Field

Embodiments of the present disclosure relate to wireless communications, and more specifically to multi-input multi-output (MIMO) communications.

Background

Due to limitations of time and frequency resources, improving resource utilization efficiency is a target which is always pursued in communication technology. MIMO technology can simultaneously transmit a plurality of data streams on a same time-frequency resource block by exploiting space domain and thereby effectively boost system throughput.

Two-dimensional (2D) MIMO transmission has been studied and adopted in some wireless communication systems, such as the 3rd Generation Partnership Project (3 GPP) Long Term Evolution (LTE). As for the 2D-MIMO, traditional antenna arrays are arranged horizontally to form a beam in a horizontal plane. In order to explore potential gain from 3 -dimensional (3D) wireless channels, the 3D MIMO has already been discussed in the area of wireless communications, for example, in a 3GPP meeting. For example, 3D MIMO channel modeling has been discussed.

To obtain a 3D MIMO channel, the planar (2D) antenna array is going to be used to obtain the vertical spatial gain. As antenna dimensions and channel dimensions increase, the amount of feedback about channel state information (CSI) also increases. However, resources for measuring the channels and feedback channel capacity for feeding back the CSI are limited, which means that a new feedback structure needs to be proposed, and a new codebook for 3D MIMO needs to be designed, in order to facilitate an effective feedback.

In the latest 3GPP technical report, a 3D MIMO precoder structure has already been determined. The 3D MIMO precoder is divided into a long-term feedback and a short-term feedback. The long-term feedback provides a group of beams, whereas the short-term feedback selects a beam or column from the group of beams of the long-term feedback, and performs a phase adjustment between different antenna polarizations.

Embodiments of the present disclosure provide a solution related to the long-term feedback.

Summary

A brief summary of embodiments is presented below to provide basic understanding of some aspects of various embodiments. The summary is not intended to identify key points of key elements or describe the scope of various embodiments. Its sole purpose is to present some concepts in a simplified form as a prelude to the following more detailed depictions.

According to a first aspect of the present disclosure, there is provided a method for multi-input multi-output MIMO communication in a wireless communication network. The method comprises: receiving, from a device, long-term precoding information for the MIMO communication, the long-term precoding information indicating a set of three-dimensional (3D) beams, wherein elements in the set of 3D beams are obtained by combining a column of a first matrix representing a vertical beam group with a corresponding column of a second matrix representing a horizontal beam group; receiving, from the device, short-term precoding information for the MIMO communication, the short-term precoding information indicating a beam selected from the set of 3D beams; constructing a precoding matrix for the MIMO communication according to the long-term precoding information and short-term precoding information; and transmitting data encoded according to the precoding matrix to the device.

In an embodiment, the combining a column of a first matrix representing a vertical beam group with a corresponding column of a second matrix representing a horizontal beam group may comprise: combining the column of the first matrix with the corresponding column of the second matrix by performing element-wise Kronecker product operation.

In another embodiment, the long-term precoding information may indicate a long-term codebook Wi with the following structure: χ^ Θ χ^ , χ^ Θ χ^ ² , · · · 0

wherein X and XL represent the first matrix and the second matrix, respectively; wherein i and j represent an index of the vertical beam group and an index of the horizontal beam group, respectively; * represents performing column-wise Kronecker product operation; x ^l y ^k and xjf represents the tf ^h column of Χ _γ ^ι and X _H ^J , respectively; and ® represents performing element-wise Kronecker product operation.

In an embodiment, the vertical beam group is selected from a set of vertical beams IV ^an d if the number of the vertical antenna ports is 2, 4 or 8, the I may be a subset of the codebook defined in Long-Term Evolution (LTE) Release 10 or Release 12 for a corresponding number of antennas.

In another embodiment, the vertical beam group is selected from a set of vertical beams IV; and the method may further comprise: obtaining, at the device or a base station, at least one of distribution of devices in the wireless communication network which need to perform MIMO communication and antenna configuration for the MIMO communication; determining the set of vertical beams IV according to the at least one of the distribution and the antenna configuration. In an embodiment, the determining the set of vertical beams IV according to the at least one of the distribution and the antenna configuration may comprise: determining a maximum downtilt and a minimum downtilt for devices in the distribution; and determining a range of beams of the set of vertical beams according to the maximum downtilt and the minimum downtilt. In another embodiment, the determining the set of vertical beams IV according to the at least one of the distribution and the antenna configuration may comprise: determining a range of zenith angles of departure of a device in the distribution; performing nonlinear quantization on zenith angles of departure within the range of zenith angles departure according to the distribution and obtaining a plurality of sections so that difference in the number of device in each of the section is minimized; determining a representative downtilt for each of the sections; and constructing the set of vertical beams IV using the representative downtilt for each of the sections.

In a further embodiment, the obtaining at least one of distribution of devices in the wireless communication network which need to perform MIMO communication and antenna configuration for the MIMO communication may comprise: selecting a plurality of distributions and a plurality of antenna configurations from a predetermined set of distributions of devices and a predetermined set of antenna configurations; and determining the set of vertical beams By according to the at least one of the distribution and the antenna configuration may comprise: generating sets of beams for the plurality of distributions and/or the plurality of antenna configurations respectively, and obtaining the set of vertical beams By by combining the sets of beams generated respectively.

In an embodiment, the vertical beam group is selected from a set of vertical beams By and ; the set of vertical beams may be different for vertical antenna configurations with different number of antenna ports; the set of vertical beams B _M . for a vertical antenna configuration with less number, M', antenna ports is a subset of the set of vertical beams I _, for a vertical antenna configuration with more number, M, antenna ports; and a beam vector b^ _M . in B^ _M ' may be a portion of a beam vector > ^k _{v M} in B _>M ·

In another embodiment, the horizontal beam group is selected from a set of horizontal beams B _H , and if the number of the horizontal antenna ports is 2, 4 or 8, the B _H may be a subset of the codebook defined in LTE Release 10 or Release 12 for a corresponding number of antennas. In another embodiment, the horizontal beam group is selected from a set of horizontal beams B and if the number of the horizontal antenna ports is six, B _# ma lement with an index of / is b H , , L represents a

total number of the beams in the set of horizontal domain beams, and T represents an oversampling factor for a DFT matrix of the horizontal domain, and Tr represents transposition of the matrix.

In some embodiments, the vertical beam group is selected from a set of vertical beams By, the set of vertical beams By may be predefined or informed by the device/to the device via high-layer signaling, or downloaded by the device prior to use; and/or the horizontal beam group is selected from a set of horizontal beams B _H , and the set of horizontal beams B _H may be predefined or informed by the device/to the device via high-layer signaling, or downloadable prior to use.

According to a second aspect of the present disclosure, there is provided a method for multi-input multi-output (MIMO) communication in a wireless communication network, comprising: transmitting, to a device, long-term precoding information for the MIMO communication, the long-term precoding information indicating a set of three-dimensional (3D) beams, wherein elements in the set of 3D beams are obtained by combining a column of a first matrix representing a vertical beam group with a corresponding column of a second matrix representing a horizontal beam group; transmitting, to the device, short-term precoding information for the MIMO communication, the short-term precoding information indicating a beam selected from the set of 3D beams; wherein the long-term precoding information and the short-term precoding information are used by the device to construct a precoding matrix for the MIMO communication; and receiving, from the device, data encoded according to the precoding matrix.

In an embodiment, the combining a column of a first matrix representing a vertical beam group with a corresponding column of a second matrix representing a horizontal beam group may comprise: combining the column of the first matrix with a corresponding column of the second matrix by performing element-wise Kronecker product operation.

In another embodiment, the long-term precoding information may indicate a long-term codebook Wi with the following structure:

wherein Χ _γ ^ι and X _H ^J are respectively the first matrix and the second matrix, wherein i and j respectively represents an index of the vertical beam group and an index of the horizontal beam group; * represents performing column-wise Kronecker product operation; x ^l y ^k and xjf respectively represents the tf ^h column of Χ _γ ^ι and X _H ^J ; ® represents performing element-wise Kronecker product operation.

In an embodiment, the vertical beam group may be selected from a set of vertical beams I ; the B^ may be identical with that described in the first aspect of the present disclosure.

In an embodiment, the horizontal beam group may be selected from a set of horizontal beams B _# ; the B _# may be identical with that described in the first aspect of the present disclosure.

According to a third aspect of the present disclosure, there is provided an apparatus for multi-input multi-output (MIMO) communication in a wireless communication network, comprising: a first receiving unit configured to receive from a device long-term precoding information for MIMO communication, the long-term precoding information indicating a set of 3D beams, wherein elements in the set of 3D beams are obtained by combining a column of a first matrix representing a vertical beam group with a corresponding column of a second matrix representing a horizontal beam group; a second receiving unit configured to receive, from the device, short-term precoding information for the MIMO communication, the short-term precoding information indicating a beam selected from the set of 3D beams; a precoding matrix constructing unit configured to construct a precoding matrix for the MIMO communication according to the long-term precoding information and short-term precoding information; and a transmitting unit configured to transmit data encoded according to the precoding matrix to the device.

In an embodiment, the vertical beam group may be selected from a set of vertical beams IV; the IV may be identical with that described in the first aspect of the present disclosure.

In an embodiment, the horizontal beam group may be selected from a set of horizontal beams B _# ; the B _# may be identical with that described in the first aspect of the present disclosure.

In an embodiment, the vertical beam group is selected from a set of vertical beams IV; the apparatus may further comprise: an information obtaining unit configured to obtain at least one of distribution of devices in the wireless communication network which need to perform MIMO communication and antenna configuration for the MIMO communication; and a vertical beam set determining unit configured to determine the set of vertical beams IV according to the at least one of the distribution and the antenna configuration.

According to a fourth aspect of the present disclosure, there is provided an apparatus for MIMO communication in a wireless communication network, comprising: a first transmitting unit configured to transmit, to a device, long-term precoding information for MIMO communication, the long-term precoding information indicating a set of 3D beams, wherein elements in the set of 3D beams are obtained by combining a column of a first matrix representing a vertical beam group with a corresponding column of a second matrix representing a horizontal beam group; a second transmitting unit configured to transmit, to the device, short-term precoding information for the MIMO communication, the short-term precoding information indicating a beam selected from the set of 3D beams; wherein the long-term precoding information and the short-term precoding information are used by the device to construct a precoding matrix for the MIMO communications; and a receiving unit configured to receive, from the device, data encoded according to the precoding matrix. In an embodiment, the vertical beam group is selected from a set of vertical beams _v ; the B _v may be identical with that described in the first aspect of the present disclosure. In an embodiment, the horizontal beam group may be selected from a set of horizontal beams B _# ; and the B _# may be identical with that described in the first aspect of the present disclosure.

In an embodiment, the vertical beam group is selected from a set of vertical beams B^; and the apparatus may further comprise: an information obtaining unit configured to obtain at least one of: distribution of devices in the wireless communication network which need to perform MIMO communication and antenna configuration for the MIMO communication; and a vertical beam set determining unit configured to determine the set of vertical beams B^ according to the at least one of the distribution and the antenna configuration.

The method or apparatus according to embodiments of the present disclosure can improve efficiency of long-term precoding information feedback and improve the performance of 3D MIMO.

Although specific embodiments are illustrated in the drawings by way of example, it should be appreciated that descriptions of specific embodiments herein are not intended to limit the embodiments to the specific forms disclosed.

Brief Description of Drawings

Objects, advantages and other features of the present disclosure will become more apparent from the disclosure and claims below. Just for illustration purpose, non-limiting descriptions of example embodiments are presented with reference to figures, in which:

Fig. 1 illustrates a schematic diagram of an example wireless communication system in which a method according to an embodiment of the present disclosure can be implemented; Figs. 2a-2c illustrate flow charts of methods implemented at a transmitter of MIMO according to an embodiment of the present disclosure;

Figs. 3a-3b illustrate flow charts of methods implemented at a receiver of MIMO according to an embodiment of the present disclosure;

Fig. 4 illustrates a block diagram of an apparatus implemented at a transmitter of MIMO according to an embodiment of the present disclosure;

Fig. 5 illustrates a block diagram of an apparatus implemented at a receiver of MIMO according to an embodiment of the present disclosure.

Detailed Description of Preferred Embodiments

A lot of details are presented in the following description for illustration purpose. However, those skilled in the art will appreciate that embodiments of the present disclosure may be implemented without using these specific details. Hence, the present disclosure is not intended to be limited by the illustrated embodiments, but be endowed the broadest scope consistent with principles and features described herein.

It should be appreciated that the terms "first", "second" and the like are only used to distinguish one element from another. In practice, the first element can also be called the second element, or vice versa. In addition, it should be appreciated that "comprising" and "including" are only used to illustrate existence of the stated features, elements, functions and components, and do not exclude existence of one or more other features, elements, functions or components.

For ease of illustration, embodiments of the present disclosure will be introduced in the context of 3GPP LTE/LTE-Advanced (LET-A), and using specific terms in LTE/LTE-A. However, as appreciated by those skilled in the art, embodiments of the present disclosure are by no means limited to application environment of 3 GPP LTE/LTE-A, but on the contrary, may be applied to any wireless communication system where similar problems exist, for example, WLAN or other communication system to be developed in the future. Likewise, the apparatus in the present disclosure may be user device (UE) or any terminal having a wireless communication function, including but not limited to a mobile phone, computer, personal digital assistant, a gaming machine, wearable devices, sensor and the like. The term UE may be used interchangeable with mobile station, subscriber station, mobile terminal, user terminal or wireless device. In addition, the apparatus may also be a network node, such as a Node B (Node B or NB), Base Transceiver Station (BTS), base station (BS), or base station sub-system (BSS), relay, remote radio head (R H) and the like.

Fig. 1 illustrates a schematic diagram of a wireless communication system in which a method according to an embodiment of the present disclosure can be implemented. The wireless communication system 100 may include one or more network nodes 101 which for example may be in the form of a base station in this example, e.g., evolved node B (eNodeB or eNB). It will be appreciated that the network node 101 could also be in other forms, e.g., a Node B (Node B or NB), Base Transceiver Station (BTS), base station (BS), or base station sub-system (BSS), relay or the like. The network node 101 may provide radio connection for a plurality of wireless devices (e.g., UE102-104) located within its coverage.

The network node 101 may be equipped with a 2D antenna array (e.g., an antenna array with M rows and N columns and with crossed polarization) to provide 3D MIMO communication with the UE. At 3GPP RANI meeting, it has already been determined that the following five antenna configurations have higher priority levels: 8 transmit radio units (TXRUs) arranged in two rows and two columns and dual polarized (represented as V2H2P2), 12 TXRUs arranged in two rows and three columns and dual polarized or arranged in three rows and two columns and dual polarized (represented as V2H3P2, V3H2P2), and 16 TXRUs arranged in two rows and four columns and dual polarized or arranged in four rows and two columns and dual polarized (represented as V2H4P2, V4H2P2).

The MIMO communication may be applied to a downlink direction from the base station to the UE, or to an uplink direction from the UE to the base station. Regarding the downlink MIMO, the UE may estimate channel state information (CSI) based on, for example, a downlink pilot signal and feedback it to the base station, for the base station side to obtain proper transmission parameters for downlink MIMO communication.

Precoding Matrix Index (PMI) is a kind of important channel state feedback information in MIMO communication system, and it specifies an index of a precoding matrix for MIMO. At 3GPP meetings, it has already been agreed that a codebook W _3D of 3D MIMO adopts the following basic structure:

= W ₁ W ₂ ( 1 ) , wherein the λ is a long-term and wide-band precoding matrix (or called codebook) feedback which indicates a group of beams according to the long-term CSI, W ₂ is a short-term sub-band precoding matrix feedback which further selects a specific beam from the group of beams defined by λ , i.e. select a specific column from the group of beams of the long-term codebook, and adjusts phase between different antenna polarizations. Wi codebook and W ₂ codebook are selected from respective codebook set respectively and fed back to a transmitter side of MIMO.

At present, during 3 GPP standardization, some special requirements are set for the design of the MIMO codebook, and these requirements include, for example, amplitude constant property and nesty property; wherein amplitude constant property requires the amplitude of the codebook to be constant, i.e., the precoding does not increase the transmission power; whereas the nesty property requires the codebook designed for a low-rank to be a subset for a codebook designed for a high-rank. However, most of the solutions related to 3D MIMO codebook currently disclosed do not comply with the above requirements set by 3GPP, and requires large implementation complexity and/or great work effort in terms of standardization. In addition, existing codebooks in 3 GPP specifications, e.g., codebooks recited in TS36.211, are based on Grid of Beams (GoB) principle, and the design of these codebooks only takes horizontal dimension, not vertical dimension into account, so they are not appropriate for 3D MIMO channels.

To exploit a potential gain in a vertical domain of the 3D MIMO channel, design of the long-term codebook should take into account the channel properties of the vertical domain. Further, a new long-term codebook and beam set should be designed for the horizontal domain and vertical domain, for number of antenna ports and antenna configurations newly introduced, to use the potential performance gain of the 3D MIMO channel.

In the present disclosure, methods and apparatus for generating and feeding back a long-term codebook and a beam set for 3D MIMO channel are proposed.

As an example, a 2D antenna array of a transmitter (e.g., eNB) may have M rows and N columns of antenna ports, and a signal received at a receiver (e.g., UE) may be expressed as follows:

wherein Y represents a received signal prior to combination, s represents a transmitted signal, n represents noise and interference, H _3£) represents a 3D channel, and W _3£) represents a 3D precoder, and W _3£) may be represented by the above formula (1). As stated above, λ in formula (1) is long-term and wide-band codebook feedback which indicates a group of beams according to the long-term CSI, while W ₂ is short-term sub-band codebook feedback which further selects a specific beam according to the short-term CSI from a group of beams defined byW _{1 ?} and adjusts the phase between different antenna polarizations. The structure of may be expressed as follows:

X _F ® X;

W _{1 =} (3 )

X _F ® X; wherein ® represents the Kronecker product operation, and wherein X andX^ may be beams selected from a set of vertical beams B and a set of horizontal beams respectively.

The concept of the conventional Kronecker product used in formula (3) is element-wise multiplication, that is to say, a Kronecker product of a 2x2 matrix and another 2x2 matrix will result in a 4x4 matrix, because the four elements in the first matrix will be multiplied with four elements in the second matrix respectively. In other words, if there are x beams in X _F and y beams in X^ , ^x ' y 3D beams will be obtained using the original Kronecker product combinations. The number of the resultant 3D beams will directly affect the size of the short-term codebook, because the short-term codebook will select beam from the 3D beam set. To more control the number of the combined 3D beams in the long-term codebook precisely, the present disclosure proposes a new method of representing long-term precoding information and constructing a long-term codebook.

Now referring to Fig. 2a, it illustrates a flow chart of a method 200 for MIMO communication in a wireless communication network (e.g., a network 100) according to an embodiment of the present disclosure. The method 200 may be executed by the transmitter of MIMO, e.g., executed by eNB 101 in Fig. 1 in the case of downlink MIMO, or executed by any one of UE102-104 in Fig. 1 in the case of uplink MIMO. Only as an example, the method is executed by eNB in the following depictions.

As shown in Fig. 2a, the method 200 comprises: at block S201, the eNB receives from a device (e.g., UE102) the long-term precoding information for MIMO communication, the long-term precoding information indicating a set of three-dimensional (3D) beams, wherein elements in the set of 3D beams are obtained by combining a column of a first matrix representing a vertical beam group with a corresponding column of a second matrix representing a horizontal beam group; at block S202, the eNB receives, from the device, short-term precoding information for the MIMO communication, the short-term precoding information indicating beams selected from the set of 3D beams; at block S203, the eNB constructs the precoding matrix for the MIMO communication according to the received long-term precoding information and short-term precoding information; and at block S204, the eNB transmits data encoded according to the precoding matrix to the device.

Regarding the set of 3D beams indicated by the long-term precoding information received at the block S201, its elements are obtained by combining a column of a first matrix representing a vertical beam group with a corresponding column of a second matrix representing a horizontal beam group. such beam combining manner introduces a restriction, namely, only a column (e.g. the z ^-th column) of the first matrix and a corresponding column (also the z ^-th column) of the second matrix is allowed to be combined, thereby reducing the number of beams obtained finally, so that the short-term feedback can use a smaller codebook to select beams therefrom.

In an embodiment, a column of the first matrix is combined with a corresponding column of the second matrix by an element-wise Kronecker product operation. However, embodiments of the present disclosure are not limited to this, and instead any proper manners for combination may be employed, for example, combination by weighting and multiplying the elements.

In an embodiment of the present disclosure, the long-term precoding information received at block S201 indicates a long-term codebook Wi with the following structure:

W _{1 =}

o x *x£

wherein Χ _γ ^ι and X _H ^J are respectively the first matrix representing the vertical beam group and the second matrix representing the horizontal beam group, z and j respectively represents an index of the vertical beam group and an index of the horizontal beam group; * represents performing column-wise Kronecker product operation; and xjf respectively represents the tf ^h column of X _v ^l and X _H ^J ; ® represents performing element-wise Kronecker product operation. In the embodiment, as compared with current codebook design as shown in formula (3), column-wise Kronecker product (represented by * ) is used in place of the traditional element-wise Kronecker product operation, thereby putting a restriction on the combination of the vertical domain beam group and the horizontal domain beam group, and facilitating reducing the number of the obtained 3D beams and the overhead of the short-term feedback. Meanwhile, in the case that the number of the obtained 3D beams is given, the method according to the embodiment of the present disclosure, as compared with the traditional method, can allow for more beams in the horizontal beam group and the vertical beam group, i.e., allows for more columns in the first matrix and second matrix, which means that the horizontal beams and vertical beams can be controlled more finely to obtain the combined 3D beams.

For example, assume that Χ _γ ^ι and X _H ^J are respectively 2x4 matrix, four 3D beams can be obtained by the method according to the embodiment of the present disclosure; by contrast, according to the traditional method, to obtain four 3D beams, Χ _γ ^ι and X _H ^J can only be 2x2 matrix.

In an embodiment, the vertical beam group is selected from a set of vertical beams

Bj . For example, Χ _γ ^ι in the formula (3) or (4) may be selected from the set of vertical beams F . Hence, the design of the set of vertical beams Bj directly affects the size and efficiency of the long-term codebook Wi. Regarding an antenna configuration with the number of antenna ports 2, 4, 8, a corresponding codebook has already been defined in 3GPP standard Releases 10 and 12. Reference may be made to TS 36.211 section 6.3.4 for specific definition. However, in 3 GPP, the current traditional codebooks are optimized for several antenna configurations, for example, single polarized and placed at a close distance (with an antenna spacing being wavelength/2), crossed polarized and placed at a close distance, and crossed polarized and placed at a far distance (with an antenna spacing being four times the wavelength). In 3D MIMO, the antenna configuration may be designed as being crossed polarized and placed at a close distance. Hence, the traditional codebooks optimized for several antenna configurations are not optimal for the 3D MIMO-specific antenna configuration. Hence, the present disclosure proposes selecting from the traditional codebook a subset adapted for a specific vertical antenna configuration and using it for the set of vertical beams of 3D MIMO. Hence, in an embodiment, if the number of the vertical antenna ports is 2, 4 or 8, Bymay be a subset of the codebook defined in LTE Release 10 or Release 12 for a corresponding number of antennas. This can improve the efficiency of the codebook on the one hand, and can reuse the current design on the other hand.

In an embodiment, the vertical beam group is selected from the set of vertical beams B _v ; and B _F = {b* |i , wherein the beam with an index of k is represented as br and the m element of b is

represented as:

wherein M represents the number of the vertical antenna ports, and T represents an oversampling factor, for example, for eight vertical antenna ports and four times oversampling, TxM=32. Tr represents transposition of the matrix. In this embodiment, the set of vertical beams is an oversample of a Discrete Fourier Transformation (DFT) matrix.

The DFT matrix is a good approximation for a scenario with a large angle spread, for example, a horizontal domain, but might not be optimal for a vertical domain with a small angle spread. Hence, in another embodiment of the present disclosure, a new codebook design may be employed to improve the performance, for example, further reduce the feedback overhead and use the limited codebook size to increase the codebook resolution.

Fig. 2b illustrates a flow chart of another example of the method 200. As shown in Fig. 2b, the method 200 may further comprise constructing the set of vertical beams By in the following manner: at block S205, obtaining at least one of distribution of devices in the wireless communication network which need to perform MIMO communication and antenna configuration for the MIMO communication; and at block S206, determining the set of vertical beams By according to the at least one of the distribution and the antenna configuration. The constructing (namely blocks S205 and S206) may be performed at the device performing the method 200, for example, the device is eNB in the case of downlink MIMO; the constructing may also be performed by a device sending the long-term codebook, for example, the device may be eNB in the case of uplink MIMO.

In another embodiment, a specific distribution model or antenna configuration model may be assumed at block S205, and _v is constructed at the block S206 according to the assumption.

In a further embodiment, at block S205, a plurality of distributions and a plurality of antenna configurations are selected from a predetermined set of device distributions and a predetermined set of antenna configurations; and at block S206, beam sets are generated respectively for the plurality of distributions and/or the plurality of antenna configurations, and the set of vertical beams I is obtained by combining the beam sets generated respectively. This can avoid defining a plurality of codebooks for different user distribution models and different antenna configurations.

In addition, in formula (5), k represents an index of a beam in the set of vertical beams, and m represents an index of row in the vertical domain of the 2D antenna array (2D AAA). The inventor of the present disclosure noticed that since the angle spread of the vertical domain is small, not all beams will be selected to constitute the set of vertical beams. In addition, the beam spacing is related to the number of the vertical antenna ports (the number of lines of 2D AAA). Hence, it is feasible to further control the beam spacing and the range of the beams to reduce the long term codebook, improve the codebook efficiency and reduces the feedback overhead.

From point of view of a downtilt Θ of a beam, weight of the vertical beam may be represented as:

The following formulas may be obtained with reference to formula (5):

¹ =— ^d v - ^■ cos Θ a, ( 7 )

TxM λ

and

wherein # _max and θ^ _η represent a maximum downtilt and a minimum downtilt, d _v represents an antenna spacing between the vertical antenna ports, λ represents a wavelength of a carrier frequency, k _max represents an index of a beam in the set of beams with the maximum downtilt # _max , whereas k^ _n represents an index of a beam in the set of beams with the minimum downtilt θ^ _η .

In an embodiment of the present disclosure, the determining the set of vertical beams I according to the at least one of the device distribution and the antenna configuration comprises: for example, at block S206, determining the maximum downtilt # _max and the minimum downtilt θ^ _η for a device in the distribution; and determining a range of beams of the set of vertical beams according to the maximum downtilt and the minimum downtilt.

In an embodiment, # _max and may be found by using a fixed and/or predetermined user distribution model; in another embodiment, # _max and may be obtained at the base station based on an actual distribution of the UE.

In an embodiment, the range of beams between # _max and θ^ _η may be uniformly divided into respective beams to obtain the set of vertical beams βγ. In another embodiment, an un-uniform distribution of beams may be used, which depends on the distribution situation of the users.

It is assumed that the beam towards a UE has a zenith angle of departure (ZOD) 6? (0≤ 0 _i ≤ π) . The precoding vector of the corresponding beam may be expressed as below:

That is to say, design of the codebook is equivalent to design of a group of downtilt

® = {0 _i \l≤i≤K) , which requires performing quantization for the UE's ZOD and minimizing quantization distortion. Regarding the uniformly-distributed UEs, it is appropriate to find an optimal downtilt© using linear quantization, and calculate the vertical codebook according to the preceding procedure. However, the UEs are usually un-uniformly distributed in the vertical domain, and it is not optimal to perform equal quantization for this domain. In the present disclosure, an embodiment of using nonlinear quantization in the design of the downtilt is provided.

An example of constructing the set of vertical beams _v with an un-uniform beam distribution is presented below. In this example, the determining the set of vertical beams I according to the at least one of the device distribution and the antenna configuration is implemented for example by block S206. As shown in fig. 2c, the block S206 may further comprise:

At block S2061 , determining a range of zenith angle of departure of devices in the distribution; for example, the range of ZOD is assumed as [¾p ₅ ¾,wn ] _> ^a starting point and an ending point of the range are respectively ·½ ^{= x} up and ^X K ^{= x} down - Here ·½ ^{= x} up represents the highest beam toward the UE, while

^X K ^{= x} down represents the lowest beam toward the UE.

At block S2062, performing nonlinear quantization of the zenith angle of departure in the range of zenith angle departure according to the distribution and obtaining a plurality of sections so that difference in the number of device in each of the sections is minimized; assume that the range is divided into K= 2 ⁵ sections,

X = , χ _λ , · ^■ · , ½· ] , and as an example, sections may be calculated as follows, such that the number of UEs in each of the sections is equal as much as possible, but the embodiments of the present disclosure are not limited to this approach for calculation: X = f _x (x) dx

wherein f _x (x) represents the number of the UE with the downtilt · .

At the block S2603, determining a representative downtilt for each of the sections; the representative downtilt Θ for each of the sections may be obtained for example by using the following equation, so that sum of a difference between the representative downtilt and the downtilt of each UE in the section is minimized:

c

6 _k = arg min∑(0 - _c ) ( 12 )

V0 _c= i wherein z¾ represents the ZOD of the c ^th UE in the section , x _i+1 ) . It should be appreciated that embodiments of the present disclosure are not limited to obtaining the representative downtilt of each of the sections in any specific calculation manner/formula, but any proper approximation manner may be used for determining a group of representative downtilts for the set of beams.

Furthermore, at block S2604, the set of vertical beams _v is constructed by using the representative downtilt of each of the sections. The vertical codebook constructed in this way is optimized for the UE distribution.

As stated above, by using a beam space-based method, the vertical codebook with rank 1 may be described as: B^ , wherein the k ^th codebook b _T may be expressed as: b _v = k,M

V wherein

-j2n(m-\ d _v

r k,m _ J ^2n: ( ^{m !} ) ^C0S ¾ . In an embodiment, the set of vertical beams B _v for the

O — Q

vertical antenna configurations with different number of antenna ports may be different, and the set of vertical beams for M vertical antenna ports may be expressed as , whereas the set of vertical beams for M' vertical antenna ports may be expressed as B _>M ,■

In an embodiment, the set of vertical beams ^ν,Μ' for a vertical antenna configuration with less, namely, M' antenna ports is a subset of the set of vertical beams B _;M for a vertical antenna configuration with more, namely, M antenna ports; for example, B _F = .

Meanwhile, a beam vector b _{v M} , in B _>M , is a portion of a beam vector b _{v M} m ^'

_; M , for example, b _v

In another embodiment, the horizontal beam group is selected from a set of horizontal beams B _# . There already exist the codebook definitions for 2, 4 or 8 antenna ports in 3 GPP. However, the current traditional codebooks in 3 GPP are optimized for several antenna configurations, for example, single polarized and placed at a close distance(with an antenna spacing being wavelength/2), cross polarized and placed at a close distance, and cross polarized and placed at a far distance (with an antenna spacing being four times the wavelength). In 3D MIMO, the antenna configuration may be set as a specific configuration, e.g., cross polarized and placed at a close distance. Hence, reusing the traditional codebook is not optimal for the 3D MIMO-specific antenna configuration. If the traditional codebook is directly reused as the set of horizontal beams, it will result in redundancy the low overall long-term encoding efficiency. Hence, in an embodiment of the present disclosure, to lower the whole long-term codebook size increased by introducing the vertical domain beam group, several entries in the traditional horizontal codebook should be removed to obtain the set of horizontal beams B _# . That is to say, in an embodiment, if the number of the horizontal antenna ports is 2, 4 or 8, B _# may be a subset of the codebook defined in LTE Release 10 or Release 12 for a corresponding number of antennas.

In the current 3 GPP standard, the antenna configuration with six antenna ports is not supported. However, in 3D MIMO, since there might be a 2D antenna array with six horizontal antenna ports, and therefore, the present disclosure further provides a design scheme for six horizontal antenna ports.

In an embodiment, the horizontal beam group is selected from a set of horizontal beams B _# and if the number of the horizontal antenna ports is six, B _# may be expressed as

= |l < / < j , wherein an element with an index of / is b ^l _H = b ¹ ' ¹ b ^l? b H and b _j ^l f = _e ^j2;r ( ^~1 )(' ^2~1 V ^{( 3r)} _? L represents a total number of the beams in the set of beams in the horizontal domain, and T represents the oversampling factor for the DFT matrix of the horizontal domain.

It should be appreciated that although some embodiments provide method steps for constructing the set of vertical beams βγ, the set of vertical beams B^ may also be pre-defined in some other embodiments. In an embodiment, the set of vertical beams B^ may be informed by the base station to the UE via high-layer signaling. In another embodiment, Ey may be downloaded by the device (e.g., eNB, UE) prior to use.

Similarly, the set of horizontal beams _H may also be pre-defined or informed by the device/to the device via high-layer signaling, or downloadable prior to use.

As can be appreciated by those skilled in the art, the method 200 may further comprise other operations not shown in Figs. 2a-2c, e.g., an operation of constructing the long-term codebook according to the set of vertical beams βγ. The constructing may be performed for example by a conventional method, so it will not be described in the present disclosure in detail.

According to embodiments of the present disclosure, indication of long-term precoding information can be effectively provided. In some embodiments, the set of beams can be designed based on device distribution, to make the whole long-term codebook more effective, and meanwhile reduce the feedback overhead and improve the performance of 3D MIMO.

Now reference is made to Fig. 3a which illustrates a flow chart of a method 300 for MIMO communications in a wireless communication network (e.g., network 100) according to an embodiment of the present disclosure. The method 300 corresponds to the method 200 and may be performed by a receiver of MIMO, for example, performed by UE 102 in Fig. 1 in the case of downlink MIMO, or performed by eNB 101 in Fig. 1 in the case of the uplink MIMO. Only as an example, the method is performed by the UE in the depictions below.

As shown in Fig. 3a, the method 300 comprises: at block S301 , transmitting to a device (e.g., eNB) the long-term precoding information for MIMO communication, the long-term precoding information indicating a set of 3D beams, wherein elements in the set of 3D beams are obtained by combining a column of a first matrix representing a vertical beam group with a corresponding column of a second matrix representing a horizontal beam group; at block S302, transmitting to the device the short-term precoding information for the MIMO communication, the short-term precoding information indicating a beam selected from the set of 3D beams; wherein the long-term precoding information and the short-term precoding information may be used by the device (e.g., eNB, at block S203 of the method 200) to construct a precoding matrix for the MIMO communications; and at block S303, receiving, from the device, data encoded according to the precoding matrix. The data for example may be sent by the eNB at the block S204 of the method 200.

The UE performing the example method 300 may communicate with the eNB performing the example method 200, including performing transmission and reception of data, and transmission and reception of feedback information related to MIMO. Hence, the long-term precoding information transmitted at the block S301 may be the long-term precoding information received at the block S201 in the method 200 depicted with reference to Fig. 2. Hence, the depictions regarding the long-term precoding information, long-term codebook Wi, the set of vertical beams and the set of horizontal beams B _# with reference to Fig. 2 and method 200 also apply here and therefore are not repeated any longer. For example, in an embodiment, the long-term precoding information may indicate the long-term codebook Wi with the structure as shown in the formula (4).

In another embodiment, the vertical beam group represented by the first matrix may be selected from a set of vertical beams βγ. In an example, may be a subset of the codebook defined in LTE Release 10 or Release 12 for a corresponding number of antennas. In another example, may be constructed according to the distribution of the UE. For example, it may be constructed according to the blocks S205 and S206 as described with reference to Fig. 2b, and the block S206 may include operations in S2061-S2064 as described with reference to Fig. 2c. In the case of uplink MIMO, the apparatus for implementing the method 300 may be eNB, so in an embodiment the method 300 may also include blocks S205-S206 in Fig. 2b; and in another embodiment, the block S206 may further include operations in S2061-S2064 in Fig. 2c, as shown in Fig. 3b.

Fig. 4 illustrates an example block diagram of an apparatus 400 for multi-input multi-output (MIMO) communications in a wireless communication network according to an embodiment of the present disclosure. In an embodiment, the apparatus 400 may be implemented as a transmitter in MIMO communication (e.g., eNB 101 or UE 102) or a part thereof. The apparatus 400 is operable to perform the method 200 depicted with reference to Figs. 2a-2c, and any other processing and methods. It should be appreciated that the method 200 is not limited to being performed by the apparatus 400, and at least some blocks of the method 200 may also be executed by other means or entities.

As shown in Fig. 4, the apparatus 400 comprises a first receiving unit 401 configured to receive, from a device, the long-term precoding information for MIMO communication, the long-term precoding information indicating a set of 3D beams, wherein elements in the set of 3D beams are obtained by combining a column of a first matrix representing a vertical beam group with a corresponding column of a second matrix representing a horizontal beam group; a second receiving unit 402 configured to receive, from the device, short-term precoding information for the MIMO communication, the short-term precoding information indicating beams selected from the set of 3D beams; a precoding matrix constructing unit 403 configured to construct a precoding matrix for the MIMO communication according to the long-term precoding information and short-term precoding information; and a transmitting unit 404 configured to transmit data encoded according to the precoding matrix to the device.

Since the apparatus 400 is operable to perform the method 200 as depicted with reference to Figs. 2a-2c, the depictions regarding the long-term precoding information, long-term codebook Wi, the set of vertical beams By and the set of horizontal beams BH with regards to Figs. 2a-2c and method 200 also apply here and therefore are not repeated any longer. For example, the apparatus 400 may further comprise an information obtaining unit 405 configured to obtain at least one of distribution of devices in the wireless communication network which need to perform MIMO communication and antenna configuration for the MIMO communication; and a vertical beam set determining unit 406 configured to determine the set of vertical beams By according to the at least one of the distribution and the antenna configuration.

As appreciated by those skilled in the art, the apparatus 400 may further comprise other units not shown in Fig. 4, e.g., a unit for constructing the long-term codebook according to the set of vertical beams By.

Fig. 5 illustrates an example block diagram of an apparatus 500 for multi-input multi-output (MIMO) communications in a wireless communication network according to an embodiment of the present disclosure. In an embodiment, the apparatus 500 may be implemented as a receiver in MIMO communication (e.g., eNB 101 or UE 102) or a portion thereof, and may perform MIMO communication with the apparatus 400. The apparatus 500 is operable to execute the method 300 depicted with reference to Fig. 3, and any other processing and methods. It should be appreciated that the method 300 is not limited to being performed by the apparatus 500, and at least some blocks of the method 300 may also be executed by other means or entities.

As shown in Fig. 5, the apparatus 500 comprises: a first transmitting unit 501 configured to transmit to a device (e.g., eNB) long-term precoding information for MIMO communication, the long-term precoding information indicating a set of 3D beams, wherein elements in the set of 3D beams are obtained by combining a column of a first matrix representing a vertical beam group with a corresponding column of a second matrix representing a horizontal beam group; a second transmitting unit 502 configured to transmit to the device short-term precoding information for the MIMO communication, the short-term precoding information indicating a beam selected from the set of 3D beams; wherein the long-term precoding information and the short-term precoding information are used by the device (e.g., eNB) to construct a precoding matrix for the MIMO communication; and a receiving unit 503 configured to receive, from the device, data encoded according to the precoding matrix.

Since the apparatus 500 is operable to perform the method 300 as depicted with reference to Fig. 3 and communicate with the apparatus 400, the depictions regarding the long-term precoding information, the short-term precoding information, long-term codebook the set of vertical beams and the set of horizontal beams B _# with reference to methods 200 and 300 also apply here and therefore are not repeated any longer. For example, in an embodiment, the apparatus is implemented as a portion of a base station (e.g., for UL MIMO), and the apparatus may further comprise an information obtaining unit 504 configured to obtain at least one of distribution of devices in the wireless communication network which need to perform MIMO communication and antenna configuration for the MIMO communication; and a vertical beam set determining unit 505 configured to determine the set of vertical beams according to the at least one of the distribution and the antenna configuration. In an embodiment, the vertical beam set determining unit 505 is configured to perform at least partial functions of blocks S206, S2061-S2064.

Advantages of the method and apparatus proposed by the present disclosure comprise at least one of the following:

- can effectively design the long-term codework for 3D MIMO;

- make the long-term codebook suitable for a specific UE distribution;

- reduce the size and feedback overhead of the long-term codebook; and

- achieve improvement of the 3D-MIMO system performance.

Those skilled in the art can easily appreciate that blocks or steps in the above methods may be performed by a programmed computer. In the present disclosure, some embodiments are intended to cover a program storage device such as a digital data storage medium, which stores an instruction program which is machine or computer-readable and may be executed by an coded machine or may be executed by a computer, wherein the instruction performs some or all steps of the above methods. The program storage device may be for example a digital memory, a magnetic storage medium such as a magnetic disk or magnetic tape, a hard disk driver or optically-readable digital data storage medium. The embodiments are further intended to cover a computer programmed to execute steps of the above method.

Functions of elements of the apparatus shown in the figures may be provided by using software, dedicated hardware, and hardware associated with proper software and being capable of executing software, or firmware, or a combination thereof. When provided by a processor, the functions may be provided by a single dedicated processor, a single shared processor or a plurality of separate processors. In addition, the term "processor" may include but is not limited to a digital signal processor (DSP) hardware, a network processor, an application-specific integrated circuit (ASIC), a field programmable gate array (FPGA), a Read Only Memory (ROM) for storing software, Radom Access Memory (RAM), and a non-volatile memory. It may further comprise other conventional and/or customized hardware.

Those skilled in the art should appreciate that the description and figures only illustrate the principles of the present disclosure. Hence, it should be appreciated that those skilled in the art can design various arrangements, though not explicitly described or shown herein, reflecting principles of the present disclosure and are included in the spirit and scope of the present disclosure. In addition, all examples as illustrated here are mainly intended for teaching purpose expressly to help a reader to understand principles of the present disclosure and the concept contributed by the inventor to further the field, and should be construed as not limited to these specific examples and conditions illustrated. Furthermore, all illustrations and their specific examples of the principle, aspect and embodiments of the present disclosure are also intended to cover their equivalents.