Patent Description:
<CIT> discloses a method for a channel state information (CSI) feedback, comprising receiving CSI feedback configuration information for the CSI feedback including a spatial channel information indicator based on a linear combination (LC) codebook, deriving the spatial channel information indicator using the LC codebook that indicates a weighted linear combination of a plurality of basis vectors or a plurality of basis matrices, and transmitting over an uplink channel, the CSI feedback including the spatial channel information indicator.

Samsung: "Outcome of offline session for CSI enhancement for MU-MIMO support" R1-<NUM> discusses various aspects on Type II rank <NUM>-<NUM> overhead reduction, (if applicable) each with alternatives for down selection in later RANI meetings.

Samsung: "Summary of CSI enhancement for MU-MIMO support" R1-<NUM> discusses Type II overhead reduction for rank <NUM>-<NUM>.

Intel Corporation: "Discussion on Type II CSI compression" R1-<NUM> discusses Type II overhead reduction (rank <NUM>, <NUM>) and codebook design for rank <NUM>-<NUM> PMI reporting with Type II-like PMI structure based on linear combination of spatial DFT beams.

In accordance with the present invention, there is provided a method of wireless communication performed by a user equipment (UE) as set out in claim <NUM>, a method of wireless communication performed by a base station as set out in claim <NUM>, and apparatuses for performing those methods in claims <NUM> and <NUM><NUM>. Other aspects of the invention can be found in the dependent claims.

Controller/processor <NUM> of base station <NUM>, controller/processor <NUM> of UE <NUM>, and/or any other component(s) of <FIG> may perform one or more techniques associated with Type II feedback for channel state information, as described in more detail elsewhere herein. For example, controller/processor <NUM> of base station <NUM>, controller/processor <NUM> of UE <NUM>, and/or any other component(s) of <FIG> may perform or direct operations of, for example, process <NUM> of <FIG>, process <NUM> of <FIG>, and/or other processes as described herein. Memories <NUM> and <NUM> may store data and program codes for base station <NUM> and UE <NUM>, respectively.

In some aspects, UE <NUM> may include means for performing CSI measurements on one or more reference signal transmissions from a base station; means for determining, based at least in part on the CSI measurements, a set of non-zero linear combination complex coefficients for weighting and co-phasing for a linear combination of a plurality of frequency domain basis vectors and a plurality of spatial domain basis vectors; means for transmitting a CSI feedback, wherein a first part of the CSI feedback includes at least an indication of a number of the set of non-zero linear combination complex coefficients with regard to all layers of a communication link between the UE and the base station; and/or the like. In some aspects, such means may include one or more components of UE <NUM> described in connection with <FIG>.

In some aspects, base station <NUM> may include means for receiving CSI feedback for a communication link with a UE, wherein a first part of the CSI feedback includes at least an indication of a number of a set of non-zero linear combination complex coefficients with regard to all layers of the communication link, wherein the set of non-zero linear combination complex coefficients is for weighting and co-phasing for a linear combination of a plurality of frequency domain basis vectors and a plurality of spatial domain basis vectors; means for performing a communication on the communication link based at least in part on the CSI feedback; and/or the like. In some aspects, such means may include one or more components of base station <NUM> described in connection with <FIG>.

A BS (e.g., BS <NUM>) may transmit a plurality of beams. For example, the BS may generate the beams using an antenna panel that generates beams at a spatial and/or phase displacement from each other. The BS and a corresponding UE may select a set of beams that are to be used for communication between the BS and the UE. For example, the set of beams transmitted from the BS to the UE may be referred to herein as a communication link. The communication link between the BS and the UE may propagate in a medium and/or through various geometric paths, which are collectively referred to herein as a channel between the BS and the UE.

In some aspects, the UE may select a set of beams for communication with the BS. For example, the UE may select the set of beams based at least in part on the set of beams being associated with favorable characteristics (e.g., a satisfactory receive power, a satisfactory signal to interference plus noise (SINR) value, and/or the like). The UE may generate a codeword using a codebook and based at least in part on performing channel estimation of the channel between the UE and the BS. The codeword may indicate the set of beams and parameters to be used.

One such codebook is the Type II codebook, prescribed in <NUM>/NR. The Type II codebook may use a two-stage procedure to generate the codeword: a first stage wherein the set of beams is selected for a wideband of the communication link (e.g., sometimes referred to herein as W1), and a second stage wherein linear combination is performed, for a set of subbands, using the set of beams for each set of subbands. The codeword may be based at least in part on the linear combination, and may indicate the set of beams and/or respective amplitudes, phase coefficients, and/or the like. Thus, the UE may provide an indication of channel state at the UE and may request the set of beams to be used for the UE. The Type II codebook may provide more precise specification of the channel state than a Type I codebook, which may provide a predefined codeword-based approach to specifying selected beams. Thus, the Type II codebook may be referred to as a high resolution codebook in comparison to the Type I codebook. The Type II codebook may improve multi-user multiple input multiple output (MU-MIMO) performance on the communication link.

For one type of Type II codebook (e.g., the codebook specified in Release <NUM> of the 3GPP standard for <NUM>/NR), the precoder of the codebook is based at least in part on a linear combination of discrete Fourier transform (DFT) beams. The linear combination codebook may define the precoder W as W = W<NUM>W<NUM> , wherein the spatial domain compression matrix <MAT>, wherein <MAT> are L spatial domain basis vectors of dimension N<NUM>N<NUM> × <NUM> (mapped to the two polarizations, so <NUM>L in total), P = <NUM>N<NUM>N<NUM> indicates a number of dimensions (sometimes represented as D), and the combination coefficient matrix W<NUM> is composed of K = <NUM>Lv linear combination coefficients, where v indicates a total number of layers. Each column in W<NUM> indicates the linear combination of complex coefficients (i.e., amplitude and phase) for one layer, wherein the amplitude coefficient is given by <MAT> for l = <NUM>,. , v - <NUM> and <MAT> and <MAT> are the wideband and subband coefficients, respectively. The phase coefficient is given by <MAT> for l = <NUM>,. , v - <NUM> and ci is one of the <NUM> phase shift keying (8PSK) or the quadrature phase shift keying (QPSK) constellation points.

The UE may report the above values and/or other values associated with channel estimation using channel state information (CSI) feedback. CSI feedback for the Type II codebook may include two parts: a first part, sometimes referred to as CSI Part I, and a second part, sometimes referred to as CSI Part II. In some cases, the first part may have a smaller payload than the second part, and/or may have a fixed payload. For example, the first part may have a payload size of less than approximately <NUM> bits, whereas the second part may have a variable payload size that may be dependent on the first part. In some cases, the second part may have a payload size of approximately <NUM> bits to <NUM> bits, although other values may be used.

In some cases, the first part may identify one or more of: a rank indicator (RI) (e.g., <NUM> bit to indicate one layer v = <NUM> or two layers v = <NUM> when the configured maximum rank is <NUM>); wideband and subband differential channel quality indicators (CQI), for which a total payload size may be dependent on the number of subbands (e.g., approximately <NUM> + <NUM> × <NUM> = <NUM> bits for <NUM> subbands); an indication of the number of non-zero wideband amplitude coefficients Ql for each layer; and/or the like. In some cases, the second part may identify one or more of: wideband and/or subband precoding matrix indicators (PMIs) including a spatial basis vector selection indication; wideband and subband amplitude coefficients; subband phase coefficients; and/or the like.

In some cases, the Type II CSI feedback may use a compressed Type II precoder. This may reduce overhead of Type II CSI feedback. The compressed precoder may exploit the sparsity of the spatial domain and/or the frequency domain. For example, an example of a compressed Type II precoder W is given by W = <MAT>, wherein the precoder matrix W has P = <NUM>N<NUM>N<NUM> rows (representing the spatial domain and the number of ports) and N<NUM> columns (wherein N<NUM> is a frequency-domain compression unit of resource blocks or reporting subbands). The W<NUM> matrix, described above, is the spatial basis consisting of L beams per polarization group (hence a total of <NUM>L beams). The W̃<NUM> matrix indicates all of the required linear combination complex coefficients (amplitude and co-phasing), similarly to what is described above. The Wf matrix is composed of the basis vectors used to perform compression in frequency domain, Wf = [f<NUM> f<NUM>. fM-<NUM>], where <MAT> are M size-N<NUM> × <NUM> orthogonal DFT vectors for each spatial basis i = <NUM>,. , <NUM>L - <NUM>. The above Type II CSI feedback may be referred to in some cases as enhanced or modified Type II CSI feedback (e.g., enhanced relative to the approach that does not use basis vectors in the spatial and frequency domains to compress feedback size).

The CSI feedback for this modified Type II CSI feedback may include a spatial domain basis vector selection that is similar to the approach described in connection with the legacy Type II CSI feedback configuration. The CSI feedback may further include a frequency-domain (FD) basis subset selection (wherein M out of a total N<NUM> basis vectors are selected). In some cases, common FD basis vectors for all the <NUM> spatial beams may be used, which is referred to herein as Alternative <NUM>. In these cases, M basis vectors are dynamically selected and reported. The value of M may be configured by the network or reported by the UE. In other cases, independent FD basis vectors may be used for each spatial domain basis vector, with potentially different numbers and/or selections of FD basis vectors for each spatial domain basis vector. The total number of FD basis vectors across all the <NUM>L spatial beams may be configured.

The modified Type II CSI feedback may further include the FD coefficients (e.g., amplitude and phase) in W̃<NUM>. For Alternative <NUM>, which is the common FD basis vector subset selection, the modified Type II CSI feedback may report only a subset K<NUM> < K = <NUM>LM of the coefficients. For Alternative <NUM>, which is the independent basis subset selection, the modified Type II CSI feedback may report <MAT> amplitude and phase coefficients, wherein Mi is the number of FD basis vectors associated with one spatial beam.

A variety of quantization and reporting options may be used, two examples of which are provided below. As a first example, for each of the K or K<NUM> FD coefficients, the modified Type II CSI feedback may use <NUM>-bit amplitude and QPSK or 8PSK phase. As a second example, the modified Type II CSI feedback may report a <NUM>-bit wideband amplitude for each beam or spatial domain basis vector, a <NUM>-bit or <NUM>-bit differential amplitude for each FD coefficient, and a QPSK or 8PSK phase.

It may be desirable to reduce the overhead associated with the modified Type II CSI feedback. One way to achieve this is to report the number of non-zero FD complex coefficients of W̃<NUM> in the first part of the CSI feedback. This may be particularly beneficial when a larger number of complex coefficients (i.e., K = <NUM>LM) is configured. Another way to achieve this may be to provide an indication of the number of beams or spatial domain basis vectors with at least one non-zero FD complex coefficient. However, the number of non-zero FD complex coefficients can be different in each layer of the communication link, and the number of beams with at least one non-zero FD complex coefficient can be layer-specific. Because of these concerns, it may be undesirable to report these values individually for these layers (referred to as per-layer reporting), since CSI Part <NUM> may be increased to an undesired or unsustainable level. As an example, for rank <NUM>, K = <NUM>, and L = <NUM>, per layer reporting may require (<NUM> + <NUM>) * <NUM> = <NUM> bits (i.e., <NUM> bits for the number of non-zero FD complex coefficients and <NUM> bits for the number of beams with non-zero FD coefficients), which represents a <NUM>% increase to the size of CSI Part <NUM>.

Some techniques and apparatuses described herein provide joint reporting of a total number of non-zero FD complex coefficients (e.g., Ktotal) across all layers of the communication link. Similarly, if the number of beams with non-zero FD complex coefficients (e.g., Qtotal) is to be reported, then the number of beams may be reported jointly across all layers. This may reduce CSI Part <NUM> overhead relative to per-layer reporting. More particularly, for rank <NUM>, K = <NUM>, and L = <NUM>, the joint reporting may use a total of <NUM> + <NUM> = <NUM> bits (i.e., log<NUM> (<NUM> × <NUM>) = <NUM> bits for Qtotal and log<NUM> (<NUM> × <NUM>) = <NUM> bits for Ktotal). In this example, <NUM> - <NUM> = <NUM> bits are saved compared to per-layer reporting.

Furthermore, some techniques and apparatuses described herein provide constrained FD basis vector subset selection and reporting in CSI Part <NUM> (e.g., based at least in part on an adjacency-based approach, described in more detail elsewhere herein), which further reduces CSI feedback overhead. In some aspects, techniques and apparatuses described herein provide per-layer reporting of the number of non-zero FD complex coefficients in CSI Part <NUM> based at least in part on the rank that is reported in CSI Part <NUM>. When the rank is <NUM> (e.g., a rank indicator of <NUM>), positions of the non-zero FD complex coefficients for all layers and values of the coefficients may be reported for all layers. When the rank is greater than <NUM> (e.g., a rank indicator of <NUM> or more), a number of non-zero FD coefficients for a subset of layers may be reported, thereby further reducing overhead. In the above cases, when the beam selection is to be reported, non-zero beam selections may be reported for all layers (e.g., as opposed to individually), which even further reduces overhead. In this way, signaling resources of the communication link are conserved and communication performance between the UE and the BS is improved based at least in part on increased efficiency in signaling the Type II CSI feedback.

By reducing signaling overhead of the Type II CSI feedback, efficiency of the communication link may be improved and radio resources of the UE and the BS may be conserved. This, in turn, may allow for the signaling of more robust CSI feedback (e.g., with higher specificity about the coefficients to be used for the beams) than would otherwise be practical with less efficient signaling overhead.

<FIG> is a diagram illustrating an example <NUM> of modified Type II CSI feedback in accordance with various aspects of the present disclosure. As shown, example <NUM> includes a UE <NUM> and a BS <NUM> that are associated with a communication link. As further shown, the communication link may be associated with a channel. For example, the communication link may be referred to as the channel, or may propagate via the channel.

As shown in <FIG>, and by reference number <NUM>, the BS <NUM> may transmit reference signal transmissions to the UE <NUM>. The reference signal transmissions may include, for example, a CSI reference signal, a demodulation reference signal, and/or the like. As shown by reference number <NUM>, the UE <NUM> may perform CSI measurements on the reference signal transmissions to determine CSI feedback, as described below.

As shown by reference number <NUM>, the UE <NUM> may determine a set of non-zero FD complex coefficients based at least in part on the CSI measurements. For example, the UE <NUM> may determine CSI feedback based at least in part on the CSI measurements. A more detailed description of the determination of the CSI feedback and the corresponding reporting of the CSI feedback is provided in connection with the description of the CSI feedback below. A non-zero FD complex coefficient is also referred to as a non-zero linear combination complex coefficient herein.

As shown by reference number <NUM>, the UE may transmit CSI feedback based at least in part on the set of non-zero FD complex coefficients. The CSI feedback is described in more detail in connection with reference numbers <NUM> through <NUM> (e.g., the first part of the CSI feedback) and reference numbers <NUM> through <NUM> (e.g., the second part of the CSI feedback).

As shown by reference number <NUM>, the first part may identify a reported rank of the communication link. For example, the first part may identify a rank indicator (RI), which may indicate that the communication link is associated with one layer (e.g., v = <NUM>), two layers (e.g., v = <NUM>), or more layers.

As shown by reference number <NUM>, the first part may identify a number of non-zero FD complex coefficients of the set of non-zero FD complex coefficients. For example, the first part may identify a total number of non-zero FD complex coefficients (e.g., Ktotal) across all layers of the communication link. In some aspects, Ktotal may be in a range of <NUM> to K*vmax (inclusive), wherein vmax indicates a maximum number of allowed layers and K is a maximum number of the non-zero FD complex coefficients for one layer. K may be defined as the minimum of (Kc, 2LM), wherein Kc is a configured value (e.g., configured by a higher layer of UE <NUM> or BS <NUM>) and M is the number of FD basis vectors. By using vmax instead of the reported rank, a fixed payload size or bitwidth (e.g., independent of the RI) for CSI Part <NUM> is ensured. In some aspects, the bitwidth for reporting Ktotal may be fixed (e.g., based at least in part on the configured maximum rank). In some aspects, the bitwidth of Ktotal may be based at least in part on the reported rank and may be padded to a fixed width.

As shown by reference number <NUM>, the first part may identify a number of beams or spatial domain basis vectors with non-zero FD complex coefficients across all layers of the communication link. For example, the first part may identify Qtotai, wherein Qtotai is between <NUM> and <NUM>vmax (inclusive). The dotted line around the block indicated by reference number <NUM> indicates that this operation is optional. In some aspects, the UE may selectively provide the information identifying the number of beams with non-zero FD complex coefficients based at least in part on a size of Qtotal. For example, Qtotal may only reduce overhead when Qtotal is lower than a threshold, as described in more detail elsewhere herein. Therefore, the UE <NUM> may provide information identifying Qtotal only when Qtotal is lower than the threshold.

As shown by reference number <NUM>, the second part may identify a per-layer number of non-zero FD complex coefficients based at least in part on the reported rank. In some aspects, information indicating the per-layer number of non-zero FD complex coefficients may relate to multiple, different layers. For example, if the reported rank is larger than one (e.g., v > <NUM>), then the number of the non-zero FD coefficients for the first v - <NUM> layers may be reported. For example, the UE <NUM> may report the number of non-zero FD complex coefficients as Kl, l = <NUM>,<NUM>,. , v - <NUM> with <NUM> ≤ Kl ≤ K. Thus, the UE <NUM> may conserve signaling resources that would be used to report the non-zero FD complex coefficients for the v-th layer, since this value can be determined from Ktotal in CSI Part <NUM> and the values of the other layers.

As shown by reference number <NUM>, the second part may identify a selected FD basis vector subset. For example, the second part may identify MFD basis vector subset. In some aspects, the UE <NUM> may select M from the set of all FD basis vectors N<NUM>. For example, the UE <NUM> may perform an unconstrained selection of M. In such a case, the UE <NUM> may report a combination index with the value range <MAT>.

In some aspects, the UE <NUM> may perform a constrained selection of M FD basis vector subset, which conserves processing resources used to select M and enables a reduction in size of CSI Part <NUM>. For example, the UE <NUM> may select M based at least in part on a set of adjacent or almost-adjacent basis vectors. More particularly, the UE <NUM> may determine and report a first index i<NUM> from N<NUM> (e.g., <NUM> ≤ i<NUM> < N<NUM>) as a first basis vector of M. The UE <NUM> may determine and report a second index for selection of the remaining M - <NUM> basis vectors from a subset of adjacent N' basis vectors, <NUM> ≤ i<NUM> < <MAT> where N' < N<NUM> is either configured or predefined (e.g., fixed in the standards) based at least in part on N<NUM> and the subband size of the CSI feedback. The index of the N' basis vectors is given by mod(i<NUM> + d, N<NUM>) for d = <NUM>,<NUM>,. , N' - <NUM>. The constrained selection may have less complexity than the unconstrained selection, particularly for larger values of N<NUM>. For example, the FD complex coefficient may be equivalent to the tap coefficient of the channel time domain response for which a very large delay spread for each spatial domain beam may not be common, i.e. unconstrained selection of M.

As shown by reference number <NUM>, the second part may identify a per-layer or joint reporting of locations or positions of non-zero FD complex coefficients. For example, the CSI feedback may identify the positions or selections of the coefficients of Ktotal corresponding to the <NUM> spatial domain basis vectors and the MFD basis vectors. In some aspects, the UE <NUM> may determine and report positions of the non-zero FD complex coefficients for each layer using a <NUM>LM × v bit bitmap (e.g., <NUM>LM bits bitmap for each of a total v layers), which may reduce complexity in comparison to determining whether to reduce non-zero FD complex coefficient locations. In some aspects, the UE <NUM> may determine and report positions of zero-value coefficients or of non-zero coefficients. For example, the UE <NUM> may determine and jointly report, across all layers, the location of the Ktotal non-zero FD coefficients or the Kv - Ktotal zero-value FD coefficients with <MAT> bits for each coefficient. Thus, the UE <NUM> may reduce the number of bits required to report the locations or positions to the minimum of <MAT> bits where K = min(Kc, 2LM). Then, based at least in part on the reporting of per-layer Kl in CSI Part <NUM>, the mapping of the position of the non-zero FD coefficients to each layer can be determined.

As an example, the down-selection of the two options (e.g., whether to report the non-zero FD complex coefficients (or the zero-value FD complex coefficients) individually per layer or jointly across all the layers) can be based at least in part on the option associated with the smaller payload. For example, the smaller payload may be identified based at least in part on Ktotal and v for a given L and M. As a more particular example, for L = <NUM>, M = <NUM>, K = <NUM>, v = <NUM>:.

As shown by reference number <NUM>, the UE <NUM> may aggregate and report the set of non-zero FD complex coefficients. For example, the UE <NUM> may aggregate and report the amplitude and phase coefficients of the non-zero FD complex coefficients of all layers of the communication link according to layer index. This may enable a recipient (e.g., the BS <NUM>) to map the coefficients to each layer based at least in part on the number and position of non-zero FD complex coefficients of each layer and the basis vectors at a reduced CSI feedback size relative to individual reporting of coefficients per layer.

As shown by reference number <NUM>, the second part may identify per-layer reporting of beams or spatial domain basis vectors with non-zero FD complex coefficients. For example, if the total number of spatial beams with non-zero FD complex coefficients across all layers Qtotal is reported in CSI Part <NUM>, then the indication of the position of the non-zero FD coefficients using the individual (per-layer) reporting technique described above can be performed using a bitmap of, for example, Qtotal × M bits. In such a case, CSI Part <NUM> may further indicate the selection of beams with non-zero FD coefficients for each layer, which may use a total <NUM>L × v bits bitmap (e.g., <NUM>L bits bitmap for each of a total v layers). Therefore, there may be overhead reduction only when Qtotal is not too large. For example, L = <NUM>, M = <NUM>, v = <NUM>, <NUM> ≤ Qtotal ≤ <NUM>, overhead is reduced only for Qtotal < <NUM>, corresponding to <NUM>% beams with non-zero FD coefficients. Thus, the UE <NUM> may selectively determine whether Qtotal is to be reported, and if the per-layer reporting of beams is to be performed as indicated by reference number <NUM>, based at least in part on whether the overhead of CSI Part <NUM> is reduced by reporting Qtotal.

As shown by reference number <NUM>, the BS <NUM> (and/or the UE <NUM>) may perform communication on the communication link based at least in part on the CSI feedback. For example, the BS <NUM> may generate one or more beamforming for the UE <NUM> using the phase and amplitude FD coefficients, one or more spatial domain basis vectors, one or more frequency domain basis vectors, and/or other information included in the CSI feedback. In this way, the UE <NUM> and the BS <NUM> reduce overhead associated with CSI feedback by signaling information identifying at least a number of non-zero FD coefficients across all layers of the communication link between the UE <NUM> and the BS <NUM>, thereby conserving computing resources.

<FIG> is a diagram illustrating an example process <NUM> performed, for example, by a UE, in accordance with various aspects of the present disclosure. Example process <NUM> is an example where a UE (e.g., UE <NUM>) performs CSI feedback identifying at least a number of non-zero FD coefficients across all layers of a communication link.

As shown in <FIG>, in some aspects, process <NUM> may include performing channel state information (CSI) measurements on one or more reference signal transmissions from a base station (block <NUM>). For example, the UE (e.g., using antenna <NUM>, DEMOD <NUM>, MIMO detector <NUM>, receive processor <NUM>, controller/processor <NUM>, and/or the like) may perform CSI measurements on one or more reference signal transmissions from a base station (e.g., BS <NUM>), as described elsewhere herein.

As shown in <FIG>, in some aspects, process <NUM> may include determining, based at least in part on the CSI measurements, a set of non-zero linear combination complex coefficients for weighting and co-phasing for a linear combination of a plurality of frequency domain basis vectors and a plurality of spatial domain basis vectors (block <NUM>). For example, the UE (e.g., using controller/processor <NUM> and/or the like) may determine a set of non-zero linear combination complex coefficients. The set of non-zero linear combination complex coefficients may be for weighting and co-phasing for a linear combination of a plurality of frequency domain basis vectors and a plurality of spatial domain basis vectors.

As shown in <FIG>, in some aspects process <NUM> may include transmitting a CSI feedback, wherein a first part of the CSI feedback includes at least an indication of a number of the set of non-zero linear combination complex coefficients with regard to all layers of a communication link between the UE and the base station (block <NUM>). For example, the UE (e.g., using controller/processor <NUM>, transmit processor <NUM>, TX MIMO processor <NUM>, MOD <NUM>, antenna <NUM>, and/or the like) may transmit CSI feedback. A first part of the CSI feedback may include at least an indication of a number of non-zero linear combination complex coefficients, of the set of non-zero linear combination complex coefficients, with regard to all layers of the communication link.

Process <NUM> may include additional aspects, such as any single aspect and/or any combination of aspects described below and/or in connection with one or more other processes described elsewhere herein.

In a first aspect, the indication of the number of the set of non-zero linear combination complex coefficients has a fixed bitwidth based at least in part on a configured maximum number of layers of the communication link.

In a second aspect, alone or in combination with the first aspect, a bitwidth of the indication is independent of a rank indicator of the CSI feedback. In a third aspect, alone or in combination with one or more of the first and second aspects, if a number of layers of the CSI feedback is equal to v and is greater than <NUM>, a second part of the CSI feedback indicates how many non-zero linear combination complex coefficients are included in a set of v minus <NUM> layers of the communication link. In a fourth aspect, alone or in combination with one or more of the first through third aspects, a second part of the CSI feedback includes an index identifying a selected subset of frequency domain basis vectors from a set of potential frequency domain basis vectors. In a fifth aspect, alone or in combination with one or more of the first through fourth aspects, the index is a combination index with a value between zero and a set of all basis vectors of the frequency domain basis vectors.

In a sixth aspect, alone or in combination with one or more of the first through fifth aspects, a second part of the CSI feedback includes: a first index identifying a particular frequency domain basis vector of a selected subset of frequency domain basis vectors, and a second index identifying remaining frequency domain basis vectors of the selected subset of frequency domain basis vectors. In a seventh aspect, alone or in combination with one or more of the first through sixth aspects, the remaining frequency domain basis vectors are selected from a subset of a set of potential frequency domain basis vectors, and the subset of the set of potential frequency domain basis vectors are adjacent to the particular frequency domain basis vector. In an eighth aspect, alone or in combination with one or more of the first through seventh aspects, a size of the subset of the set of potential frequency domain basis vectors is based at least in part on a subband size of the CSI feedback.

In a ninth aspect, alone or in combination with one or more of the first through eighth aspects, a second part of the CSI feedback identifies positions of the set of non-zero linear combination complex coefficients, or of a set of zero-value linear combination complex coefficients. In a tenth aspect, alone or in combination with one or more of the first through ninth aspects, the positions are identified using a respective bitmap for each layer of the CSI feedback. In an eleventh aspect, alone or in combination with one or more of the first through tenth aspects, the positions of the set of non-zero linear combination complex coefficients, or of the set of zero-value linear combination complex coefficients, are identified by individually reporting the position of each frequency domain complex coefficient of the set of non-zero linear combination complex coefficients or the set of zero-value linear combination complex coefficients.

In a twelfth aspect, alone or in combination with one or more of the first through eleventh aspects, the second part of the CSI feedback identifies the positions based at least in part on a payload size of the CSI feedback when identifying positions of the set of non-zero linear combination complex coefficients in comparison to a payload size of the CSI feedback when identifying positions of the set of zero-value linear combination complex coefficients. In a thirteenth aspect, alone or in combination with one or more of the first through twelfth aspects, the first part of the CSI feedback includes information indicating a number of spatial domain basis vectors, of a set of selected spatial domain basis vectors, that have non-zero linear combination complex coefficients, and the second part of the CSI feedback identifies the positions of the set of non-zero linear combination complex coefficients based at least in part on a bitmap, wherein the bitmap is based at least in part on the number of spatial domain basis vectors that have non-zero linear combination complex coefficients and a number of a set of selected frequency domain basis vectors.

In a fourteenth aspect, alone or in combination with one or more of the first through thirteenth aspects, the second part of the CSI feedback identifies respective subsets of spatial domain basis vectors, of the number of spatial domain basis vectors that have non-zero linear combination complex coefficients, for each layer of the communication link. In a fifteenth aspect, alone or in combination with one or more of the first through fourteenth aspects, a second part of the CSI feedback jointly identifies respective amplitude and phase values of the set of non-zero linear combination complex coefficients for all layers of the communication link.

Although <FIG> shows example blocks of process <NUM>, in some aspects process <NUM> may include additional blocks, fewer blocks, different blocks, or differently arranged blocks than those depicted in <FIG>.

<FIG> is a diagram illustrating an example process <NUM> performed, for example, by a base station, in accordance with various aspects of the present disclosure. Example process <NUM> is an example where a base station (e.g., BS <NUM>) performs CSI feedback identifying at least a number of non-zero FD coefficients across layers of a communication link.

As shown in <FIG>, in some aspects, process <NUM> may include receiving channel state information (CSI) feedback for a communication link with a UE, wherein a first part of the CSI feedback includes at least an indication of a number of a set of non-zero linear combination complex coefficients with regard to all layers of the communication link, wherein the set of non-zero linear combination complex coefficients is for weighting and co-phasing for a linear combination of a plurality of frequency domain basis vectors and a plurality of spatial domain basis vectors (block <NUM>). For example, the base station (e.g., using antenna <NUM>, DEMOD <NUM>, MIMO detector <NUM>, receive processor <NUM>, controller/processor <NUM>, and/or the like) may receive CSI feedback for a communication link (e.g., a channel) with a UE. A first part of the CSI feedback may include at least an indication of a number of a set of non-zero linear combination complex coefficients with regard to all layers of the communication link. In some aspects, the set of non-zero linear combination complex coefficients is for weighting and co-phasing for a linear combination of a plurality of frequency domain basis vectors and a plurality of spatial domain basis vectors.

As shown in <FIG>, in some aspects, process <NUM> may include performing a communication on a communication link between the UE and the base station based at least in part on the CSI feedback (block <NUM>). For example, the base station (e.g., using controller/processor <NUM>, transmit processor <NUM>, TX MIMO processor <NUM>, MOD <NUM>, antenna <NUM>, and/or the like) may perform a communication on the communication link based at least in part on the CSI feedback.

In a first aspect, the indication of the number of the set non-zero linear combination complex coefficients has a fixed bitwidth based at least in part on a configured maximum number of layers of the communication link. In a second aspect, alone or in combination with the first aspect, if a number of layers of the CSI feedback is equal to v and is greater than <NUM>, a second part of the CSI feedback indicates how many non-zero linear combination complex coefficients are included in a set of v minus <NUM> layers of the communication link. In a third aspect, alone or in combination with one or more of the first and second aspects, a second part of the CSI feedback includes an index identifying a selected subset of frequency domain basis vectors from a set of potential frequency domain basis vectors.

In a fourth aspect, alone or in combination with one or more of the first through third aspects, a second part of the CSI feedback includes a first index identifying a particular frequency domain basis vector of a selected subset of frequency domain basis vectors, and a second index identifying remaining frequency domain basis vectors of the selected subset of frequency domain basis vectors.

In a fifth aspect, alone or in combination with one or more of the first through fourth aspects, the remaining frequency domain basis vectors are selected from a subset of a set of potential frequency domain basis vectors, and wherein the subset of the set of potential frequency domain basis vectors are adjacent to the particular frequency domain basis vector. In a sixth aspect, alone or in combination with one or more of the first through fifth aspects, a size of the subset of the set of potential frequency domain basis vectors is based at least in part on a subband size of the CSI feedback. In a seventh aspect, alone or in combination with one or more of the first through sixth aspects, a second part of the CSI feedback identifies positions of the set of non-zero linear combination complex coefficients, or of a set of zero-value linear combination complex coefficients. In an eighth aspect, alone or in combination with one or more of the first through seventh aspects, the positions are identified using a respective bitmap for each layer of the CSI feedback. In a ninth aspect, alone or in combination with one or more of the first through eighth aspects, the positions of the set of non-zero linear combination complex coefficients, or of the set of zero-value linear combination complex coefficients, are identified by individually reporting the position of each frequency domain complex coefficient of the set of non-zero linear combination complex coefficients or the set of zero-value linear combination complex coefficients. In a tenth aspect, alone or in combination with one or more of the first through ninth aspects, the second part of the CSI feedback identifies the positions based at least in part on a payload size of the CSI feedback when identifying positions of the set of non-zero linear combination complex coefficients in comparison to a payload size of the CSI feedback when identifying positions of the set of zero-value linear combination complex coefficients. In an eleventh aspect, alone or in combination with one or more of the first through tenth aspects, the first part of the CSI feedback includes information indicating a number of spatial domain basis vectors, of a set of selected spatial domain basis vectors, that have non-zero linear combination complex coefficients, and the second part of the CSI feedback identifies the positions of the set of non-zero linear combination complex coefficients based at least in part on a bitmap, wherein the bitmap is based at least in part on the number of spatial domain basis vectors that have non-zero linear combination complex coefficients and a number of a set of selected frequency domain basis vectors.

In a twelfth aspect, alone or in combination with one or more of the first through eleventh aspects, the second part of the CSI feedback identifies respective subsets of spatial domain basis vectors, of the number of spatial domain basis vectors that have non-zero linear combination complex coefficients, for each layer of the communication link. In a thirteenth aspect, alone or in combination with one or more of the first through twelfth aspects, a second part of the CSI feedback jointly identifies respective amplitude and phase values of the set of non-zero linear combination complex coefficients for all layers of the communication link.

As used herein, satisfying a threshold may, depending on context, refer to a value being greater than the threshold, greater than or equal to the threshold, less than the threshold, less than or equal to the threshold, equal to the threshold, not equal to the threshold, and/or the like.

Claim 1:
A method of wireless communication performed by a user equipment, UE (<NUM>), comprising:
performing channel state information, CSI, measurements on one or more reference signal transmissions (<NUM>) from a base station (<NUM>);
determining, based at least in part on the CSI measurements, a set of non-zero linear combination complex coefficients for weighting and co-phasing for a linear combination of a plurality of frequency domain basis vectors and a plurality of spatial domain basis vectors (<NUM>); and
transmitting a CSI feedback, wherein a first part of the CSI feedback includes at least an indication of a number of non-zero linear combination complex coefficients of the set of non-zero linear combination complex coefficients with regard to all layers of a communication link (<NUM>) between the UE (<NUM>) and the base station (<NUM>).