Hybrid common/independent FD-basis for type II CSI enhancement

1. A method performed by a wireless device for transmitting a channel state information (CSI) report for a downlink channel, the CSI report indicating a plurality of precoder vectors, wherein each of the precoder vectors corresponds to a frequency sub-band of the bandwidth of the downlink channel, a precoder vector being expressed as a linear combination of spatial-domain components and frequency-domain components, and wherein indicating the plurality of precoder vectors comprises indicating the frequency-domain components and a set of linear combination coefficients, the method comprising:

determining a set of spatial-domain components;

determining a second set of candidate frequency-domain components as a subset of a first set of candidate frequency-domain components, wherein the second set of candidate frequency-domain components is smaller than the first set of candidate frequency-domain components;

determining, for each spatial-domain component of the set of spatial-domain components, a spatial-domain component-specific set of one or more frequency-domain components by selecting a subset of the candidate frequency-domain components from the second set of candidate frequency-domain components to be used for that spatial-domain component, wherein the subset of the candidate frequency-domain components from the second set is smaller than the second set and, wherein each frequency-domain component in the spatial-domain component-specific set of one or more frequency-domain components is associated with a non-zero coefficient of the set of linear combination coefficients; and

transmitting, to a network node, the CSI report comprising an indication of: the determined second set of candidate frequency-domain components; the determined spatial-domain component-specific sets of one or more frequency-domain components; and the non-zero coefficients of the set of linear combination coefficients.

2. The method of claim 1 , wherein, for at least one spatial-domain component, the spatial-domain component-specific set of one or more frequency-domain components includes fewer frequency-domain components than the second set of candidate frequency-domain components.

3. The method of any claim 1 , wherein the candidate frequency-domain components are orthogonal basis vectors.

4. The method of claim 2 , wherein the orthogonal basis vectors are discrete Fourier transform vectors.

5. The method of claim 1 , wherein a size of the frequency sub-band is an integer number of physical resource blocks (PRBs).

6. The method of claim 1 , wherein determining the second set of candidate frequency-domain components as the subset of the first set of candidate frequency-domain components comprises:

creating a common basis for all the spatial-domain components.

7. A method performed by a network node for receiving a channel state information (CSI) report for a downlink channel, the CSI report indicating a plurality of precoder vectors, wherein each of the precoder vectors corresponds to a frequency sub-band of the bandwidth of the downlink channel, a precoder vector being expressed as a linear combination of spatial-domain components and frequency-domain components, and wherein indicating the plurality of precoder vectors comprises indicating the frequency-domain components and a set of linear combination coefficients, the method comprising:

receiving, from a wireless device, the channel state information (CSI) report comprising an indication of: a second set of candidate frequency-domain components being a subset of a first set of candidate frequency-domain components, wherein the second set of candidate frequency-domain components is smaller than the first set of candidate frequency-domain components; one or more spatial-domain component-specific sets of one or more frequency-domain components which are selected as one or more subsets of the second set of candidate frequency-domain components, wherein at least one of the selected one or more subsets of the second set is smaller than the second set each frequency-domain component in the one or more spatial-domain component-specific sets of one or more frequency-domain components being associated with a non-zero coefficient of the set of linear combination coefficients; and the non-zero coefficients; and

determining one or more precoder vectors based upon the indication received in the CSI report.

8. The method of claim 7 , wherein, for at least one spatial-domain component, the spatial-domain component-specific set of one or more frequency-domain components includes fewer frequency-domain components than the second set of candidate frequency-domain components.

9. The method of claim 7 , further comprising:

using one of the determined one or more precoder vectors to perform a transmission to the wireless device.

10. The method of claim 7 , wherein a size of the frequency sub-band is an integer number of physical resource blocks (PRBs).

11. A wireless device configured to transmit a channel state information (CSI) report for a downlink channel, the CSI report indicating a plurality of precoder vectors, wherein each of the precoder vectors corresponds to a frequency sub-band of the bandwidth of the downlink channel, a precoder vector being expressed as a linear combination of spatial-domain components and frequency-domain components, and wherein indicating the plurality of precoder vectors comprises indicating the frequency-domain components and a set of linear combination coefficients, the wireless device comprising:

a receiver;

a transmitter; and

processing circuitry coupled to the receiver and the transmitter, the processing circuitry configured to:

determine a set of spatial-domain components;

determine a second set of candidate frequency-domain components as a subset of a first set of candidate frequency-domain components, wherein the second set of candidate frequency-domain components is smaller than the first set of candidate frequency-domain components;

determine, for each spatial-domain component of the set of spatial-domain components, a spatial-domain component-specific set of one or more frequency-domain components by selecting a subset of the candidate frequency-domain components from the second set of candidate frequency-domain components to be used for that spatial-domain component, wherein the subset of candidate frequency-domain components from the second set is smaller than the second set, wherein each frequency-domain component in the spatial-domain component-specific set of one or more frequency-domain components is associated with a non-zero coefficient of the set of linear combination coefficients; and

transmit, to a network node, the CSI report comprising an indication of: the determined second set of candidate frequency-domain components; the determined spatial-domain component-specific sets of one or more frequency-domain components; and the non-zero coefficients of the set of linear combination coefficients.

12. The wireless device of claim 11 , wherein, for at least one spatial-domain component, the spatial-domain component-specific set of one or more frequency-domain components includes fewer frequency-domain components than the second set of candidate frequency-domain components.

13. The wireless device of claim 11 , wherein the candidate frequency-domain components are orthogonal basis vectors.

14. The wireless device of claim 13 , wherein the orthogonal basis vectors are discrete Fourier transform vectors.

15. The wireless device of claim 11 , wherein a size of the frequency sub-band is an integer number of physical resource blocks (PRBs).

16. The wireless device of claim 11 , wherein the processing circuitry configured to determine the second set of candidate frequency-domain components as the subset of the first set of candidate frequency-domain components is further configured to:

create a common basis for all the spatial-domain components.

17. A network node configured to receive a channel state information (CSI) report for a downlink channel, the CSI report indicating a plurality of precoder vectors, wherein each of the precoder vectors corresponds to a frequency sub-band of the bandwidth of the downlink channel, a precoder vector being expressed as a linear combination of spatial-domain components and frequency-domain components, and wherein indicating the plurality of precoder vectors comprises indicating the frequency-domain components and a set of linear combination coefficients, the network node comprising:

a receiver;

a transmitter; and

processing circuitry coupled to the receiver and the transmitter, the processing circuitry configured to:

receive, from a wireless device, the channel state information (CSI) report comprising an indication of: a second set of candidate frequency-domain components being a subset of a first set of candidate frequency-domain components, wherein the second set of candidate frequency-domain components is smaller than the first set of candidate frequency-domain components; one or more spatial-domain component-specific sets of one or more frequency-domain components which are selected as a subset of the second set of candidate frequency-domain components, wherein the subset of the second set is smaller than the second set, each frequency-domain component in the one or more spatial-domain component-specific sets of one or more frequency-domain components being associated with a non-zero coefficient of the set of linear combination coefficients; and the non-zero coefficients; and

determine one or more precoder vectors based upon the indication received in the CSI report.

18. The network node of claim 17 , wherein, for at least one spatial-domain component, the spatial-domain component-specific set of one or more frequency-domain components includes fewer frequency-domain components than the second set of candidate frequency-domain components.

19. The network node of claim 17 , wherein the processing circuitry is further configured to:

use one of the determined one or more precoder vectors to perform a transmission to the wireless device.

20. The network node of claim 17 , wherein a size of the frequency sub-band is an integer number of physical resource blocks (PRBs).