel delays are small.

15 (a) shows an example in which the CDM scheme is applied to antenna pairing when the RS frequency interval is 6 according to another embodiment of the present invention. 15 (b) is a representation in the time domain of Fig. 15 (a).

16 (a) shows an example of applying the CDM scheme to antenna pairing when the frequency spacing of the RS is 3 and 6 according to another embodiment of the present invention. 16 (b) and 16 (c) show the time domain in FIG. 16 (a).

17 shows an example of applying the CDM scheme to antenna pairing when the frequency interval of the RS is 3 according to another embodiment of the present invention.

18 (a) and 18 (b) show an RS pattern to which the CDM scheme is applied when the positions of the RSs for the antennas 0 to 3 are changed according to another embodiment of the present invention.

19 shows the channel delay spreading of the RS observed in the time domain when the RS pattern of FIG. 18 is applied in an environment where the channel delay is large.

20 shows the channel delay spreading of the RS observed in the time domain when the RS pattern of FIG. 18 is applied in an environment with a small channel delay.

21 is a flowchart illustrating a method of allocating the RS of the antenna 4-7 to the DRS position according to the CDM scheme according to an embodiment of the present invention.

FIG. 22 is a flowchart illustrating a method of estimating a channel in a terminal when an RS of a 5-8 Tx antenna is transmitted using a DRS position according to another embodiment of the present invention.

23A and 23B show a structure in which a demodulation reference signal RS is allocated to a subframe using a normal CP and an extended CP in 3GPP LTE.

24 shows an example of allocating a DRS position to a subframe using an extended CP according to an embodiment of the present invention.

25 shows another example of allocating a DRS position to a subframe using an extended CP according to another embodiment of the present invention.

26 shows another example of assigning a DRS position to a subframe using an extended CP according to another embodiment of the present invention.

FIG. 27 shows another example of allocating a DRS position to a subframe using an extended CP according to another embodiment of the present invention.

28 shows another example of allocating a DRS position to a subframe using an extended CP according to another embodiment of the present invention.

FIG. 29 shows another example of allocating a DRS position to a subframe using an extended CP according to another embodiment of the present invention.

30 shows another example of allocating a DRS position to a subframe using an extended CP according to another embodiment of the present invention.

31 shows another example of allocating a DRS position to a subframe using an extended CP according to another embodiment of the present invention.

32 shows another example of assigning a DRS position to a subframe using an extended CP according to another embodiment of the present invention.

33 shows another example of allocating a DRS position to a subframe using an extended CP according to another embodiment of the present invention.

Claims (40)

delete A method for downlink transmission of a reference signal in a wireless communication system having multiple antennas, Allocating a reference signal for a first antenna group to a subframe including a specific area having a different accessibility according to a terminal capability; Mapping a reference signal for a second antenna group to the first antenna group in a specific region and assigning the reference signal to the first antenna group in a code division multiplexing manner when the rank is equal to or greater than a predetermined value; And And transmitting the downlink data of the subframe. 3. The method of claim 2, Wherein the specific region is defined such that a terminal supporting only four or less multiple antennas can not be read. 3. The method of claim 2, Wherein the subframe includes a multimedia broadcast / multicast single frequency network (MBSFN) subframe. 3. The method of claim 2, Wherein the reference signal is a common reference signal (CRS) or a dedicated reference signal (DRS). 3. The method of claim 2, And the reference signal for the first antenna group is allocated using the pattern 1-1 or 1-2 in the specific region. [Pattern 1-1]

[Pattern 1-2]

Here, 1 denotes an OFDM symbol index, numbers in the table indicate subcarrier indices in a resource block, '-' denotes that a reference signal is not allocated, Here, antenna ports 0-3 may be permutated, and the reference signal may be shifted along the frequency axis or the time axis. 3. The method of claim 2, Wherein the second antenna group includes one to four antennas. 3. The method of claim 2, Wherein the number of reference signals for each antenna belonging to the second antenna group is independently four or less. 3. The method of claim 2, Wherein the reference signal for each antenna belonging to the second antenna group has a frequency interval of 3 or 6 independently. 3. The method of claim 2, Wherein a reference signal for each antenna belonging to the second antenna group is cyclically delayed in a time domain and a cyclic delay value is independently selected from a value equal to or greater than an allowed channel delay. / RTI > delete delete delete delete delete delete delete delete delete delete delete delete delete delete delete delete delete delete delete delete delete delete delete delete delete delete delete delete delete delete

Images