Hybrid resource mapping for RAR

1. A method of wireless communication performed by a user equipment (UE), the method comprising:

receiving, from a base station (BS), a random access channel (RACH) response message as part of a RACH procedure;

decoding a physical downlink control channel (PDCCH) in the RACH response message transmitted in a common search space to obtain downlink control information (DCI) from the PDCCH;

determining a scheduled physical downlink shared channel (PDSCH) from among a plurality of PDSCH channels in the DCI and a downlink modulation reference signal (DMRS) resource configuration for the UE from among a plurality of DMRS resource configurations in a configuration field of the DCI, each DMRS resource configuration corresponding to a different PDSCH channel from among the plurality of PDSCH channels;

demodulating the scheduled PDSCH based on the DMRS resource configuration identified by the DCI; and

decoding, based on the determining, each scheduled PDSCH with a corresponding distinct flag asserted in the configuration field of the DCI until decoding the scheduled PDSCH for the UE to obtain a random access response (RAR) message from the BS.

2. The method of claim 1 , further comprising:

interpreting each consecutive bit in the configuration field of the DCI as a distinct flag bit for a different DMRS resource based on a system information message from the BS before the RACH response message indicating information about the configuration field of the DCI;

and

ignoring each PDSCH with a corresponding distinct flag left unasserted in the configuration field of the DCI.

3. The method of claim 1 , wherein the scheduled PDSCH comprises a unicast RAR message for the UE, the method further comprising:

descrambling the RAR message with a UE-specific signature.

4. The method of claim 1 , wherein the scheduled PDSCH comprises a multicast RAR message for a plurality of UEs including the UE, the method further comprising:

descrambling the multicast RAR message with a temporary group identifier; and

locating the RAR message for the UE from among a plurality of UE-specific RAR messages multiplexed in the multicast RAR message.

5. The method of claim 1 , wherein the RACH procedure comprises a two-step RACH procedure and the RACH response message comprises message B based on the BS having detected one or both of a preamble and a payload from message A from the UE.

6. The method of claim 5 , further comprising:

descrambling a cyclic redundancy check (CRC) of the PDCCH with a message B radio network temporary identifier (msgB-RNTI).

7. The method of claim 5 , wherein the decoding further comprises:

obtaining, from the scheduled PDSCH, a first RAR message type as the RAR message based on the BS detecting the preamble from message A, the first RAR message type comprising a PUSCH grant; and

obtaining, from the scheduled PDSCH, a second RAR message type as the RAR message based on the BS detecting the preamble and the payload from message A.

8. The method of claim 7 , further comprising:

retransmitting, using the PUSCH grant, the payload to the BS based on obtaining the first RAR message type, wherein the RACH procedure comprises a two-step RACH procedure.

9. A method of wireless communication performed by a base station (BS), the method comprising:

receiving, from a user equipment (UE), a first random access channel (RACH) message as part of a RACH procedure;

determining, in response to successfully decoding at least a portion of the first RACH message, a PDSCH channel from among a plurality of PDSCH channels and downlink modulation reference signal (DMRS) resource configuration for the UE;

including a plurality of DMRS resource configurations including the DMRS resource configuration for the UE in downlink control information (DCI) in a physical downlink control channel (PDCCH), each DMRS resource configuration corresponding to a different PDSCH channel from among the plurality of PDSCH channels;

configuring each consecutive bit in a configuration field of the DCI as a distinct flag for a different DMRS resource;

including a random access response (RAR) message for the UE in the determined PDSCH channel; and

transmitting, to the UE, a second RACH message as part of the RACH procedure, the second RACH message comprising the PDCCH and the determined PDSCH channel scheduled by the PDCCH.

10. The method of claim 9 , wherein the scheduled PDSCH comprises a unicast RAR message for the UE, the method further comprising:

scrambling the RAR message with a UE-specific signature.

11. The method of claim 9 , wherein the scheduled PDSCH comprises a multicast RAR message for a plurality of UEs including the UE, the method further comprising:

placing the RAR message for the UE within a plurality of UE-specific RAR messages multiplexed in the multicast RAR message; and

scrambling the multicast RAR message with a temporary group identifier.

12. The method of claim 9 , wherein the RACH procedure comprises a two-step RACH procedure and the RAR response message comprises message B based on the BS having detected one or both of a preamble and a payload from message A from the UE, the method further comprising:

scrambling a cyclic redundancy check (CRC) of the PDCCH with a message B radio network temporary identifier (msgB-RNTI).

13. The method of claim 12 , further comprising:

including, in the scheduled PDSCH, a first RAR message type as the RAR message based on the BS detecting the preamble from message A, the first RAR message type comprising a PUSCH grant; and

including, in the scheduled PDSCH, a second RAR message type as the RAR message based on the BS detecting the preamble and the payload from message A.

14. The method of claim 13 , further comprising:

re-receiving the payload from the UE using the PUSCH grant based on the UE receiving the first RAR message type, wherein the RACH procedure comprises a two-step RACH procedure.

15. A user equipment, comprising:

a memory;

a transceiver; and

at least one processor coupled to the memory and the transceiver, wherein the user equipment is configured to:

receive, from a base station (BS), a random access channel (RACH) response message as part of a RACH procedure;

decode a physical downlink control channel (PDCCH) in the RACH response message transmitted in a common search space to obtain downlink control information (DCI) from the PDCCH;

determine a scheduled physical downlink shared channel (PDSCH) from among a plurality of PDSCH channels in the DCI and a downlink modulation reference signal (DMRS) resource configuration for the user equipment from among a plurality of DMRS resource configurations in a configuration field of the DCI, each DMRS resource configuration corresponding to a different PDSCH channel from among the plurality of PDSCH channels;

demodulate the scheduled PDSCH based on the DMRS resource configuration identified by the DCI; and

decode, based on the determining, each scheduled PDSCH with a corresponding distinct flag asserted in the configuration field of the DCI until decoding the scheduled PDSCH for the UE to obtain a random access response (RAR) message from the BS.

16. The user equipment of claim 15 , wherein the user equipment is further configured to:

interpret each consecutive bit in the configuration field of the DCI as a distinct flag bit for a different DMRS resource based on a system information message from the BS before the RACH response message indicating information about the configuration field of the DCI;

and

ignore each PDSCH with a corresponding distinct flag left unasserted in the configuration field of the DCI.

17. The user equipment of claim 15 , wherein the scheduled PDSCH comprises a unicast RAR message for the user equipment, the user equipment being further configured to:

descramble the RAR message with a user equipment-specific signature.

18. The user equipment of claim 15 , wherein the scheduled PDSCH comprises a multicast RAR message for a plurality of user equipment including the user equipment, the user equipment being further configured to:

descramble the multicast RAR message with a temporary group identifier; and

locate the RAR message for the user equipment from among a plurality of user equipment-specific RAR messages multiplexed in the multicast RAR message.

19. The user equipment of claim 15 , wherein the RACH procedure comprises a two-step RACH procedure and the RACH response message comprises message B based on the BS having detected one or both of a preamble and a payload from message A from the user equipment.

20. The user equipment of claim 19 , wherein the user equipment is further configured to:

descramble a cyclic redundancy check (CRC) of the PDCCH with a message B radio network temporary identifier (msgB-RNTI).

21. The user equipment of claim 19 , wherein the user equipment is further configured to:

obtain, from the scheduled PDSCH, a first RAR message type as the RAR message based on the BS detecting the preamble from message A, the first RAR message type comprising a PUSCH grant; and

obtain, from the scheduled PDSCH, a second RAR message type as the RAR message based on the BS detecting the preamble and the payload from message A.

22. The user equipment of claim 21 , wherein the user equipment is further configured to:

retransmit, using the PUSCH grant, the payload to the BS based on obtaining the first RAR message type, wherein the RACH procedure comprises a two-step RACH procedure.

23. A base station, comprising:

a memory;

a transceiver; and

at least one processor coupled to the memory and the transceiver, wherein the base station is configured to:

receive, from a user equipment (UE), a first random access channel (RACH) message as part of a RACH procedure;

determine, in response to successfully decoding at least a portion of the first RACH message, a PDSCH channel from among a plurality of PDSCH channels and downlink modulation reference signal (DMRS) resource configuration for the UE;

include a plurality of DMRS resource configurations including the DMRS resource configuration for the UE in downlink control information (DCI) in a physical downlink control channel (PDCCH), each DMRS resource configuration corresponding to a different PDSCH channel from among the plurality of PDSCH channels;

configure each consecutive bit in a configuration field of the DCI as a distinct flag for a different DMRS resource;

include a random access response (RAR) message for the UE in the determined PDSCH channel; and

transmit, to the UE, a second RACH message as part of the RACH procedure, the second RACH message comprising the PDCCH and the determined PDSCH channel scheduled by the PDCCH.

24. The base station of claim 23 , wherein the scheduled PDSCH comprises a unicast RAR message for the UE, wherein the base station is further configured to:

scramble the RAR message with a UE-specific signature.

25. The base station of claim 23 , wherein the scheduled PDSCH comprises a multicast RAR message for a plurality of UEs including the UE, wherein the base station is further configured to:

place the RAR message for the UE within a plurality of UE-specific RAR messages multiplexed in the multicast RAR message; and

scramble the multicast RAR message with a temporary group identifier.

26. The base station of claim 23 , wherein the RACH procedure comprises a two-step RACH procedure and the RAR response message comprises message B based on the base station having detected one or both of a preamble and a payload from message A from the UE, the base station being further configured to:

scramble a cyclic redundancy check (CRC) of the PDCCH with a message B radio network temporary identifier (msgB-RNTI).

27. The base station of claim 26 , wherein the base station is further configured to:

include, in the scheduled PDSCH, a first RAR message type as the RAR message based on the base station detecting the preamble from message A, the first RAR message type comprising a PUSCH grant; and

include, in the scheduled PDSCH, a second RAR message type as the RAR message based on the base station detecting the preamble and the payload from message A.

28. The base station of claim 27 , wherein the base station is further configured to:

re-receive the payload from the UE using the PUSCH grant based on the UE receiving the first RAR message type, wherein the RACH procedure comprises a two-step RACH procedure.