WO2017008268A1

WO2017008268A1 - Method and apparatus for performing beamforming

Info

Publication number: WO2017008268A1
Application number: PCT/CN2015/084097
Authority: WO
Inventors: Zhennian SUN; Chuangxin JIANG; Gang Wang; Pan Chen; Shengqian HAN; Chenyang Yang
Original assignee: Nec Corporation
Priority date: 2015-07-15
Filing date: 2015-07-15
Publication date: 2017-01-19

Abstract

Embodiments of the disclosure provide a method and apparatus for performing beamforming in a MIMO system. In the MIMO system, a BS includes a first antenna array containing a plurality of antennas. The method may comprise： transforming a first channel matrix from an array domain to an angle domain, as a first initial angle domain channel matrix， wherein the first channel matrix characterizes a channel between a UE and the first antenna array of the BS； determining, based on the first initial angle domain channel matrix, a first optimal transmission angle domain channel matrix: generating a first angle domain precoder based on the first optimal angle domain channel matrix; and determining a target precoder in the array domain based on the first angle domain precoder.

Description

METHOD AND APPARATUS FOR PERFORMING BEAMFORMING

FIELD OF THE INVENTION

Embodiments of the present invention generally relate to communication techniques. More particularly， embodiments of the present invention relate to a method and apparatus for performing beamforming.

BACKGROUND OF THE INVENTION

Multiple Input Multiple Output (MIMO) is a key feature of Long Term Evolution (LTE) /LTE-Advanced (LTE-A) system. Conventionally， one-dimensional (horizontal) antenna array provides flexible beamforming adaption in the azimuth domain. It has been recently found that the full MIMO capability can be exploited through leveraging a two dimensional antenna system (AAS) ， such that it is possible to implement user-specific elevation beamforming and spatial multiplexing in the vertical domain.

Conventionally， for performing beamforming in the MIMO system， eigenvector (s) obtained by eigenvalue decomposition have been used as beamforming weight (s) or precoder (s) in transmission/reception of a signal， in order to improve radio characteristics. However， due to the large number of antenna elements in the AAS， complexity of conventional decomposition of channel covariance matrix is extremely high.

Therefore， there is a need for reducing complexity in determination of the precoder， especially in the AAS.

SUMMARY OF THE INVENTION

The present invention proposes a solution for performing beamforming with low complexity in determination of the precoder.

According to a first aspect of embodiments of the present invention， embodiments of the invention provide a method of performing beamforming in a MIMO system， wherein a base station (BS) in the MIMO system includes a first antenna array containing a plurality of antennas. The method comprises： transforming a first channel matrix from an array domain to an angle domain， as a first initial angle domain channel matrix， wherein the first channel matrix characterizes a channel between user equipment (UE) and the first antenna array of the BS； determining， based on the first initial angle domain channel matrix， a first optimal transmission angle domain channel matrix； generating a first angle domain precoder based on the first optimal angle domain channel matrix； and determining a target precoder in the array domain based on the first angle domain precoder.

According to a second aspect of embodiments of the present invention， embodiments of the invention provide an apparatus for performing beamforming in a MIMO system， wherein a BS in the MIMO system includes a first antenna array containing a plurality of antennas. The apparatus comprises： a transforming unit configured to transform a first channel matrix from an array domain to an angle domain， as a first initial angle domain channel matrix， wherein the first channel matrix characterizes a channel between a UE and the first antenna array of the BS； a first determining unit configured to determine， based on the first initial angle domain channel matrix， a first optimal transmission angle domain channel matrix； a generating unit configured to generate a first angle domain precoder based on the first optimal angle domain channel matrix； and a second determining unit configured to determine a target precoder in the array domain based on the first angle domain precoder.

Other features and advantages of the embodiments of the present invention will also be apparent from the following description of specific embodiments when read in conjunction with the accompanying drawings， which illustrate， by way of example， the principles of embodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention are presented in the sense of examples and their advantages are explained in greater detail below， with reference to the accompanying drawings， where

FIG. 1 illustrates a schematic diagram of an antenna array according to embodiments of the invention；

FIG. 2 illustrates a flow chart of a method for performing beamforming according to embodiments of the invention；

FIG. 3 illustrates a flow chart of a method for performing beamforming according to further embodiments of the invention；

FIG. 4A illustrates a schematic diagram of a cross-polarized AAS according to embodiments of the invention；

FIG. 4B illustrates a schematic diagram of two cross-polarized antenna arrays according to embodiments of the invention；

FIG. 5 illustrates a flow chart of a method for performing beamforming according to embodiments of the invention； and

FIG. 6 illustrates a schematic diagram of an apparatus for performing beamforming according to embodiments of the invention.

Throughout the figures， same or similar reference numbers indicate same or similar elements.

DETAILED DESCRIPTION OF EMBODIMENTS

The subject matter described herein will now be discussed with reference to several example embodiments. It should be understood these embodiments are discussed only for the purpose of enabling those skilled persons in the art to better understand and thus implement the subject matter described herein， rather than suggesting any limitations on the scope of the subject matter.

The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of example embodiments. As used herein， the singular forms “a， ” “an” and “the” are intended to include the plural forms as well， unless the context clearly indicates otherwise. It will be further understood that the terms “comprises， ” “comprising， ” “includes” and/or “including， ” when used herein， specify the presence of stated features， integers， steps， operations， elements and/or components， but do not preclude the presence or addition of one or more other features， integers， steps， operations， elements， components and/or groups thereof.

It should also be noted that in some alternative implementations， the functions/acts noted may occur out of the order noted in the figures. For example， two functions or acts shown in succession may in fact be executed concurrently or may sometimes be executed in the reverse order， depending upon the functionality/acts involved.

As used herein， the term “base station” or “BS” represents a node B (NodeB or NB) ， an evolved NodeB (eNodeB or eNB) ， a Remote Radio Unit (RRU) ， a radio header (RH) ， a remote radio head (RRH) ， a relay， a low power node such as a femto， a pico， and so forth.

As used herein， the term “user equipment” or “UE” refers to any device that is capable of communicating with the BS. By way of example， the UE may include a terminal， a Mobile Terminal (MT) ， a Subscriber Station (SS) ， a Portable Subscriber Station， a Mobile Station (MS) ， or an Access Terminal (AT) .

Embodiments of the present invention may be applied in various communication systems， including but not limited to a Long Term Evolution (LTE) system or a Long Term Evolution Advanced (LTE-A) system. Given the rapid development in communications， there will of course also be future type wireless communication technologies and systems with which the present invention may be embodied. It should not be seen as limiting the scope of the invention to only the aforementioned system.

Now some exemplary embodiments of the present invention will be described below with reference to the figures. Reference is first made to FIG. 1， which illustrates a schematic diagram of an antenna array 100 according to embodiments of the invention.

In a MIMO system， a BS usually transmits signals to a UE by using multiple transmitting antennas and the UE receives the signals by using multiple receiving antennas， vice verses. In other words， both the BS and the UE may use multiple antennas in the communication. In practice， the multiple antennas may be arranged in one dimension， for example， in a line. In some cases， a large number of antennas may be arranged in two dimensions， which is called as an antenna array. FIG. 1 exemplarily shows such an antenna array according to embodiments of the invention， which is a co-polarized AAS. As shown in FIG. 1， the antenna array has M antennas in the vertical direction and has N antennas in the horizontal direction. In other words， the antenna array shown in FIG. 1 includes M×N antennas， wherein each of M and N is a natural number.

Conventionally， in the determination of a precoder (sometimes called as “beamforming matrix” or “beamforming parameters” ) for performing beamforming， one or more channel covariance matrices need to be decomposed. Since the decomposition complexity with respect to a large-size antenna array is extremely high， embodiments of the present invention propose a solution to reduce the complexity. In this way， the precoder may be determined in a fast and efficient way.

Reference is now made to FIG. 2， which illustrates a flow chart of a method 200 for performing beamforming according to embodiments of the invention. In embodiments with respect to FIG. 2， a BS communicates via a first antenna array with a UE. The first antenna array may be implemented in several ways. By way of example， it may be implemented as the two dimensional antenna array illustrated in FIG. 1， including M×N antennas. In such cases， in the downlink transmission， the BS may use N_T＝M×N transmit antennas to transmit signals to the UE， and the UE may use N_R receive antennas to receive signals from the BS， wherein N_R is a natural number larger than or equal to 1.

The method 200 starts at step S210， in which a first channel matrix is transformed from an array domain to an angle domain， as a first initial angle domain channel matrix， wherein the first channel matrix characterizes a channel between a UE and a first antenna array of a BS.

According to embodiments of the present invention， a channel matrix may be obtained based on reference signals received from the UE via the first antenna array. Then， the obtained channel matrix is transformed from an array domain to the angle domain.

At step 220， a first optimal transmission angle domain channel matrix is determined based on the first initial angle domain channel matrix.

According to embodiments of the present invention， a selection factor may be determined first. The selection factor may include elements with large values in the first initial angle domain channel matrix. In some embodiments， the selection factor may be a matrix (also called as “selection matrix” ) having the same size as the first initial angle domain channel matrix， and may have the same elements as those belonging to the first initial angle domain channel matrix and larger than a first predetermined threshold. Other elements in the selection matrix may be set as 0. By way of example， in an embodiment， the selection factor may be obtained by determining a set of elements in the first initial angle domain channel matrix， wherein each of the set of elements has a value exceeding the first predetermined threshold； and constructing the selection factor based on positions of elements of the set in the first initial angle domain channel matrix.

Based on the first initial angle domain channel matrix obtained at step 210 and the determined selection factor， the first optimal transmission angle domain channel matrix may be calculated at step 220.

At step 230， a first angle domain precoder is generated based on the first optimal angle domain channel matrix.

According to embodiments of the present invention， a first channel vector may be determined based on the first optimal transmission angle domain channel matrix obtained at step 220. Based on the first channel vector， it may be determined a second channel factor that is composed of a set of elements of the first channel vector， wherein each of the set of elements has a value exceeding a second predetermined threshold. Then， the first angle domain precoder may be determined based on the second channel vector.

According to embodiments of the present invention， the first angle domain precoder based on the second channel vector may be determined in various ways. In some embodiments， Eigen Value Decomposition (EVD) may be carried out on the second channel factor. Then， a beamforming matrix which maximizes results of the EVD may be determined as the first angle domain precoder.

At step 240， a target precoder in the array domain is determined based on the first angle domain precoder.

In some embodiments， after the first and second channel vectors are determined step 230， a scaling factor may be calculated based on the first and second channel vectors. According to embodiments of the present invention， the scaling factor may be a matrix including multiple elements. Then， the target precoder may be determined based on the first angle domain precoder and the scaling factor.

In some alternatively embodiments， in addition to the first antenna array， the BS may further comprise a second antenna array which is cross-polarized with the first antenna array and contains a plurality of antennas. In this case， the BS may determine a second first angle domain precoder associated with the second antenna array， and determine a target precoder in the array domain based on both the first angle domain precoder and the second angle domain precoder.

More specifically， in an embodiment， a second channel matrix may be transformed from an array domain to an angle domain， as a second initial angle domain channel matrix， wherein the second channel matrix characterizes a channel between the UE and the second antenna array of the BS. Next， a second optimal transmission angle domain channel matrix may be determined based on the second initial angle domain channel matrix. Then， a second angle domain precoder may be generated based on the second optimal angle domain channel matrix. By means of the second angle domain precoder， as well as the first angle domain precoder obtained at step 230， the target precoder in the array domain can be calculated with less complexity.

In some embodiments， steps 210-240 may be performed per RB. It is to be noted that， this is an example rather than limitation. Alternatively， in some embodiments， step 210 may be performed per RB， while steps 220 to 240 may be performed within a predetermined frequency band including one or more RBs. In this way， the target precoder may be determined in a more accurate way.

Reference is now made to FIG. 3， which illustrates a flow chart of a method 300 for performing beamforming according to further embodiments of the invention. The method 300 may be considered as a specific implementation of the method 200 described above with reference to Fig. 2. However， it is noted that this is only for the purpose of illustrating the principles of the present invention， rather than limiting the scope thereof.

Similar to embodiments of FIG. 2， in embodiments of FIG. 3， a BS transmitting signals to a UE via a first antenna array as illustrated in FIG. 1. In the downlink communication， the BS transmits signals by the first antenna array including N_T＝M×N transmit antennas， and the UE receives these signals by N_R receive antennas.

At step 310， a channel matrix is obtained based on reference signals received from the UE via the first antenna array.

The channel matrix comprises a plurality of channel parameters characterizing the channel between the BS and the UE. It is to be noted that the channel matrix may be in the form of a matrix， but it is just an example rather than limitation. Those skilled in the art will appreciate that the channel matrix may have some other suitable forms. For example， the channel matrix may be a set including multiple groups of channel parameters， and each group comprises a certain number of channel parameters.

According to embodiments of the present invention， the channel matrix may be obtained in several ways. In some embodiment， the UE may send reference signals such as sounding reference signals (SRSs) to the BS， for example， according to preset configuration (s) . The BS may measure the reference signals and determine the channel matrix based thereon. Since there have been ways for determining a channel matrix based on the reference signals， relevant details are omitted here.

At step 320， the obtained channel matrix is transformed from an array domain to the angle domain.

Assuming that the channel matrix obtained at step 310 is denoted as “H” ， it may be transformed to the angle domain by equation (1) ：

where U_v and U_h are DFT matrices respectively， and H^a represents the first initial angle domain channel matrix.

At step 330， a selection factor is determined， wherein the selection factor includes elements with large values in the first initial angle domain channel matrix.

According to embodiments of the present invention， the selection factor may be determined in several ways. In some embodiments， a first predetermined threshold may be preset for defining a value reference for elements of the first initial angle domain channel matrix. Then， each element in the first initial angle domain channel matrix may be compared with the first predetermined threshold， and element (s) which have larger value (s) than the first predetermined threshold may be obtained. Accordingly， the position (s) of the larger element (s) in the first initial angle domain channel matrix may be determined. Next， element (s) of the section factor which have the same position (s) may be set as 1， and other element (s) of the section factor may be set as 0.

Alternatively， in some other embodiments， a first predetermined threshold may be preset for defining the number of larger elements of the first initial angle domain channel matrix. By way of example， first predetermined threshold may be 10. For a first initial angle domain channel matrix of 8*8， there are 64 elements in total. By sorting the elements in the first initial angle domain channel matrix may be sorted in descending order of their values， the first 10 elements may be determined. Accordingly， the positions of the first 10 elements in the first initial angle domain channel matrix may be determined. According to embodiments of the present invention， a position of an element may include the column index and the row index of the element. Next， elements of the section factor which have the same positions may be set as 1， and other element (s) of the section factor may be set as 0.

For sorting the elements of a matrix， according to embodiments of the present invention， there are multiple ways. For example， absolute values or the square of the absolute values may be calculated to sort these elements.

In some embodiments， the first channel matrix may be obtained in a predetermined frequency band or a predetermined time period. For example， the first channel matrix may be obtained in a plurality of resource blocks (RBs) . In this case， a plurality of first initial angle domain channel matrices may be obtained in the plurality of RBs. With respect to one element in the respective first initial angle domain channel matrices， the absolute value of the element may be calculated by averaging respective absolute values of the element in the respective first initial angle domain channel matrices.

More specifically， in an embodiment， a temporary matrix B may be determined by calculating：

where p and q are indices of elements of the first initial angle domain channel matrix H^a； N_RB represents the number of the RBs； |·|represents the operation of calculating an absolute value； and [B] _pq represents an element in the p^th row and q^th column of the temporary matrix B.

Next， large elements in the temporary matrix B may be selected and their positions (for example， the row index and the column index) in the temporary matrix B may be determined. By setting elements of the section factor which have the same positions in the selection matrix as 1， and other elements of the section factor as 0， the selection matrix can be obtained.

At step 340， the first optimal transmission angle domain channel matrix is calculated based on the first initial angle domain channel matrix and the selection factor.

According to embodiments of the present invention， the first optimal transmission angle domain channel matrix is a matrix which only comprises large elements of the first initial angle domain channel matrix. In an embodiment， the first optimal transmission angle domain channel matrix may be calculated by multiplexing the first initial angle domain channel matrix and the selection factor as follows：

where

represents the first optimal transmission angle domain channel matrix.

At step 350， a first channel vector is determined based on the first optimal transmission angle domain channel matrix.

The first channel vector may be obtained by vectorizing the first optimal transmission angle domain channel matrix. In some embodiments， the first channel vector may be obtained by：

where vec (·) is a vectorization operation， g represents the first channel vector.

Since the size of the first initial channel matrix is M×N， g is a vector including N_T elements， wherein N_T＝M×N. By way of example， assuming that

is a 8×8 matrix， that is， M＝8， N＝8， and N_T＝M×N＝64， the size of the first channel vector is 64×1.

At step 360， a second channel factor is determined， wherein the second channel factor is composed of a set of elements of the first channel vector， and each of the set of elements has a value exceeding a second predetermined threshold.

According to embodiments of the present invention， elements of the second channel factor may be a subset of elements of the first channel factor. The second predetermined threshold may be preset as any suitable value， for example， 0. Thus， all the non-zero elements of first channel vector may be selected as the elements of second channel vector. For example， ifthe size of the first channel vector is 64×1 and there are 10 elements of the first channel vector are not 0， the size of the second channel vectoris 10×1.

In some embodiments， the second channel vector， denoted as

may be determined as

where

and N_z represents the number of non-zero elements of the first channel vector g.

At step 370， the first angle domain precoder is determined based on the second channel vector.

Assuming that the first channel vector is demoted as g， and the second channel vector is denoted as

the scaling factor P may be determined by：

where

Since h can be expressed as：

The covariance matrix needs to be decomposed may be expressed as follows：

Thus， the first angle domain precoder T may be obtained by computing the EVD of

as follows：

At step 380， a target precoder in the array domain is determined based on the first angle domain precoder.

According to embodiments of the present invention， a scaling factor may be calculated based on the first and second channel vectors as discussed at step 370. As an alternatively， the scaling factor may be obtained from the process of step 370. Then， the target precoder may be determined based on the first angle domain precoder and the scaling factor.

For example， based on the first angle domain precoder T and the scaling factor P， the target precoder W may be obtained as follows：

The above embodiments of the present invention are described with respect to one antenna array of the BS. With respect to cross-polarized AAS， the present invention is also applicable. FIG. 4A illustrates a schematic diagram of a cross-polarized AAS 400 according to embodiments of the invention. In the MIMO system as shown with respect to FIG. 4A， a BS comprises two antenna arrays which are cross-polarized and are arranged at +45° and -45° respectively. As shown， each of the two antenna arrays comprises M antennas in the vertical direction and N antennas in the horizontal direction. In other words， each of the antenna arrays includes M×N antennas. FIG. 4B illustrates a schematic diagram of the two cross-polarized antenna arrays according to embodiments of the invention. The two cross-polarized antenna arrays comprise a first antenna array 410 and a second antenna array 420.

FIG. 5 illustrates a flow chart of a method 500 for performing beamforming according to embodiments of the invention. The method 500 may be considered as a specific implementation of the method 200 described above with reference to Fig. 2. Additionally， in the embodiments of FIG. 5， cross-polarized antenna arrays (such as the first antenna array 410 and the second antenna array 420) are arranged at the BS， and the target precoder is determined from channel information about each of the cross-polarized antenna arrays. However， it is noted that this is only for the purpose of illustrating the principles of the present invention， rather than limiting the scope thereof.

The method 500 starts at step S510， in which a first channel matrix is transformed from an array domain to an angle domain， as a first initial angle domain channel matrix， wherein the first channel matrix characterizes a channel between a UE and a first antenna array of a BS.

At step 520， a first optimal transmission angle domain channel matrix is determined based on the first initial angle domain channel matrix. At step 530， a first angle domain precoder is generated based on the first optimal angle domain channel matrix. Steps 510-530 are similar to steps 210-230， thus related details are not repeated here.

At step 540， a second channel matrix is transformed from an array domain to an angle domain， as a second initial angle domain channel matrix， wherein the second channel matrix characterizes a channel between the UE and the second antenna array of the BS. At step 550， a second optimal transmission angle domain channel matrix is determined based on the second initial angle domain channel matrix. At step 560， a second angle domain precoder is generated based on the second optimal angle domain channel matrix.

According to embodiments of the present invention， steps 540-560 are similar to steps 510-530， except that the first channel matrix is associated with the first antenna array but the second channel matrix is associated with the second antenna array， thus details steps 540-560 may be derived from descriptions about steps 510-530. By way of example， the second channel matrix may be obtained based on reference signals received from the UE via the second antenna array. Then， the second initial angle domain channel matrix may be obtained by transforming the second channel matrix from an array domain to the angle domain.

At step 570， a target precoder in the array domain is determined based on the first angle domain precoder and the second angle domain precoder.

In some embodiments， assuming that the first angle domain precoder obtained at step 530 is denoted as T₁， and the second angle domain precoder obtained at step 560 is denoted as T₂， the target precoder W may be determined as follows：

where β is a phase factor of the first channel matrix and the second channel matrix.

It is to be noted that in some alternative embodiments， the first angle domain precoder and the second angle domain precoder may be determined out of the order noted in FIG. 5. For example， steps 510-530 and 540-560 may in fact be executed concurrently or may sometimes be executed in the reverse order， as long as the first angle domain precoder and the second angle domain precoder are obtained before determining the target precoder in the array domain.

FIG. 6 illustrates a schematic diagram of an apparatus 600 for performing beamforming according to embodiments of the invention. According to embodiments of the present invention， the apparatus 600 may be implemented at a BS in a MIMO system， and the BS may comprise a first antenna array containing a plurality of antennas.

As shown in FIG. 6， the apparatus 600 comprises a transforming unit 610 configured to transform a first channel matrix from an array domain to an angle domain， as a first initial angle domain channel matrix， wherein the first channel matrix characterizes a channel between a UE and the first antenna array of the BS； a first determining unit 620 configured to determine， based on the first initial angle domain channel matrix， a first optimal transmission angle domain channel matrix； a generating unit 630 configured to generate a first angle domain precoder based on the first optimal angle domain channel matrix； and a second determining unit 640 configured to determine a target precoder in the array domain based on the first angle domain precoder.

According to embodiments of the present invention， the transforming unit 610 may comprise： a channel matrix obtaining unit configured to obtain a channel matrix based on reference signals received from the UE via the first antenna array； and wherein the transforming unit 610 may be further configured to transform the obtained channel matrix from an array domain to the angle domain.

According to embodiments of the present invention， the first determining unit 620 may comprise： a selection factor determining unit configured to determine a selection factor that includes elements with large values in the first initial angle domain channel matrix； and an optimization calculating unit configured to calculate the first optimal transmission angle domain channel matrix based on the first initial angle domain channel matrix and the selection factor.

According to embodiments of the present invention， the selection factor determining unit may comprise： an element selecting unit configured to determine a set of elements in the first initial angle domain channel matrix， wherein each of the set of elements has a value exceeding a first predetermined threshold； and a constructing unit configured to construct the selection factor based on positions of elements of the set in the first initial angle domain channel matrix.

According to embodiments of the present invention， the generating unit 630 may comprise： a first channel vector determining unit configured to determine a first channel vector based on the first optimal transmission angle domain channel matrix； a second channel vector determining unit configured to determine a second channel factor that is composed of a set of elements of the first channel vector， wherein each of the set of elements has a value exceeding a second predetermined threshold； and an angle domain precoder determining unit configured to determine the first angle domain precoder based on the second channel vector.

According to embodiments of the present invention， the angle domain precoder determining unit may comprise： a decomposing unit configured to perform Eigen Value Decomposition (EVD) on the second channel factor； and a beamforming matrix determining unit configured to determine a beamforming matrix which maximizes results of the EVD as the first angle domain precoder.

According to embodiments of the present invention， the second determining unit 640 may comprise： a scaling factor calculating unit configured to calculate a scaling factor based on the first and second channel vectors； and wherein the second determining unit is further configured to determine the target precoder based on the first angle domain precoder and the scaling factor.

According to embodiments of the present invention， the BS may further include a second antenna array containing a plurality of antennas， and the second antenna array and the first antenna array are cross-polarized. In the embodiments， the transforming unit 610 may be further configured to transform a second channel matrix from an array domain to an angle domain， as a second initial angle domain channel matrix， wherein the second channel matrix characterizes a channel between the UE and the second antenna array of the BS； the first determining unit 620 may be further configured to determine， based on the second initial angle domain channel matrix， a second optimal transmission angle domain channel matrix； the generating unit 630 may be further configured to generate a second angle domain precoder based on the second optimal angle domain channel matrix； and the second determining unit 640 may be further configured to determine a target precoder in the array domain based on the first angle domain precoder and the second angle domain precoder.

It is also to be noted that the apparatus 600 may be respectively implemented by any suitable technique either known at present or developed in the future. Further， a single device shown in FIG. 6 may be alternatively implemented in multiple devices separately， and multiple separated devices may be implemented in a single device. The scope of the present invention is not limited in these regards.

It is noted that the apparatus 600 may be configured to implement functionalities as described with reference to FIGs. 2， 3 and 5. Therefore， the features discussed with respect to the

method

200， 300 or 500 may apply to the corresponding components of the apparatus 600. It is further noted that the components of the apparatus 600 may be embodied in hardware， software， firmware， and/or any combination thereof. For example， the components of the apparatus 600 may be respectively implemented by a circuit， a processor or any other appropriate device. Those skilled in the art will appreciate that the aforesaid examples are only for illustration not limitation.

In some embodiment of the present disclosure， the apparatus 600 may comprise at least one processor. The at least one processor suitable for use with embodiments of the present disclosure may include， by way of example， both general and special purpose processors already known or developed in the future. The apparatus 600 may further comprise at least one memory. The at least one memory may include， for example， semiconductor memory devices， e.g.， RAM， ROM， EPROM， EEPROM， and flash memory devices. The at least one memory may be used to store program of computer executable instructions. The program can be written in any high-level and/or low-level compliable or interpretable programming languages. In accordance with embodiments， the computer executable instructions may be configured， with the at least one processor， to cause the apparatus 600 to at least perform according to the

method

200， 300， or 500 as discussed above.

Based on the above description， the skilled in the art would appreciate that the present disclosure may be embodied in an apparatus， a method， or a computer program product. In general， the various exemplary embodiments may be implemented in hardware or special purpose circuits， software， logic or any combination thereof. For example， some aspects may be implemented in hardware， while other aspects may be implemented in firmware or software which may be executed by a controller， microprocessor or other computing device， although the disclosure is not limited thereto. While various aspects of the exemplary embodiments of this disclosure may be illustrated and described as block diagrams， flowcharts， or using some other pictorial representation， it is well understood that these blocks， apparatus， systems， techniques or methods described herein may be implemented in， as non-limiting examples， hardware， software， firmware， special purpose circuits or logic， general purpose hardware or controller or other computing devices， or some combination thereof.

The various blocks shown in FIGs. 2， 3 and 5 may be viewed as method steps， and/or as operations that result from operation of computer program code， and/or as a plurality of coupled logic circuit elements constructed to carry out the associated function (s) . At least some aspects of the exemplary embodiments of the disclosures may be practiced in various components such as integrated circuit chips and modules， and that the exemplary embodiments of this disclosure may be realized in an apparatus that is embodied as an integrated circuit， FPGA or ASIC that is configurable to operate in accordance with the exemplary embodiments of the present disclosure.

While this specification contains many specific implementation details， these should not be construed as limitations on the scope of any disclosure or of what may be claimed， but rather as descriptions of features that may be specific to particular embodiments of particular disclosures. Certain features that are described in this specification in the context of separate embodiments can also be implemented in combination in a single embodiment. Conversely， various features that are described in the context of a single embodiment can also be implemented in multiple embodiments separately or in any suitable sub-combination. Moreover， although features may be described above as acting in certain combinations and even initially claimed as such， one or more features from a claimed combination can in some cases be excised from the combination， and the claimed combination may be directed to a sub-combination or variation of a sub-combination.

Similarly， while operations are depicted in the drawings in a particular order， this should not be understood as requiring that such operations be performed in the particular order shown or in sequential order， or that all illustrated operations be performed， to achieve desirable results. In certain circumstances， multitasking and parallel processing may be advantageous. Moreover， the separation of various system components in the embodiments described above should not be understood as requiring such separation in all embodiments， and it should be understood that the described program components and systems can generally be integrated together in a single software product or packaged into multiple software products.

Various modifications， adaptations to the foregoing exemplary embodiments of this disclosure may become apparent to those skilled in the relevant arts in view of the foregoing description， when read in conjunction with the accompanying drawings. Any and all modifications will still fall within the scope of the non-limiting and exemplary embodiments of this disclosure. Furthermore， other embodiments of the disclosures set forth herein will come to mind to one skilled in the art to which these embodiments of the disclosure pertain having the benefit of the teachings presented in the foregoing descriptions and the associated drawings.

Therefore， it is to be understood that the embodiments of the disclosure are not to be limited to the specific embodiments disclosed and that modifications and other embodiments are intended to be included within the scope of the appended claims. Although specific terms are used herein， they are used in a generic and descriptive sense only and not for purpose of limitation.

Claims

A method of performing beamforming in a Multiple Input Multiple Output (MIMO) system， wherein a base station (BS) in the MIMO system includes a first antenna array containing a plurality of antennas， comprising：

transforming a first channel matrix from an array domain to an angle domain， as a first initial angle domain channel matrix， wherein the first channel matrix characterizes a channel between user equipment (UE) and the first antenna array of the BS；

determining， based on the first initial angle domain channel matrix， a first optimal transmission angle domain channel matrix；

generating a first angle domain precoder based on the first optimal angle domain channel matrix； and

determining a target precoder in the array domain based on the first angle domain precoder.
The method of Claim 1， wherein transforming a first channel matrix from an array domain to an angle domain， as a first initial angle domain channel matrix comprises：

obtaining a channel matrix based on reference signals received from the UE via the first antenna array； and

transforming the obtained channel matrix from an array domain to the angle domain.
The method of Claim 1， wherein determining a first optimal transmission angle domain channel matrix comprises：

determining a selection factor that includes elements with large values in the first initial angle domain channel matrix； and

calculating the first optimal transmission angle domain channel matrix based on the first initial angle domain channel matrix and the selection factor.
The method of Claim 3， wherein determining a selection factor that indicates elements with large values in the first initial angle domain channel matrix comprises：

determining a set of elements in the first initial angle domain channel matrix， wherein each of the set of elements has a value exceeding a first predetermined threshold； and

constructing the selection factor based on positions of elements of the set in the first initial angle domain channel matrix.
The method of Claim 1， wherein generating a first angle domain precoder based on the first optimal angle domain channel matrix comprises：

determining a first channel vector based on the first optimal transmission angle domain channel matrix；

determining a second channel factor that is composed of a set of elements of the first channel vector， wherein each of the set of elements has a value exceeding a second predetermined threshold； and

determining the first angle domain precoder based on the second channel vector.
The method of Claim 5， wherein determining the first angle domain precoder based on the second channel vector comprises：

performing Eigen Value Decomposition (EVD) on the second channel factor； and

determining a beamforming matrix which maximizes results of the EVD as the first angle domain precoder.
The method of Claim 5， wherein determining a target precoder in the array domain based on the first angle domain precoder comprises：

calculating a scaling factor based on the first and second channel vectors； and

determining the target precoder based on the first angle domain precoder and the scaling factor.
The method of Claim 1， wherein the BS further includes a second antenna array containing a plurality of antennas， and the second antenna array and the first antenna array are cross-polarized， and wherein determining a target precoder in the array domain based on the first angle domain precoder comprises：

transforming a second channel matrix from an array domain to an angle domain， as a second initial angle domain channel matrix， wherein the second channel matrix characterizes a channel between the UE and the second antenna array of the BS；

determining， based on the second initial angle domain channel matrix， a second optimal transmission angle domain channel matrix；

generating a second angle domain precoder based on the second optimal angle domain channel matrix； and

determining a target precoder in the array domain based on the first angle domain precoder and the second angle domain precoder.
An apparatus of performing beamforming in a Multiple Input Multiple Output (MIMO) system， wherein a base station (BS) in the MIMO system includes a first antenna array containing a plurality of antennas， comprising：

a transforming unit configured to transform a first channel matrix from an array domain to an angle domain， as a first initial angle domain channel matrix， wherein the first channel matrix characterizes a channel between user equipment (UE) and the first antenna array of the BS；

a first determining unit configured to determine， based on the first initial angle domain channel matrix， a first optimal transmission angle domain channel matrix；

a generating unit configured to generate a first angle domain precoder based on the first optimal angle domain channel matrix； and

a second determining unit configured to determine a target precoder in the array domain based on the first angle domain precoder.
The apparatus of Claim 9， wherein the transforming unit comprises：

a channel matrix obtaining unit configured to obtain a channel matrix based on reference signals received from the UE via the first antenna array； and

wherein the transforming unit is further configured to transform the obtained channel matrix from an array domain to the angle domain.
The apparatus of Claim 9， wherein the first determining unit comprises：

a selection factor determining unit configured to determine a selection factor that includes elements with large values in the first initial angle domain channel matrix； and

an optimization calculating unit configured to calculate the first optimal transmission angle domain channel matrix based on the first initial angle domain channel matrix and the selection factor.
The apparatus of Claim 11， wherein the selection factor determining unit comprises：

an element selecting unit configured to determine a set of elements in the first initial angle domain channel matrix， wherein each of the set of elements has a value exceeding a first predetermined threshold； and

a constructing unit configured to construct the selection factor based on positions of elements of the set in the first initial angle domain channel matrix.
The apparatus of Claim 9， wherein the generating unit comprises：

a first channel vector determining unit configured to determine a first channel vector based on the first optimal transmission angle domain channel matrix；

a second channel vector determining unit configured to determine a second channel factor that is composed of a set of elements of the first channel vector， wherein each of the set of elements has a value exceeding a second predetermined threshold； and

an angle domain precoder determining unit configured to determine the first angle domain precoder based on the second channel vector.
The apparatus of Claim 13， wherein the angle domain precoder determining unit comprises：

a decomposing unit configured to perform Eigen Value Decomposition (EVD) on the second channel factor； and

a beamforming matrix determining unit configured to determine a beamforming matrix which maximizes results of the EVD as the first angle domain precoder.
The apparatus of Claim 13， wherein the second determining unit comprises：

a scaling factor calculating unit configured to calculate a scaling factor based on the first and second channel vectors； and

wherein the second determining unit is further configured to determine the target precoder based on the first angle domain precoder and the scaling factor.
The apparatus of Claim 9， wherein the BS further includes a second antenna array containing a plurality of antennas， and the second antenna array and the first antenna array are cross-polarized， and wherein：

the transforming unit is further configured to transform a second channel matrix from an array domain to an angle domain， as a second initial angle domain channel matrix， wherein the second channel matrix characterizes a channel between the UE and the second antenna array of the BS；

the first determining unit is further configured to determine， based on the second initial angle domain channel matrix， a second optimal transmission angle domain channel matrix；

the generating unit is further configured to generate a second angle domain precoder based on the second optimal angle domain channel matrix； and

the second determining unit is further configured to determine a target precoder in the array domain based on the first angle domain precoder and the second angle domain precoder.