WO2021046783A1

WO2021046783A1 - Channel state information feedback

Info

Publication number: WO2021046783A1
Application number: PCT/CN2019/105482
Authority: WO
Inventors: Hao Liu
Original assignee: Nokia Shanghai Bell Co., Ltd.; Nokia Solutions And Networks Oy; Nokia Technologies Oy
Priority date: 2019-09-11
Filing date: 2019-09-11
Publication date: 2021-03-18
Also published as: CN114342281A

Abstract

Embodiments of the present disclosure relate to method, device, apparatus and computer readable storage media for channel state information (CSI) feedback. The first device determines a first indicator of a first CSI mode based on current measurement of a channel between the first device and a second device. The first device determines a second indicator of a second CSI mode at least in part based on previous CSI feedback. Then, the first device selects, from the first CSI mode and the second CSI mode, a target CSI mode for current CSI feedback based on the first indicator and the second indicator. With such a dynamic design of an explicit CSI feedback, it is possible to lead to a trade-off between system performance and CSI feedback payload, especially for new radio system, by flexibly switching between different CSI modes. As such, a save of CSI payload and a gain of cell average throughout can be achieved.

Description

CHANNEL STATE INFORMATION FEEDBACK

FIELD

Embodiments of the present disclosure generally relate to the field of telecommunication and in particular, to a method, device, apparatus and computer readable storage medium for channel state information (CSI) feedback.

BACKGROUND

In 3GPP NR Rel-15, type-2 codebook has been introduced, due to a superior performance gains over that can be achieved in 3GPP LTE Rel-14. In 3GPP NR Rel-16, type-2 codebook is further designed to specify and support frequency domain compression techniques, which can significantly reduce CSI feedback overhead per sub-band level without any performance loss.

With the arrival of 5G New Radio (NR) , a concept of massive Multi-Input Multi-Output (MIMO) , which is one of the key technologies raised in NR, has been introduced to enhance the system performance of Multi-User Multi-Input Multi-Output (MU-MIMO) . Current systems generally require a finer granularity and a higher accuracy for channel feedback, as well as a reasonable feedback overhead. In this case, the CSI feedback with a coarse frequency granularity may unavoidably restrict MU pre-coding accuracy and MU-MIMO scheduling performance improvement.

SUMMARY

In general, example embodiments of the present disclosure provide a solution for CSI feedback.

In a first aspect, there is provided a first device. The first device comprises at least one processor; and at least one memory including computer program codes; the at least one memory and the computer program codes are configured to, with the at least one processor, cause the first device to, determine a first indicator of a first CSI mode based on current measurement of a channel between the first device and a second device; determine a second indicator of a second CSI mode at least in part based on previous CSI feedback; and select, from the first CSI mode and the second CSI mode, a target CSI mode for current CSI feedback based on the first indicator and the second indicator.

In a second aspect, there is provided a second device. The second device comprises at least one processor; and at least one memory including computer program codes; the at least one memory and the computer program codes are configured to, with the at least one processor, cause the second device to determine a target CSI mode from a first part of current CSI feedback for a channel between a first device and the second device, the target CSI mode being selected from a first CSI mode and a second CSI mode for current CSI feedback by the first device; and receive, based on the target CSI mode, a second part of the current CSI feedback.

In a third aspect, there is provided a method implemented at a first device. The method comprises: determining a first indicator of a first CSI mode based on current measurement of a channel between the first device and a second device; determining a second indicator of a second CSI mode at least in part based on previous CSI feedback; and selecting, from the first CSI mode and the second CSI mode, a target CSI mode for current CSI feedback based on the first indicator and the second indicator.

In a fourth aspect, there is provided a method implemented at a second device. The method comprises determining a target CSI mode from a first part of current CSI feedback for a channel between a first device and the second device, the target CSI mode being selected from a first CSI mode and a second CSI mode for current CSI feedback by the first device; and receiving, based on the target CSI mode, a second part of the current CSI feedback.

In a fifth aspect, there is provided an apparatus comprising: means for determining a first indicator of a first CSI mode based on current measurement of a channel between the first device and a second device; means for determining a second indicator of a second CSI mode at least in part based on previous CSI feedback; and means for selecting, from the first CSI mode and the second CSI mode, a target CSI mode for current CSI feedback based on the first indicator and the second indicator.

In a sixth aspect, there is provided an apparatus comprising: means for determining a target CSI mode from a first part of current CSI feedback for a channel between a first device and the second device, the target CSI mode being selected from a first CSI mode and a second CSI mode for CSI feedback; and means for receiving, based on the target CSI mode, a second part of the current CSI feedback.

In a seventh aspect, there is provided a non-transitory computer readable medium comprising program instructions for causing an apparatus to perform at least the method according to the above third or fourth aspect.

It is to be understood that the summary section is not intended to identify key or essential features of embodiments of the present disclosure, nor is it intended to be used to limit the scope of the present disclosure. Other features of the present disclosure will become easily comprehensible through the following description.

BRIEF DESCRIPTION OF THE DRAWINGS

Some example embodiments will now be described with reference to the accompanying drawings, where:

FIG. 1 shows an example communication network in which example embodiments of the present disclosure may be implemented;

FIG. 2 illustrates a flowchart of a method implemented at a first device according to some embodiments of the present disclosure;

Fig. 3 illustrates a flowchart of a method implemented at a second device according to some embodiments of the present disclosure;

FIG. 4 shows a diagram of a signaling flow of the transmission of CSI according to some example embodiments of the present disclosure;

Fig. 5 illustrates a simplified block diagram of an apparatus that is suitable for implementing embodiments of the present disclosure; and

Fig. 6 illustrates a block diagram of an example computer readable medium in accordance with some embodiments of the present disclosure.

Throughout the drawings, the same or similar reference numerals represent the same or similar element.

DETAILED DESCRIPTION

Principle of the present disclosure will now be described with reference to some example embodiments. It is to be understood that these embodiments are described only for the purpose of illustration and help those skilled in the art to understand and implement the present disclosure, without suggesting any limitation as to the scope of the disclosure. The disclosure described herein can be implemented in various manners other than the ones described below.

In the following description and claims, unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skills in the art to which this disclosure belongs.

References in the present disclosure to “one embodiment, ” “an embodiment, ” “an example embodiment, ” and the like indicate that the embodiment described may include a particular feature, structure, or characteristic, but it is not necessary that every embodiment includes the particular feature, structure, or characteristic. Moreover, such phrases are not necessarily referring to the same embodiment. Further, when a particular feature, structure, or characteristic is described in connection with an embodiment, it is submitted that it is within the knowledge of one skilled in the art to affect such feature, structure, or characteristic in connection with other embodiments whether or not explicitly described.

It shall be understood that although the terms “first” and “second” etc. may be used herein to describe various elements, these elements should not be limited by these terms. These terms are only used to distinguish one element from another. For example, a first element could be termed a second element, and similarly, a second element could be termed a first element, without departing from the scope of example embodiments. As used herein, the term “and/or” includes any and all combinations of one or more of the listed terms.

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

As used in this application, the term “circuitry” may refer to one or more or all of the following:

(a) hardware-only circuit implementations (such as implementations in only analog and/or digital circuitry) and

(b) combinations of hardware circuits and software, such as (as applicable) :

(i) a combination of analog and/or digital hardware circuit (s) with software/firmware and

(ii) any portions of hardware processor (s) with software (including digital signal processor (s) ) , software, and memory (ies) that work together to cause an apparatus, such as a mobile phone or server, to perform various functions) and

(c) hardware circuit (s) and or processor (s) , such as a microprocessor (s) or a portion of a microprocessor (s) , that requires software (e.g., firmware) for operation, but the software may not be present when it is not needed for operation.

This definition of circuitry applies to all uses of this term in this application, including in any claims. As a further example, as used in this application, the term circuitry also covers an implementation of merely a hardware circuit or processor (or multiple processors) or portion of a hardware circuit or processor and its (or their) accompanying software and/or firmware. The term circuitry also covers, for example and if applicable to the particular claim element, a baseband integrated circuit or processor integrated circuit for a mobile device or a similar integrated circuit in server, a cellular network device, or other computing or network device.

As used herein, the term “communication network” refers to a network following any suitable communication standards, such as Long Term Evolution (LTE) , LTE-Advanced (LTE-A) , Wideband Code Division Multiple Access (WCDMA) , High-Speed Packet Access (HSPA) , Narrow Band Internet of Things (NB-IoT) and so on. Furthermore, the communications between a terminal device and a network device in the communication network may be performed according to any suitable generation communication protocols, including, but not limited to, the first generation (1G) , the second generation (2G) , 2.5G, 2.75G, the third generation (3G) , the fourth generation (4G) , 4.5G, the future fifth generation (5G) communication protocols, and/or any other protocols either currently known or to be developed in the future. Embodiments of the present disclosure may be applied in various communication systems. Given the rapid development in communications, there will of course also be future type communication technologies and systems with which the present disclosure may be embodied. It should not be seen as limiting the scope of the present disclosure to only the aforementioned system.

As used herein, the term “first device” refers to any end device that may be capable of wireless communication. In some embodiments, the first device may be a terminal device. By way of example rather than limitation, a terminal device may also be referred to as a communication device, user equipment (UE) , a Subscriber Station (SS) , a Portable Subscriber Station, a Mobile Station (MS) , or an Access Terminal (AT) . The terminal device may include, but not limited to, a mobile phone, a cellular phone, a smart phone, voice over IP (VoIP) phones, wireless local loop phones, a tablet, a wearable terminal device, a personal digital assistant (PDA) , portable computers, desktop computer, image capture terminal devices such as digital cameras, gaming terminal devices, music storage and playback appliances, vehicle-mounted wireless terminal devices, wireless endpoints, mobile stations, laptop-embedded equipment (LEE) , laptop-mounted equipment (LME) , USB dongles, smart devices, wireless customer-premises equipment (CPE) , an Internet of Things (IoT) device, a watch or other wearable, a head-mounted display (HMD) , a vehicle, a drone, a medical device and applications (e.g., remote surgery) , an industrial device and applications (e.g., a robot and/or other wireless devices operating in an industrial and/or an automated processing chain contexts) , a consumer electronics device, a device operating on commercial and/or industrial wireless networks, and the like. In the following description, the terms “terminal device” , “communication device” , “terminal” , “user equipment” and “UE” may be used interchangeably.

The term “second device” refers to a node in a communication network via which a terminal device accesses the network and receives services therefrom. In some embodiments, the second device may be a network device. The network device may refer to a base station (BS) or an access point (AP) , for example, a node B (NodeB or NB) , an evolved NodeB (eNodeB or eNB) , a NR NB (also referred to as a gNB) , a Remote Radio Unit (RRU) , a radio header (RH) , a remote radio head (RRH) , a relay, a low power node such as a femto, a pico, and so forth, depending on the applied terminology and technology.

An explicit CSI feedback design is proposed for exploiting time domain channel sparsity property to reduce CSI feedback overhead and improve CSI accuracy. According to a certain compression criterion, such as orthogonal matching pursuit (OMP) , significant channel taps among all the channel taps in the time domain channel are selected to form a sparsifying basis matrix which includes for example, discrete Fourier transform (DFT) vectors. Since the DFT basis design, i.e., a predetermined set of DFT vectors, is known to both the first device (e.g., the UE) and the second device (e.g., the gNB) side, the first device only needs to report the indices of DFT vector corresponding to the significant channel taps. Then, the second device may reconstruct the sparsifying basis matrix by the indices of the DFT vectors. In this case, the CSI feedback overhead mainly depends on quantization of the compressed linear combination (LC) coefficients, which is related to the number of significant channel taps between the first device and the second device.

Another CSI feedback design is proposed to provide a better channel compression behavior by using channel statistics based on Karhunen-Loeve transform (KLT) . In such a design, a sparsifying basis matrix is built from dominant eigenvectors of frequency domain (FD) covariance matrix with a size of N _f×N _f, where N _f denotes the number of active subcarriers. In this case, since the sparsifying basis matrix is unknown to the second device, a higher feedback overhead is needed for quantization of LC coefficients as well as quantization of sparsifying basis matrix.

As mentioned above, MU-MIMO technology raises higher requirement for system performances. Hence, there is a need for a new CSI feedback design which can achieve the finer granularity and higher accuracy for channel feedback, as well as the reasonable feedback overhead.

Embodiments of the present disclosure provide a dynamic scheme for explicit CSI feedback. In the dynamic scheme, a target CSI mode for current CSI feedback may be switched between different CSI modes by using a high layer configured parameter, such as a switching factor α. For example, in each feedback instance, the first device (e.g., the UE) may select the target CSI mode such that an optimal trade-off between system performance and CSI feedback payload can be realized. By flexibly switching between different CSI modes, the scheme is capable of saving the CSI payload and increasing the cell average throughout.

In communication networks where a number of network devices are jointly deployed in a geographical area to serve respective cells, a terminal device may have an active connection with a network device when being located within the corresponding cell. In the active connection, the terminal device may communicate with that network device on the frequency band in both an uplink (UL) and a downlink (DL) . The terminal device may need to switch a link in one direction such as the UL to a further network device due to various reasons such as quality degradation in the UL. Some example embodiments of the present disclosure will be described below with reference to the figures. However, those skilled in the art would readily appreciate that the detailed description given herein with respect to these figures is for explanatory purpose as the present disclosure extends beyond theses limited embodiments.

FIG. 1 shows an example communication network 100 in which implementations of the present disclosure can be implemented. The communication network 100 includes a first device 110 and a second device 120. For example, the network 100 can provide one or more cells to serve the first device 110. It is to be understood that the numbers of first device, second device and/or the cells are given for the purpose of illustration without suggesting any limitations to the present disclosure. The communication network 100 may include any suitable number of network devices, terminal devices and/or cells adapted for implementing implementations of the present disclosure.

In the communication network 100, the first device 110 may communicate data and CSI feedback to the second device 120, and the second device 120 may allocate resource for the first device 110 and receive CSI feedback from the first device 110. A link from the first device 110 to the second device 120 is referred to as an uplink (UL) , while a link from the second device 120 to the first device 110 is referred to as a downlink (DL) .

The CSI may ensure reliability of the wireless communication between the first device 110 and the second device 120. The process for reporting the CSI is also called as “CSI feedback” . In order to obtain CSI of a communication channel between the first device 110 and the second device 120, the second device 120 may allocate resource for the first device 110. Then, the first device 110 may report the CSI to the second device 120 on the PUSCH.

The communications in the network 100 may conform to any suitable standards including, but not limited to, Global System for Mobile Communications (GSM) , Long Term Evolution (LTE) , LTE-Evolution, LTE-Advanced (LTE-A) , Wideband Code Division Multiple Access (WCDMA) , Code Division Multiple Access (CDMA) , GSM EDGE Radio Access Network (GERAN) , and the like. Furthermore, the communications may be performed according to any generation communication protocols either currently known or to be developed in the future. Examples of the communication protocols include, but not limited to, the first generation (1G) , the second generation (2G) , 2.5G, 2.75G, the third generation (3G) , the fourth generation (4G) , 4.5G, the fifth generation (5G) communication protocols.

The CSI feedback includes a set of CSI feedback parameters, which may be split into two parts, i.e., CSI part 1 and CSI part 2. CSI part 1 has a fixed payload size, and CSI part 2 has a variable payload size depending on the parameters included in the CSI part 1. Such a two-part CSI structure will be discussed in details below.

FIG. 2 illustrates a flowchart of a method 200 implemented at a first device according to some example embodiments of the present disclosure. For the purpose of discussion, the process 200 will be described with reference to FIG. 1. The process 200 may involve the first device 110 and the second device 120 as illustrated in FIG. 1.

As noted above, the first device 110 may measure the channel between the first device 110 and the second device 120 from the current measurement of the downlink channel, for example, measurement of CSI reference signal (CSI-RS) . The first device 110 may then determine a channel matrix H _FD for characterizing the channel based on the current measurement of the channel. In other words, the channel matrix H _FD may represent the measured channel.

In some example embodiments, the channel matrix H _FD may be a frequency domain matrix obtained based on downlink measurement with a dimension of N _f×N _p, and the channel matrix H _FD may be represented as follows.

where N _f is the number of active subcarriers, N _p=N _tx×N _rx is the number of channel paths each linking a transmit port of the second device 120 and a receive port of the first device 110, N _tx is the number of transmit ports of the second device 120, and N _rx is the number of receive ports of the first device 110.

In some example embodiments, the number of the transmit ports may be the number of transmit antennas of the second device, in the case that there is no spatial-domain compression. If the spatial-domain compression is performed, spatial domain beams may be formed and the number of the transmit ports may be the number of the spatial domain beams.

The number of the receive ports may be the number of receive antennas of the first device, if there is no spatial-domain compression. If the spatial-domain transformation is performed, spatial domain beams may be also formed and the number of the receive ports may be the number of the spatial domain beams.

In some example embodiments, a set of discrete Fourier Transform (DFT) vectors may be predetermined and known by both the first device 110 and the second device 120. The first device 110 may select at least a part of DFT vectors form a first basis matrix

with a size of N _f×N _p, where N _tap is the number of dominant taps. The at least a part of DFT vectors may be determined by for example OMP searching rule. Then, the channel matrix H _FD may be compressed into a first transformed matrix H _TD1. The first transformed matrix H _TD1 may be determined by the formula

which consists of a set of compressed linear combination (LC) coefficients with a lower dimension N _tap×N _p, since the number of dominant taps N _tap is far less than the number of active subcarriers N _f. In the embodiment, the first basis matrix

is common to all N _p channel paths, and may be represented as follows.

The first transformed matrix H _TD1 may then be quantized, and thus a quantization version of the first transformed matrix H _TD1, denoted as

is obtained. Next, a quantized channel matrix

which represents the estimated channel, may be restored by leveraging the formula

Finally, a normalized mean square error (NMSE) between the estimated channel

and currently measured channel H _FD may be calculated as follows.

Now referring to FIG. 2, at 210, the first device 110 may determine a first indicator of a first CSI mode based on the current measurement of the channel. In this embodiment, the first indicator may be NMSE _DFT.

In some example embodiments, the first CSI mode may be designed to feedback the following CSI parameters, including but not limited to, a selected set of tap indices in the first basis matrix

and N _tap×N _p LC coefficients in a quantization version of the first transformed matrix H _TD1, which is denoted as

It should be understood that the first CSI mode may include other necessary CSI parameters for reconstructing the channel information.

At 220, the first device 120 may determine a second indicator of a second CSI mode at least in part based on previous CSI feedback. In some example embodiments, the second indicator of the second CSI mode is determined based on previous CSI feedback and the current measurement of the channel.

In addition to the current measurement of the channel, the second CSI mode may further consider the previous CSI feedback. For example, the second CSI mode may take the advantage of Karhunen-Loeve transform (KLT) which leads to a very limited CSI feedback overhead.

In this embodiment, the estimated matrix

is reconstructed by exploiting the previous CSI feedback information, which has been shared to both the first device 110 and the second device 120. The estimated matrix

may be represented as follows.

where

is an estimated channel vector in the preceding feedback instance with a size of N _f×1 for channel path n, and n=1, …, N _p.

As an example, according to KLT operation, for channel path n, only one tap is considered since all the energies of the FD channel are integrated in this tap after KLT operation. A second basis vector

related to the channel path n may be obtained through the following two options.

In option 1, a covariance matrix R _H of the estimated matrix

and its eigen decomposition (ED) may be determined, and the second basis vector

is the dominant eigenvector from U as follows.

where the covariance matrix R _H has a dimension of N _f×N _f, and U is formed by eigenvectors and ∑ is composed of eigenvalues along the main diagonal.

Alternatively, in option 2, the estimated channel vector

is normalized and the normalization version of the channel vector

is directly used as the second basis vector

which is represented as below.

Accordingly, for channel path n, the second basis vector

may then be used for FD compression in the currently measured channel vector

where

is the n-th column of the current channel matrix H _FD. After FD compression, a corresponding LC coefficient is calculated for channel path n as follows.

The respective LC coefficients

for all the channel paths may be quantized as

and formed into a quantization version of the second transformed matrix H _TD2, which is denoted as

and thus the estimated channel matrix may be obtained as follows.

Similarly, a normalized mean square error (NMSE) between the estimated channel

and currently measured channel H _FD may be calculated as follows.

In this embodiment, the second indicator may be NMSE _KLT, and the second CSI mode may be designed to feedback 1×N _p LC coefficients in the quantized second transformed matrix

and have no need to feedback the second basis matrix including a set of basis vectors since they are determined from the previous CSI feedback which have been known in both the first device 110 and the second device 120. It should be understood that the second CSI mode may include other necessary CSI parameters for reconstructing the channel information.

At 230, the first device 110 may select, from the first CSI mode and the second CSI mode, a target CSI mode for the current CSI feedback based on the first indicator and the second indicator.

In some example embodiments, the first device 110 may receive a switching factor α for selecting the target CSI mode via a high layer RRC signaling message from the second device 120. The switching factor α ranging from 0 to 1 (i.e., α∈ [0, 1] ) is defined for adjusting and comparing the first indicator and the second indicator, such the NMSE values of the above two CSI modes. Then, the target CSI mode for current CSI feedback may be determined based on for example whether the following condition is satisfied

NMSE _KLT×α≤NMSE _DFT (11)

In this embodiment, the first CSI mode is the DFT based CSI mode, and the second CSI mode is the KLT based CSI mode. According to formula (11) , the second indicator is adjusted by the switching factor α. If the adjusted second indicator NMSE _KLT, such as the product of the value NMSE _KLT and the switching factor α is greater than the first indicator NMSE _DFT, the first CSI mode is selected as the target CSI mode. And, if the adjusted second indicator is less than or equal to the first indicator NMSE _DFT, the second CSI mode is selected as the target CSI mode.

Generally, the first CSI mode corresponding to DFT based CSI mode utilizes only the current measurement of the channel for CSI feedback, and thus has a higher feedback accuracy; while the second CSI mode corresponding to the KLT based CSI mode utilizes both the previous CSI feedback and the currently measured channel information for CSI feedback, and thus has a significantly lower CSI feedback overhead. According to the example embodiments of the present disclosure, since the switching probabilities of different CSI modes are effectively adjusted by the switching factor α, the payload and system performance of explicit CSI feedback can be flexibly controlled.

In some example embodiments, the first device 110 may transmit the current CSI feedback indicating the target CSI mode to the second device 120. The current CSI feedback may include a CSI mode indicator for indicating the target CSI mode. This aspect will be discussed in details below.

FIG. 3 illustrates a flowchart of a method 300 implemented at a second device according to some other embodiments of the present disclosure. The method 300 can be implemented by the second device 120 as shown in FIG. 1. For the purpose of discussion, the process 300 will be described with reference to FIG. 1.

Upon receiving the current CSI feedback from the first device 110, the second device 120 may determine the target CSI mode from a first part of current CSI feedback for the channel between the first device 110 and the second device 120, at 310. For example, the second device 120 may determine the target CSI mode from the CSI mode indicator included in the first part of current CSI feedback.

At 320, the second device 120 may receive, at least in part based on the target CSI mode, a second part of the current CSI feedback. The CSI parameters included in the two-part CSI structure are listed in Table 1 below.

Table 1-CSI parameters of the CSI feedback

In the above table, in the CSI part 1,

1) the CSI mode indicator is defined to indicate the target CSI mode of the current CSI feedback;

2) the number of non-zero (NZ) linear combination (LC) coefficients K. Typically, a maximum number of NZ LC coefficients K ₀ is defined as an RRC configured parameter, where K ₀≤N _tap×N _p and N _tap≥1 is also a RRC configured parameter. Hence, the number of NZ LC coefficients may be quantized as

bits.

In the CSI part 2:

3) the selection of spatial domain (SD) beams is performed in transmit ports, for example like Rel. 15 Type II CSI, and occupies

bits;

4) the selection of channel taps is transmitted only in a case that the first CSI mode, i.e., DFT based CSI mode is selected, and DFT vectors corresponding to tap indices are selected from DFT matrix with dimension of N _f×N _f. Hence, the indication of the section of a set of taps occupies

bits;

5) a bitmap of LC coefficient occupies different bits depending on the target CSI mode. If the first CSI mode corresponding to the DFT based explicit CSI is selected, the bitmap is defined with N _tap×2L×N _rx bits which indicate the locations and types of LC coefficients (e.g., zero or non-zero LC coefficients) . For example, “1” denotes non-zero coefficient and “0” denotes zero coefficient. There is no need to report the amplitude and phase value of a zero coefficient in terms of bitmap indication. If the second CSI mode corresponding to KLT based explicit CSI is selected is selected, a corresponding bitmap is defined with 2L×N _rx bits.

6) for the strongest LC coefficient, the index of the strongest NZ LC coefficient is signalled using

bits.

7) for LC coefficients, totally K NZ LC coefficients are signalled in terms of amplitude and phase quantization. The strongest LC coefficient may have different quantization bit length and quantization set from the other LC coefficients.

It should be understood that the above CSI parameters are described only for the purpose of illustration, without suggesting any limitation as to the scope of the disclosure, and the CSI feedback may include other necessary CSI parameters for reconstructing the channel information.

In some example embodiments, in a case of a determination of the target CSI mode being the first CSI mode, the second device 120 may obtain at least a bitmap of a first transformed matrix H _TD1 and tap indices of at least part of taps associated with a channel matrix for characterizing the channel. In this example, the first transformed matrix H _TD1 is determined based on at least a part of the current CSI feedback, such as the CSI mode indicator, the number of NZ LC coefficients K, the bitmap of LC coefficients, the strongest NZ LC coefficient, and amplitude and phase quantization of NZ LC coefficients. The bitmap indicates locations and types of LC coefficients in the first transformed matrix H _TD1, and a size of the bitmap is associated with the number of the at least part of taps and the number of receive ports in the first device 110 and the number of transmit ports in the second device 120.

In some example embodiments, in a case of a determination of the target CSI mode being the second CSI mode, the second device 120 may obtain at least the bitmap of a second transformed matrix H _TD2. In this example, the second transformed matrix H _TD2 is determined based on at least a part of the current CSI feedback, and the bitmap indicates locations and types of LC coefficients in the second transformed matrix H _TD2, and a size of the bitmap is associated with the number of receive ports in the first device 110 and the number of transmit ports in the second device 120.

In some example embodiments, the second device 120 may determine a first basis matrix

based on the tap indices of at least part of taps included in second part of the current CSI feedback and a set of vectors shared by the first device 110 and the second device 120. The second device 120 may reconstruct the channel matrix, that is, the estimated channel matrix

at least in part based on the first basis matrix

and the first transformed matrix

for example by the formula

In some example embodiments, the second device 120 may determine the second basis matrix

based on previous CSI feedback, and then reconstruct the channel matrix, that is the estimated channel matrix

at least in part based on the second basis matrix

and the second transformed matrix

for example by

According to the example embodiments of the present disclosure, the first CSI mode may utilize a DFT operation, and the second CSI mode may utilize a Karhunen-Loeve transform-based operation.

Statistics and comparison for the payload size of different CSI modes

In this section, respective payload sizes of different CSI modes, such as DFT based explicit CSI, KLT based explicit CSI and dynamic explicit CSI, Rel-16 Type II CSI and Rel-15 Type II CSI are discussed and compared. It is to be understood that the above CSI modes are discussed with respect to simulations below for purpose of illustrations, rather than limitation. The parameters or configuration can be changed according to different requirements for the simulations. For example, in some of the simulations discussed below, parameters in CSI part 1 are preset as default values, which may be determined according to system requirements, experimental values, and so on. Additionally, it is to be understood that the bitmap can be also set as a predefined bitmap for purpose of simulation.

Now assume that

● The number of transmit ports is 16 with (N ₁, N ₂) = (4, 2) and oversampling (O ₁, O ₂) = (4, 4) ;

● The number of receive ports is 2;

● The number of spatial domain (SD) beam is L=4 for a polarization; and

● The number of active subcarriers is N _f=600.

For sake of simplicity, assuming that the number of NZ LC coefficients equals to the total number of LC coefficients, N _tap×2L×N _rx, and thus the bitmap indication as well as the CSI part 1 are not considered in the statistic of payload below for explicit CSI.

Payload size of DFT based CSI mode

The selection of SD beams is performed in transmit ports like Rel-15 Type II CSI. Hence, the selection of SD beams is signaled by using

bits. Assuming that the number of the dominant taps is N _tap=5, and DFT matrix has a dimension of 600 x 600. Hence, the indication of the selection of a set of taps occupies

bits.

After the selection of SD beams and the channel taps, LC coefficients have a total number of N _tap×2L×2=80. The strongest one of all the LC coefficients is signaled using

bits for indicating the location and 4 bits for phase/amplitude quantization respectively. Other LC coefficients are first normalized by the strongest coefficient, and then quantized separately in terms of 4-bit phase and 3-bit amplitude. The detailed statistic of payload is listed in Table 2 below.

Table 2 –statistic of the payload for DFT based CSI mode

Payload size of KLT based CSI mode

According to KLT operation, there is only one channel tap for each channel path, and thus the total number of LC coefficients is 1×2L×2=16. The strongest one of all the LC coefficients is signaled by using

bits for location indication and 4 bits for phase/amplitude quantization respectively. Other LC coefficients are first normalized by the strongest coefficient, and then quantized separately in terms of 4-bit phase and 3-bit amplitude. The sparsifying basis vectors, for example, the second basis vectors, have no need to be reported since they can be obtained according to the previous CSI feedback information and have been known in both the first device 110 and the second device 120 side. The detailed statistics of the payload for KLT based CSI mode is listed in Table 3.

Table 3 -statistic of payload for KLT based CSI mode

Payload size of dynamic explicit CSI

For dynamic explicit CSI, the payload size for each CSI mode is calculated separately as above, and then according to the average switching ratio statistic, i.e. γ for KLT based CSI mode selection probability, the average payload size is determined as (1-γ) ×619+γ×128 for dynamic explicit CSI feedback.

Payload size of Rel-16 Type II CSI

For Rel-16 Type II CSI, assuming that:

· The number of configured PMI sub-bands is N ₃=13;

· The number of layers is RI=2;

· The number of FD basis components is M=8;

· The total number of non-zero (NZ) LC coefficients across layers is K _NZ=36;

· Reference amplitude quantization is 4 bits for strongest FD coefficient in the weak polarization;

· Differential amplitude quantization is 3 bits for rest of FD coefficients;

· Phase quantization of LC coefficients is 3 bits.

The statistic of the payload for Rel-16 CSI is listed in Table 4.

Table 4 -statistic of payload for Rel-16 Type II CSI

Payload size of Rel-15 Type II CSI

For Rel-15 Type II CSI, assuming that:

· The number of configured PMI sub-bands is N ₃=13;

· The number of layers is RI=2;

· Wideband amplitude quantization of LC coefficients is 3 bits;

· Sub-band differential amplitude quantization of LC coefficients is 1 bits;

· Sub-band phase quantization of LC coefficients is 3 bits;

The statistic of payload for Rel-15 CSI is listed in Table 5.

Table 5 -statistic of payload for Rel-15 Type II CSI

Table 6 shows a comparison of respective total payload of the above CSI schemes.

Table 6 -comparison of the payload size for different CSI schemes

CSI schemes	Total payload (bit)
Rel-15 Type II CSI	787
Rel-16 Type II CSI	377
DFT based explicit CSI	619
Dynamic explicit CSI	(1-γ) ×619+γ×128

Comparison for System performance

For performance evaluation of the dynamic explicit CSI scheme provided according to the example embodiments of the present disclosure, the full buffer system level evaluations are carried out in LTE 3D UMA scenario. The results are provided for 16 transmit ports with (N1, N2) = (4, 2) in the horizontal and vertical dimension respectively. The relevant simulation parameters are listed in Table 7 below. Rel-15 and Rel-16 Type II CSI is used as performance reference. The simulation results are shown in Table 8.

Table 7 -Simulation assumptions for system level evaluation

Table 8 -System level evaluation of different CSI schemes

As shown in Table 8, if the switching factor α is set to 1.0, only 18%of CSI modes are selected as KLT based explicit CSI, and thus dynamic explicit CSI has very similar system performance to DFT based explicit CSI without mode switching and has only 14%payload reduction.

If the switching factor α is set to 0.9, 62%of CSI modes are switched as KLT based explicit CSI exploiting the preceding CSI feedback information, and thus dynamic explicit CSI has significantly reduced feedback overhead by 49%compared with fixed DFT based CSI mode while has only limited performance loss. Comparing with Rel. 16 CSI, dynamic explicit CSI still has 16%of payload saving capability and above 10%gain of cell average throughput. Therefore, adjusting the switching factor α can realize the tradeoff between system performance and feedback payload for dynamic explicit CSI feedback, and it provides an effective implementation method for explicit CSI in Rel-17 NR MIMO.

Reference is now made to FIG. 4, which shows a diagram of a signaling flow of the transmission of CSI according to some example embodiments of the present disclosure. The process 400 can be implemented between the first device 110 and the second device 120 as shown in FIG. 1. For the purpose of discussion, the process 400 will be described with reference to FIG. 1.

As shown in FIG. 4, the first device 110 may determine 405 a first indicator of a first CSI mode based on current measurement of a channel between the first device and a second device, and determine 410 a second indicator of a second CSI mode at least in part based on previous CSI feedback. The first device 110 may then select 415, from the first CSI mode and the second CSI mode, a target CSI mode for current CSI feedback based on the first indicator and the second indicator. The first device 110 may transmit 420, to the second device 120, the current CSI feedback indicating the target CSI mode. The current CSI feedback also includes other CSI parameters, such as, the number of NZ LC coefficients K, the selection of SD beams, the selection of channel taps, the bitmap of LC coefficients and so on.

The second device 120, upon receiving the current CSI feedback, may determine 425 the target CSI mode from a first part of current CSI feedback. The second device 120 may receive 430, at least in part based on the target CSI mode, a second part of the current CSI feedback. Then, the second device 120 may reconstruct 435 the channel matrix for charactering the channel.

In some embodiments, an apparatus capable of performing any of the method 200 (for example, the first device 110) may comprise means for performing the respective steps of the method 200. The means may be implemented in any suitable form. For example, the means may be implemented in a circuitry or software module.

In some embodiments, the apparatus comprises means for determining a first indicator of a first CSI mode based on current measurement of a channel between the first device and a second device; means for determining a second indicator of a second CSI mode at least in part based on previous CSI feedback; and means for selecting, from the first CSI mode and the second CSI mode, a target CSI mode for current CSI feedback based on the first indicator and the second indicator CSI parameter set indicating at least the payload of the first portion of the target CSI report.

In some embodiments, the apparatus further comprises means for performing other steps in some embodiments of the method 200. In some embodiments, the means comprises at least one processor; and at least one memory including computer program code, the at least one memory and computer program code configured to, with the at least one processor, cause the performance of the apparatus.

In some embodiments, the apparatus further comprises means for receiving, from the second device, a switching factor for selecting the target CSI mode.

In some embodiments, the apparatus further comprises means for adjusting the second indicator by the switching factor; means for in response to the adjusted second indicator is greater than the first indicator, selecting the first CSI mode as the target CSI mode; and means for in response to the adjusted second indicator is less than or equal to the first indicator, selecting the second CSI mode as the target CSI mode.

In some embodiments, the apparatus further comprises means for generating a channel matrix for characterizing the channel based on the current measurement of the channel; means for determining a first basis matrix indicating tap indices of the at least part of taps of the channel matrix, from a set of vectors shared by the first device and the second device; means for determining a first transformed matrix based on the first basis matrix and the channel matrix; means for determining an estimated channel matrix based on the first transformed matrix and the first basis matrix; and means for determining the first indicator based on a difference between the channel matrix and the estimated channel matrix.

In some embodiments, the apparatus further comprises means for generating a channel matrix for characterizing the channel based on the current measurement of the channel; means for determining a second basis matrix based on the previous CSI feedback; means for determining a second transformed matrix based on the second basis matrix and the channel matrix ; means for determining an estimated channel matrix based on the second transformed matrix and the second basis matrix; and means for determining the second indicator based on a difference between the channel matrix and the estimated channel matrix.

In some embodiments, the apparatus further comprises means for obtaining, from the previous CSI feedback, one or more estimated channel vectors corresponding to one or more channel paths between the first device and the second device; means for determining a corresponding covariance matrix for each of the one or more channel paths based on the corresponding estimated channel vector; and means for determining the second basis matrix based on a corresponding dominant eigenvector of the covariance matrix for each of the one or more channel paths.

In some embodiments, the apparatus further comprises means for obtaining, from the previous CSI feedback, one or more estimated channel vectors corresponding to one or more channel paths between the first device and the second device; and means for determining the second basis matrix by normalizing each of the one or more estimated channel vectors for the corresponding channel path.

In some embodiments, the apparatus further comprises means for transmitting, to the second device, the current CSI feedback indicating the target CSI mode.

In some embodiments, if the target CSI mode is the first CSI mode, the current CSI feedback at least comprises a bitmap of the first transformed matrix and tap indices of at least part of taps associated with the channel matrix, the bitmap indicates locations and types of linear combination coefficients in the first transformed matrix, and a size of the bitmap is associated with the number of the at least part of taps and the number of receive ports in the first device and the number of transmit ports in the second device.

In some embodiments, if the target CSI mode is the second CSI mode, the current CSI feedback at least comprises a bitmap of the second transformed matrix, the bitmap indicates locations and types of linear combination coefficients in the second transformed matrix, and a size of the bitmap is associated with the number of receive ports in the first device and the number of transmit ports in the second device.

In some embodiments, the current CSI feedback comprises a CSI mode indicator for indicating the target CSI mode.

In some embodiments, the first CSI mode utilizes a discrete Fourier transform-based operation.

In some embodiments, the second CSI mode utilizes a Karhunen-Loeve transform-based operation.

In some embodiments, the first device is a terminal device and the second device is a network device.

In some embodiments, an apparatus capable of performing any of the process 400 (for example, the second device 120) may comprise means for performing the respective steps of the process 400. The means may be implemented in any suitable form. For example, the means may be implemented in a circuitry or software module.

In some embodiments, the apparatus comprises: means for determining a target CSI mode from a first part of current CSI feedback for a channel between a first device and the second device, the target CSI mode being selected from a first CSI mode and a second CSI mode for current CSI feedback by the first device; and receiving, at least in part based on the target CSI mode, a second part of the current CSI feedback.

In some embodiments, the apparatus further comprises means for performing other steps in some embodiments of the process 400. In some embodiments, the means comprises at least one processor; and at least one memory including computer program code, the at least one memory and computer program code configured to, with the at least one processor, cause the performance of the apparatus.

In some embodiments, the apparatus further comprises means for in response to determining that the target CSI mode is the first CSI mode, obtaining at least a bitmap of a first transformed matrix and tap indices of at least part of taps associated with a channel matrix for characterizing the channel, wherein the first transformed matrix is determined based on at least a part of the current CSI feedback, and wherein the bitmap indicates locations and types of linear combination coefficients in the first transformed matrix, and a size of the bitmap is associated with the number of the at least part of taps and the number of receive ports in the first device and the number of transmit ports in the second device.

In some embodiments, the apparatus further comprises means for determining a first basis matrix based on the tap indices of at least part of taps included in second part of the current CSI feedback and a set of vectors shared by the first device and the second device; and means for reconstructing the channel matrix at least in part based on the first basis matrix and the bitmap of the first transformed matrix.

In some embodiments, the apparatus further comprises means for in response to determining that the target CSI mode is the second CSI mode, obtaining at least a bitmap of a second transformed matrix, wherein the second transformed matrix is determined based on at least a part of the current CSI feedback, and wherein the bitmap indicates locations and types of linear combination coefficients in the second transformed matrix, and a size of the bitmap is associated with the number of receive ports in the first device and the number of transmit ports in the second device.

In some embodiments, the apparatus further comprises means for determining a second basis matrix based on previous CSI feedback; and means for reconstructing the channel matrix based on the second basis matrix and the second transformed matrix.

Fig. 5 is a simplified block diagram of a device 500 that is suitable for implementing embodiments of the present disclosure. The device 500 may be provided to implement the communication device, for example the first device 110 or the second device 120 as shown in Fig. 1. As shown, the device 500 includes one or more processors 510, one or more memories 520 coupled to the processor 510, and one or more communication modules 540 coupled to the processor 510.

The communication module 540 is for bidirectional communications. The communication module 540 has at least one antenna to facilitate communication. The communication interface may represent any interface that is necessary for communication with other network elements.

The processor 510 may be of any type suitable to the local technical network and may include one or more of the following: general purpose computers, special purpose computers, microprocessors, digital signal processors (DSPs) and processors based on multicore processor architecture, as non-limiting examples. The device 500 may have multiple processors, such as an application specific integrated circuit chip that is slaved in time to a clock which synchronizes the main processor.

The memory 520 may include one or more non-volatile memories and one or more volatile memories. Examples of the non-volatile memories include, but are not limited to, a Read Only Memory (ROM) 524, an electrically programmable read only memory (EPROM) , a flash memory, a hard disk, a compact disc (CD) , a digital video disk (DVD) , and other magnetic storage and/or optical storage. Examples of the volatile memories include, but are not limited to, a random access memory (RAM) 522 and other volatile memories that will not last in the power-down duration.

A computer program 530 includes computer executable instructions that are executed by the associated processor 510. The program 530 may be stored in the ROM 524. The processor 510 may perform any suitable actions and processing by loading the program 530 into the RAM 522.

The embodiments of the present disclosure may be implemented by means of the program 530 so that the device 500 may perform any process of the disclosure as discussed with reference to Figs. 2 and 3. The embodiments of the present disclosure may also be implemented by hardware or by a combination of software and hardware.

In some embodiments, the program 530 may be tangibly contained in a computer readable medium which may be included in the device 500 (such as in the memory 520) or other storage devices that are accessible by the device 500. The device 500 may load the program 530 from the computer readable medium to the RAM 522 for execution. The computer readable medium may include any types of tangible non-volatile storage, such as ROM, EPROM, a flash memory, a hard disk, CD, DVD, and the like. Fig. 6 shows an example of the computer readable medium 600 in form of CD or DVD. The computer readable medium has the program 530 stored thereon.

Generally, various embodiments of the present disclosure may be implemented in hardware or special purpose circuits, software, logic or any combination thereof. Some aspects may be implemented in hardware, while other aspects may be implemented in firmware or software which may be executed by a controller, microprocessor or other computing device. While various aspects of embodiments of the present disclosure are illustrated and described as block diagrams, flowcharts, or using some other pictorial representations, it is to be understood that the block, apparatus, system, technique or method described herein may be implemented in, as non-limiting examples, hardware, software, firmware, special purpose circuits or logic, general purpose hardware or controller or other computing devices, or some combination thereof.

The present disclosure also provides at least one computer program product tangibly stored on a non-transitory computer readable storage medium. The computer program product includes computer-executable instructions, such as those included in program modules, being executed in a device on a target real or virtual processor, to carry out the

method

300 or 400 as described above with reference to Figs. 3-4. Generally, program modules include routines, programs, libraries, objects, classes, components, data structures, or the like that perform particular tasks or implement particular abstract data types. The functionality of the program modules may be combined or split between program modules as desired in various embodiments. Machine-executable instructions for program modules may be executed within a local or distributed device. In a distributed device, program modules may be located in both local and remote storage media.

Program code for carrying out methods of the present disclosure may be written in any combination of one or more programming languages. These program codes may be provided to a processor or controller of a general purpose computer, special purpose computer, or other programmable data processing apparatus, such that the program codes, when executed by the processor or controller, cause the functions/operations specified in the flowcharts and/or block diagrams to be implemented. The program code may execute entirely on a machine, partly on the machine, as a stand-alone software package, partly on the machine and partly on a remote machine or entirely on the remote machine or server.

In the context of the present disclosure, the computer program codes or related data may be carried by any suitable carrier to enable the device, apparatus or processor to perform various processes and operations as described above. Examples of the carrier include a signal, computer readable medium, and the like.

The computer readable medium may be a computer readable signal medium or a computer readable storage medium. A computer readable medium may include but not limited to an electronic, magnetic, optical, electromagnetic, infrared, or semiconductor system, apparatus, or device, or any suitable combination of the foregoing. More specific examples of the computer readable storage medium would include an electrical connection having one or more wires, a portable computer diskette, a hard disk, a random access memory (RAM) , a read-only memory (ROM) , an erasable programmable read-only memory (EPROM or Flash memory) , an optical fiber, a portable compact disc read-only memory (CD-ROM) , an optical storage device, a magnetic storage device, or any suitable combination of the foregoing.

Further, while operations are depicted in a particular order, this should not be understood as requiring that such operations be performed in the particular order shown or in sequential order, or that all illustrated operations be performed, to achieve desirable results. In certain circumstances, multitasking and parallel processing may be advantageous. Likewise, while several specific implementation details are contained in the above discussions, these should not be construed as limitations on the scope of the present disclosure, but rather as descriptions of features that may be specific to particular embodiments. Certain features that are described in the context of separate embodiments may also be implemented in combination in a single embodiment. Conversely, various features that are described in the context of a single embodiment may also be implemented in multiple embodiments separately or in any suitable sub-combination.

Although the present disclosure has been described in languages specific to structural features and/or methodological acts, it is to be understood that the present disclosure defined in the appended claims is not necessarily limited to the specific features or acts described above. Rather, the specific features and acts described above are disclosed as example forms of implementing the claims.

Claims

A method, implemented at a first device, comprising:

determining a first indicator of a first CSI mode based on current measurement of a channel between the first device and a second device;

determining a second indicator of a second CSI mode at least in part based on previous CSI feedback; and

selecting, from the first CSI mode and the second CSI mode, a target CSI mode for current CSI feedback based on the first indicator and the second indicator.
The method of claim 1, further comprising:

receiving, from the second device, a switching factor for selecting the target CSI mode.
The method of claim 2, wherein the selecting the target CSI mode comprises:

adjusting the second indicator by the switching factor;

in response to the adjusted second indicator is greater than the first indicator, selecting the first CSI mode as the target CSI mode; and

in response to the adjusted second indicator is less than or equal to the first indicator, selecting the second CSI mode as the target CSI mode.
The method of claim 1, wherein the determining the first indicator of the first CSI mode comprises:

generating a channel matrix for characterizing the channel based on the current measurement of the channel;

determining a first basis matrix indicating tap indices of the at least part of taps of the channel matrix, from a set of vectors shared by the first device and the second device;

determining a first transformed matrix based on the first basis matrix and the channel matrix;

determining an estimated channel matrix based on the first transformed matrix and the first basis matrix; and

determining the first indicator based on a difference between the channel matrix and the estimated channel matrix.
The method of claim 1, wherein the determining the first indicator of the first CSI mode comprises:

generating a channel matrix for characterizing the channel based on the current measurement of the channel;

determining a second basis matrix based on the previous CSI feedback;

determining a second transformed matrix based on the second basis matrix and the channel matrix;

determining an estimated channel matrix based on the second transformed matrix and the second basis matrix; and

determining the second indicator based on a difference between the channel matrix and the estimated channel matrix.
The method of claim 5, wherein the determining the second basis matrix comprises:

obtaining, from the previous CSI feedback, one or more estimated channel vectors corresponding to one or more channel paths between the first device and the second device;

determining a corresponding covariance matrix for each of the one or more channel paths based on the corresponding estimated channel vector; and

determining the second basis matrix based on a corresponding dominant eigenvector of the covariance matrix for each of the one or more channel paths.
The method of claim 5, wherein the determining the second basis matrix comprises:

obtaining, from the previous CSI feedback, one or more estimated channel vectors corresponding to one or more channel paths between the first device and the second device; and

determining the second basis matrix by normalizing each of the one or more estimated channel vectors for the corresponding channel path.
The method of claim 1, further comprising:

transmitting, to the second device, the current CSI feedback indicating the target CSI mode.
The method of claim 8, wherein if the target CSI mode is the first CSI mode, the current CSI feedback at least comprises a bitmap of the first transformed matrix and tap indices of at least part of taps associated with the channel matrix, the bitmap indicates locations and types of linear combination coefficients in the first transformed matrix, and a size of the bitmap is associated with the number of the at least part of taps and the number of receive ports in the first device and the number of transmit ports in the second device.
The method of claim 8, wherein if the target CSI mode is the second CSI mode, the current CSI feedback at least comprises a bitmap of the second transformed matrix, the bitmap indicates locations and types of linear combination coefficients in the second transformed matrix, and a size of the bitmap is associated with the number of receive ports in the first device and the number of transmit ports or spatial-domain beams in the second device.
The method of claim 1, wherein the current CSI feedback comprises a CSI mode indicator for indicating the target CSI mode.
The method of claim 1, wherein the first CSI mode utilizes a discrete Fourier transform-based operation, and/or the second CSI mode utilizes a Karhunen-Loeve transform-based operation.
The method of any of claims 1-12, wherein the first device is a terminal device and the second device is a network device.
A method, implemented at a second device, comprising:

determining a target CSI mode from a first part of current CSI feedback for a channel between a first device and the second device, the target CSI mode being selected from a first CSI mode and a second CSI mode for current CSI feedback by the first device; and

receiving, at least in part based on the target CSI mode, a second part of the current CSI feedback.
The method of claim 14, further comprising:

in response to determining that the target CSI mode is the first CSI mode, obtaining at least a bitmap of a first transformed matrix and tap indices of at least part of taps associated with a channel matrix for characterizing the channel, wherein the first transformed matrix is determined based on at least a part of the current CSI feedback, and

wherein the bitmap indicates locations and types of linear combination coefficients in the first transformed matrix, and a size of the bitmap is associated with the number of the at least part of taps and the number of receive ports in the first device and the number of transmit ports in the second device.
The method of claim 15, further comprising:

determining a first basis matrix based on the tap indices of at least part of taps included in second part of the current CSI feedback and a set of vectors shared by the first device and the second device; and

reconstructing the channel matrix at least in part based on the first basis matrix and the first transformed matrix.
The method of claim 14, further comprising:

in response to determining that the target CSI mode is the second CSI mode, obtaining at least a bitmap of a second transformed matrix, wherein the second transformed matrix is determined based on at least a part of the current CSI feedback, and

wherein the bitmap indicates locations and types of linear combination coefficients in the second transformed matrix, and a size of the bitmap is associated with the number of receive ports in the first device and the number of transmit ports in the second device.
The method of claim 17, further comprising:

determining a second basis matrix based on previous CSI feedback; and

reconstructing the channel matrix based on the second basis matrix and the second transformed matrix.
The method of claim 14, wherein the first CSI mode utilizes a discrete Fourier transform-based operation, and/or the second CSI mode utilizes a Karhunen-Loeve transform-based operation.
The method of any of claims 14-19, wherein the first device is a terminal device and the second device is a network device.
A first device comprising:

at least one processor; and

at least one memory including computer program codes;

the at least one memory and the computer program codes are configured to, with the at least one processor, cause the first device to perform the method according to any of claims 1 to 13.
A second device comprising:

at least one processor; and

at least one memory including computer program codes;

the at least one memory and the computer program codes are configured to, with the at least one processor, cause the first device to perform the method according to any of claims 14 to 20.
An apparatus comprising:

means for determining a first indicator of a first CSI mode based on current measurement of a channel between the first device and a second device;

means for determining a second indicator of a second CSI mode based on previous CSI feedback; and

means for selecting, from the first CSI mode and the second CSI mode, a target CSI mode for current CSI feedback based on the first indicator and the second indicator.
An apparatus comprising:

means for determining a target CSI mode from a first part of current CSI feedback for a channel between a first device and the second device, the target mode being selected from a first CSI mode and a second CSI mode for CSI feedback; and

means for receiving, at least in part based on the target mode, a second part of the current CSI feedback including at least one CSI parameter.
A computer readable storage medium comprising program instructions stored thereon, the instructions, when executed by a processor of a device, causing the device to perform the method of any of claims 1-13 and 14-20.