Channel construction for unsounded MIMO channel

1. A method for use in a network node for constructing an unsounded channel in a wireless network wherein the network node comprises more antennas than a wireless device and the wireless device comprises more than one receive antenna, the method comprising:

receiving channel state information (CSI) feedback and a sounded reference signal from the wireless device;

estimating a sounded channel from the sounded reference signal;

deriving a partial downlink channel based on the sounded reference signal;

constructing an unsounded channel as a function of the CSI feedback and the partial downlink channel, wherein constructing the unsounded channel as a function of the CSI feedback and the partial downlink channel comprises:

ordering columns of a precoding matrix indicator (PMI), or a variant of the PMI, included in the received CSI feedback;

down-selecting a subset of the ordered columns;

projecting the down-selected columns in a nulling space of the partial downlink channel;

scaling the projection so that constructed unsounded channel is power comparable with the partial downlink channel;

concatenating the sounded channel and unsounded channel into a full channel; and

transmitting a wireless signal to the wireless device using the full channel for beamforming or link adaptation.

2. The method of claim 1 , wherein ordering the columns of the PMI, or the variant of the PMI, comprises ordering each PMI, or variant of the PMI, column based on its distance from the sounded channel.

3. The method of claim 1 , wherein down-selecting the subset of ordered columns comprises filtering out PMI columns associated with the partial downlink channel.

4. A network node operable to construct an unsounded channel in a wireless network wherein the network node comprises more antennas than a wireless device and the wireless device comprises more than one receive antenna, the network node comprising processing circuitry operable to:

receive channel state information (CSI) feedback and a sounded reference signal from the wireless device;

estimate a sounded channel from the sounded reference signal;

derive a partial downlink channel based on the sounded reference signal;

construct an unsounded channel as a function of the CSI feedback and the partial downlink channel, wherein the processing circuitry operable to construct the unsounded channel as a function of the CSI feedback and the partial downlink channel is operable to:

order columns of a precoding matrix indicator (PMI), or a variant of the PMI, included in the received CSI feedback;

down-select a subset of the ordered columns;

project the down-selected columns in a nulling space of the partial downlink channel;

scale the projection so that constructed unsounded channel is power comparable with the partial downlink channel;

concatenate the sounded channel and unsounded channel into a full channel; and

transmit a wireless signal to the wireless device using the full channel for beamforming or link adaptation.

5. The network node of claim 4 , wherein the processing circuitry is operable to order the columns of the PMI, or the variant of the PMI, by ordering each PMI, or variant of the PMI, column based on its distance from the sounded channel.

6. The network node of claim 4 , wherein the processing circuitry is operable to down-select the subset of ordered columns by filtering out PMI columns associated with the partial downlink channel.

7. A method for use in a network node for constructing an unsounded channel in a wireless network wherein the network node comprises more antennas than a wireless device and the wireless device comprises more than one receive antenna, the method comprising:

receiving channel state information (CSI) feedback and a sounded reference signal from the wireless device;

estimating a sounded channel from the sounded reference signal;

deriving a partial downlink channel based on the sounded reference signal;

constructing an unsounded channel as a function of the CSI feedback and the partial downlink channel, wherein constructing the unsounded channel as a function of the CSI feedback and the partial downlink channel comprises:

ordering columns of a precoding matrix indicator (PMI), or a variant of the PMI, included in the received CSI feedback;

down-selecting a subset of the ordered columns;

obtaining a channel characteristic based on the sounded channel;

acquiring an approximation of right singular vectors based on the obtained channel characteristic;

constructing an intermediate matrix based on a subset of the approximation of right singular vectors and the down-selected columns;

decomposing the intermediate matrix to obtain linear combination weights;

determining the unsounded channel based on the subset of the approximation of right singular vectors and the obtained linear combination weights;

concatenating the sounded channel and unsounded channel into a full channel; and

transmitting a wireless signal to the wireless device using the full channel for beamforming or link adaptation.

8. The method of claim 7 , wherein the channel characteristic comprises a transmitter correlation matrix.

9. The method of claim 8 , wherein the transmitter correlation matrix is based on an estimated downlink channel for a first polarization.

10. The method of claim 8 , wherein the transmitter correlation matrix is based on an estimated downlink channel for a first polarization and a second polarization.

11. The method of claim 8 , wherein acquiring the approximation of the right singular vectors comprises acquiring the right singular vectors by singular value decomposition (SVD) or eigen value decomposition.

12. The method of claim 8 , wherein acquiring the approximation of the right singular vectors comprises acquiring a subset of vectors comprising the first dominant singular vectors.

13. A network node operable to construct an unsounded channel in a wireless network wherein the network node comprises more antennas than a wireless device and the wireless device comprises more than one receive antenna, the network node comprising processing circuitry operable to:

receive channel state information (CSI) feedback and a sounded reference signal from the wireless device;

estimate a sounded channel from the sounded reference signal;

derive a partial downlink channel based on the sounded reference signal;

construct an unsounded channel as a function of the CSI feedback and the partial downlink channel, wherein the processing circuitry operable to construct the unsounded channel as a function of the CSI feedback and the partial downlink channel is operable to:

order columns of a precoding matrix indicator (PMI), or a variant of the PMI, included in the received CSI feedback;

down-select a subset of the ordered columns;

obtain a channel characteristic based on the sounded channel;

acquire an approximation of right singular vectors based on the obtained channel characteristic;

construct an intermediate matrix based on a subset of the approximation of right singular vectors and the down-selected columns;

decompose the intermediate matrix to obtain linear combination weights;

determine the unsounded channel based on the subset of the approximation of right singular vectors and the obtained linear combination weights;

concatenate the sounded channel and unsounded channel into a full channel; and

transmit a wireless signal to the wireless device using the full channel for beamforming or link adaptation.

14. The network node of claim 13 , wherein the channel characteristic comprises a transmitter correlation matrix.

15. The network node of claim 14 , wherein the transmitter correlation matrix is based on an estimated downlink channel for a first polarization.

16. The network node of claim 14 , wherein the transmitter correlation matrix is based on an estimated downlink channel for a first polarization and a second polarization.

17. The network node of claim 14 , wherein the processing circuitry is operable to acquire the approximation of the right singular vectors by acquiring the right singular vectors by singular value decomposition (SVD) or eigen value decomposition.

18. The network node of claim 14 , wherein the processing circuitry is operable to acquire the approximation of the right singular vectors by acquiring a subset of vectors comprising the first dominant singular vectors.