Uplink non-codebook based precoder selection using DMRS measurement

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

memory; and

at least one processor coupled to the memory and configured to:

receive, from a base station (BS), downlink control information (DCI), a media access control (MAC) control element (CE) (MAC-CE), or radio resource control (RRC) signaling that indicates a length of a time window;

receive, from the BS, an uplink grant configured to schedule at least one uplink reference signal;

receive, from the BS, at least one downlink reference signal comprising at least one demodulation reference signal (DMRS) and at least one channel state information reference signal (CSI-RS) via at least one downlink channel, the at least one downlink channel comprising at least one of a physical downlink shared channel (PDSCH) or a physical downlink control channel (PDCCH);

measure, based on the at least one downlink reference signal and based on the at least one downlink channel being within the length of the time window from the uplink grant, the at least one downlink channel, the measuring being based jointly at least in part on the at least one DMRS and the at least one CSI-RS;

select at least one first uplink precoder for at least one uplink channel corresponding to the at least one downlink channel based on the measuring of the at least one downlink channel, the at least one first uplink precoder being non-codebook based;

transmit, to the BS, the at least one uplink reference signal based on the at least one first uplink precoder for the at least one uplink channel;

receive, from the BS, an indication of a schedule for a set of additional downlink reference signals after transmission of the at least one uplink reference signal, wherein a quantity of the at least one uplink reference signal is less than a specified quantity; and

transmit, to the BS based on a measurement of the set of additional downlink reference signals, a set of additional uplink reference signals associated with a set of additional uplink precoders.

2. The apparatus of claim 1 , wherein the at least one uplink reference signal is indicative of at least one second uplink precoder for the at least one uplink channel, the at least one second uplink precoder being a subset of the at least one first uplink precoder.

3. The apparatus of claim 2 , the at least one processor being further configured to:

receive, from the BS, a scheduling grant indicative of at least one third uplink precoder for uplink transmission, the at least one third uplink precoder being a subset of the at least one second uplink precoder; and

transmit, to the BS, data based on the at least one third uplink precoder indicated in the scheduling grant.

4. The apparatus of claim 1 , wherein to select the at least one first uplink precoder, the at least one processor is configured to estimate the at least one uplink channel corresponding to the at least one downlink channel based on the measuring of the at least one downlink channel.

5. The apparatus of claim 1 , wherein the at least one uplink reference signal comprises at least one sounding reference signal (SRS).

6. The apparatus of claim 1 , wherein the at least one processor is further configured to receive from the BS an indication of a quasi co-location (QCL) relationship between the at least one DMRS and the at least one CSI-RS for uplink precoder selection through a transmission configuration indicator (TCI) state indication.

7. The apparatus of claim 1 , wherein the at least one first uplink precoder corresponds to a first quantity of layers, at least one second uplink precoder corresponds to a second quantity of layers, the at least one DMRS corresponds to a fourth quantity of layers, the at least one CSI-RS corresponds to a fifth quantity of layers, the fourth quantity is less than the first quantity, the first quantity is equal to the second quantity, and a sum of the fourth quantity and the fifth quantity is equal to the first quantity.

8. The apparatus of claim 1 , wherein the at least one first uplink precoder corresponds to a first quantity of layers, at least one second uplink precoder corresponds to a second quantity of layers, the at least one DMRS corresponds to a fourth quantity of layers, the at least one CSI-RS corresponds to a fifth quantity of layers, the fourth quantity is equal to or greater than the second quantity, the first quantity is greater than the second quantity, a sum of the fourth quantity and the fifth quantity is equal to the first quantity, and the at least one second uplink precoder is selected from the at least one first uplink precoder based on one or more layer quality metrics or a configuration received from the BS.

9. The apparatus of claim 1 , wherein the at least one first uplink precoder corresponds to a first quantity of layers, at least one second uplink precoder corresponds to a second quantity of layers, the at least one DMRS corresponds to a fourth quantity of layers, and the fourth quantity is equal to the first quantity and is greater than the second quantity.

10. The apparatus of claim 9 , further comprising a transceiver coupled to the at least one processor, wherein the at least one second uplink precoder is selected from the at least one first uplink precoder based on one or more layer quality metrics or a configuration received from the BS.

11. A method of wireless communication of a user equipment (UE), comprising:

receiving, from a base station (BS), downlink control information (DCI), a media access control (MAC) control element (CE) (MAC-CE), or radio resource control (RRC) signaling that indicates a length of a time window;

receiving, from the BS, an uplink grant configured to schedule at least one uplink reference signal;

receiving, from the BS, at least one downlink reference signal comprising at least one demodulation reference signal (DMRS) and at least one channel state information reference signal (CSI-RS) via at least one downlink channel, the at least one downlink channel comprising at least one of a physical downlink shared channel (PDSCH) or a physical downlink control channel (PDCCH);

measuring, based on the at least one downlink reference signal and based on the at least one downlink channel being within the length of the time window from the uplink grant, the at least one downlink channel, the measuring being based jointly at least in part on the at least one DMRS and the at least one CSI-RS;

selecting at least one first uplink precoder for at least one uplink channel corresponding to the at least one downlink channel based on the measuring of the at least one downlink channel, the at least one first uplink precoder being non-codebook based;

transmitting, to the BS, the at least one uplink reference signal based on the at least one first uplink precoder for the at least one uplink channel;

receiving, from the BS, an indication of a schedule for a set of additional downlink reference signals after transmission of the at least one uplink reference signal, wherein a quantity of the at least one uplink reference signal is less than a specified quantity; and

transmitting, to the BS based on a measurement of the set of additional downlink reference signals, a set of additional uplink reference signals associated with a set of additional uplink precoders.

12. An apparatus for wireless communication at a base station (BS), comprising:

memory; and

at least one processor coupled to the memory and configured to:

transmit, to a user equipment (UE), downlink control information (DCI), a media access control (MAC) control element (CE) (MAC-CE), or radio resource control (RRC) signaling that indicates a length of a time window;

transmit, to the UE, an uplink grant configured to schedule at least one uplink reference signal;

transmit, to the UE, at least one downlink reference signal comprising at least one demodulation reference signal (DMRS) and at least one channel state information reference signal (CSI-RS) via at least one downlink channel, the at least one downlink channel being within the length of the time window from the uplink grant;

receive, from the UE, the at least one uplink reference signal indicative of at least one second uplink precoder for at least one uplink channel corresponding to the at least one downlink channel, the at least one second uplink precoder being a subset of at least one first uplink precoder for the at least one uplink channel, the at least one first uplink precoder being non-codebook based;

transmit, to the UE based on a quantity of the at least one uplink reference signal being less than a specified quantity, an indication of a schedule for a set of additional downlink reference signals after transmission of the at least one uplink reference signal; and

receive, from the UE based on the set of additional downlink reference signals, a set of additional uplink reference signals associated with a set of additional uplink precoders.

13. The apparatus of claim 12 , the at least one processor being further configured to:

select at least one third uplink precoder usable for uplink transmission, the at least one third uplink precoder being a subset of the at least one second uplink precoder.

14. The apparatus of claim 13 , the at least one processor being further configured to:

transmit, to the UE, a scheduling grant indicative of the at least one third uplink precoder usable for uplink transmission; and

receive, from the UE, data based on the at least one third uplink precoder indicated in the scheduling grant.

15. The apparatus of claim 14 , wherein the at least one uplink reference signal comprises at least one sounding reference signal (SRS).

16. The apparatus of claim 12 , wherein the at least one processor is further configured to transmit to the UE an indication of a quasi co-location (QCL) relationship between the at least one DMRS and the at least one CSI-RS for uplink precoder selection through a transmission configuration indicator (TCI) state indication.

17. The apparatus of claim 12 , wherein the at least one first uplink precoder corresponds to a first quantity of layers, the at least one second uplink precoder corresponds to a second quantity of layers, the at least one DMRS corresponds to a fourth quantity of layers, the at least one CSI-RS corresponds to a fifth quantity of layers, the fourth quantity is less than the first quantity, the first quantity is equal to the second quantity, and a sum of the fourth quantity and the fifth quantity is equal to the first quantity.

18. The apparatus of claim 12 , wherein the at least one first uplink precoder corresponds to a first quantity of layers, at least one second uplink precoder corresponds to a second quantity of layers, the at least one DMRS corresponds to a fourth quantity of layers, the at least one CSI-RS corresponds to a fifth quantity of layers, the fourth quantity is equal to or greater than the second quantity, the first quantity is greater than the second quantity, a sum of the fourth quantity and the fifth quantity is equal to the first quantity, and the at least one second uplink precoder is selected from the at least one first uplink precoder based on one or more layer quality metrics or a configuration received from the BS.

19. The apparatus of claim 12 , wherein the at least one first uplink precoder corresponds to a first quantity of layers, the at least one second uplink precoder corresponds to a second quantity of layers, the at least one DMRS corresponds to a fourth quantity of layers, and the fourth quantity is equal to the first quantity and is greater than the second quantity.

20. A method of wireless communication of a base station (BS), comprising:

transmitting, to a user equipment (UE), downlink control information (DCI), a media access control (MAC) control element (CE) (MAC-CE), or radio resource control (RRC) signaling that indicates a length of a time window;

transmitting, to the UE, an uplink grant configured to schedule at least one uplink reference signal;

transmitting, to the UE, at least one downlink reference signal comprising at least one demodulation reference signal (DMRS) and at least one channel state information reference signal (CSI-RS) via at least one downlink channel, the at least one downlink channel being within the length of the time window from the uplink grant;

receiving, from the UE, the at least one uplink reference signal indicative of at least one second uplink precoder for at least one uplink channel corresponding to the at least one downlink channel, the at least one second uplink precoder being a subset of at least one first uplink precoder for the at least one uplink channel, the at least one first uplink precoder being non-codebook based;

transmitting, to the UE based on a quantity of the at least one uplink reference signal being less than a specified quantity, an indication of a schedule for a set of additional downlink reference signals after transmission of the at least one uplink reference signal; and

receiving, from the UE based on the set of additional downlink reference signals, a set of additional uplink reference signals associated with a set of additional uplink precoders.