Low complexity and accurate cluster of beams selection

1. A method at user equipment ‘UE’, the method comprising:

determining one or more radio channel estimates from received signals;

selecting a precoding index fulfilling an optimisation criterion based on the one or more radio channel estimates and a reduced precoding matrix, wherein the reduced precoding matrix comprises properties of a single beam for each of one or more polarisation planes, and the reduced precoding matrix corresponds to an expanded precoding matrix comprising properties of a plurality of beams for each of one or more polarisation planes, and wherein the properties of the single beam for each polarisation plane define a beam centred on the plurality of beams for each polarisation plane defined in the corresponding expanded precoding matrix; and

transmitting a precoding matrix indicator ‘PMI’ based on the selected precoding index.

2. The method of claim 1 , wherein the plurality of beams for each polarisation plane is a cluster of beams.

3. The method of claim 1 , wherein the properties of the single beam are based on the Discrete Fourier Transform ‘DFT’.

4. The method of claim 1 , wherein the precoding index fulfilling the optimisation criterion is a precoding index defining a reduced precoding matrix such that a maximum mutual information of transmission link over an effective channel is achieved, wherein the effective channel is a combination of the radio channel estimates and reduced precoding matrices.

5. The method of claim 1 , wherein the precoding index defines one reduced precoding matrix out of a plurality of reduced precoding matrices stored in a codebook.

6. The method of claim 5 , wherein the precoding index fulfilling the optimisation (criterion is a precoding index k such that k=argmax Σ n=1 n H trace((H n ·W 1 (k) )·(H n ·W 1 (k) ) H ), wherein H n is one of the radio channel estimates and W 1 (k) is a reduced precoding matrix selected from the codebook.

7. The method of claim 1 , wherein the received signals are received from a plurality of transmitter antennas at a base station for transmitting signals within one or more polarisation planes.

8. The method of claim 7 , wherein the reduced precoding matrix is of the form [X 0; 0 X] wherein X is a T×1 matrix, where T is the number of the transmitter antennas.

9. The method of claim 7 , wherein the reduced precoding matrix is a (P*T)×P matrix, where T is the number of transmitter antennas and P is the number of different polarisation planes.

10. The method of claim 1 , wherein the determining one or more radio channel estimates comprises estimating a channel matrix from received reference signal data.

11. The method of claim 1 , wherein the PMI comprises the selected precoding index.

12. The method of claim 11 , wherein the PMI further comprises a second precoding index corresponding to a second precoding matrix W 2 .

13. A user equipment ‘UE’ comprising:

one or more processors; and

memory comprising instructions which when executed by the one or more processors cause the UE to:

determine one or more radio channel estimates from received signals;

select a precoding index fulfilling an optimisation criterion based on the one or more radio channel estimates and a reduced precoding matrix, wherein the reduced precoding matrix comprises properties of a single beam for each of one or more polarisation planes, and the reduced precoding matrix corresponds to an expanded precoding matrix comprising properties of a plurality of beams for each of one or more polarisation planes, and wherein the properties of the single beam for each polarisation plane define a beam centred on the plurality of beams for each polarisation plane defined in the corresponding expanded precoding matrix; and

transmit a precoding matrix indicator ‘PMI’ based on the selected precoding index.

14. The UE of claim 13 , wherein the plurality of beams for each polarisation plane is a cluster of beams.

15. The UE of claim 13 , wherein the properties of the single beam are based on the Discrete Fourier Transform ‘DFT’.

16. The UE of claim 13 , wherein the precoding index fulfilling the optimisation criterion is a precoding index defining a reduced precoding matrix such that a maximum mutual information of transmission link over an effective channel is achieved, wherein the effective channel is a combination of the radio channel estimates and reduced precoding matrices.

17. The UE of claim 13 , wherein the precoding index defines one reduced precoding matrix out of a plurality of reduced precoding matrices stored in a codebook.

18. The UE of claim 17 , wherein the precoding index fulfilling the optimisation criterion is a precoding index k such that k=argmax Σ n=1 n H trace((H n ·W 1 (k) )·(H n ·W 1 (k) ) H ), wherein H n is one of the radio channel estimates and W 1 (k) is a reduced precoding matrix selected from the codebook.

19. The UE of claim 13 , wherein the received signals are received from a plurality of transmitter antennas at a base station for transmitting signals within one or more polarisation planes.

20. The UE of claim 19 , wherein the reduced precoding matrix is of the form [X 0; 0 X] wherein X is a T×1 matrix, where T is the number of the transmitter antennas.

21. The UE of claim 13 , wherein the determining one or more radio channel estimates comprises estimating a channel matrix from received reference signal data.

22. The UE of claim 13 , wherein the PMI comprises the selected precoding index.

23. The UE of claim 22 , wherein the PMI further comprises a second precoding index corresponding to a second precoding matrix W 2 .