Multi-user multiple-input/multiple-output transmissions in millimeter wave systems

1. A method for wireless communication at a user equipment (UE), comprising:

identifying a set of transmit beams having a channel characteristic above a first threshold, the set of transmit beams comprising an ordered list of transmit beams based on the associated channel characteristic of each transmit beam;

determining, based at least in part on the identified channel characteristic of the set of transmit beams, a cross-correlation metric between each receive beam of a set of receive beams, the set of receive beams comprising an ordered list of receive beam configurations with channel characteristic above a second threshold; and

transmitting a message that identifies the cross-correlation metric between each of the receive beams in the set of receive beams.

2. The method of claim 1 , further comprising:

performing a beam training phase between the UE and a base station to identify the set of transmit beams and the set of receive beams.

3. The method of claim 2 , further comprising:

performing the beam training phase using a first channel; and

transmitting the message using a second channel, wherein the second channel is associated with a lower data rate with respect to the first channel.

4. The method of claim 3 , wherein:

the first channel comprises a millimeter wave channel and the second channel comprises a sub 6 GHz channel.

5. The method of claim 3 , wherein:

the first channel comprises a millimeter wave data channel and the second channel comprises a millimeter wave control channel.

6. The method of claim 1 , further comprising:

transmitting a codebook entry to indicate the identifier of each beam in the set of transmit beams, the channel characteristic for each beam in the set of transmit beams, and the cross-correlation metric between each of the receive beams in the set of receive beams.

7. The method of claim 1 , further comprising:

identifying one or more of a received signal reference power (RSRP), a received signal reference quality (RSRQ), a signal-to-noise ratio (SNR), a signal-to-interference-and noise ratio (SINR), and a channel quality indicator (CQI) associated with each beam in the set of transmit beams, wherein the channel characteristic comprises at least one of the RSRP, the RSRQ, the SNR, the SINR, or the CQI.

8. The method of claim 1 , further comprising:

identifying a number of antenna subarrays of the UE, wherein each subarray has an associated set of receive beams.

9. The method of claim 8 , further comprising:

determining that at least one antenna subarray does not have any receive beams above the second threshold; and

refraining from identifying the set of receive beams for the at least one antenna subarray.

10. The method of claim 1 , further comprising:

transmitting the message further comprising an identifier that identifies each beam in the set of transmit beams and the channel characteristic for each beam in the set of transmit beams.

11. An apparatus for wireless communication at a user equipment (UE), comprising:

a processor,

memory in electronic communication with the processor; and

instructions stored in the memory and executable by the processor to cause the apparatus to:

identify a set of transmit beams having a channel characteristic above a first threshold, the set of transmit beams comprising an ordered list of transmit beams based on the associated channel characteristic of each transmit beam;

determine, based at least in part on the identified channel characteristic of the set of transmit beams, a cross-correlation metric between each receive beam of a set of receive beams, the set of receive beams comprising an ordered list of receive beam configurations with channel characteristic above a second threshold; and

transmit a message that identifies the cross-correlation metric between each of the receive beams in the set of receive beams.

12. The apparatus of claim 11 , wherein the instructions are further executable by the processor to cause the apparatus to:

perform a beam training phase between the UE and a base station to identify the set of transmit beams and the set of receive beams.

13. The apparatus of claim 12 , wherein the instructions are further executable by the processor to cause the apparatus to:

perform the beam training phase using a first channel; and

transmit the message using a second channel, wherein the second channel is associated with a lower data rate with respect to the first channel.

14. The apparatus of claim 13 , wherein the first channel comprises a millimeter wave channel and the second channel comprises a sub 6 GHz channel.

15. The apparatus of claim 13 , wherein the first channel comprises a millimeter wave data channel and the second channel comprises a millimeter wave control channel.

16. The apparatus of claim 11 , wherein the instructions are further executable by the processor to cause the apparatus to:

transmit a codebook entry to indicate the identifier of each beam in the set of transmit beams, the channel characteristic for each beam in the set of transmit beams, and the cross-correlation metric between each of the receive beams in the set of receive beams.

17. The apparatus of claim 11 , wherein the instructions are further executable by the processor to cause the apparatus to:

identify one or more of a received signal reference power (RSRP), a received signal reference quality (RSRQ), a signal-to-noise ratio (SNR), a signal-to-interference-and-noise ratio (SINR), and a channel quality indicator (CQI) associated with each beam in the set of transmit beams, wherein the channel characteristic comprises at least one of the RSRP, the RSRQ, the SNR, the SINR, or the CQI.

18. The apparatus of claim 11 , wherein the instructions are further executable by the processor to cause the apparatus to:

identify a number of antenna subarrays of the UE, wherein each subarray has an associated set of receive beams.

19. The apparatus of claim 18 , wherein the instructions are further executable by the processor to cause the apparatus to:

determine that at least one antenna subarray does not have any receive beams above the second threshold; and

refrain from identifying the set of receive beams for the at least one antenna subarray.

20. The apparatus of claim 11 , wherein the instructions are further executable by the processor to cause the apparatus to:

transmit the message further comprising an identifier that identifies each beam in the set of transmit beams and the channel characteristic for each beam in the set of transmit beams.

21. A method for wireless communication, comprising:

receiving, from a user equipment (UE), a message that identifies a cross-correlation metric between each receive beam in a set of receive beams based at least in part on a channel characteristic of a set of transmit beams;

constructing, based at least in part on the message, a channel matrix for a channel used for communicating with the UE; and

communicating with the UE according to the channel matrix.

22. The method of claim 21 , further comprising:

receiving the message from a plurality of UEs;

constructing, based at least in part on the message, a channel matrix for each UE of the plurality of UEs; and

scheduling, based at least in part on the channel matrix, one or more sets of UEs for communications according to the associated channel matrix for the UE.

23. The method of claim 22 , further comprising:

selecting a subset of transmit beams to use for the scheduled communications with the one or more sets of UEs.

24. The method of claim 21 , further comprising:

identifying, based at least in part on the message, a channel quality metric for the UE, wherein the channel quality metric comprises information associated with the UE being scheduled for communications along with at least one other UE; and

constructing the channel matrix based at least in part on the channel quality metric.

25. The method of claim 24 , wherein:

the channel quality metric comprises a signal-to-interference-and-noise ratio (SINR) metric.

26. An apparatus for wireless communication, comprising:

a processor,

memory in electronic communication with the processor; and

instructions stored in the memory and executable by the processor to cause the apparatus to:

receive, from a user equipment (UE), a message that identifies a cross-correlation metric between each receive beam in a set of receive beams based at least in part on a channel characteristic of a set of transmit beams;

construct, based at least in part on the message, a channel matrix for a channel used for communicating with the UE; and

communicate with the UE according to the channel matrix.

27. The apparatus of claim 26 , wherein the instructions are further executable by the processor to cause the apparatus to:

receive the message from a plurality of UEs;

construct, based at least in part on the message, a channel matrix for each UE of the plurality of UEs; and

schedule, based at least in part on the channel matrix, one or more sets of UEs for communications according to the associated channel matrix for the UE.

28. The apparatus of claim 27 , wherein the instructions are further executable by the processor to cause the apparatus to:

select a subset of transmit beams to use for the scheduled communications with the one or more sets of UEs.

29. The apparatus of claim 26 , wherein the instructions are further executable by the processor to cause the apparatus to:

identify, based at least in part on the message, a channel quality metric for the UE, wherein the channel quality metric comprises information associated with the UE being scheduled for communications along with at least one other UE; and

construct the channel matrix based at least in part on the channel quality metric.

30. The apparatus of claim 29 , wherein the channel quality metric comprises a signal-to-interference-and-noise ratio (SINR) metric.