RACH conveyance of DL synchronization beam information for various DL-UL correspondence states

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

receiving a downlink (DL) signal from a base station on one or more DL beams;

identifying a selected DL beam of the one or more DL beams for communications from the base station to the UE;

receiving an indication, from the base station, that beam correspondence is absent between the one or more DL beams from the base station and one or more uplink (UL) receive beams at the base station, the indication being based at least in part on a preamble format;

identifying that beam correspondence is absent between the one or more DL beams from the base station and the one or more UL receive beams at the base station based at least in part on the indication; and

transmitting a random access channel (RACH) message to the base station based at least in part on the beam correspondence being absent and using at least one of a resource or a RACH waveform selected based at least in part on the selected DL beam.

2. The method of claim 1 , further comprising:

receiving the indication of the absent beam correspondence in a master information block (MIB) or a minimum system information block (SIB).

3. The method of claim 1 , wherein the absent beam correspondence is associated with the one or more DL beams having a different channel propagation characteristic than the one or more UL receive beams, wherein the one or more DL beams are one or more DL transmit beams.

4. The method of claim 1 , wherein identifying that beam correspondence is absent comprises:

identifying a mapping of the DL beams to the resource or RACH waveform from the base station; and

identifying the absent beam correspondence based at least in part on the mapping.

5. The method of claim 1 , wherein the resource comprises a time resource, a frequency resource, or a time-frequency resource.

6. The method of claim 1 , further comprising:

selecting the resource or RACH waveform based at least in part on an index of the selected DL beam.

7. The method of claim 1 , further comprising:

selecting the resource or RACH waveform based at least in part on a symbol of a subframe of the DL signal of the selected DL beam.

8. The method of claim 1 , wherein transmitting the RACH message comprises:

transmitting the RACH message during an entire duration of a corresponding random access subframe.

9. The method of claim 1 , further comprising:

selecting the RACH waveform by selecting a RACH preamble, a cyclic shift, a spreading code, or combinations thereof, based at least in part on an index of the selected DL beam.

10. The method of claim 9 , wherein the spreading code comprises an orthogonal cover code across different symbols of the RACH message.

11. The method of claim 1 , wherein the DL signal comprises a synchronization signal or a reference signal.

12. A method for wireless communication at a base station, comprising:

transmitting a downlink (DL) signal on one or more DL beams;

transmitting an indication that is based at least in part on a preamble format, the indication indicating that beam correspondence is absent between the one or more DL beams from the base station and one or more uplink (UL) receive beams at the base station, wherein the indication is transmitted in a physical broadcast channel (PBCH) signal, a minimum system information (SI) signal, or combinations thereof;

receiving, on at least one of a resource or a random access channel (RACH) waveform, a RACH message from the UE;

identifying, based at least in part on the resource or the RACH waveform and on the indication of the absent beam correspondence, a selected DL beam of the one or more DL beams for communications from the base station to the UE; and

transmitting one or more subsequent messages to the UE using the selected DL beam.

13. The method of claim 12 , wherein the resource comprises a frequency resource, a time resource, or a time-frequency resource.

14. The method of claim 12 , wherein identifying the selected DL beam comprises:

associating the resource or the RACH waveform with an index of the selected DL beam.

15. The method of claim 12 , wherein identifying the selected DL beam comprises:

associating the resource or the RACH waveform with a symbol of a subframe of the DL signal of the selected DL beam.

16. The method of claim 12 , wherein receiving the RACH message from the UE comprises:

receiving the RACH message with a RACH preamble, a cyclic shift, a spreading code, or combinations thereof selected based at least in part on an index of the selected DL beam.

17. The method of claim 16 , wherein the spreading code comprises an orthogonal cover code across different symbols of the RACH message.

18. A method for wireless communication at a base station, comprising:

transmitting a downlink (DL) signal on one or more DL beams;

transmitting an indication that is based at least in part on a preamble format, the indication indicating that beam correspondence is absent between the one or more DL beams from the base station and one or more uplink (UL) receive beams at the base station;

receiving, on at least one of a resource or a random access channel (RACH) waveform, a RACH message from a user equipment (UE), wherein the RACH message is based at least in part on the indication that beam correspondence between the one or more DL beams from the base station and the one or more uplink receive beams at the base station is absent;

identifying, based at least in part on the received resource or the RACH waveform, a preferred DL beam for communications from the base station to the UE; and

transmitting one or more subsequent messages to the UE using the preferred DL beam.

19. The method of claim 18 , wherein the resource comprises a time resource, a frequency resource, or a time-frequency resource.

20. The method of claim 18 , wherein the RACH message comprises a RACH message 1 (msg1).

21. The method of claim 18 , wherein the RACH message comprises an identification or index of the preferred DL beam.

22. The method of claim 18 , wherein identifying the preferred DL beam comprises:

associating the resource or the RACH waveform with an index of the preferred DL beam.

23. The method of claim 22 , further comprising:

measuring a quality of the RACH message received; and

determining a preferred UL receive beam for communications from the UE to the base station based at least in part on the quality.

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

a processor;

memory in electronic communication with the processor; and

instructions stored in the memory and operable, when executed by the processor, to cause the apparatus to:

receive a downlink (DL) signal from a base station on one or more DL beams;

identify a selected DL beam of the one or more DL beams for communications from the base station to the UE;

receive an indication, from the base station, that beam correspondence is absent between the one or more DL beams from the base station and one or more uplink (UL) receive beams at the base station, the indication being based at least in part on a preamble format;

identify that beam correspondence is absent between the one or more DL beams from the base station and the one or more UL receive beams at the base station based at least in part on the indication; and

transmit a random access channel (RACH) message to the base station based at least in part on the beam correspondence being absent and using at least one of a resource or a RACH waveform selected based at least in part on the selected DL beam.

25. The apparatus of claim 24 , wherein the resource comprises a time resource, a frequency resource, or a time-frequency resource.

26. The apparatus of claim 24 , wherein the instructions are further executable by the processor to cause the apparatus to:

select the resource or RACH waveform based at least in part on an index of the selected DL beam.