Message 1 of a two-step random access procedure

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

identifying that the UE is configured to use a two-step random access channel (RACH) procedure, the two-step RACH procedure including an uplink request message and a downlink response;

selecting a preamble sequence from a plurality of predefined sequences associated with two-step RACH procedures;

transmitting the uplink request message as part of the two-step RACH procedure during a RACH occasion, the uplink request message including a preamble portion comprising the preamble sequence and a payload portion that comprises a waveform of a physical uplink shared channel, the payload portion being transmitted during a time resource that is configured to be multiplexed with one or more payload portions from one or more additional UEs, and the RACH occasion being based at least in part on a mapping between the set the plurality of predefined sequences and a plurality of RACH occasions including the RACH occasion; and

receiving the downlink response as part of the two-step RACH procedure and in response to the uplink request message.

2. The method of claim 1 , further comprising:

receiving an indication of the preamble portion before transmitting the uplink request message.

3. The method of claim 1 , wherein transmitting the uplink request message comprises:

transmitting the payload portion using a payload size that is based, at least in part, on whether the two-step RACH procedure is for initial access or for handover.

4. The method of claim 1 , wherein transmitting the uplink request message comprises:

transmitting the payload portion using a fixed payload size.

5. The method of claim 1 , wherein transmitting the uplink request message comprises:

transmitting the preamble portion of the uplink request message back-to-back with the payload portion of the uplink request message without use of cyclic prefixes between resource elements of the preamble portion.

6. The method of claim 1 , further comprising:

receiving, via remaining minimum system information or radio resource control signaling, an association between the preamble portion of the uplink request message and the payload portion of the uplink request message.

7. The method of claim 6 , wherein transmitting the uplink request message comprises:

transmitting the preamble portion of the uplink request message during a first transmission time interval; and

transmitting, in accordance with the received association, the payload portion of the uplink request message during a second transmission time interval.

8. The method of claim 7 , wherein transmitting the uplink request message further comprises:

transmitting the preamble portion of the uplink request message and the payload portion of the uplink request message without an intervening transmission time interval between the first transmission time interval and the second transmission time interval.

9. The method of claim 7 , wherein transmitting the uplink request message further comprises:

transmitting the preamble portion of the uplink request message and the payload portion of the uplink request message with an intervening transmission time interval between the first transmission time interval and the second transmission time interval, wherein the intervening transmission time interval is available for non-RACH transmissions.

10. The method of claim 1 ,

wherein only a portion of the plurality of predefined sequences are associated with two-step RACH procedures.

11. The method of claim 1 ,

wherein the selected preamble sequence shares a resource association with another preamble sequence from the plurality of predefined sequences; and

wherein the method further comprises:

applying an additional differentiating factor to the uplink request message to allow differentiation of the payload portion of the uplink request message.

12. The method of claim 11 , wherein the additional differentiating factor is one or more of use of different demodulation reference signal ports or use of different scrambling identifications.

13. The method of claim 1 ,

wherein the selected preamble sequence has a resource association with more than one payload resource; and

wherein the method further comprises:

applying an additional identifying factor to the uplink request message to allow identification of the payload portion of the uplink request message.

14. The method of claim 13 , wherein the additional identifying factor is one or more of use of different demodulation reference signal ports or use of different scrambling identifications.

15. The method of claim 1 , wherein transmitting the uplink request message comprises:

transmitting the payload portion during the time resource that is time-multiplexed with payload portions from additional UEs.

16. The method of claim 1 , wherein transmitting the uplink request message comprises:

transmitting the payload portion during the time resource that is code division-multiplexed with payload portions from additional UEs.

17. The method of claim 1 , wherein the mapping maps two or more predefined sequences of the plurality of predefined sequences and an associated RACH occasion of the plurality of RACH occasions.

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

receiving, as part of a two-step random access channel (RACH) procedure, an uplink request message during a RACH occasion from a user equipment (UE), the uplink request message including a preamble portion comprising a preamble sequence from a plurality of predefined sequences associated with two-step RACH procedures and a payload portion that comprises a waveform of a physical uplink shared channel, the payload portion being received during a time resource that is configured to be multiplexed with one or more payload portions from one or more additional UEs, the RACH occasion being based at least in part on a mapping between the plurality of predefined sequences and a plurality of RACH occasions including the RACH occasion; and

transmitting a downlink response as part of the two-step RACH procedure and in response to the uplink request message.

19. The method of claim 18 , further comprising:

transmitting an indication of the preamble portion to the UE before receiving the uplink request message.

20. The method of claim 18 , wherein receiving the uplink request message comprises:

receiving the payload portion using a payload size that is based, at least in part, on whether the two-step RACH procedure is for initial access or for handover.

21. The method of claim 18 , wherein receiving the uplink request message comprises:

receiving the payload portion using a fixed payload size.

22. The method of claim 18 , wherein receiving the uplink request message comprises:

receiving the preamble portion of the uplink request message back-to-back with the payload portion of the uplink request message without use of cyclic prefixes between resource elements of the preamble portion.

23. The method of claim 18 , further comprising:

transmitting, via remaining minimum system information or radio resource control signaling, an association between the preamble portion of the uplink request message and the payload portion of the uplink request message.

24. The method of claim 23 , wherein receiving the uplink request message comprises:

receiving the preamble portion of the uplink request message during a first transmission time interval; and

receiving, in accordance with the transmitted association, the payload portion of the uplink request message during a second transmission time interval.

25. The method of claim 18 ,

wherein only a portion of the plurality of predefined sequences are associated with two-step RACH procedures.

26. The method of claim 18 ,

wherein the preamble sequence shares a resource association with another preamble sequence from the plurality of predefined sequences; and

wherein receiving the uplink request message further comprises:

differentiating the payload portion of the uplink request message via a differentiating factor.

27. The method of claim 26 , wherein the differentiating factor is one or more of use of different demodulation reference signal ports or use of different scrambling identifications.

28. The method of claim 18 ,

wherein the preamble sequence has a resource association with more than one payload resource; and

wherein receiving the uplink request message further comprises:

identifying the payload portion of the uplink request message via an identifying factor.

29. The method of claim 28 , wherein the identifying factor is one or more of use of different demodulation reference signal ports or use of different scrambling identifications.

30. The method of claim 18 , wherein the mapping maps two or more predefined sequences of the plurality of predefined sequences and an associated RACH occasion of the plurality of RACH occasions.

31. 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 that the UE is configured to use a two-step random access channel (RACH) procedure, the two-step RACH procedure including an uplink request message and a downlink response;

select a preamble sequence from a plurality of predefined sequences associated with two-step RACH procedures;

transmit the uplink request message as part of the two-step RACH procedure during a RACH occasion, the uplink request message including a preamble portion comprising the preamble sequence and a payload portion that comprises a waveform of a physical uplink shared channel, the payload portion being transmitted during a time resource that is configured to be multiplexed with one or more payload portions from one or more additional UEs, and the RACH occasion being based at least in part on a mapping between the plurality of predefined sequences and a plurality of RACH occasions including the RACH occasion; and

receive the downlink response as part of the two-step RACH procedure and in response to the uplink request message.

32. The apparatus of claim 31 , wherein the mapping maps two or more predefined sequences of the plurality of predefined sequences and an associated RACH occasion of the plurality of RACH occasions.

33. An apparatus for wireless communication at a base station, 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, as part of a two-step random access channel (RACH) procedure, an uplink request message during a RACH occasion from a user equipment (UE), the uplink request message including a preamble portion comprising a preamble sequence from a plurality of predefined sequences associated with two-step RACH procedures and a payload portion that comprises a waveform of a physical uplink shared channel, the payload portion being received during a time resource that is configured to be multiplexed with one or more payload portions from one or more additional UEs, the RACH occasion being based at least in part on a mapping between the plurality of predefined sequences and a plurality of RACH occasions including the RACH occasion; and

transmit a downlink response as part of the two-step RACH procedure and in response to the uplink request message.

34. The apparatus of claim 33 , wherein the mapping maps two or more predefined sequences of the plurality of predefined sequences and an associated RACH occasion of the plurality of RACH occasions.