METHOD AND APPARATUS FOR INFORMATION FEEDBACK AND PRECODING FIELD OF THE INVENTION [0001] Embodiments of the present invention generally relate to wireless communication systems, and more particularly to methods and apparatuses for downlink information feedback and precoding. BACKGROUND OF THE- INVENTION 100021 In Multiple Input Multiple Output (MIMO) wireless communication systems, both of the transmitter and the receiver use antenna arrays lo provide a rich diversity and a large communication capacity. Spatial multiplexing is a common space-time modulation technique for MIMO communication systems where independent data streams are transmitted via different transmit antennas. Unfortunately, the spatial multiplexing is very sensitive to the poor condition of the channels. For this end, a precoding technique is employed for improving the adaptability of the spatial multiplexing. [0003] The function of the precoding is to pre-process the data streams to be transmitted and to map the data streams to respective transmitting antennas, based on the channel conditions. In Long Term Evolution (LTE) systems and LTE-Advanced (LTE-A) systems, a codebook based limited feedback precoding technique is used at the transmitting side. [0004] For the limited feedback precoding, there are at least two feedback mechanisms, i.e., explicit feedback and implicit feedback. In the explicit feedback, the receiver feeds back information about the channel condition to the transmitter, and then the transmitter precodes the data streams to be transmitted based on the feedback channel condition. The implicit feedback defines different feedback modes for different assumptions, such as single-user MIMO (SU-MIMO) or multi-user MIMO (MU-MIMO), etc.. Generally, in the implicit feedback, the receiver selects an optimal precoder from a finite codebook known to both the transmitter and the receiver based on the channel condition, and then feeds back information (e.g., an index) about the optimal precoder to the transmitter. [0005] So far, implicit feedback solutions where single-user one-rank correlation adaptability is implemented by using the precoding codebook defined in the existing specifications have been proposed by Intel., Huawei, etc. "Correlation adaptability" means modifying the predefined prccoding codebook by using downlink spatial correlation matrix, and "rank" refers to the number of the duta streams to be transmitted at the transmitter. I lowevcr, the proposed implicit feedback solutions are only for (he single-user Ml MO single-stream cases. For multi-rank cases the system performance will rapidly deteriorate. Y ot in the existing implicit feedback system, it Is required to transmit multiple data streams for multiple users and to transmit multiple data streams per user. [0006| In the current standardization process of the LTH-A, the limited feedback precoding technique is still in discussion and research. As yet, a fixed precoding codebook is still used for the multiple users and multiple data streams cases, but the implicit feedback solutions with spatial correlation adaptability merely adapt to the single-user single-stream cases. SUMMARY OF THE INVENTION [0007] Thus, there is a need in the art for a spatial correlation adaptable implicit feedback solution which can adapt to multiple streams cases. [0008] According to one exemplary aspect of the present invention, a method for processing communication data at a user equipment in a wireless communication system is provided. The method comprises: deriving a spatial correlation matrix R of multiple transmit antennas of a base station based on an obtained downlink channel transmission matrix H; transforming a precoding codebook F according to the spatial correlation matrix R; selecting a precoding matrix Fs based on the transformed precoding codebook; and feeding back information about the spatial correlation matrix R and information about the selected precoding matrix Fs to the base station. [0009] In one embodiment, the deriving may comprise averaging the downlink channel transmission matrix H in time and/or frequency to obtain the spatial correlation matrix R. [0010] In one embodiment, the transforming may comprise: quantizing the spatial correlation matrix R and transforming the precoding codebook with the quantized spatial correlation matrix. Each codeword Fk in the precoding codebook is transformed to obtain the transformed precoding matrix FR)kaccording to R,K k, where k=l,...,K, and K is a position integer. [0011] In one embodiment, the selecting may comprise singular value decomposing the downlink channel transmission matrix H to obtain H = ULVH ; taking the first m columns of elements in Ihe unitary singulur matrix V obtained from the singular value deeomposition as an ideal prccoding matrix V where m is the number of data streams transmitted to the user equipment; and selecting the prccoding matrix Fs from the prccoding eodebook such that the distance between a prccoding matrix FR transformed via the spatial correlation matrix and the ideal precoding matrix Vm is minimized. The distance may be , where k=l,...,K, K is a position integer, H denotes conjugate transpose, | |- | |F denotes matrix Frobenius norms, abs() denotes the modular of a matrix, and tr() denotes the trace of a matrix. Alternatively, the distance may be chosen from a group consisted of the chordal distance, the projection two-norm distance and the Fubini-Study distance. [0012] In one embodiment, the information about the selected precoding matrix Fs is an index of the selected precoding matrix Fs in the precoding eodebook; and the information about the spatial correlation matrix R is an index of the spatial correlation matrix R in a spatial correlation matrix eodebook. [0013] According to another exemplary aspect of the present invention, a method for data precoding at a base station in a wireless communication system is provided. The method comprises: obtaining, from a user equipment, information about a spatial correlation matrix R of multiple transmit antennas of the base station and information about a precoding matrix Fs selected by the user equipment; determining a desired precoding matrix FRs based on the obtained information and a precoding eodebook; and precoding downlink data to be transmitted to the user equipment with the desired precoding matrix FRS. [0014] In one embodiment, the determining may comprise: retrieving the selected precoding matrix Fs from the precoding eodebook based on the information about the selected precoding matrix Fs, wherein the information about the selected precoding matrix includes an index of the selected precoding matrix in the precoding eodebook; retrieving the spatial correlation matrix R from a spatial correlation matrix eodebook based on the information about the spatial correlation matrix R, wherein the information about the spatial correlation matrix R includes an index of the spatial correlation matrix R in the spatial correlation matrix eodebook; and transforming the selected precoding matrix Fs with the spatial correlation matrix R to obtain the desired precoding matrix FR)S. [0015] In one embodiment, the precoding may comprise: taking the conjugate transpose of the desired preocoding matrix FRjS as an approximate effective channel matrix of the user equipment; and precoding the downlink data to be transmitted to the user equipment based on the approximate effective channel matrix. (0016] In another embodiment, in a case where there are multiple user equipments, the precoding may comprise: scheduling user equipments of the multiple user equipments whose desired precoding matrices FRiS are orthogonal to each other precoding matrices. [0017] According to yet another exemplary aspect of the present invention, an apparatus for processing communication data at a user equipment in a wireless communication system is provided. The apparatus may comprise: a deriving module, configured to derive a spatial correlation matrix R of multiple transmit antennas of a base station based on an obtained downlink channel transmission matrix H; a transforming module, configured to transform a precoding codebook F according to the spatial correlation matrix R; a selecting module, configured to select a precoding matrix Fs based on the transformed precoding codebook; and a feedback module, configured to feed back information about the spatial correlation matrix R and information about the selected precoding matrix Fs to the base station. [0018] According to a further exemplary aspect of the present invention, an apparatus for data precoding at a base station in a wireless communication system is provided. The apparatus may comprise: an obtaining module, configured to obtain, from a user equipment, information about a spatial correlation matrix R of multiple transmit antennas of the base station and information about a precoding matrix Fs selected by the user equipment; a determining module, configured to determine a desired precoding matrix FRJS based on the obtained information and a precoding codebook; and a precoding module, configured to precode downlink data to be transmitted to the user equipment with the desired precoding matrix FR,S. [0019] Embodiments of the present invention may be adaptable to both single user SU-MIMO and multiple users MU-MIMO. Moreover, for each user equipment, it may have a single stream or multiple streams. Compared with the prior art, because the spatial correlation information specific to each user equipment (UE) has been fed back to the base station (eNB), significant performance gain may be achieved by using the spatial correlation adaptable codebook than using a fixed codebook. In addition, the solutions as proposed in the invention are easy to implement. For example, the current LTE Release 8 4-Tx feedback codebook can be used as the baseline codebook for correlation adaptation. The only extra signaling overhead would be used for feeding back the spatial correlation matrix. Furthe r, the computational complexity of the embodiments of the present invention is quite low. BRIEF DESCRIPTION OF THE DRAWINGS [0020] The above and other aspects, features, and benefits of various embodiments of the invention will become more fully apparent, by way of example, from the following detailed description and the accompanying drawings, in which: [0021] Fig. 1 illustrates an exemplary environment of wireless communication systems where embodiments of the present invention may be implemented; [0022] Fig. 2 illustrates an exemplary logic flow chart of a method for processing communication data at a user equipment in a wireless communication system according to one embodiment of the present invention; [0023] Fig. 3 illustrates an exemplary logic flow chart of a method for precoding data at a base station in a wireless communication system according to one embodiment of the present invention; [0024] Fig. 4 illustrates a schematic structure diagram of an apparatus for processing communication data at a user equipment in a wireless communication system according to one embodiment of the present invention; [0025] Fig. 5 illustrates a schematic structure diagram of an apparatus for precoding data at a base station (eNB) in a wireless communication system according to one embodiment of the present invention; and [0026] Figs. 6-15 illustrate the simulation result comparison diagrams between the solutions according to embodiments of the present invention and the solutions in the prior art. [0027] Like reference numbers and designations in the various drawings indicate like elements. DETAILED DESCRIPTION OF EMBODIMENTS [0028] Hereinafter, exemplary description of the embodiments of the present invention will be described in detail with reference to the attached drawings. [0029] With reference to Fig. 1, an example of wireless communication network environment 100 where embodiments of the present invention may be implemented is shown. As illustrated in Fig. 1, in the wireless communication network environment 100, there may be a transmitter 101 and several receivers 102-1, 102-2, ...102-L, where L is an integer greater than or equal to 1. The transmitter 101 has M transmit antennas, and each receiver has N receive antennas, where either of M and N is an integer greater than 1. The transmitter transmits m data streams to each of the receivers, where m < min (M, N). In various embodiments, the transmitter may bo for example a base station (BS), or known as an eNB in LTE and LTE-A systems. The receivers 102 may be for example user equipments (UEs). In the following descriptions, a base station (eNB) and multiple user equipments (UEs) are taken as examples for illustration. [0030] Because the uplink and downlink channels between a user equipment (UE) and a base station (eNB) are not symmetric, the eNB needs information about downlink fed back by the UE, based on which the data to be transmitted to the UE may be precoded. [0031] Now with reference to Fig. 2, it illustrates an exemplary logic flow chart of a method for processing communication data at a user equipment UE in a wireless communication system according to one embodiment of the present invention. In the following, the flow of Fig. 2 will be described in detail in conjunction with the wireless communication network environment 100 as shown in Fig. 1. [0032] As shown in Fig. 2, at the step S201, at each user equipment (UE), a spatial correlation matrix R of M transmit antennas of the base station (eNB) is derived based on an obtained downlink channel transmission matrix H, where H is a two-dimension matrix of N*M, and R is a two-dimension matrix of MxM. [0033] Generally, the user equipment (UE) may perform channel estimation according to the downlink channel signal received from the base station (eNB), so as to obtain the downlink channel transmission matrix H. Specifically, it is well known in the art as how to perform channel estimation. Reference may be made to "Digital Communication", John G. Proakis, and the description thereof is omitted herein. [0034] In one embodiment, the obtained downlink channel transmission matrix H is averaged in time and/or frequency to get the spatial correlation matrix R of M transmit antennas of the base station. For example, , where H denotes conjugate transpose. In other words, R is the mean value of H • H on mul tiple time points and/or on multiple subcarriers. [0035] The spatial correlation matrix R of multiple transmit anntenas of the base station (eNB) is a physical q uanlity changing slowly over time. Therefore, the downlink channel transmission matrix II may be averaged over a long period, such as more than 20 ms. [0036| Then, ill the step S202, a precoding codebook F is transformed according to the spatial correlation matrix R. [0037| In the limited feedback precoding technique, there is a precoding codebook F which is known to both the user equipment (UE) and the busc station (eNB). The precoding codebook is consisted of a limited number of codewords, e.g., K codewords, where each codeword Fk is a precoding matrix of Mxm, where k=l,...,K, and K is a position integer. Such a precoding codebook may be for example the precoding matrix index (PMI) codebook defined in LTE release 8, the codebook defined in IEEE 802.16m standards, etc.. [0038] m is the number of data streams to be transmitted from the base station (eNB) to a certain user equipment (UE). The number of data streams to be transmitted may be set in advance by the communication system, or it may be determined dynamically by the base station based on the real-time channel conditions. [0039] In one embodiment, the spatial correlation matrix R may be quantized. Then the precoding codebook F maybe be transformed with the quantized spatial correlation matrix R to get the transformed precoding codebook FR. The subscript R denotes being transformed with the spatial correlation matrix, i.e., being processed by the spatial correlation adaption. [0040] Many manners may be adopted to quantize the spatial correlation matrix R. In one embodiment, the spatial correlation matrix R may be quantized according to a spatial correlation matrix codebook. Similarly to the precoding codebook, the spatial correlation matrix codebook is also a codebook consisted of a limited number of matrices whichis known to or synchronized to both the user equipment (UE) and the base station (eNB). Various manners may be adopted to design the spatial correlation matrix codebook. In various embodiment of the present invention, only the designed spatial correlation matrix will be utilized, and thus the design manners thereof will not be described in detail here. [0041] In one embodiment, each codeword \\ in the precoding codebook F is transformed to obtain the corresponding transformed precoding matrix Fuji according to; [0042| Next, in (lie slep S203, a precoding matrix Fs is selected based on the transformed precoding codebook FR. [0043] At each user equipment i (i=l, .... I-). a precoding matrix which is desired to use may be selected from the precoding codebook based on different selection criterions. These selection criteria may be for example the maximum likelihood criterion (ML-SC), the minimum singular value selection criterion (MSV), the minimum mean squared error selection criterion (MMSE-SC), and capacity selection criterion (Capacity-SC), etc. [0044] Study shows that for single user precoding, all these criteria are in fact equivalent. The optimal selected precoding matrix by the user equipment (UE) is the conjugate transpose of the primary eigen mode of the downlink channel transmission matrix H. For example, reference may be made to D.J.Love, R.W.Heath, Jr., "Limited feedback Unitary Precodings for Spatial Multiplexing Systems," IEEE Transactions on Information Theory, Vol. 51, No. 8, 2005, pp2967-2976. [0045] Specifically, for UE i, its obtained downlink channel transmission matrix Hi is singular value decomposed (SVD) to get H, = U,E,V,H. The first m columns of elements of V, are taken to construct a two-dimension matrix Vm, of Mxm, wherein V, is a two-dimension unitary matrix of MxM at the right side obtained by the singular value decomposition. At the UE i, the optimal precoding matrix is: Fopt, = Vm,. [0046] With the above result, in one embodiment of the present invention, a precoding matrix closest to the preferred precoding matrix is selected from the transformed precoding codebook FR as obtained in step 202. Bee ause each precoding matrix FRin the transformed precoding codebook FR may be expressed as FRk = RFk , from another angle, a precoding matrix Fs is selected from the fixed precoding codebook F, such that the distance between the precoding matrix being transformed with the spatial correlation matrix R and the optimal precoding matrix is minimized. [0047] Specifically, in one embodiment, for each user equipment UH, the obtained downlink channel transmission matrix H is subjected to the singular value decomposition (SVD), deriving " = UΣV11 . || should be noted that similar processing is performed at each user equipment Uli, the following description omits the subscript i representing Hie user equipment. |0048| Taking the first in columns of elements of the right side unitary singular matrix V derived from the above singular value decomposition as the optimal precoding matrix Vm, wherein m denotes the number of data streams transmitted to the user equipment. From the transformed precoding codebook FR, a precoding matrix FR(S with the minimal distance from the optimal precoding matrix Vm is selected, wherein the subscript s denotes the selected precoding matrix. Such selected precoding matrix FRIS is just the precoding matrix which the user equipment expects the eNB to use. T he above selection process may be denoted as: wherein " denotes conjugate transpose, | | • | | F denotes matrix Frobenius norms, abs() denotes the modular of a matrix, and tr() denotes the trace of a matrix. [0050] In another embodiment, the distance function may adopt other form, for example, the chordal distance, the projection two-norm distance, the Fubini-Study distance, etc. [0051] Because the precoding matrix FR,k in the transformed precoding codebook FR may be expressed as ,, the above selection process may also be expressed as: [0052] In other words, a precoding matrix Fg is selected from the fixed precoding codebook F, such that the distance between the precoding matrix FR,, being transformed with the spatial correlation matrix K and the optimal precoding matrix V„, is minimized. |0053| Finally, in step S204, the user equipment may feed back the relevant information of the spatial correlation matrix R and the relevant information of the selected precoding matrix F„ to the base station eNB. |0054| In one embodiment, the relevant information of spatial correlation matrix R comprises an index of the spatial correlation matrix R in the spatial correlation matrix codebook when the spatial correlation matrix R is quantized in step S202, while the relevant information of the selected precoding matrix Fs comprises an index of the selected precoding matrix Fs in the fixed precoding codebook F. [0055| Besides, as mentioned above, the spatial correlation matrix R is a physical quantity slowly varying with time, and thus the feedback information may also be called as a long-term wideband precoding matrix index PMI. In contrast, the downlink channel transmission channel H is a physical quantity rapidly changing with time, and thus its feedback information may also be called as a short-term narrowband PMI. F or example, the feedback period of the long-term wideband PMI may be 20ms above, while the feedback period of the short-term narrowband PMI is about 5ms. [0056] Because the user equipment UE and the base station eNB have known the spatial correlation matrix codebook and the precoding codebook, the user equipment UE only needs to feed back the corresponding indexes to the base station eNB. Then the base station eNB may obtain the spatial correlation matrix R and the selected precoding matrix Fs. [0057] Now, referring to Fig. 3, Fig. 3 illustrates an exemplary logic flow chart of a method for precoding data at a base station in a wireless communication system according to one embodiment of the present invention. H ereinafter, the flow of Fig. 3 will be described in detail in conjunction with the wireless communication network environment 100 as shown in Fig. 1. [0058] First, in step S301, the base station eNB obtains from the user equipment UE relevant information of a spatial correlation matrix R of M transmit antennas of the base station and relevant information of a precoding matrix Fs as selected by the user equipment UE. A s the above mentioned, for example, the above relevant Information may be for example an index of the spatial correlation malrix K in the spatial correlation matrix codebook and an index of the selected precoding matrix F, in the fixed precoding codebook F. [0059| Next, in step S302, the base station cNU determines a desired precoding matrix FR based on the obtained information and the precoding codebook F. [0060] Specifically, in one embodiment, the base station eNB retrieves the selected precoding matrix Fg from the precoding codebook F based on the index of the precoding matrix Fs as obtained in step S30I and retrieves the spatial correlation matrix R from the spatial correlation matrix codebook based on the index of the spatial correlation malrix R. [0061] Then, by transforming the selected precoding matrix Fs with the retrieved spatial correlation matrix R, the desired precoding matrix FR,S may be obtained. The desired precoding matrix can compensate for the channel condition and improve channel performances because it considers the spatial correlation between transmit antennas as fed back by the user equipment. [0062] In one embodiment, corresponding to the transformation of the precoding codebook F performed by the user equipment UE in step S202 of Fig. 2, the base station eNB transforms the selected precoding matrix Fs according to the following equation so as to obtain the desired precoding matrix FRIS: FR,S = RFS. [0063] Finally, in step S303, the base station eNB precodes downlink data to be transmitted to the user equipment with the desired precoding matrix FR,S. [0064] By using the desired precoding matrix FR,S, the downlink data may be precoded in a plurality of manners. [0065] In one embodiment, the base station eNB regards the conjugate transpose of the desired precoding matrix FR s as an approximate effective channel matrix H 0f the user equipment UE. H denote s a two-dimension matrix of mxM and can be expressed as: H = F?S= (RFS)" = FsHRH = FSHR [0066] Then, the downlink data of cacli user equipment may be precoded using the derived approximate effeetive eh annel matrix of the user equipment UK. for example, a zero-forcing (/>') preeoding can be performed. Such precoding manner is not only suitable for the single user SU-MIM() but also suitable for the multi-user MU-MIMO. [0067] In anolhcr embodiment, in a case where there are multiple user equipments, the base station eNB obtains information fed back from each user equipment UE. Thus, the base station eNB may obtain a desired precoding matrix Frs of each user equipment, respectively, wherein the subscript i denotes a user equipment. B y using a possible orthogonal feature between the precoding matrixes FR,S,i of the user equipments, the user equipments whose desired precoding matrixes FR,S,I are mutually orthogonal can be scheduled. [0068] 1 lowcver, in this precoding manner, each user equipment UE has transformed the precoding codebook using the derived spatial correlation matrix in step S202 of Fig. 2. Thus, such a scenario might occur, where it is possible that no orthogonal pairs exist between the desired precoding matrixes FRS of respective user equipments as derived by the base station eNB in step S302 of Fig. 3. Therefore, it is possible to reduce the probability of pairing between multiple users, thereby limiting the performance of the multi-user MU-MIMO. [0069] In view of the above, at the base station eNB, the preceding precoding manner is preferably used to precode the downlink data. [0070] Each time the user equipment derives a new spatial correlation matrix and feeds it back to the base station eNB, the user equipment UE and the base station eNB update their respective spatial correlation matrix so as to be available during the transformation process. [0071] Fig. 4 illustrates a schematic structure diagram of an apparatus 400 for processing communication data at a user equipment in a wireless communication system according to one embodiment of the present invention. [0072] As illustrated in Fig. 4, the apparatus 400 may comprise a deriving module 401, a transforming module 402, a selecting module 403, and a feedback module 404. [0073] The deriving module 401 may derive a spatial correlation matrix R of M transmit antennas in the base station eNB based on an obtained downlink channel transmission matrix H, wherein H is a two-dimension matrix of NxM, R is the two-dimension matrix of M* M. [0074] Generally, the user equipment UE may perform channel estimation based on the downlink channel signal it receives from the base station eNB to thereby obtain the downlink chennal transmission matrix H. [0075| In one embodiment, the deriving module 401 is configured to average the downlink channel transmission matrix H in time and/or frequency, thereby deriving the spatial correlation nuilrix R of (he M transmit antennas in the base station. For example, wherein 11 denotes conjugate transpose, i.e., k is the average value on multiple time points and/or multiple subcarricrs. [0076] The transforming module 402 may transform the prccoding codebook F based on the spatial correlation matrix R as derived by the deriving module 401. The precoding codebook is a codebook having a finite number of matrixes which is known to or synchronized at both the user equipment UE and the base station eNB. [0077] In one embodiment, the transforming module 402 is configured to quantize the spatial correlation matrix R, and then transforms the precoding codebook F usin g the quantized spatial correlation matrix, thereby obtaining the transformed precoding codebook FR. The subscript R denotes that transformation has been performed using the spatial correlation matrix, i.e., spatial correlation adaptation has been conducted. [0078] The spatial correlation matrix R may be quantized in a plurality of manners. In one embodiment, the spatial correlation matrix R is quantized based on a spatial correlation matrix codebook. Simi lar to the precoding codebook, the spatial correlation matrix codebook is also a codebook having a finite number of matrixes which is known to or synchronized at both the user equipment UE and the base station eNB. [0079] In one embodiment, the transforming module 402 may be configured to transform each codeword Fk in the precoding codebook F according to the following expression so as to obtain the corresponding transformed precoding matrix FR,K FRS. 21. The apparatus of claim 19, wherein the precoding module is configured to: take the conjugate transpose of the desired preocoding matrix FR>S as an approximate effective channel matrix of the user equipment; and precode the downlink data to be transmitted to the user equipment based on the approximate effective channel matrix. 22. The apparatus of claim 19, wherein in a case where there are multiple user equipments, the precoding module is configured to: schedule user equipments of the multiple user equipments whose desired precoding matrices FRS are orthogonal to each other.