Methods and apparatuses for enhancement on basis subset indication for two-codebook based CSI reporting

1. A method performed by a User Equipment (UE), the method comprising:

receiving from a network node, (gNB) a radio signal via a Multiple Input Multiple Output (MIMO) channel, wherein the radio signal contains at least one DownLink (DL) reference signal according to a DL reference signal configuration;

estimating the MIMO channel based on the received at least one DL reference signal for configured subbands;

calculating a precoder matrix or a Channel State Information, (CSI) matrix for a number of antenna ports of the gNB and configured subbands, N 3 , the precoder matrix being based on a first codebook and on a second codebook and a set of combining coefficients for complex scaling/combining one or more vectors selected from the first codebook and the second codebook;

where,

the first codebook contains one or more transmit-side spatial beam components of the precoder matrix or CSI matrix; and

the second codebook contains one or more delay components of the precoder matrix or CSI matrix;

determining a common delay domain (CDD) basis subset of the selected vectors associated with the delay components across all layers of the precoder matrix or CSI matrix, wherein the CDD basis subset is defined by a parameter D representing the number of elements of the CDD basis subset, and a parameter D in representing the first index of the D delay vectors from the second codebook, wherein the value range of the parameter D in is reduced from N 3 to D in values; and

reporting, to the gNB, a CSI feedback report and/or a Precoder matrix Indicator (PMI) and/or a PMI/Rank Indicator (PMI/RI) used to indicate the precoder matrix or the CSI matrix for the configured antenna ports and configured subbands, wherein the selected vectors from the CDD basis subset are indicated in the reporting, for each layer, by a layer-specific delay domain (LDD) basis subset indicator; and the parameter D in is reported to the gNB.

2. The method according to claim 1 , further comprising receiving a configuration from the gNB, said configuration comprising a CSI report configuration including at least one higher-layer parameter, D (l) , representing a number of delays per layer used for the configuration of the precoder matrix or CSI matrix.

3. The method according to claim 2 , wherein the number of delays D (l) is layer-independent and identical for all layers, such that D (l) =D, and configuring the UE with only a single parameter D for the configuration of the precoder matrix or CSI matrix.

4. The method according to claim 1 , further comprising performing a N 3 -modulo shift operation on all delay indices associated with the selected delay vectors indicated by the LDD basis subset indicator and on all non-zero combining coefficients of a layer with respect to a delay index associated with the strongest coefficient indicator (SCI) of the layer.

5. The method according to claim 4 , wherein the SCI is configured to be rank-dependent, wherein for RI=1 transmission, the SCI is given by a ┌log 2 (K NZ )-bit indicator, and for RI>1 transmission, the SCI is given by ┌log 2 (2U)-bit indicator.

6. The method according to claim 1 , wherein the parameter D in is selected, such that the first delay vector associated with index 0 is included in the CDD basis sub set.

7. The method according to claim 1 , wherein the value range of the parameter D in is defined by:

D in =mod(d SCI − D +1+n,N 3 ), n∈[0, D −1],wherein d SCI is the index of the Frequency Domain (FD) basis vector associated with the SCI, and the UE is configured to report D in using a ┌log 2 D ┐-bit indicator.

8. The method according to claim 1 , wherein the value range of the parameter D in is defined by:

D in =mod (N 3 − D +1+n,N 3 ), n∈[0, D −1], and the UE is configured to report D in using a ┌log 2 D ┐-bit indicator.

9. The method according to claim 1 , wherein the parameter D , defining the CDD basis subset size, depends on the transmission rank.

10. A User Equipment (UE) comprising a processor ( 310 ) and a memory said memory containing instructions executable by said processor whereby said UE is configured to:

receive from a network node (gNB) a radio signal via a Multiple Input Multiple Output (MIMO) channel, wherein the radio signal contains at least one DownLink (DL) reference signal according to a DL reference signal configuration;

estimate the MIMO channel based on the received at least one DL reference signal for configured subbands;

calculate a precoder matrix or a Channel State Information (CSI) matrix for a number of antenna ports of the gNB and configured subbands, N 3 the precoder matrix being based on a first codebook and on a second codebook and a set of combining coefficients for complex scaling/combining one or more vectors selected from the first codebook and the second codebook;

where,

the first codebook contains one or more transmit-side spatial beam components of the precoder matrix or CSI matrix; and

the second codebook contains one or more delay components of the precoder matrix or CSI matrix;

determine a common delay domain (CDD) basis subset of the selected vectors associated with the delay components across all layers of the precoder matrix or CSI matrix, wherein the CDD basis subset is defined by a parameter D representing the number of elements of the CDD basis subset, and a parameter D in representing the first index of the D delay vectors from the second codebook, wherein the value range of the parameter D in is reduced from N 3 to D values; and

report, to the gNB, a CSI feedback report and/or a Precoder matrix Indicator (PMI) and/or a PMI/Rank Indicator (PMI/RI) used to indicate the precoder matrix or the CSI matrix for the configured antenna ports and configured subbands; wherein the selected vectors from the CDD basis subset are indicated in the reporting, for each layer, by a layer-specific delay domain (LDD) basis subset indicator; and the parameter D in is reported to the gNB.

11. A method performed by a network node (gNB) the method comprising:

transmitting to a User Equipment (UE) a radio signal via a Multiple Input Multiple Output (MIMO) channel, wherein the radio signal contains at least one DownLink (DL) reference signal according to a DL reference signal configuration, for enabling the UE to:

estimate the MIMO channel based on the received at least one DL reference signal for configured subbands;

calculate a precoder matrix or a Channel State Information (CSI) matrix for a number of antenna ports of the gNB and configured subbands, N 3 ; the precoder matrix being based on a first codebook and on a second codebook and a set of combining coefficients for complex scaling/combining one or more vectors selected from the first codebook and the second codebook;

where,

the first codebook contains one or more transmit-side spatial beam components of the precoder matrix or CSI matrix; and

the second codebook contains one or more delay components of the precoder matrix or CSI matrix;

determine a common delay domain (CDD) basis subset of the selected vectors associated with the delay components across all layers of the precoder matrix or CSI matrix, wherein the CDD basis subset is defined by a parameter D representing the number of elements of the CDD basis subset, and a parameter D in representing the first index of the D delay vectors from the second codebook, wherein the value range of the parameter D in is reduced from N 3 to D values; and

receiving, from the UE, a CSI feedback report and/or a Precoder matrix Indicator (PMI) and/or a PMI/Rank Indicator (PMI/RI) used to indicate the precoder matrix or the CSI matrix for the configured antenna ports and configured subbands; wherein the selected vectors from the CDD basis subset are indicated in the report, for each layer, by a layer-specific delay domain (LDD) basis subset indicator; and the parameter D in is reported to the gNB.

12. A network node comprising a processor and a memory, said memory containing instructions executable by said processor whereby said network node is configured to:

transmit to a User Equipment (UE) a radio signal via a Multiple Input Multiple Output (MIMO) channel, wherein the radio signal contains at least one DownLink (DL) reference signal according to a DL reference signal configuration; for enabling the UE to:

estimate the MIMO channel based on the received at least one DL reference signal for configured subbands;

calculate a precoder matrix or a Channel State Information (CSI) matrix for a number of antenna ports of the gNB and configured subbands, N 3 ; the precoder matrix being based on a first codebook and on a second codebook and a set of combining coefficients for complex scaling/combining one or more vectors selected from the first codebook and the second codebook;

where,

the first codebook contains one or more transmit-side spatial beam components of the precoder matrix or CSI matrix,

the second codebook contains one or more delay components of the precoder matrix or CSI matrix;

determine a common delay domain (CDD) basis subset of the selected vectors associated with the delay components across all layers of the precoder matrix or CSI matrix, wherein the CDD basis subset is defined by a parameter D representing the number of elements of the CDD basis subset, and a parameter D in representing the first index of the D delay vectors from the second codebook, wherein the value range of the parameter D in is reduced from N 3 to D values; and

receive, from the UE, a CSI feedback report and/or a Precoder matrix Indicator (PMI) and/or a PMI/Rank Indicator (PMI/RI) used to indicate the precoder matrix or the CSI matrix for the configured antenna ports and configured subbands; wherein the selected vectors from the CDD basis subset are indicated in the report, for each layer, by a layer-specific delay domain (LDD) basis subset indicator; and the parameter D in is reported to the gNB.