Method for transmitting dedicated reference signal, and method for receiving dedicated reference signal

Abstract

Provided are a method of transmitting a dedicated reference signal (DRS), a method of receiving a DRS, and a feedback method of a terminal. The method of transmitting a DRS includes determining a DRS transmitting resource for at least one terminal which is a target of transmission, and transmitting the DRS using the determined transmission resource and notifying the terminal of information about layer used by the terminal. The method of receiving a DRS includes determining a DRS receiving resource, receiving information about layer used by a terminal from a serving cell base station, and receiving the DRS for the terminal using the determined reception resource and the information about layer. Accordingly, a terminal can find the position and sequence of its DRS. In particular, in the case of multi-user multiple input multiple output (MU-MIMO) or joint scheduling, it is possible to prevent or remove signal interference using the DRS of another terminal.

Description

Here, s denotes a symbol vector that has a magnitude of L×1 and is transmitted to a terminal by a serving cell, and n denotes a noise vector that has a magnitude of M_R×1 and includes both of neighboring cell interference and Gaussian thermal noise.

When a channel covariance matrix is expressed by R_S=H_S^HH_S and a matrix consisting of eigenvectors having an eigenvalue other than 0 is indicated by V_S¹, V_S¹ can be obtained from an eigen decomposition R_S=V_SΛ_SV_S^H of R_S=H_S^HH_S or a singular value decomposition (SVD) H_S=U_SΣ_SV_S^H (Σ_S^HΣ_S=Λ_S) of H_S.

$\begin{array}{cc}{R}_S={\left[\begin{array}{cc}{V}_S^1& V_S^0\end{array}\right]\left[\begin{array}{cc}{\Lambda }_S^1& 0\\ 0& 0\end{array}\right]\left[\begin{array}{cc}{V}_S^1& V_S^0\end{array}\right]}^H=V_S^1{\Lambda }_S^1{V}_S^{1^H}(H_S=U_S^1{\Sigma }_S^1{V}_S^{1^H})& \left[\mathrm{Equation}4\right]\end{array}$

When a transmitting side uses V_S¹ as a precoder, the reception signal of the terminal is expressed by y=H_SV_S¹s+n. By multiplying the reception signal by a channel estimation value estimated from a DRS, the following valid reception signal can be obtained:

$\begin{array}{cc}\begin{array}{c}\tilde{y}={\left(\stackrel{\_}{H_S{V}_S^1}\right)}^H·y\\ ={\left(\stackrel{\_}{H_S{V}_S^1}\right)}^H{H}_S{V}_S^{1}s+{\left(\stackrel{\_}{H_S{V}_S^1}\right)}^{H}n\\ \approx {\Lambda }_1·s+{\left(\stackrel{\_}{H_S{V}_S^1}\right)}^{H}n.\end{array}& \left[\mathrm{Equation}5\right]\end{array}$

Such a transmitting and receiving method has been known as an excellent method capable of achieving a channel capacity. To enable the aforementioned single cell transmission and reception, the following can be used in particular as a feedback for precoding at a transmitting side among feedbacks that a terminal needs to transmit in a frequency-division duplex (FDD) system:

(i) a channel coefficient matrix H_S

(ii) a channel covariance matrix R_S=H_S^HH_S

(iii) a main eigenmatrix V_S¹ of a channel.

While these three types of matrices are used for explicit channel feedback, a matrix that is the most similar to the eigenmatrix of (iii) is found from a predefined codebook, and the corresponding precoding matrix index (PMI) is fed back for implicit channel feedback. When the codebook is indicated by S^(A)={w_n, n=1, 2, . . . }, the PMI can be expressed as Equation 6 below.

$\begin{array}{cc}{W}_S=\arg \begin{array}{c}\min \\ w\in S^{\left(A\right)}\end{array}D\left(V_S^1,w\right)& \left[\mathrm{Equation}6\right]\end{array}$

Here, D(V_S¹, w) denotes a “distance” between V_S¹ and w.

(2) The Case of Coordinated Scheduling Transmission

When a terminal receives coordinated scheduling transmission from two cells as illustrated in FIG. 2, a signal received by the terminal can be expressed by Equation 7 below.
y=H_SP_Ss_S+H_AP_As_A+n  [Equation 7]

When H_i (i=S, A) denotes the channel coefficient matrix of a terminal and cell i and P_i (i=S, A) denotes a precoder applied to cell i, an effective reception signal obtained by multiplying the received signal by a channel estimation value estimated from a DRS can be expressed by Equation 8 below.

$\begin{array}{cc}\begin{array}{c}\tilde{y}={\stackrel{\_}{H_S{P}_S}}^H·y\\ =\left({\stackrel{\_}{H_S{P}_S}}^H{H}_S{P}_S{s}_s+{\stackrel{\_}{H_S{P}_S}}^H{H}_A{P}_A{s}_A\right)+{\stackrel{\_}{H_S{P}_S}}^H·n\end{array}& \left[\mathrm{Equation}8\right]\end{array}$

When P_S is an eigenmatrix consisting of eigenvectors of a channel H_S, an intra-cell term H_SP_S^HH_SP_Ss_S related to only a serving cell becomes approximately a diagonal matrix. In other words, H_SP_S^HH_SP_Ss_S≈(V_S^H)(V_SΣ_S²V_S^H)(V_S)=Σ_S².

On the other hand, in order to cause little interference with the terminal, cell A can select P_A to belong to a null space of H_A or a space formed by eigenvectors corresponding to 0 or very small eigenvalues among eigenvectors corresponding to eigenvalues obtained by SVD of H_A. In other words, H_SP_S^HH_AP_As_A≈0.

To implement the aforementioned coordinated scheduling transmission, the terminal can use one of the following as a feedback:

(i) channel coefficient matrices H_S and H_A

(ii) channel covariance matrices R_S=H_S^HH_S and R_A=H_A^HH_A and

(iii) main eigenmatrices V_S and V_A of a channel (obtained from eigen decompositions R_S=V_SΛ_SV_S^H and R_A=V_AΛ_AV_A^H).

These three types of matrices are used for explicit channel feedback, and the eigenmatrices of (iii) can be applied to codebook-based implicit channel feedback. In this case when the codebook is indicated by, S⁽ⁱ⁾={w_n, n=1, 2, . . . } (i=S, A) a PMI can be expressed by Equation 9 below.

$\begin{array}{cc}{W}_S=\arg \begin{array}{c}\min \\ w\in S^{\left(S\right)}\end{array}D\left(V_S,w\right)W_A=\arg \begin{array}{c}\min \\ w\in S^{\left(A\right)}\end{array}D\left(V_A,w\right)& \left[\mathrm{Equation}9\right]\end{array}$

(3) The Case of Joint Processing—Coherent Joint Processing

In coherent joint processing, cells participating in joint processing artificially adjust the phases of signals transmitted by the respective cells to obtain the maximum effect in consideration of the phases. On the other hand, in non-coherent joint transmission, a phase between internal antennas of each cell is adjusted without consideration of phase adjustment between cells.

When a terminal receives joint processing from two cells as shown in FIG. 3, a signal received by the terminal can be expressed by Equation 10 below.
y=H_SP_Ss+H_AP_As+n  [Equation 10]

When H_i (i==S, A) denotes a channel coefficient matrix of a terminal and cell i, and P_i (i=S, A) denotes a precoder applied to cell i, an effective reception signal obtained by multiplying the received signal by a channel estimation value estimated from a DRS can be expressed by Equation 11 below.

$\begin{array}{cc}\begin{array}{c}\tilde{y}={\left(\stackrel{\_}{H_S{P}_S+H_A{P}_A}\right)}^H·y\\ =\left(\begin{array}{c}{\stackrel{\_}{H_S{P}_S}}^H{H}_S{P}_S+{\stackrel{\_}{H_A{P}_A}}^H{H}_A{P}_A+\\ {\stackrel{\_}{H_A{P}_A}}^H{H}_S{P}_S+{\stackrel{\_}{H_S{P}_S}}^H{H}_A{P}_A\end{array}\right)s+\\ {\left(\stackrel{\_}{H_S{P}_S+H_A{P}_A}\right)}^H·n\end{array}& \left[\mathrm{Equation}11\right]\end{array}$

The following two coherent precoding methods can be used.

(A) Global Precoding

An eigenvector matrix V is obtained from a joint channel

$H=\left[\begin{array}{c}{H}_A^H\\ H_B^H\end{array}\right]$
of two cells. V is used as a precoder.

$\begin{array}{cc}\begin{array}{c}R=V\Lambda V^H\\ =\left[\begin{array}{c}{H}_S^H\\ H_A^H\end{array}\right]\left[\begin{array}{cc}{H}_S& H_A\end{array}\right]\\ =\left[\begin{array}{cc}{R}_S& R_{\mathrm{SA}}\\ R_{\mathrm{SA}}^H& R_A\end{array}\right]\end{array}& \left[\mathrm{Equation}12\right]\end{array}$

Here, R_ij=H_i^HH_j, and the other transmission and reception process is similar to the case of single cell transmission.

(B) Local Precoding Plus Phase Correction

Cells participating in joint processing determine precoding in consideration of only wireless channels between the respective cells and the terminal. Phase correction is determined on the assumption of determined precoding of the respective cells in consideration of the wireless channels between all the cells participating in joint processing and the terminal. Precoding at cell i (i=S, A) can be expressed by Equation 13 below.
P_S=V_SD_S
P_A=V_AD_A  [Equation 13]

Here, D_S^HD_S=1 and D_A^HD_A=1 and (unitary). Intra-cell terms related to only the channels of respective individual cells become approximately diagonal matrices irrelevant to selection of D_S and D_A when D_S and D_A are limited to diagonal matrices. In other words, the intra-cell terms are expressed by Equation 14 below.

$\begin{array}{cc}{\stackrel{\_}{H_S{P}_S}}^H{H}_S{P}_S\approx \left(D_S^H{V}_S^H\right)\left(V_S{\Sigma }_S^2{V}_S^H\right)\left(V_S{D}_S\right)& \left[\mathrm{Equation}14\right]\\ =\left(D_S^H{\Sigma }_S^2{D}_S\right)& \\ ={\Sigma }_S^2& \end{array}$

Likewise, H_AP_A^HH_AP_A≈≈(D_A^HΣ_A²D_A)=Σ_A².

The matrices D_S and D_A for phase correction may be determined so that an inter-cell term H_AP_A^HH_SP_S+H_SP_S^HH_AP_A can provide the highest data transmission rate.

To implement the aforementioned joint processing, the terminal can use one of the following as an explicit channel feedback:

(i) channel coefficient matrices H_S and H_A

(ii) channel covariance matrices R_S=H_S^HH_S, R_A=H_A^HH_A, and an inter-cell matrix R_SA=H_S^HH_A.

(iii) a main eigenmatrix V (obtained from an eigen decomposition

$\begin{array}{c}R=V\Lambda V^H\\ =\left[\begin{array}{c}{H}_S^H\\ H_A^H\end{array}\right]\left[\begin{array}{cc}{H}_S& H_A\end{array}\right]).\end{array}$

(iv) main eigenmatrices V_S and V_A, and phase correction matrices D_S and D_A.

In the case of implicit feedback, (iii) and (iv) are corrected, precoders that are the most similar to the corrected matrices are found, and the corresponding indices are fed back. When a codebook for global precoding is indicated by S^(SA), a codebook for local precoding of cell i is indicated by S⁽ⁱ⁾(i=S, A), and a codebook for phase correction is indicated by X⁽ⁱ⁾(i=S, A), an implicit feedback of the terminal can be expressed as follows:

(v) a precoding matrix for global precoding is

$W=\arg \begin{array}{c}\min \\ w\in S^{\left(\mathrm{SA}\right)}\end{array}D\left(V,w\right)$

(vi) precoding matrices for local precoding and phase correction are

$W_S=\arg \begin{array}{c}\min \\ w\in S^{\left(S\right)}\end{array}D\left(V_S,w\right)\mathrm{and}{W}_A=\arg \begin{array}{c}\min \\ w\in S^{\left(A\right)}\end{array}D\left(V_A,w\right),$
and

a matrix index for phase correction is

$\left(X_S,X_A\right)=\arg \begin{array}{c}\min \\ x_S\in X^{\left(S\right)},x_A\in X^{\left(A\right)}\end{array}R\left(H,x_S,x_A❘W_S,W_A\right).$

Here, R denotes an overall data transmission rate.

Preferable Feedback Structure of Terminal

Consequently, in a terminal employing a DM-RS transmitting method according to an exemplary embodiment of the present invention, feedback of the terminal for downlink transmission such as single cell transmission, coordinated scheduling transmission, and coherent joint processing has characteristics as described below.

First, coordinated scheduling transmission and non-coherent joint processing can use the same terminal feedback.

Second, when terminal feedback used for single cell transmission is simply scaled up to a plurality of cells, the terminal feedback can be used for coordinated scheduling and non-coherent joint processing.

Third, terminal feedback for coherent joint processing can be designed to include coordinated scheduling transmission or non-coherent joint processing. Local precoding and phase correction have the same content as terminal feedback for coordinated scheduling transmission or non-coherent joint processing except for a diagonal matrix for phase correction.

Terminal feedback may have the following two characteristics.

A first characteristic is scalability, which denotes that a transmitting side needs to be able to perform transmission using a part of content fed back by a terminal. For example, when a terminal performs feedback for joint processing in which three cells participate, a transmitting side may successfully receive feedbacks for only two cells. In this case, when the feedbacks have scalability, joint processing for the two cells can be performed using the feedbacks. Also, the transmitting side may use an additional terminal feedback to scale up the joint processing so that more cells can participate in the joint processing. For example, when a terminal transmits a feedback to a transmitting side for joint processing in which three cells participate and then additionally transmits a feedback for one cell, the transmitting side should be able to perform joint processing in which the four cells participate using the successfully received feedback information of the four cells.

A second characteristic is flexibility, which denotes that a plurality of transmission methods can be selected from a terminal feedback causing high overhead. For example, from a terminal feedback for coherent joint processing, a transmitting side should be able to select and perform coherent joint processing, non-coherent joint processing, coordinated scheduling transmission, or single cell transmission.

In Table 1 and Table 2, merits and demerits of the aforementioned explicit channel feedback and implicit channel feedback are arranged according to transmission methods.

TABLE 1
Explicit Channel Feedback
	CoMP
			Non-
			Coherent
Explicit	Single Point	Coordinated	Joint	Coherent Joint
Feedback	Transmission	Scheduling	Processing	Processing
(i) Channel	HS	Hi(i = S, A, . . . )	Hi(i = S, A, . . . )	Hi(i = S, A, . . . )	High
Coefficient	HS				Overhead,
Matrix					Complete
					Scalability
					and
					Flexibility
(ii) Channel	RS	Ri(i = S, A, . . . )	Ri(i = S, A, . . . )	Ri(i = S, A, . . . ) +	High
Covariance	RS			Rij(i≠j, i, j = S, A, . . . )	Overhead,
Matrix					Complete
					Scalability
					and
					Flexibility
(iii)	VS	Vi(i = S, A, . . . )	Vi(i = S, A, . . . )	Vjoint	Low
Eigenmatrix	VS				Overhead,
for Global					Partial
Precoding					Scalability
					and
					Flexibility
(iv) Local	VS	Vi(i = S, A, . . . )	Vi(i = S, A, . . . )	Vi(i = S, A, . . . ) +	Medium
Precoding	VS			Di(i = S, A, . . . )	Overhead,
Eigenmatrix					Complete
and Diagonal					Scalability
Matrix for					and
Phase					Flexibility
Correction

TABLE 2
Implicit Channel Feedback
	CoMP
			Non-Coherent
Implicit	Single Point	Coordinated	Joint	Coherent Joint
Feedback	Transmission	Scheduling	Processing	Processing
(v) PMI for	WS	Wi(i = S, A, . . . )	Wi(i = S, A, . . . )	Wjoint	Multi-Cell
Global					Codebook
Precoding					Is Needed.
					Partial
					Scalability
					and
					Flexibility
(vi) PMI for	WS	Wi(i = S, A, . . . )	Wi(i = S, A, . . . )	Wi(i = S, A, . . . ) +	Single Cell
Local				Xi(i = S, A, . . . )	Codebook
Precoding					Is Needed.
and Phase					Complete
Correction					Scalability
					and
					Flexibility

Referring to Table 1 and Table 2 above, (i), (ii) and (iv) of Table 1 and (vi) of Table 2 satisfy both of scalability and flexibility. Also, (i) and (ii) of Table 1 increase the amount of feedback of a terminal, and, in comparison with (i) and (ii) of Table 1, (iv) of Table 1 has scalability and flexibility and maintains an appropriate amount of feedback. Furthermore, (vi) of Table 2 is based on the codebook of each cell and thus has scalability and flexibility while reducing the amount of feedback.

While the example embodiments of the present invention and their advantages have been described in detail, it should be understood that various changes, substitutions and alterations may be made herein without departing from the scope of the invention.

Claims

1. A communication method for a base station to transmit a reference signal to a terminal, the method comprising:

transmitting, from the a base station, a downlink control information (DCI) to a terminal, wherein the DCI includes virtual terminal-specific information to the terminal;

generating, by the base station, a sequence for the reference signal for the terminal by using based on a physical layer cell identity (PCI) of a serving cell of the terminal and the virtual terminal-specific information; and

generating, by the base station, a reference signal based on the sequence; and

transmitting, from the base station, the reference signal using the sequence to the terminal,

wherein the virtual terminal-specific information is transmitted to the terminal by using downlink control information (DCI), wherein the reference signal is a dedicated reference signal for the terminal.

2. The method of claim 1, further comprising transmitting information about layer used by the terminal to the terminal wherein the reference signal is a user equipment specific (UE-specific) reference signal.

3. The method of claim 1, wherein the virtual terminal-specific information is shared by at least two terminals including the terminal and selected among plural values predefined for the at least two terminals the DCI is also transmitted to a second terminal and the reference signal is also transmitted to the second terminal.

4. The method of claim 1, wherein the virtual terminal-specific information is used for generating the sequence instead of a radio network temporary identifier (RNTI) of the terminal further comprising:

transmitting, from the base station, a virtual cell identity of the serving cell to the terminal;

generating, by the base station, another sequence based on the virtual cell identity and the virtual terminal-specific information;

generating, by the base station, another reference signal based on the another sequence; and

transmitting, from the base station, the another reference signal to the terminal.

5. The method of claim 1, wherein the base station performs a single point transmission for the terminal.

6. A communication method for a terminal to receive a reference signal from a base station, the method comprising:

receiving, from the base station by a terminal, a downlink control information (DCI) from a base station, wherein the DCI includes virtual terminal-specific information from the base station;

generating a sequence for the reference signal for the terminal by using a physical cell identity (PCI) of a serving cell of the terminal and the virtual terminal-specific information; and

receiving, from the base station by the terminal, the a reference signal by using the sequence from the base station,; and

obtaining, by the terminal, user data based on the reference signal, the virtual terminal-specific information and a physical layer cell identity (PCI) of a serving cell,

wherein the virtual terminal-specific information is received from the base station by using downlink control information (DCI), wherein the reference signal is a dedicated reference signal for the terminal.

7. The method of claim 6, further comprising receiving information about layer used by the terminal from the base station wherein the reference signal is a user equipment specific (UE-specific) reference signal.

8. The method of claim 6, wherein the virtual terminal-specific information is shared by at least two terminals including the terminal and selected among plural values predefined for the at least two terminals the DCI is also transmitted to a second terminal and the reference signal is also transmitted to the second terminal.

9. The method of claim 6, wherein the virtual terminal-specific information is used for generating the sequence instead of a radio network temporary identifier (RNTI) of the terminal further comprising:

receiving, by the terminal, a virtual cell identity of the serving cell from the base station;

receiving, by the terminal, another reference signal from the base station; and

obtaining, by the terminal, other user data based on the another reference signal, the virtual terminal-specific information and the virtual cell identity of the serving cell.

10. The method of claim 6, wherein the terminal performs a single point reception from the base station.

11. A method for a base station to transmit a reference signal to a terminal, the method comprising:

transmitting, from the base station, a virtual cell-specific information and a virtual terminal-specific information to the terminal;

generating a sequence for the reference signal for the terminal by using the virtual cell-specific information and the virtual terminal-specific information; and

transmitting, from the base station, the reference signal using the sequence to the terminal,

wherein the virtual cell-specific information is transmitted to the terminal by using downlink control information (DCI),

wherein the reference signal is a dedicated reference signal for the terminal.

12. The method of claim 11, wherein the virtual cell-specific information is indicated by the down link control information among a plurality of cell identities.

13. The method of claim 11, wherein the virtual terminal-specific information is transmitted to the terminal by using a downlink control information (DCI).

14. The method of claim 11, further comprising transmitting information about layer used by the terminal to the terminal.

15. The method of claim 11, wherein the virtual cell-specific information is shared by at least two cells including a serving cell of the terminal.

16. The method of claim 11, wherein the virtual terminal-specific information is shared by at least two terminals including the terminal and selected among plural values predefined for the at least two terminals.

17. The method of claim 11, wherein

the virtual cell-specific information is identical to or different from a physical cell identity (PCI) of a serving cell of the terminal and

the virtual terminal-specific information is different from a radio network temporary identifier (RNTI) of the terminal.

18. The method of claim 11, wherein the base station performs a coordinated multi-point transmission for the terminal with other base station.

19. A method for a terminal to receive a reference signal from a base station, the method comprising:

receiving, from the base station, a virtual cell-specific information and a virtual terminal-specific information from the base station;

generating a sequence for the reference signal for the terminal by using the virtual cell-specific information and the virtual terminal-specific information; and

receiving, from the base station, the reference signal by using the sequence from the base station,

wherein the virtual cell-specific information is received from the base station by using downlink control information (DCI),

wherein the reference signal is a dedicated reference signal for the terminal.

20. The method of claim 19, wherein the virtual cell-specific information is indicated by the down link control information among a plurality of cell identities.

21. The method of claim 19, wherein the virtual terminal-specific information is received from the terminal by using a downlink control information (DCI).

22. The method of claim 19, further comprising receiving information about layer used by the terminal from the base station.

23. The method of claim 19, wherein the virtual cell-specific information is shared by at least two cells including a serving cell of the terminal.

24. The method of claim 19, wherein the virtual terminal-specific information is shared by at least two terminals including the terminal and selected among plural values predefined for the at least two terminals.

25. The method of claim 19, wherein

the virtual cell-specific information is identical to or different from a physical cell identity (PCI) of a serving cell of the terminal and

the virtual terminal-specific information is different from a radio network temporary identifier (RNTI) of the terminal.

26. The method of claim 19, wherein the terminal performs a coordinated multi-point reception from a plurality of cells including a serving cell of the terminal.