Low overhead PMI and CQI feedback and pairing schemes for MU-MIMO

1. A method for searching for a pairing precoding matrix indicator (PMI), comprising:

estimating a multiple input/multiple output (MIMO) wireless propagation channel from an access point transmitter to a receiver at a user equipment (UE);

quantizing the estimated channel with a codebook consisting of a plurality of codewords, the quantization result being an actual single user MIMO (SU-MIMO) PMI of a plurality of potential SU-MIMO PMIs currently applicable to the UE;

estimating a channel quality corresponding to the actual SU-MIMO PMI, resulting in at least one channel quality indicator (CQI) for the UE;

constructing a plurality of subsets from the codebook of codewords, wherein each subset comprises potential multi-user MIMO (MU-MIMO) PMIs that represent channels that have reduced interference with a channel represented by one of the plurality of potential SU-MIMO PMIs;

associating each of the plurality of potential SU-MIMO PMIs with a subset of the plurality of subsets that comprises the potential MU-MIMO PMIs that represent channels that have reduced interference with the channel represented by the each of the plurality of potential SU-MIMO PMIs;

searching for an appropriate potential MU-MIMO PMI and an associated CQI for the UE only in the subset associated with the each of the plurality of potential SU-MIMO PMIs that is the same as the actual SU-MIMO PMI for the UE; and

pairing the appropriate potential MU-MIMO PMI with the actual SU-MIMO PMI for the UE.

2. The method of claim 1 , wherein each subset of the plurality of subsets comprises one of:

codewords that are orthogonal to one another and to the codeword represented by the each of the plurality of potential SU-MIMO PMIs associated with the subset;

codewords that are orthogonal to the codeword represented by the each of the plurality of potential SU-MIMO PMIs associated with the subset but that are not necessarily orthogonal to one another and;

codewords that have a chordal distance to the codeword indicated by the each of the plurality of potential SU-MIMO PMIs that is greater than a threshold, wherein the threshold is predefined such that channels represented by the codewords will have reduced interference with the channel represented by the each of the plurality of potential SU-MIMO PMIs associated with the subset.

3. The method of claim 1 , wherein, when the appropriate potential MU-MIMO PMI and the actual SU-MIMO PMI for the UE are paired, the paired potential MU-MIMO PMI and the actual SU-MIMO PMI are fed back to an access node.

4. The method of claim 3 , wherein the feedback to the access node comprises a designation of the each of the plurality of potential SU-MIMO PMIs associated with the subset that includes the paired potential MU-MIMO PMI and further comprises a designation of which MU-MIMO PMI within the subset is the paired MU-MIMO PMI.

5. The method of claim 4 , wherein the feedback to the access node further comprises an actual channel quality indication (CQI) calculated based on the actual SU-MIMO PMI and further comprises a paired potential MU-MIMO CQI specified as a differential from the actual SU-MIMO CQI.

6. The method of claim 5 , wherein the access node uses the actual SU-MIMO PMI, the paired potential MU-MIMO PMI, the actual SU-MIMO CQI, and the paired potential MU-MIMO CQI to assign a modulation and coding scheme to a MU-MIMO transmission to the UE and to a second UE associated with the paired potential MU-MIMO PMI.

7. The method of claim 6 , wherein the UE and the second UE are of different ranks.

8. The method of claim 6 , wherein the UE selects and feeds back to the access node a plurality of paired potential MU-MIMO PMIs.

9. The method of claim 8 , wherein at least one additional UE is paired with the UE and the second UE in the MU-MIMO transmission.

10. The method of claim 3 , wherein the access node specifies the configuration the UE is to use for the feedback.

11. The method of claim 3 , wherein the access node, based on considerations of feedback accuracy and feedback overhead, informs the UE whether to search for the paired potential MU-MIMO PMI in one of the subsets or in the entire codebook.

12. A user equipment (UE), comprising:

a processor configured to:

estimate a multiple input/multiple output (MIMO) wireless propagation channel from an access point transmitter to a receiver at the UE;

quantize the estimated channel with a codebook consisting of a plurality of codewords, the quantization result being an actual single user MIMO (SU-MIMO) precoding matrix indicator (PMI) currently applicable to the UE;

estimate a channel quality corresponding to the actual SU-MIMO PMI, resulting in at least one channel quality indicator (CQI) for the UE;

construct a plurality of subsets from the codebook of codewords, wherein each subset comprises potential multi-user MIMO (MU-MIMO) PMIs that represent channels that have reduced interference with a channel represented by one of the plurality of potential SU-MIMO PMIs;

associate each of the plurality of potential SU-MIMO PMIs with a subset of the plurality of subsets that comprises the potential MU-MIMO PMIs that represent channels that have reduced interference with the channel represented by the each of the plurality of potential SU-MIMO PMIs;

search for an appropriate potential MU-MIMO PMI and an associated CQI for the UE only in the subset associated with the each of the plurality of potential SU-MIMO PMIs that is the same as the actual SU-MIMO PMI for the UE; and

pair the appropriate potential MU-MIMO PMI with the actual SU-MIMO PMI for the UE.

13. The UE of claim 12 , wherein each subset of the plurality of subsets comprises one of:

codewords that are orthogonal to one another and to the codeword represented by the each of the plurality of potential SU-MIMO PMI associated with the subset;

codewords that are orthogonal to the codeword represented by the each of the plurality of potential SU-MIMO PMIs associated with the subset but that are not necessarily orthogonal to one another and;

codewords that have a chordal distance to the codeword indicated by the each of the plurality of potential SU-MIMO PMIs that is greater than a threshold, wherein the threshold is predefined such that channels represented by the codewords will have reduced interference with the channel represented by the each of the plurality of potential SU-MIMO PMIs associated with the subset.

14. The UE of claim 12 , wherein, when the appropriate potential MU-MIMO PMI and the actual SU-MIMO PMI for the UE are paired, the appropriate potential MU-MIMO PMI and the actual SU-MIMO PMI are fed back to an access node.

15. The UE of claim 14 , wherein the feedback to the access node comprises a designation of the each of the potential SU-MIMO PMIs associated with the subset that includes the paired potential MU-MIMO PMI and further comprises a designation of which MU-MIMO PMI within the subset is the paired potential MU-MIMO PMI.

16. The UE of claim 15 , wherein the feedback to the access node further comprises an actual channel quality indication (CQI) calculated based on the actual SU-MIMO PMI and further comprises a potential MU-MIMO CQI specified as a differential from the actual SU-MIMO CQI.

17. The UE of claim 16 , wherein the access node uses the actual SU-MIMO PMI, the paired potential MU-MIMO PMI, the actual SU-MIMO CQI, and the potential MU-MIMO CQI to assign a modulation and coding scheme to a MU-MIMO transmission to the UE and to a second UE associated with the pairing PMI.

18. The UE of claim 17 , wherein the UE and the second UE are of different ranks.

19. The UE of claim 17 , wherein the UE selects and feeds back to the access node a plurality of appropriate potential MU-MIMO PMIs.

20. The UE of claim 19 , wherein at least one additional UE is paired with the UE and the second UE in the MU-MIMO transmission.