WO2020069459A1

WO2020069459A1 - Joint orthogonal compression and quantization for type ii channel state information feedback

Info

Publication number: WO2020069459A1
Application number: PCT/US2019/053679
Authority: WO
Inventors: Filippo Tosato; Marco MASO; Nhat-Quang NHAN
Original assignee: Nokia Technologies Oy; Nokia Of America Corporation
Priority date: 2018-09-28
Filing date: 2019-09-27
Publication date: 2020-04-02

Abstract

A method for transmitting channel state information feedback by a user equipment (UE) to a base station includes receiving channel state information feedback configuration information; determining, based on the channel state information feedback configuration information, whether to report a first type or a second type of channel state information feedback; determining in response to determining to report the second type of channel state information feedback, the second type of channel state information feedback based on normalizing each column of a matrix by a phase of a corresponding element on a main diagonal such that elements on the main diagonal are real-valued, the matrix containing subband linear combination coefficients, and representing each normalized element below the main diagonal of the matrix by two angular values, the angular values including a phase rotation coefficient and a Givens rotation coefficient; and transmitting the determined second type of channel state information feedback.

Description

JOINT ORTHOGONAL COMPRESSION AND QUANTIZATION FOR

TYPE II CHANNEL STATE INFORMATION FEEDBACK

RELATED APPLICATIONS

[0001] This application claims priority to and the benefit of U.S. Provisional Patent

Application No. 62/738,562, filed September 28, 2018, entitled“JOINT ORTHOGONAL COMPRESSION AND QUANTIZATION FOR TYPE II CHANNEL STATE INFORMATION FEEDBACK,” the disclosure of which is hereby incorporated by reference in its entirety.

TECHNICAL FIELD

[0002] This description relates to wireless communications, and in particular, to transmission of channel state information (CSI) feedback in a wireless network.

BACKGROUND

[0003] A communication system may be a facility that enables communication between two or more nodes or devices, such as fixed or mobile communication devices. Signals can be carried on wired or wireless carriers.

[0004] An example of a cellular communication system is an architecture that is being standardized by the 3rd Generation Partnership Project (3GPP). A recent development in this field is often referred to as the long-term evolution (LTE) of the Universal Mobile Telecommunications System (UMTS) radio-access technology. E-UTRA (evolved UMTS Terrestrial Radio Access) is the air interface of 3GPP's Long Term Evolution (LTE) upgrade path for mobile networks. In LTE, base stations or access points (APs), which are referred to as enhanced Node AP (eNBs), provide wireless access within a coverage area or cell. In LTE, mobile devices, or mobile stations are referred to as user equipments (UE). LTE has included a number of improvements or developments.

[0005] 5G New Radio (NR) development is part of a continued mobile broadband evolution process to meet the requirements of 5G, similar to earlier evolution of 3G & 4G wireless networks. In addition, 5G is also targeted at the new emerging use cases in addition to mobile broadband. A goal of 5G is to provide significant improvement in wireless performance, which may include new levels of data rate, latency, reliability, and security. 5G NR may also scale to efficiently connect the massive Internet of Things (IoT), and may offer new types of mission-critical services. Ultra-reliable and low-latency communications (URLLC) devices may require high reliability and very low latency.

SUMMARY

[0006] According to an example implementation, a method is provided of transmitting channel state information (CSI) feedback by a user equipment (UE) to a base station, the method comprising: receiving, by the user equipment (UE) from the base station, channel state information (CSI) feedback configuration information; determining, by the user equipment (UE) based on the channel state information (CSI) feedback configuration information, whether to report a first type or a second type of channel state information (CSI) feedback; determining, by the user equipment (UE) in response to determining to report the second type of channel state information (CSI) feedback, the second type of channel state information (CSI) feedback based on: normalizing each column of a matrix by a phase of a corresponding element on a main diagonal such that elements on the main diagonal are real valued, the matrix containing subband linear combination coefficients, and representing each normalized element below the main diagonal of the matrix by two angular values, the angular values including a phase rotation coefficient and a Givens rotation coefficient; and transmitting, by the user equipment (UE), the determined second type of channel state information (CSI) feedback to the base station.

[0007] According to an example implementation, a method is provided of transmitting channel state information (CSI) feedback by a user equipment (UE) to a base station, the method comprising: receiving, by the user equipment (UE) from the base station, channel state information (CSI) feedback configuration information; determining, by the user equipment (UE) based on the channel state information (CSI) feedback configuration information, whether to report a first type or a second type of channel state information (CSI) feedback; determining, by the user equipment (UE) in response to determining to report the second type of channel state information (CSI) feedback, the second type of channel state information (CSI) feedback based on: normalizing each column of a matrix by a phase of a corresponding element on a main diagonal such that elements on the main diagonal are real-valued, the matrix containing subband linear combination coefficients, and representing each normalized element below the main diagonal of the matrix by two angular values, the angular values including a phase rotation coefficient and a Givens rotation coefficient; and transmitting, by the user equipment (UE), the determined second type of channel state information (CSI) feedback to the base station, the second type of channel state information (CSI) feedback includes Givens rotation coefficients on a wideband basis.

[0008] According to an example implementation, a method is provided of transmitting channel state information (CSI) feedback by a user equipment (UE) to a base station, the method comprising: receiving, by the user equipment (UE) from the base station, channel state information (CSI) feedback configuration information; determining, by the user equipment (UE) based on the channel state information (CSI) feedback configuration information, whether to report a first type or a second type of channel state information (CSI) feedback; determining, by the user equipment (UE) in response to determining to report the second type of channel state information (CSI) feedback, the second type of channel state information (CSI) feedback based on: normalizing each column of a matrix by a phase of a corresponding element on a main diagonal such that elements on the main diagonal are real valued, the matrix containing subband linear combination coefficients, and representing each normalized element below the main diagonal of the matrix by two angular values, the angular values including a phase rotation coefficient and a Givens rotation coefficient; and transmitting, by the user equipment (UE), the determined second type of channel state information (CSI) feedback to the base station, the second type of channel state information (CSI) feedback includes Givens rotation coefficients on a wideband basis and differential Givens rotation coefficients on a subband basis, a differential Givens rotation coefficient being reported when a corresponding wideband Givens rotation coefficient, before or after quantization, is a non-zero value.

[0009] According to an example implementation, a method is provided of transmitting channel state information (CSI) feedback by a user equipment (UE) to a base station, the method comprising: receiving, by the user equipment (UE) from the base station, channel state information (CSI) feedback configuration information; determining, by the user equipment (UE) based on the channel state information (CSI) feedback configuration information, whether to report a first type or a second type of channel state information (CSI) feedback; determining, by the user equipment (UE) in response to determining to report the second type of channel state information (CSI) feedback, the second type of channel state information (CSI) feedback based on: normalizing each column of a matrix by a phase of a corresponding element on a main diagonal such that elements on the main diagonal are real valued, the matrix containing subband linear combination coefficients, and representing each normalized element below the main diagonal of the matrix by two angular values, the angular values including a phase rotation coefficient and a Givens rotation coefficient; and transmitting, by the user equipment (UE), the determined second type of channel state information (CSI) feedback to the base station, the second type of channel state information (CSI) feedback includes Givens rotation coefficients on a wideband basis, differential Givens rotation coefficients on a subband basis, a differential Givens rotation coefficient being reported when a corresponding wideband Givens rotation coefficient, before or after quantization, is a non-zero value, and phase rotation coefficients on a subband basis, a phase rotation coefficient being reported when a corresponding wideband Givens rotation coefficient, before or after quantization, is a non-zero value.

[0010] The details of one or more examples of implementations are set forth in the accompanying drawings and the description below. Other features will be apparent from the description and drawings, and from the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011] FIG. 1 is a block diagram of a wireless network according to an example implementation.

[0012] FIG. 2 describes an example implementation of the enhanced CSI feedback mechanism for reducing the signalling overhead, according to an example aspect.

[0013] FIG. 3 is a flow chart illustrating reporting of Type II CSI feedback from a user equipment, according to an example aspect.

[0014] FIG. 4 is a block diagram of a node or wireless station (e.g., base station/access point or mobile station/user device/UE), according to an example implementation.

DETAILED DESCRIPTION

[0015] FIG. 1 is a block diagram of a wireless network 130 according to an example implementation. In the wireless network 130 of FIG. 1, user devices 131, 132, 133 and 135, which may also be referred to as mobile stations (MSs) or user equipment (UEs), may be connected (and in communication) with a base station (BS) 134, which may also be referred to as an access point (AP), an enhanced Node B (eNB) or a network node. At least part of the functionalities of an access point (AP), base station (BS) or (e)Node B (eNB) may also be carried out by any node, server or host which may be operably coupled to a transceiver, such as a remote radio head. BS (or AP) 134 provides wireless coverage within a cell 136, including to user devices 131, 132, 133 and 135. Although only four user devices are shown as being connected or attached to BS 134, any number of user devices may be provided.

BS 134 is also connected to a core network 150 via a Sl interface 151. This is merely one simple example of a wireless network, and others may be used.

[0016] A user device (user terminal, user equipment (UE)) may refer to a portable computing device that includes wireless mobile communication devices operating with or without a subscriber identification module (SIM), including, but not limited to, the following types of devices: a mobile station (MS), a mobile phone, a cell phone, a smartphone, a personal digital assistant (PDA), a handset, a device using a wireless modem (alarm or measurement device, etc.), a laptop and/or touch screen computer, a tablet, a phablet, a game console, a notebook, and a multimedia device, as examples, or any other wireless device. It should be appreciated that a user device may also be a nearly exclusive uplink only device, of which an example is a camera or video camera loading images or video clips to a network.

[0017] In LTE (as an example), core network 150 may be referred to as Evolved Packet Core (EPC), which may include a mobility management entity (MME) which may handle or assist with mobility /handover of user devices between BSs, one or more gateways that may forward data and control signals between the BSs and packet data networks or the Internet, and other control functions or blocks.

[0018] In addition, by way of illustrative example, the various example

implementations or techniques described herein may be applied to various types of user devices or data service types, or may apply to user devices that may have multiple applications running thereon that may be of different data service types. New Radio (5G) development may support a number of different applications or a number of different data service types, such as for example: machine type communications (MTC), enhanced machine type communication (eMTC), Internet of Things (IoT), and/or narrowband IoT user devices, enhanced mobile broadband (eMBB), and ultra-reliable and low-latency communications (URLLC).

[0019] IoT may refer to an ever-growing group of objects that may have Internet or network connectivity, so that these objects may send information to and receive information from other network devices. For example, many sensor type applications or devices may monitor a physical condition or a status, and may send a report to a server or other network device, e.g., when an event occurs. Machine Type Communications (MTC, or Machine to Machine communications) may, for example, be characterized by fully automatic data generation, exchange, processing and actuation among intelligent machines, with or without intervention of humans. Enhanced mobile broadband (eMBB) may support much higher data rates than currently available in LTE.

[0020] Ultra-reliable and low-latency communications (URLLC) is a new data service type, or new usage scenario, which may be supported for New Radio (5G) systems. This enables emerging new applications and services, such as industrial automations, autonomous driving, vehicular safety, e-health services, and so on. 3GPP targets in providing connectivity with reliability corresponding to block error rate (BLER) of 10-5 and up to 1 ms U-Plane (user/data plane) latency, by way of illustrative example. Thus, for example, URLLC user devices/UEs may require a significantly lower block error rate than other types of user devices/UEs as well as low latency (with or without requirement for simultaneous high reliability). Thus, for example, a URLLC UE (or URLLC application on a UE) may require much shorter latency, as compared to a eMBB UE (or an eMBB application running on a UE).

[0021] The various example implementations may be applied to a wide variety of wireless technologies or wireless networks, such as LTE, LTE-A, 5G, cmWave, and/or mmWave band networks, IoT, MTC, eMTC, eMBB, URLLC, etc., or any other wireless network or wireless technology. These example networks, technologies or data service types are provided only as illustrative examples.

[0022] Multiple Input, Multiple Output (MIMO) may refer to a technique for increasing the capacity of a radio link using multiple transmit and receive antennas to exploit multipath propagation. MIMO may include the use of multiple antennas at the transmitter and/or the receiver. MIMO may include a multi-dimensional approach that transmits and receives two or more unique data streams through one radio channel. For example, MIMO may refer to a technique for sending and receiving more than one data signal simultaneously over the same radio channel by exploiting multipath propagation. According to an illustrative example, multi-user multiple input, multiple output (multi-user MIMIO, or MU-MIMO) enhances MIMO technology by allowing a base station (BS) or other wireless node to simultaneously transmit multiple streams to different user devices or UEs, which may include simultaneously transmitting a first stream to a first UE, and a second stream to a second UE, via a same (or common or shared) set of physical resource blocks (PRBs) (e.g., where each PRB may include a set of time-frequency resources).

[0023] Also, a BS may use precoding to transmit data to a UE (based on a precoder matrix or precoder vector for the UE). For example, a UE may receive reference signals or pilot signals, and may determine a quantized version of a DL channel estimate, and then provide the BS with an indication of the quantized DL channel estimate. The BS may determine a precoder matrix based on the quantized channel estimate, where the precoder matrix may be used to focus or direct transmitted signal energy in the best channel direction for the UE. Also, each UE may use a decoder matrix may be determined, e.g., where the UE may receive reference signals from the BS, determine a channel estimate of the DL channel, and then determine a decoder matrix for the DL channel based on the DL channel estimate. For example, a precoder matrix may indicate antenna weights (e.g., an amplitude/gain and phase for each weight) to be applied to an antenna array of a

transmitting wireless device. Likewise, a decoder matrix may indicate antenna weights (e.g., an amplitude/gain and phase for each weight) to be applied to an antenna array of a receiving wireless device.

[0024] For example, according to an example aspect, a receiving wireless user device may determine a precoder matrix using Interference Rejection Combining (IRC) in which the user device may receive reference signals (or other signals) from a number of BSs (e.g., and may measure a signal strength, signal power, or other signal parameter for a signal received from each BS), and may generate a decoder matrix that may suppress or reduce signals from one or more interferers (or interfering cells or BSs), e.g., by providing a null (or very low antenna gain) in the direction of the interfering signal, in order to increase a signal-to interference plus noise ratio (SINR) of a desired signal. In order to reduce the overall interference from a number of different interferers, a receiver may use, for example, a Linear Minimum Mean Square Error Interference Rejection Combining (LMMSE-IRC) receiver to determine a decoding matrix. The IRC receiver and LMMSE-IRC receiver are merely examples, and other types of receivers or techniques may be used to determine a decoder matrix. After the decoder matrix has been determined, the receiving UE/user device may apply antenna weights (e.g., each antenna weight including an amplitude and a phase) to a plurality of antennas at the receiving UE or device based on the decoder matrix. Similarly, a precoder matrix may include antenna weights that may be applied to antennas of a transmitting wireless device or node.

[0025] Downlink multiple-input and multiple-output (MIMO) operations in 4G and 5G radios are supported by channel state information (CSI) feedback from a user equipment (UE) to a base station. 5G New Radio (NR) development introduces a new feedback type, Type II CSI feedback. The Type II CSI feedback includes reporting of the dominant Eigenvectors (e.g., R dominant Eigenvectors) of the channel measured over the subbands of a wideband. The subbands of the wideband are configured at the UE for communication with the base stations. However, this reporting mechanism is not always efficient because of the large amount of signalling overhead when reporting the Type II CSI feedback

(e.g., amount of signalling being transmitted from the UE to the base station).

[0026] Several compression techniques have been considered to reduce the size of the feedback being reported to the base station. However, the compression techniques were not efficient due to various limitations. Some example limitations include:

a) no compression is applied in the layer domain because the layers are compressed independently; b) orthogonality between the Eigenvectors is not preserved in the presence of quantization errors; and c) two different quantizations are applied to reported coefficients - a logarithmic quantizer applied to amplitudes and an uniform quantizer applied to phases.

[0027] The present disclosure proposes an enhanced (or improved) Type II CSI feedback mechanism to reduce (or further reduce) the overhead associated with the reporting of the Type II CSI feedback. The proposed mechanism improves the compression of the Type II CSI feedback by: a) omitting reporting of coefficients that are quantized to zero or/and b) orthogonalizing the matrix of the remaining coefficients. In addition to reducing the overhead, the proposed mechanism achieves the improvement while supporting the quantization framework of Rel-l5 Type II CSI feedback. For example, in an aspect, the proposed mechanism reduces the signalling overhead associated with the reporting of Type II CSI feedback by applying compression in the layer domain, for example, by exploiting the orthogonality of the Eigenvectors being reported.

[0028] One important consideration in the enhancement of MIMO operations, in particular multi-user (MU)-MIMO, in new radio (NR) standardization is improving the trade-off between overhead reduction and performance in reporting channel state information (CSI) feedback from the UEs to the base station (e.g., gNB). For a CSI feedback report scheduled at a given time, this overhead reduction can be achieved through compression of the channel in one or more of the available domains, i.e. transmit antenna ports, frequency, or reported layers.

[0029] In an aspect, the present disclosure achieves better reduction of the signalling overhead by reducing the number of reported coefficients by selectively reporting the coefficients that are related to non-zero quantized elements. This can be achieved by normalizing the columns of the matrix by the phase of their main diagonal elements before the Givens decomposition, as described below in detail. Although, the disclosure may be described in the context of orthogonal vectors, the proposed mechanism applies to non- orthogonal vectors. [0030] For example, the CSI feedback (e.g., CSI feedback payload) can be reduced by performing joint orthogonabzation and quantization of the CSI feedback vectors such that: a) the number of coefficients to quantize is reduced by exploiting the orthogonality of vectors, b) the orthogonabzation process yields the compressed representation coefficients as a by-product, c) the number of signaled coefficients is further reduced by dropping certain coefficients that cause a smaller quantization error, d) no additional signaling required to indicate which coefficients are signaled and which coefficients are not signaled, e) orthogonality at the base station being maintained in the presence of quantization error.

[0031] These goals are achieved by performing the orthogonabzation of a matrix of channel coefficients by means of Givens (or Jacobi) rotations which provide a representation of the orthogonalized vectors (or, alternatively, of the already orthogonal vectors) in terms of two sets of angles: one set with values in [0, 2p) and one in [0, p/2). This algorithm is an algebraic technique that yields a representation of the orthogonal vectors with the minimum possible number of real-valued coefficients. The present disclosure reduces the number of coefficients in the quantized representation by omitting selected coefficients such that the quantization error is minimized and no additional signalling is required to identify the omitted coefficients in the reconstruction.

[0032] FIG. 2 describes an example implementation of the enhanced CSI feedback mechanism for reducing the signalling overhead, according to one example implementation.

[0033] A base station (e.g., base station 134 of Fig. 1) may be equipped with N cross- polarized antennas and may provide N CSI reference signals (CSI-RS) for channel acquisition by a UE (e.g., any UE of Fig. 1) mapped to N antenna ports (e.g., N different logical entities distinguished by their reference signals, such that the channel over which a symbol on the antenna port is conveyed can be inferred from the channel over which another symbol on the same antenna port is conveyed). The UE, communicating with the base station, may be configured to report Type II CSI feedback associated with the base station. The UE may report the Type II CSI feedback for a wideband and/or may also include information on a subband basis (e.g., for K subbands).

[0034] In one aspect, the UE receives CSI feedback configuration information from the base station. The CSI feedback configuration information indicates to the UE the type of CSI feedback to be reported to the base station. For example, the feedback configuration information may indicate whether the UE should report Type I CSI feedback, Type II CSI feedback, or some other type of CSI feedback. The UE may receive this information via a radio resource configuration (RRC) configuration message from the base station, or any other suitable configuration message. The CSI feedback configuration information may also indicate to the UE to report CSI feedback for a defined number (R) of layers (or data streams) transmitted on at least one configured subband (K > 1), configured at the UE for

communication with the base station. The CSI feedback for the R layers being determined based on selecting of Rstrongest Eigenvectors of the channel measured over the subbands.

[0035] In one aspect, the matrix V®, obtained by stacking the R strongest Eigenvectors measured over the subband k are approximated by a linear combination, as shown below:

y^(k) = Wi * W2^(k), . (1)

with k = 0 to k = K-l, where Wi is a wideband (WB) approximation of the vector space spanned by [V(0), . , V(K-l)] and \W^k| contains subband linear combination coefficients, based on the projection of V^(k) on Wi.

[0036] For example, Wi may contain L orthogonal beams (i.e., vectors) per polarization, selected from a two-dimensional grid of oversampled DFT beams, such that:

[0037] B is aN x L orthonormal matrix containing the selected subset of DFT beams. The selected subset of beams is signalled by indexing all the possible combinations of L orthogonal beams in the oversampled grid. In one example implementation, the matrices are shown at 210. For example, the subbands may include subbands k = 0 to k = K-l. Each of the matrices has R layers (e.g., layers 0-3) and 2L spatial beams.

[0038] In Rel-l5, the complex coefficients of W2 ^ik| are quantized in amplitude and phase and these quantization levels form part of the PMI (precoding matrix indicator) feedback message. Because there are 2L x R complex coefficients in \VV^k| for each SB, a total of 4KLR real-valued coefficients need reporting to indicate CSI over the K subbands. However, to reduce signalling overhead, amplitudes are reported in a wideband fashion, and possibly, a subband differential refinement whilst phases are reported in a subband fashion. However, this type of compressed representation of the linear combination coefficients has some limitations, such as: a) layers are compressed independently, and therefore no compression is applied to the layer domain b) orthogonality between the Eigenvectors on the left-hand side of (1) is not preserved in the presence of quantization errors even if span (V®) c span (Wi), where span denotes the vector space spanned by the columns of the matrix given as argument, c) two different quantizations are applied to the reported coefficients, with a logarithmic and uniform quantizer applied to amplitudes and phases, respectively. [0039] The proposed enhanced CSI feedback mechanism overcomes the drawbacks of existing CSI feedback mechanisms by reducing the number of coefficients being reported as part of the Type II CSI feedback and achieves higher (better) compression.

[0040] The W2 ^(k) matrices 210 (e.g., matrices 212, 214, etc.) are inputs to the normalization process 220. During the normalization process 220, the columns of a matrix are normalized by the phase of the respective elements on the main diagonal. This makes the phase values of the elements on the main diagonal of a matrix real-valued. The normalization process can be described using one matrix as an example, for instance, matrix 212 as input and matrix 222 as the output (or normalized matrix).

[0041] The normalization can be defined as subtracting the phase of each element in a column by a phase of the element on the main diagonal. The main diagonal of a matrix 321 is a collection of elements of the matrix whose row index and column index coincide.

The main diagonal of a matrix (e.g., W2 ^ik|) can be also defined as including elements whose row and column numbers coincide, or whose elements he on the diagonal that runs from top left to bottom right of the matrix (W2 ^(k)), or whose elements start in the upper left comer of the matrix and proceeds down and to the right. For instance, in one example aspect, the elements on the main diagonal of the matrix 222 are represented by elements 225, 226, 227, and 228 (shaded portions of the matrix 222).

[0042] In one aspect, the phase of the element 225 is subtracted from the phase of each element in column 0, the phase of the element 226 is subtracted from the phase of each element in column 1, and so on. The normalization process reduces the phases of the elements (at least) on the main diagonal to real-values (e.g., phase values of zero). Moreover, there is no need to report the phase normalization values (e.g., to the base station) because the design of a precoder is not affected by phase rotations applied to the columns of the matrix.

[0043] The next step is the process of orthogonalization, which is performed on the normalized matrices, on a column by column basis from left to right. For each column r, one element is considered at a time from top to bottom below the main diagonal and the elements above the main diagonal in the columns are ignored.

[0044] Ignoring the elements below the main diagonal of a r^th column reduces the number of retained rows from 2L to 2L-r-l (r represents the column index, or layer index) as shown at 230. In other words, and as shown at 230 of Fig. 2, the number of retained elements for each column goes down (e.g., decreases) as you move from left to right. For example, column 0 in 232 (and 234) contains 2L-1 elements and column R-l in 232 (and 234) contains 2L-R elements. More explicitly, for the matrix 232, the number of elements for column 0 is reduced to 7 (from 8), the number of elements for column 1 is reduced to 6 (from 8), the number of elements for column 2 is reduced to 5 (from 8), and the number of elements for column 3 is reduced to 4 (from 8).

[0045] In an aspect, each element in each column below the main diagonal is then represented by its phase (or phase rotation) [f]ί, _r £ [0, 2p] and Givens rotation, [y]ί, r £ [0, p/2]. In particular, [f]ί, _r is first identified as the rotation that would make the element in the position (i, r) real. Conversely, [y]i, r is the rotation that would make the element in the position (i, r) go to zero after a phase rotation by [f]ί, r is performed over the element in the position (i, r).

[0046] The joint effect of ignoring the elements above the main diagonal of a r* column and the use of two angular values (e.g., phase rotation and Givens rotation) to represent each remaining element, results in further compressing of normalized matrices 220 as the normalized matrices 220 are compressed (to produce matrices 230) in the layer domain by orthogonalizing the representation of V ^(k).

[0047] In particular, the normalized complex matrices 220 have (2 L— 1) xR complex elements plus R real elements. Conversely, the representation by means of appropriate [f]ί, r and [y]ί_{, r}values for each element in the position (i, r), with r = 0, ... R— 1 and i = r +

1, ... ,21— 1, allows to achieve a representation of each initial normalized matrix 220 with R * (4L-R-1) real-valued elements. This number can be obtained by subtracting from the 4L-1 real degrees of freedom of each column of each normalized matrix 220 vector of length 2L, the R-l conditions on orthogonality and one condition on the unit norm, for a total of 4L-R-1 remaining degrees of freedom per column (that is per layer). This ensures that such representation uses the minimum number of real-valued elements for representing the orthogonalized (W2 ^ik|)·

In Fig. 2, 240 represents a visualization of the output of the orthogonalization process, for example, for a layer r, with each column containing 2L-r-l elements.

[0048] The next step is the process of quantization. During the quantization process, for example, if a Givens rotation coefficient of an element is zero or close to zero, the quantized value of that element can be considered as zero, and the corresponding element of the orthogonalized matrix has near zero amplitude. Therefore, only the phase rotation coefficients corresponding to the nonzero values of Givens rotation coefficients are reported.

[0049] In one aspect, the Givens rotation coefficients are reported for the wideband.

A differential subband refinement is added to the non-zero Givens rotation coefficients.

In another aspect, the choice of reported coefficients can vary. For example, the Givens rotation coefficients may also be reported for subbands. [0050] In one example implementation, the Givens rotation may be reported for the wideband as shown by 252. The differential Givens and Phase rotations may be reported per subband as shown at 254 for subband 0 and 256 for subband K-l, respectively for non-zero wideband Given rotations only. In other words, differential Givens and phase rotations for subbands with zero wideband Givens rotations are not reported. The reporting of Givens rotations on a wideband basis and the reporting of differential Givens and phase rotations on a subband basis (for non-zero wideband Givens rotations) not only provides the feedback information on a subband basis but compresses the feedback as some of the values (zero values) are omitted resulting in smaller overhead for reporting of the Type II CSI feedback.

[0051] Therefore, the diagonal phase normalization followed by orthogonabzation and quantization provides a technological solution that allows reporting the wideband coefficients and the related subband coefficients associated with nonzero amplitude wideband coefficients.

[0052] FIG. 3 is a flow chart illustrating reporting of Type II CSI feedback from a user equipment according to an example aspect.

[0053] At block 310, a user equipment may receive, from the base station, channel state information (CSI) feedback configuration information.

[0054] At block 320, the user equipment may determine, based on the channel state information (CSI) feedback configuration information, whether to report a first type or a second type of channel state information (CSI) feedback.

[0055] At block 330, the user equipment may determine, in response to determining to report the second type of channel state information (CSI) feedback, the second type of channel state information (CSI) feedback. In some aspects, for example, at block 332, the determining may be based on normalizing each column of a matrix by a phase of a corresponding element on a main diagonal such that elements on the main diagonal are real valued, the matrix containing subband linear combination coefficients. The determining, at block 334, may be further based on representing each normalized element below the main diagonal of the matrix by two angular values, the angular values including a phase rotation coefficient and a Givens rotation coefficient.

[0056] At block 340, the user equipment may transmit the determined second type of channel state information (CSI) feedback to the base station.

[0057] Some additional aspects are now described.

[0058] Example 1. A method of transmitting channel state information (CSI) feedback by a user equipment (UE) to a base station, the method comprising receiving, by the user equipment (UE) from the base station, channel state information (CSI) feedback configuration information; determining, by the user equipment (UE) based on the channel state information (CSI) feedback configuration information, whether to report a first type or a second type of channel state information (CSI) feedback; determining, by the user equipment (UE) in response to determining to report the second type of channel state information (CSI) feedback, the second type of channel state information (CSI) feedback based on: normalizing each column of a matrix by a phase of a corresponding element on a main diagonal such that elements on the main diagonal are real-valued, the matrix containing subband linear combination coefficients, and representing each normalized element below the main diagonal of the matrix by two angular values, the angular values including a phase rotation coefficient and a Givens rotation coefficient; and transmitting, by the user equipment (UE), the determined second type of channel state information (CSI) feedback to the base station.

[0059] Example 2. The method of Example 1, wherein a phase rotation coefficient is between 0 and 2p and the Givens rotation coefficient is between 0 and p/2.

[0060] Example 3. The method of any combination of Examples 1-2, and further comprising compressing the matrix in a layer domain based on orthogonal representation of columns in the matrix using appropriate angular values.

[0061] Example 4. The method of any combination of Examples 1-3, wherein the second type of channel state information (CSI) feedback includes reporting Givens rotation coefficients on a wideband basis.

[0062] Example 5. The method of any combination of Examples 1-4, wherein the second type of channel state information (CSI) feedback includes reporting differential Givens rotation coefficients on a subband basis, a differential Givens rotation coefficient being reported when a corresponding wideband Givens rotation coefficient, before or after quantization, is a non-zero value.

[0063] Example 6. The method of any combination of Examples 1-5, wherein the second type of channel state information (CSI) feedback includes reporting phase rotation coefficients on a subband basis, a phase rotation coefficient being reported when a corresponding wideband Givens rotation coefficient, before or after quantization, is a non zero value.

[0064] Example 7. The method of any combination of Examples 1-6, wherein the first type of channel state information (CSI) feedback is associated with a first codebook and the second type of channel state information (CSI) feedback is associated with a second codebook. [0065] Example 8. The method of any combination of Examples 1-7, wherein the channel state information (CSI) feedback configuration information is received from the base station via a radio resource configuration (RRC) message.

[0066] Example 9. The method of any combination of Examples 1-8, wherein the user equipment (UE) is configured with a wideband and the wideband is configured to include a plurality of subbands, for communicating with the base station.

[0067] Example 10. The method of any combination of Examples 1-9, wherein the base station is equipped with a plurality of cross-polarized antennas.

[0068] Example 11. The method of any combination of Examples 1-10, wherein the channel state information (CSI) feedback configuration information indicates to the user equipment (UE) to report channel state information (CSI) feedback for a defined number of layers transmitted on at least one configured subband configured at the user equipment (UE) for communication with the base station, the channel state information (CSI) feedback for the defined number of layers being determined based on selecting a number of strongest

Eigenvectors of the channel measured over the subbands, the number of strongest Eigenvectors being equal to the number of defined number of layers.

[0069] Example 12. The method of any combination of Examples 1-11, wherein the matrix

matrix.

[0070] Example 13. The method of any combination of Examples 1-12, wherein the R Eigenvectors are approximated using a linear combination.

[0071] Example 14. A method of transmitting channel state information (CSI) feedback by a user equipment (UE) to a base station, the method comprising: receiving, by the user equipment (UE) from the base station, channel state information (CSI) feedback configuration information; determining, by the user equipment (UE) based on the channel state information (CSI) feedback configuration information, whether to report a first type or a second type of channel state information (CSI) feedback; determining, by the user equipment (UE) in response to determining to report the second type of channel state information (CSI) feedback, the second type of channel state information (CSI) feedback based on: normalizing each column of a matrix by a phase of a corresponding element on a main diagonal such that elements on the main diagonal are real-valued, the matrix containing subband linear combination coefficients, and representing each normalized element below the main diagonal of the matrix by two angular values, the angular values including a phase rotation coefficient and a Givens rotation coefficient; and transmitting, by the user equipment (UE), the determined second type of channel state information (CSI) feedback to the base station, the second type of channel state information (CSI) feedback includes Givens rotation coefficients on a wideband basis.

[0072] Example 15. A method of transmitting channel state information (CSI) feedback by a user equipment (UE) to a base station, the method comprising: receiving, by the user equipment (UE) from the base station, channel state information (CSI) feedback configuration information; determining, by the user equipment (UE) based on the channel state information (CSI) feedback configuration information, whether to report a first type or a second type of channel state information (CSI) feedback; determining, by the user equipment (UE) in response to determining to report the second type of channel state information (CSI) feedback, the second type of channel state information (CSI) feedback based on: normalizing each column of a matrix by a phase of a corresponding element on a main diagonal such that elements on the main diagonal are real-valued, the matrix containing subband linear combination coefficients, and representing each normalized element below the main diagonal of the matrix by two angular values, the angular values including a phase rotation coefficient and a Givens rotation coefficient; and transmitting, by the user equipment (UE), the determined second type of channel state information (CSI) feedback to the base station, the second type of channel state information (CSI) feedback includes Givens rotation coefficients on a wideband basis and differential Givens rotation coefficients on a subband basis, a differential Givens rotation coefficient being reported when a corresponding wideband Givens rotation coefficient, before or after quantization, is a non-zero value.

[0073] Example 16. A method of transmitting channel state information (CSI) feedback by a user equipment (UE) to a base station, the method comprising: receiving, by the user equipment (UE) from the base station, channel state information (CSI) feedback configuration information; determining, by the user equipment (UE) based on the channel state information (CSI) feedback configuration information, whether to report a first type or a second type of channel state information (CSI) feedback; determining, by the user equipment (UE) in response to determining to report the second type of channel state information (CSI) feedback, the second type of channel state information (CSI) feedback based on: normalizing each column of a matrix by a phase of a corresponding element on a main diagonal such that elements on the main diagonal are real-valued, the matrix containing subband linear combination coefficients, and representing each normalized element below the main diagonal of the matrix by two angular values, the angular values including a phase rotation coefficient and a Givens rotation coefficient; and transmitting, by the user equipment (UE), the determined second type of channel state information (CSI) feedback to the base station, the second type of channel state information (CSI) feedback includes Givens rotation coefficients on a wideband basis, differential Givens rotation coefficients on a subband basis, a differential Givens rotation coefficient being reported when a corresponding wideband Givens rotation coefficient, before or after quantization, is a non-zero value, and a phase rotation coefficient on a subband basis, a phase rotation coefficient being reported when a corresponding wideband Givens rotation coefficient, before or after quantization, is a non-zero value.

[0074] Example 17. An apparatus comprising at least one processor and at least one memory including computer instructions, when executed by the at least one processor, cause the apparatus to perform a method of any combination of Examples 1-16.

[0075] Example 18. An apparatus comprising means for performing a method of any combination of Examples 1-16.

[0076] Example 19. A non-transitory computer-readable storage medium having stored thereon computer executable program code which, when executed on a computer system, causes the computer system to perform the method of any combination of

Examples 1-16.

[0077] FIG. 4 is a block diagram of a wireless station (e.g., AP or user device)

400 according to an example implementation. The wireless station 400 may include, for example, one or two RF (radio frequency) or wireless transceivers 402A, 402B, where each wireless transceiver includes a transmitter to transmit signals and a receiver to receive signals. The wireless station also includes a processor or control unit/entity (controller) 404 to execute instructions or software and control transmission and receptions of signals, and a memory 406 to store data and/or instructions.

[0078] Processor 404 may also make decisions or determinations, generate frames, packets or messages for transmission, decode received frames or messages for further processing, and other tasks or functions described herein. Processor 404, which may be a baseband processor, for example, may generate messages, packets, frames or other signals for transmission via wireless transceiver 402 (402A or 402B). Processor 404 may control transmission of signals or messages over a wireless network, and may control the reception of signals or messages, etc., via a wireless network (e.g., after being down-converted by wireless transceiver 402, for example). Processor 404 may be programmable and capable of executing software or other instructions stored in memory or on other computer media to perform the various tasks and functions described above, such as one or more of the tasks or methods described above. Processor 404 may be (or may include), for example, hardware, programmable logic, a programmable processor that executes software or firmware, and/or any combination of these. Using other terminology, processor 404 and transceiver 402 together may be considered as a wireless transmitter/receiver system, for example.

[0079] In addition, referring to FIG. 4, a controller (or processor) 408 may execute software and instructions, and may provide overall control for the station 400, and may provide control for other systems not shown in FIG. 4, such as controlling input/output devices (e.g., display, keypad), and/or may execute software for one or more applications that may be provided on wireless station 400, such as, for example, an email program, audio/video applications, a word processor, a Voice over IP application, or other application or software.

[0080] In addition, a storage medium may be provided that includes stored instructions, which when executed by a controller or processor may result in the processor 404, or other controller or processor, performing one or more of the functions or tasks described above.

[0081] According to another example implementation, RF or wireless transceiver(s) 402A/402B may receive signals or data and/or transmit or send signals or data. Processor 404 (and possibly transceivers 402A/402B) may control the RF or wireless transceiver 402A or 402B to receive, send, broadcast or transmit signals or data.

[0082] The aspects are not, however, restricted to the system that is given as an example, but a person skilled in the art may apply the solution to other communication systems. Another example of a suitable communications system is the 5G concept. It is assumed that network architecture in 5G will be quite similar to that of the LTE-advanced. 5G is likely to use multiple input - multiple output (MIMO) antennas, many more base stations or nodes than the LTE (a so-called small cell concept), including macro sites operating in co-operation with smaller stations and perhaps also employing a variety of radio technologies for better coverage and enhanced data rates.

[0083] It should be appreciated that future networks will most probably utilize network functions virtualization (NFV) which is a network architecture concept that proposes virtualizing network node functions into“building blocks” or entities that may be operationally connected or linked together to provide services. A virtualized network function (VNF) may comprise one or more virtual machines running computer program codes using standard or general type servers instead of customized hardware. Cloud computing or data storage may also be utilized. In radio communications this may mean node operations may be carried out, at least partly, in a server, host or node operationally coupled to a remote radio head. It is also possible that node operations will be distributed among a plurality of servers, nodes or hosts. It should also be understood that the distribution of labor between core network operations and base station operations may differ from that of the LTE or even be non-existent.

[0084] Implementations of the various techniques described herein may be implemented in digital electronic circuitry, or in computer hardware, firmware, software, or in

combinations of them. Implementations may implemented as a computer program product, i.e., a computer program tangibly embodied in an information carrier, e.g., in a

machine-readable storage device or in a propagated signal, for execution by, or to control the operation of, a data processing apparatus, e.g., a programmable processor, a computer, or multiple computers. Implementations may also be provided on a computer readable medium or computer readable storage medium, which may be a non-transitory medium.

Implementations of the various techniques may also include implementations provided via transitory signals or media, and/or programs and/or software implementations that are downloadable via the Internet or other network(s), either wired networks and/or wireless networks. In addition, implementations may be provided via machine type communications (MTC), and also via an Internet of Things (IOT).

[0085] The computer program may be in source code form, object code form, or in some intermediate form, and it may be stored in some sort of carrier, distribution medium, or computer readable medium, which may be any entity or device capable of carrying the program. Such carriers include a record medium, computer memory, read-only memory, photoelectrical and/or electrical carrier signal, telecommunications signal, and software distribution package, for example. Depending on the processing power needed, the computer program may be executed in a single electronic digital computer or it may be distributed amongst a number of computers.

[0086] Furthermore, implementations of the various techniques described herein may use a cyber-physical system (CPS) (a system of collaborating computational elements controlling physical entities). CPS may enable the implementation and exploitation of massive amounts of interconnected ICT devices (sensors, actuators, processors

microcontrollers,...) embedded in physical objects at different locations. Mobile cyber physical systems, in which the physical system in question has inherent mobility, are a subcategory of cyber-physical systems. Examples of mobile physical systems include mobile robotics and electronics transported by humans or animals. The rise in popularity of smartphones has increased interest in the area of mobile cyber-physical systems. Therefore, various implementations of techniques described herein may be provided via one or more of these technologies.

[0087] A computer program, such as the computer program(s) described above, can be written in any form of programming language, including compiled or interpreted languages, and can be deployed in any form, including as a stand-alone program or as a module, component, subroutine, or other unit or part of it suitable for use in a computing

environment. A computer program can be deployed to be executed on one computer or on multiple computers at one site or distributed across multiple sites and interconnected by a communication network.

[0088] Method steps may be performed by one or more programmable processors executing a computer program or computer program portions to perform functions by operating on input data and generating output. Method steps also may be performed by, and an apparatus may be implemented as, special purpose logic circuitry, e.g., an FPGA (field programmable gate array) or an ASIC (application specific integrated circuit).

[0089] Processors suitable for the execution of a computer program include, by way of example, both general and special purpose microprocessors, and any one or more processors of any kind of digital computer, chip or chipset. Generally, a processor will receive instructions and data from a read only memory or a random access memory or both. Elements of a computer may include at least one processor for executing instructions and one or more memory devices for storing instructions and data. Generally, a computer also may include, or be operatively coupled to receive data from or transfer data to, or both, one or more mass storage devices for storing data, e.g., magnetic, magneto optical disks, or optical disks. Information carriers suitable for embodying computer program instructions and data include all forms of non volatile memory, including by way of example semiconductor memory devices, e.g., EPROM, EEPROM, and flash memory devices; magnetic disks, e.g., internal hard disks or removable disks; magneto optical disks; and CD ROM and DVD-ROM disks. The processor and the memory may be supplemented by, or incorporated in, special purpose logic circuitry.

Claims

WHAT IS CLAIMED IS:

1. A method of transmitting channel state information (CSI) feedback by a user equipment (UE) to a base station, the method comprising:

receiving, by the user equipment (UE) from the base station, channel state information (CSI) feedback configuration information;

determining, by the user equipment (UE) based on the channel state information (CSI) feedback configuration information, whether to report a first type or a second type of channel state information (CSI) feedback;

determining, by the user equipment (UE) in response to determining to report the second type of channel state information (CSI) feedback, the second type of channel state information (CSI) feedback based on:

normalizing each column of a matrix by a phase of a corresponding element on a main diagonal such that elements on the main diagonal are real-valued, the matrix containing subband linear combination coefficients, and

representing each normalized element below the main diagonal of the matrix by two angular values, the angular values including a phase rotation coefficient and a Givens rotation coefficient; and

transmitting, by the user equipment (UE), the determined second type of channel state information (CSI) feedback to the base station.

2. The method of claim 1, wherein a phase rotation coefficient is between 0 and 2p and a Givens rotation coefficient is between 0 and p/2.

3. The method of any combination of claims 1-2, further comprising:

compressing the matrix in a layer domain based on orthogonal representation of columns in the matrix using appropriate angular values.

4. The method of any combination of claims 1-3, wherein the second type of channel state information (CSI) feedback includes reporting Givens rotation coefficients on a wideband basis.

5. The method of any combination of claims 1-4, wherein the second type of channel state information (CSI) feedback includes reporting differential Givens rotation coefficients on a subband basis, a differential Givens rotation coefficient being reported when a corresponding wideband Givens rotation coefficient, before or after quantization, is a non zero value.

6. The method of any combination of claims 1-5, wherein the second type of channel state information (CSI) feedback includes reporting phase rotation coefficients on a subband basis, a phase rotation coefficient being reported when a corresponding wideband Givens rotation coefficient, before or after quantization, is a non-zero value.

7. The method of any combination of claims 1-6, wherein the first type of channel state information (CSI) feedback is associated with a first codebook and the second type of channel state information (CSI) feedback is associated with a second codebook.

8. The method of any combination of claims 1-7, wherein the channel state information (CSI) feedback configuration information is received from the base station via a radio resource configuration (RRC) message.

9. The method of any combination of claims 1-8, wherein the user equipment (UE) is configured with a wideband and the wideband is configured to include a plurality of subbands, for communicating with the base station.

10. The method of any combination of claims 1-9, wherein the base station is equipped with a plurality of cross-polarized antennas.

11. The method of any combination of claims 1-10, wherein the channel state information (CSI) feedback configuration information indicates to the user equipment (UE) to report channel state information (CSI) feedback for a defined number of layers transmitted on at least one configured subband configured at the user equipment (UE) for communication with the base station, the channel state information (CSI) feedback for the defined number of layers being determined based on selecting a number of strongest Eigenvectors of the channel measured over the subbands, the number of strongest Eigenvectors being equal to the number of defined number of layers.

12. The method of any combination of claims 1-11, wherein the matrix is a W2 ^ik| matrix.

13. The method of any combination of claims 1-12, wherein the R Eigenvectors are approximated using a linear combination.

14. A method of transmitting channel state information (CSI) feedback by a user equipment (UE) to a base station, the method comprising:

transmitting, by the user equipment (UE), the determined second type of channel state information (CSI) feedback to the base station, the second type of channel state information (CSI) feedback includes Givens rotation coefficients on a wideband basis.

15. A method of transmitting channel state information (CSI) feedback by a user equipment (UE) to a base station, the method comprising:

determining, by the user equipment (UE) based on the channel state information (CSI) feedback configuration information, whether to report a first type or a second type of channel state information (CSI) feedback; determining, by the user equipment (UE) in response to determining to report the second type of channel state information (CSI) feedback, the second type of channel state information (CSI) feedback based on:

transmitting, by the user equipment (UE), the determined second type of channel state information (CSI) feedback to the base station, the second type of channel state information (CSI) feedback includes:

Givens rotation coefficients on a wideband basis, and

differential Givens rotation coefficients on a subband basis, a differential Givens rotation coefficient being reported when a corresponding wideband Givens rotation coefficient, before or after quantization, is a non-zero value.

16. A method of transmitting channel state information (CSI) feedback by a user equipment (UE) to a base station, the method comprising:

representing each normalized element below the main diagonal of the matrix by two angular values, the angular values including a phase rotation coefficient and a Givens rotation coefficient; and transmitting, by the user equipment (UE), the determined second type of channel state information (CSI) feedback to the base station, the second type of channel state information (CSI) feedback includes:

Givens rotation coefficients on a wideband basis,

differential Givens rotation coefficients on a subband basis, a differential Givens rotation coefficient being reported when a corresponding wideband Givens rotation coefficient, before or after quantization, is a non-zero value, and

phase rotation coefficients on a subband basis, a phase rotation coefficient being reported when a corresponding wideband Givens rotation coefficient, before or after quantization, is a non-zero value.

17. An apparatus comprising at least one processor and at least one memory including computer instructions, when executed by the at least one processor, cause the apparatus to perform a method of any combination of claims 1-16.

18. An apparatus comprising means for performing a method of any combination of claims 1-16.

19. A non-transitory computer-readable storage medium having stored thereon computer executable program code which, when executed on a computer system, causes the computer system to perform the steps of any combination of claims 1-16.