WO2012093333A1

WO2012093333A1 - Method of channel estimation, method of selecting pilot information, user equipment, and base station

Info

Publication number: WO2012093333A1
Application number: PCT/IB2012/000062
Authority: WO
Inventors: Keying Wu; Dong Li
Original assignee: Alcatel Lucent
Priority date: 2011-01-07
Filing date: 2012-01-03
Publication date: 2012-07-12
Also published as: CN102594739B; CN102594739A; TWI449367B; TW201234804A

Abstract

The present invention relates to a method of channel estimation, a method of selecting pilot information, user equipment, and base station. There is provided a user equipment that improves channel estimation utilizing spatial correlation of channels with a base station, the user equipment comprising: pilot signal receiving unit for receiving pilot signals; first estimating unit for estimating spatial correlation of the channels at the base station based on the received pilot signals; second estimating unit for estimating spatial correlation of the channels at the user equipment based on the received pilot signals; and third estimating unit for estimating channel responses of the channels based on the received pilot signals, estimation of the spatial correlation of the channels at the base station, and estimation of spatial correlation of the channels at the user equipment. According to the present invention, with the same pilot overhead, a significant channel estimation performance gain may be achieved, or pilot overhead may be dramatically reduced without degrading the channel estimation performance.

Description

METHOD OF CHANNEL ESTIMATION, METHOD OF SELECTING PILOT INFORMATION, USER EQUIPMENT, AND BASE STATION

FIELD OF THE INVENTION

[0001 ] The present invention generally relates to MIMO channel estimation, and specifically relates to a method of improving channel estimation by utilizing spatial correlation information in an MIMO system, an associated method of selecting pilot information, a user equipment utilizing the method of channel estimation, and a base station utilizing the method of selecting pilot information.

DESCRIPTION OF THE RELATED ART

[0002] In the current channel estimation (CE) solution, channel responses between different transmit/receive antenna pairs are separately estimated. In reality, these channel responses are correlated therebetween. Such correlation, referred to as spatial correlation, may be used to improve the CE performance in a similar way as the time and frequency correlation. However, in existing CE solutions, only time and frequency correlation information are exploited, while spatial correlation information is not exploited.

[0003] Besides, in the existing pilot design, all base stations exploit pilot patterns having the same pilot overhead. However, for base stations having different spatial correlations, pilot patterns having different pilot overheads may be exploited to enhance the effectiveness of CE.

SUMMARY OF THE INVENTION

[0004] An object of the present invention lies in improving the performance of MIMO channel estimation (CE) by utilizing spatial correlation information of an MIMO system. A corresponding pilot design method is also used to adapt pilot overhead according to the spatial correlation of the MIMO channels.

[0005] According to one aspect of the present invention, there is provided a user equipment that improves channel estimation utilizing spatial correlation of the channels with a base station, the user equipment comprising: pilot signal receiving unit for receiving pilot signals; first estimating unit for estimating spatial correlation of the channels at the base station based on the received pilot signals; second estimating unit for estimating spatial correlation of the channels at the user equipment based on the received pilot signals; and third estimating unit for estimating channel responses of the channels based on the received pilot signals, estimation of the spatial correlation of the channels at the base station, and estimation of spatial correlation of the channels at the user equipment.

[0006] According to another aspect of the present invention, there is provided a method of improving channel estimation utilizing spatial correlation of the channels between a user equipment and a base station, the method comprising: receiving pilot signals; estimating spatial correlation of the channels at the base station based on the received pilot signals; estimating spatial correlation of the channels at the user equipment based on the received pilot signals; and estimating channel responses of the channels based on the received pilot signals, estimation of the spatial correlation of the channels at the base station, and estimation of spatial correlation of the channels at the user equipment.

[0007] According to another aspect of the present invention, there is provided a base station, comprising a pilot pattern selecting unit for selecting a pilot pattern based on channel environment and antenna configuration of the base station; and signaling unit for signaling the pilot pattern selected by the pilot pattern selecting unit to a user equipment.

[0008] According to another aspect of the present invention, there is provided a method of selecting pilot information, comprising the following steps: selecting a pilot pattern as pilot information based on channel environment and antenna configuration of a base station; and signaling the selected pilot pattern to a user equipment.

[0009] The present invention provides the following advantages: significant CE performance gain may be achieved with the same pilot overhead, or pilot overhead can be dramatically reduced without degrading the CE permance; the pilot design strategy dependent on spatial correlation may adaptively select an appropriate pilot overhead for MIMO systems having different spatial correlations and provide a sound compromise between pilot overhead and channel estimation accuracy under all channel conditions and environments. BRIEF DESCRIPTION OF THE DRAWINGS [0010] The above and other objects, features and advantages of the present invention will become apparent through the following description on the exemplary embodiments of the present invention with reference to the drawings, where in the drawings:

[001 1 ] Fig. 1 is a structural block diagram of a base station according to an embodiment of the present invention.

[0012] Fig. 2 is a structural block diagram of a user equipment according to an embodiment of the present invention;

[0013] Fig. 3 is a flow chart of a method of selecting pilot information according to an embodiment of the present invention, where the method is exploited by the base station;

[0014] Fig. 4 is a flow chart of a method of channel estimation according to an embodiment of the present invention, where the method is exploited by the user equipment;

[0015] Fig. 5A and Fig. 5B are diagrams of pilot patterns utilizing different overheads for an MIMO system having N_T=4 transmit antennas; and

[0016] Fig. 6A and Fig. 6B are diagrams of comparing a traditional MMSE-CE and a joint MMSE CE under the conditios of same pilot overhead and half pilot overhead.

DETAILED DESCRIPTION OF THE INVENTION

[0017] According to the embodiments of the present invention, when performing channel estimation, a joint MMSE CE solution is employed, which performs joint estimation of channel coefficients of all transmit/ receive antenna pairs based on channel spatial correlation information. However, it should be noted that estimation of channel response is not limited to this MMSE CE solution, which may also adopt other method performed based on channel spatial correlation information. When the spatial channel correlation is strong, the solution may significantly improve CE performance or equivalently reduce pilot overhead. Additionally, performance gain is augmented with spatial correlation. In other words, the higher is the spatial correlation of the MIMO system, the fewer pilots can be exploited to achieve a better CE performance. Therefore, it is further desirable to provide a pilot design strategy dependent on spatial correlation, which pilot design strategy exploits different pilot overheads for MIMO systems having different spatial correlations.

[0018] The joint MMSE-CE solution and spatial correlation-dependent pilot design strategy will be described in detail hereinafter, respectively.

[0019] Now, a base station 10 according to the present invention will be described with reference to Fig. 1. For the sake of conciseness, only one base station is illustrated here. However, it should be noted that the MIMO system of the present invention has a plurality of base stations.

[0020] In the MIMO system, the MIMO channels always have a specific spatial correlation, namely, channel responses over different transmit/ receive antenna pairs are correlated. In actuality, spatial correlation is mainly determined by the following two factors: channel environment, for example, cities with dense buildings, open countryside, line of sight (LOS), and non LOS, etc.; and antenna configuration, for example, the number of antennas, spacing, polarization, etc. Once the base station is installed, the two factors will be fixed. However, for different base stations, because the two factors are different, the spatial correlations of the different base stations are also different.

[0021 ] Fig. 1 is a structural block diagram of a base station 10 according to an embodiment of the present invention. The base station 10 may comprise a pilot pattern selecting unit 101 and a signaling unit 102. The pilot pattern selecting unit 101 of each base station 10 selects a pilot pattern based on the channel environment and antenna configuration of the base station 10, wherein each selected pilot pattern has a different pilot overhead. For example, in an urban environment with dense buildings, the pilot overhead is relatively small; while in an open countryside environment, the pilot overhead is relatively large; in an LOS environment, the pilot overhead is relatively small, while in a non-LOS environment, the pilot overhead is relatively large. Moreover, for example, the pilot overhead is augmented with the increase of the number of antennas and spacing. The signaling unit 102 signals the pilot pattern selected by the pilot pattern selecting unit 101 of the base station 10 to the user equipment. When the base station communicates with the user equipment, the user equipment may, for example, hand over from one base station to another base station. At this point, the signaling unit of the target base station 10 sends the pilot pattern selected by the base station 10 to the user equipment.

[0022] Now, a user equipment 20 according to the present invention will be described with reference to the block diagram of Fig. 2. [0023] Fig. 2 is a structural block diagram of a user equipment 20 according to an embodiment of the present invention. The user equipment 20 may perform channel estimation utilizing spatial correlation information of the channels with the base station 10. The user equipment 20 may comprise a pilot signal receiving unit 201, a pilot pattern receiving unit 202, a base station spatial correlation estimating unit 203 , a user equipment spatial correlation estimating unit 204, a temporal correlation estimating unit 205, a frequency correlation estimating unit 206, and a channel response estimating unit 207.

[0024] The pilot signal receiving unit 201 receives pilot signals. The noise in the pilot signals that are received over the channels may be an additive white Gaussian noise (AWGN). The pilot pattern receiving unit 202 may receive the selected pilot pattern sent from the base station 10 according to the present invention, so as to perform synchronization.

[0025] In order to estimate channel responses over all transmit/ receive antenna pairs between the base station 10 and user equipment 20, for example, the joint MMSE channel estimation that will be described in detail hereinafter, it is further needed to estimate spatial/ temporal/ frequency correlations.

[0026] Because spatial correlation information is always unknown to the system, this information should be first estimated.

[0027] Suppose the numbers of pilots transmitted by each antenna of the base station 10 are identical. Based on the pilot signals received by the pilot signal receiving unit 201, the base station spatial correlation estimating unit 203 and the user equipment spatial correlation estimating unit 204 may estimate the spatial correlation between different antennas at the base station 10 and the spatial correlation between different antennas at the user equipment 20 , i . e . , e s tim ating the values of r_Tx (m,m') =

) _&Μ _{frequency domain}

channel response over the k-th subcarrier in the t-th OFDM symbol between the m-th transmit antenna at the base station 10 and the n-th receive antenna at the user equipment 20.

[0028] Then, the temporal correlation estimating unit 205 and the frequency correlation estimating unit 206 may estimate the channel temporal and spatial correlations utilizing a traditional method known in the art, i.e., estimating the values of r_t (A) = {k + Α,Ϊ)^Η ) _

[0029] Finally, the channel response estimating unit 207 estimates the channel resonses of the MIMO channels based on the received pilot signals, the estimated temporal and frequency correlations, the spatial correlation between different antennas at the base station 10 and the spatial correlation between different antennas at the user equipment 20, for example, performing the joint MMSE estimation as described hereinafter.

[0030] Preferably, the channel response estimating unit 207 further comprises a first correlation matrix estimating unit 208, a second correlation matrix estimating unit 209, and a final channel response estimating unit 210. For the sake of conciseness, the three units 208, 209, and 210 are not shown in the figure.

[0031 ] The first correlation matrix estimating unit 208 may estimate the correlation matrix

^^dp as described in detail in the following detailed description on the joint MMSE CE based on the estimated spatial correlation between different antennas at the base station 10, spatial correlation between different antennas at the user equipment 20, temporal correlation, and frequency correlation, where the correlation matrix ^^dp denotes the correlation matrix between the channel responses over data subcarriers and the channel responss over pilot subcarriers.

[0032] The second correlation matrix estimating unit 209 may estimate the correlation matrix ^^pp that is described in detail in the following detailed description on the joint MMSE CE based on the estimated spatial correlation between different antennas at the base station 10, the spatial correlation between different antennas at the user equipment 20, temporal correlation, and frequency correlation, where the correlation matrix ^^pp denotes correlation matrix of channel responses over pilot subcarriers.

[0033] The final channel response estimating unit 210 may estimate channel responses based on the estimated correlation matrix ^ip between the channel responses over the data subcarriers and the channel responses over the pilot subcarriers, and the estimated correlation matrix ^pp of the channel responses over the pilot subcarriers.

[0034] Fig. 3 is a flow chart of a method 300 of selecting pilot information according to an embodiment of the present invention, where the method is exploited by the base station. The present invention needs to exploit pilot patterns having different overheads with respect to MIMO systems having different antenna configurations in different environments. Therefore, a method of selecting pilot information dependent on spatial correlation is provided. At step S301, each base station selects a pilot pattern based on its channel environment and antenna configuration that influnces its spatial correlation. Pilot overheads may be seen in the pilot patterns illustrated in Figs. 5A and 5B. At step S302, the selected pilot pattern is signaled to the user equipment.

[0035] Fig. 4 is a flow chart of a method 400 of channel estimation according to an embodiment of the present invention, where the method is exploited by the user equipment. At step S401, pilot signals are received from a base station. Then, at step S402, spatial correlation between different antennas at the base station is estimated based on the received pilot signals. At step S403 , spatial correlation between different antennas at the user equipment is estimated based on the received pilot signals. Next, at step S404, temporal and frequency correlations of the channels are estimated utilizing a traditional method known in the art. Finally, at step S405, the channel resonses of the MIMO channels are estimated based on the received pilot signals, the estimated temporal and frequency correlations, the spatial correlation between different antennas at the base station and the spatial correlation between different antennas at the user equipment, for example, by performing the joint MMSE estimation as described hereinafter.

[0036] Preferably, the step S405 may be divided into sub-steps S405-1, S405-2, and S405-3. For the sake of conciseness, these sub-steps are not illustrated in the figure.

[0037] In sub-steps S405-1 and S405-2, correlation matrix ^Rdp between channel responses over the data subcarriers and channel responses over the pilot subcarriers and correlation matrix ^Rpp of channel responses over the pilot subcarriers, as described hereinafter in detail in the detailed description on the joint MMSE CE, are estimated based on the estimated spatial correlation between different antennas at the base station, the spatial correlation between different antennas at the user equipment, temporal correlation and frequency correlation. At the sub-step S405-3, channel responses are estimated based on the estimated

R _and R

[0038] Hereinafter, first, implementation of the joint MMSE CE scheme is described in detail; then the pilot design strategy dependent on spatial correlation is described in detail, so as to understand more clearly the mutual relations between the above steps and the specific calculation manners.

Joint MMSE CE

[0039] Consider a MIMO-OFDM system that has N_T number of antennas at the transmitter (for example, the base station 101 herein) and N_R number of antennas at the receiver (for example, the terminal 102 herein). Fig. 5A and Fig. 5B are diagrams of pilot patterns utilizing different overheads for an MIMO system having N_T=4 transmit antennas. As illustrated in the figure, pilot signals are evenly interpolated in a specified radio resource block comprising K_d number of subcarriers and T number of OFDM symbols. Likewise, as illustrated in the fiture, pilots from different transmit antennas are multiplexed in an orthogonal manner. Channel responses over the data subcarriers are estimated through interpolation between samples at the pilot subcarriers. The interpolation performance may be improved for example through an MMSE scheme by utilizing temporal/ frequency/ spatial correlation information.

[0040] Let P^{m) (l<m<N_T) be the number of pilots in the specified radio resource block which are transmitted from the m-th transmit antenna, ^p be their sub-carrier

(^m) n = \~P^(m) \ \x^(m) p = \~ P^(m) \

indices, ^p be their OFDM symbol indices, ^p be the values of pilots. The noised version of the pilots received at the n-th receive antenna is:

wherein, * ^{?lm) J} j_s the noised version of the pilots that are transmitted from the m-th transmit antenna and received at the n-th receive antenna, ' ' is the frequency domain channel response over the k-th subcarrier in the t-th OFDM symbol

x^(m) ₌r_x ^(m) ..._x ^{{m) ~ T} between the m-th transmit antenna and the n-th receive antenna, ^{1 plm)} is the pilot vector transmitted at the m-th transmit antenna, ^{1 p(m)} is the additive white Gaussian noise (AWGN) vector with the mean value being 0 and variance being σ2, where the variance may be obtained through estimation by the terminal. By dividing each z^{n'^m) x^(m)

p by the corresponding pilot signal ^p , it is obtained:

wherein, , and = [ηΓ"' -η J , an d

H

p^{p p} . By using ^{" p} to denote the frequency domain channel responses at the pilot sub-carriers between the m-th transmit antenna and the n-th receive antenna, namely:

[0041 ] The object of this solution is to estimate the frequency domain chanel responses at all subcarriers within a given radio resource block for all (n, m) pairs, namely;

H = [H⁽ⁿ'^m) (0,0)· · -H <^»·^»> {K_d - 1,0)· · -H <^»·^»> (0.Γ - 1)· · -H <^»·^»> {K_d - 1,T - l)f ^ _{for y}^ ^

H^^T ..._HW^T ..._HW)^T ..._H ⁽

are defined, and then, the joint MMSE CE is implemented as

H_d =R_dp (R_{pp +} I/p)-^l y

(3)

ρ = Ε ^ )/σ ² R_dp =E(H_dH_p ^H ) wherein, is the signal-to-noise ratio (SNR) of pilot,

R = E(H H^H

and ^pp ' ^{p p} , wherein the superscript H denotes the conjugate transpose. The physical

R = E( H )

meaning of ^{ip d p} is the correlation matrix between the channel responses over the data subcarriers and the channel responses over the pilot subcarriers, and the physical

R = E(H H^H

meaning of ^{pp p p} is the correlation matrix of the channel responses over the pilot subcarriers. [0042] Thus, it may be seen from equation (3) that in order to estimate the channel responses at all subcarriers, unknown ^^dp and ^^pp must be first estimated.

[0043] Hereinafter, the estimation of the correlation matrix ^^dp and ^^pp , namely, sub-steps S405-1 and S405-2, will be described.

[0044] The following are defined:

r_t (Δ) = E(H^M (k,i)H ⁿ'^m) (k,i+A )

r_f (Δ) = E(H⁽ⁿ'^m) (k,i)H⁽ⁿ'^m) (k + A,i )

r_¾ («,«') = E(H ⁿ'^m) (k,i)H ⁿ '^m) (k,i ) r_T m,m') = E(H⁽ⁿ'^mHk,i)H⁽ⁿ'^m') (k,i )

[0045] wherein, ^r* , ^Γ , ^Vr* , and ^r¾ denote temporal correlation, frequency correlation, spatial correlation at the receiver side, and spatial correlation at the transmitter side, respectively. ^^p ^ ^tp ^ , ⁿ _P (f) ^ _an(j ^m _P ) _{are use(}j _to (j_enot_e the indices of subcarrier, symbol, the receive antenna and the transmit antenna of the i-th element in ^p . The (i, j)-th

R r (i i)

element of ^pp denoted by ^pp may be calculated as below:

, (i ) = E(//^(«^^(I)) (k_p (i),t_p ( ) · (_HW'^U)) (k_p (j),t_pp (j))f )

= r, (t_p (j) -t_p (i))r_f (k_p (j) -k_p (n_p {j),n_p (i))r_Tx (m_p (j),m_p (;))

(4)

[0046] Likewise, the (i, j)-th element of ^dp may be calculated as below:

dp

= r, (t_p (j) - t_d (i))r_f (k_p (J) - k_d (n_p (j),n_d i))r_Tx (m_p ( j),m_d (;)) ^

[0047] Because the correlation statistic data is unknown to the system, it is required to estimate the correlation statistic data before channel estimation. The temporal domain and frequency doman correlations may be estimated by the following traditional method.

5ΐ (Αω_ι1 )

¾ (Δ) =

(6)

wherein, _? x_{f 1S} the length of the OFDM symbol, fj = vf_c/c is the Dopier frequency with the velocity v, f_c is carrier frequency, and c is the light velocity, number of subcarriers in an OFDM symbol, +1, wherein W is the bandwidth and x_max is the

maximum delay spread.

[0048] The spatial domain correlation is estimated based on the following noised pilot observations. Su ose P^{m) =P for Vm, then the estimations on ^Vrx and ^r¾ are as below:

(«) MY

wherein, "J and [0049] The estimation values ^r* , ^Γ , ^Vr* , and ^r¾ are just the results of the above estimation steps S402, S403, and S404.

MMSE CE is implemented as:

[0051 ] Wherein, by replacing ^r* , ^Γ , ^Vr* , and ^r¾ with respective estimation values ^r* , ^Γ , and ^^¾ in equations (4) and (5), ^Rpp and are derived, thereby obtaining the estimation values ^rf of channel responses at all subcarriers.

Spatial correlation-dependnet pilot design strategy

[0052] The proposed j oint MMSE CE is enhanced with the increase of the spatial correlation, which may be seen from the following simulation result. It means the higher is the spatial correlation of the MIMO system, the less pilot signals to be used is required. In actuality, spatial correlation of the MIMO system is mainly determined by the following two factors: channel environment, for example, cities with dense buildings, open countryside, line of sight (LOS), and non LOS, etc.; and antenna configuration, for example, the number of antennas, spacing, etc. It needs to exploit pilot patterns having different overheads with respect to MIMO systems having different antenna configurations in different environments. Therefore, a spatial correlation-dependnet pilot design strategy is proposed.

[0053] The spatial correlation-dependnet pilot design strategy comprises the following steps:

[0054] determining an appropriate pilot overhead by simulating each combination of channel environment (for example cities with dense buildings, open countryside, line of sight (LOS)/ non LOS) and antenna configuration (for example, the number of antennas, spacing, polarization);

[0055] selecting,by each base station, a pilot pattern based on its environment and antenna configuration;

[0056] signaling, by each base station, the pilot pattern currently in use, to its terminal.

[0057] Hereinafter, the advantages of the technology according to the present invention will be evaluated using numerical results.

[0058] Consider a MIMO-OFDM system that has N_T=4 antennas at a base station and N_R=2 antennas at each terminal. 3GPP spatial channel model (SCM) is used. The following two channel scenarios that have different spatial correlations are considered:

[0059] Scenario 1 : urban micro-cell, LOS, with the antenna spacings being unanimously 0.5 wavelengths at the BS and the terminal;

[0060] Scenario 2: urbam macro-cell, non-LO S, with the antenna spacings being 4 wavelengths at the BS and 0.5 wavelengths at the terminal.

[0061 ] In the following equations (10) and (11), for these two scenarios, the spatial correlation matrixes at the BS and the terminal derived through simulation are provided. It is easily found that scenario 1 has a relatively strong spatial correlation, and scenario 2 has a much weaker spatial correlation.

[0062] The spatial correlation matrxes for scenario 1 are:

1 0.9597 0.8615 0.7552

R_T 0.9597 1 0.9597 0.8615

0.8615 0.9597 1 0.9597 1 0.591

0.7522 0.8615 0.9597 1 0.591 1

and (10)

[0063] The spatial correlation matrxes for scenario 2

1 0.3969 0.2272 0.1515

0.3969 1 0.3969 0.2272

0.2272 0.3969 1 0.3969 1 0.0836

0.1515 0.2272 0.3969 1 0.0836 1

and (11)

[0064] As illustrated in Fig. 5A and 5B, two pilot patterns that have different overheads are used in simulation. Fig. 6A and Fig. 6B compare the mean square error (MSE) of the joint MMSE channel estimator according to the present inventin and that of the traditional MMSE channel estimator. For the joint MMSE channel estimator, two pilot patterns are sused.

[0065] Joint MMSE-1 : using pilot pattern A in Fig. 5A;

[0066] Joint MMSE-2: using pilot pattern B in Fig. 5B which only has as half overhead as that in Fig. A.

[0067] For the traditional MMSE channel estimator, pilot pattern A in Fig. 5A is always used. From this figure, it may be seen that in scenario 1 as illustrated in Fig. 6A, the joint MMSE channel estimator may implement the performance similar to the traditional MMSE channel estimator. When the spatial correlation is high, pilot overhead may be significantly reduced. When the spatial correlation is low, as illustrated in scenario 2 as shown in Fig. 6B, the joint MMSE and traditional MMSE channel estimators have a similar performance and require a similar pilot overhead. This observation indicates: the pilot overhead may be adjusted based on the spatial correlation of the channel such that an optimal compromise between the pilot overhead and the channel estimation accuracy may be provided in all environments.

[0068] The present invention is described with reference to the above embodiments. However, it should be understood that those skilled in the art may amend and change the embodiments of the present invention without departing from the spirit and scope of the present invention. The scope of the present invention is only limited by the appended claims.

Claims

What Is Claimed Is:

1. A user equipment for improving channel estimation with spatial correlation of channels between the user equipment and a base station, the user equipment comprising:

pilot signal receiving unit, for receiving pilot signals;

first estimating unit, for estimating the spatial correlation of the channels at the base station based on the received pilot signals;

second estimating unit, for estimating the spatial correlation of the channels at the user equipment based on the received pilot signals; and

third estimating unit, for estimating channel responses of the channels based on the received pilot signals, estimation of the spatial correlation of the channels at the base station, and estimation of the spatial correlation of the channels at the user equipment.

2. The user equipment according to claim 1, wherein channel response estimation of the channels is the channel estimation based on a joint minimum mean-square error MMSE.

3. The user equipment according to claim 2, further comprising:

fourth estimating unit, for estimating temporal correlations of the channels; and

fifth estimating unit, for estimating frequency correlations of the channels; and

wherein, the third estimating unit further estimates channel responses of the channels based on the estimation of the temporal correlation and the estimation of the frequency correlation.

4. The user equipment according to any one of claims 1 -3, wherein when the number of pilot signals transmitted by each antenna of the base station is identical, the first estimating unit and the second estimating unit further estimate the spatial correlation of the channels at the base station and the spatial correlation of the channels at the user equipment based on the number and values of the pilot signals transmited by each antenna of the base station, respectively.

5. The user equipment according to any one of claims 1 -3, wherein the third estimating unit further estimates the channel responses of the channels based on the values of the pilot signals transmitted by the base station.

6. The user equipment according to any one of claims 1 -3, wherein the third estimating unit further estimates the channel responses of the channels based on the signal-to-noise ratios of the channels.

7. The user equipment according to claim 3, wherein the third estimating unit comprises: sixth estimating unit, for estimating a correlation matrix between channel responses over data subcarriers and channel responses over pilot subcarriers based on the estimation of the spatial correlation of the channels at the base station, the estimation of the spatial correlation of the channels at the user equipment, estimation of the temporal correlation, and estimation of the frequency correlation;

seventh estimating unit, for estimating a correlation matrix of channel responses over pilot subcarriers based on the estimation of the spatial correlation of the channels at the base station, the estimation of the spatial correlation of the channels at the user equipment, estimation of the temporal correlation, and estimation of the frequency correlation; and eighth estimating unit, for estimating channel responses of the channels based on the estimation of the correlation matrix between the channel responses over the data subcarriers and the channel responses over the pilot subcarriers and the estimation of the correlation matrix of the channel responses over the pilot subcarriers.

8. A method for improving channel estimation with spatial correlation of channels between a user equipment and a base station, comprising steps of:

receiving pilot signals;

estimating the spatial correlation of the channels at the base station based on the received pilot signals;

estimating the spatial correlation of the channels at the user equipment based on the received pilot signals; and estimating channel responses of the channels based on the received pilot signals, estimation of the spatial correlation of the channels at the base station, and estimation of the spatial correlation of the channels at the user equipment.

9. The method according to claim 8, wherein channel response estimation of the channels is channel estimation based on a joint minimum mean-square error MMSE.

10. The method according to claim 9, further comprising:

estimating temporal correlations of the channels; and

estimating frequency correlations of the channels;

wherein, the step of estimating the channel responses of the channels is further based on the estimation of the temporal correlation and the estimation of the frequency correlation.

11. The method according to any one of claims 8-10, wherein when the number of pilot signals transmitted by each antenna of the base station is identical, the step of estimating the the spatial correlation of the channels at the base station and the step of estimating the spatial correlation of the channels at the user equipment are further based on the number and values of the pilot signals transmited by each antenna of the base station.

12. The method according to any one of claims 8-10, wherein the step of estimating channel responses of the channels is further based on the values of the pilot signals transmitted by the base station.

13. The method according to any one of claims 8-10, wherein the step of estimating channel responses of the channels is further based on the signal to noise ratios of the channels.

14. The method according to claim 10, wherein the step of estimating channel responses of the channels comprises:

estimating a correlation matrix between channel responses over data subcariiers and channel responses over pilot subcarriesr based on the estimation of the spatial correlation of the channels at the base station, the estimation of the spatial correlation of the channels at the user equipment, estimation of the temporal correlation, and estimation of the frequency correlation;

estimating a correlation matrix of channel responses over pilot subcarriers based on the estimation of the spatial correlation of the channels at the base station, the estimation of the spatial correlation of the channels at the user equipment, estimation of the temporal correlation, and estimation of the frequency correlation; and

estimating channel responses of the channels based on the estimation of the correlation matrix between the channel responses over the data subcarriers and the channel responses over the pilot subcarriers and the estimation of the correlation matrix of the channel responses over the pilot subcarriers.

15. A base station, comprising:

pilot pattern selecting unit for selecting a pilot pattern based on channel environment and antenna configuration of the base station; and

signaling unit for signaling the pilot pattern selected by the pilot pattern selecting unit to a user equipment.

16. The base station according to claim 15, wherein a pilot pattern selected by the base station with respect to an urban environment has less pilot overhead than a pilot pattern selected with respect to a rural environment.

17. The base station according to claim 15 or 16, wherein a pilot pattern selected by the base station with respect to a line-of-sight environment has less pilot overhead than a pilot pattern selected with respect to a non- line-of-sight environment.

18. The base station according to any one of claims 15-17, wherein the less is the number of antennas, the lower is the pilot overhead of the pilot pattern selected by the base station.

19. The base station according to any one of claims 15-18, wherein the less is the spacing between the antennas, the lower is the pilot overhead of the pilot pattern selected by the base station.

20. A method of selecting pilot information, comprising steps of:

selecting a pilot pattern as pilot information based on the channel environment and antenna configuration of a base station; and

signaling the selected pilot pattern to a user equipment.

21. The method according to claim 20, wherein a pilot pattern selected with respect to an urban environment has less pilot overhead than a pilot pattern selected with respect to a rural environment.

22. The method according to claim 20 or 21, wherein a pilot pattern selected with respect to a line-of-sight environment has less pilot overhead than a pilot pattern selected with respect to a non-line-of-sight environment.

23. The method according to any one of claims 20-22, wherein the less is the number of antennas, the lower is the pilot overhead of the selected pilot pattern.

24. The method according to any one of claims 20-23, wherein the smaller is the spacing between antennas, the lower is the pilot overhead of the selected pilot pattern.