Message 2 of a two-step random access procedure and coexistence with four-step random access procedure
Methods, systems, and devices for wireless communications are described. The method includes identifying that the user equipment (UE) is to use a two-step random access channel (RACH) procedure that includes a first message (e.g., uplink request message) and a second message (e.g., downlink response). The methods include sending, as a first message of the two-step RACH procedure, the uplink request message to a base station, receiving, from the base station, a physical downlink control channel comprising an indication of resources of a broadcast or multicast physical downlink shared channel on which the downlink response is to be received, and receiving the downlink response from the base station on the broadcast or multicast physical downlink shared channel. The physical downlink shared channel may be configured to provide downlink responses for one or multiple UEs.
1 . A method for wireless communication at a user equipment, UE, comprising:
identifying that the UE is to use a two-step random access channel, RACH, procedure, the two-step RACH procedure including an uplink request message and a downlink response;
sending, as a first message of the two-step RACH procedure, the uplink request message to a network entity;
determining a group random access radio network temporary identifier;
receiving, from the network entity, a physical downlink control channel comprising an indication of resources of a broadcast or multicast physical downlink shared channel on which the downlink response is to be received;
decoding the physical downlink control channel based at least in part on the group random access radio network temporary identifier;
decoding the broadcast or multicast physical downlink shared channel in accordance with the indication received on the physical downlink control channel, the broadcast or multicast physical downlink shared channel being configured to provide downlink responses for multiple UEs, the downlink responses for multiple UEs multiplexed at a media access control layer and comprise a downlink response intended for the UE;
receiving the downlink response intended for the UE on the broadcast or multicast physical downlink shared channel, the downlink response intended for the UE being a second message of the two-step RACH procedure;
identifying an uplink grant in the downlink response intended for the UE, the uplink grant indicating resources for transmission by the UE of a confirmation message that confirms the UE received the downlink response intended for the UE on the broadcast or multicast physical downlink shared channel; and
transmitting the confirmation message based at least in part on the uplink grant.
2 . The method of claim 1 , further comprising:
refraining from transmitting any hybrid automatic repeat request, HARQ, feedback with respect to decoding the physical downlink control channel or decoding the broadcast or multicast physical downlink shared channel.
3 . The method of claim 1 , wherein the physical downlink control channel:
uses a larger transport block scaling than would be used if HARQ feedback were to be transmitted; or
lacks one or more of a HARQ process identifier, a new data indicator, or a redundancy version identifier.
4 . The method of claim 1 , wherein the downlink response intended for the UE includes one or more of a random access preamble index, a cell radio network temporary identifier, or a UE identifier.
5 . The method of claim 1 , wherein the broadcast or multicast physical downlink shared channel includes a mix of second messages from a four-step random access procedure and the second message from the two-step RACH procedure.
6 . The method of claim 1 , wherein the confirmation message comprises an acknowledgement.
7 . The method of claim 1 , wherein transmitting the confirmation message comprises transmitting the confirmation message using resources identified by a resource indicator of the downlink response intended for the UE or of the physical downlink control channel.
8 . An apparatus for wireless communication at a user equipment, UE, comprising:
a processor,
memory coupled with the processor; and
instructions stored in the memory and executable by the processor to cause the apparatus to:
identify that the UE is to use a two-step random access channel, RACH, procedure, the two-step RACH procedure including an uplink request message and a downlink response;
send, as a first message of the two-step RACH procedure, the uplink request message to a network entity;
determine a group random access radio network temporary identifier;
receive, from the network entity, a physical downlink control channel comprising an indication of resources of a broadcast or multicast physical downlink shared channel on which the downlink response is to be received;
decode the physical downlink control channel based at least in part on the group random access radio network temporary identifier;
decode the broadcast or multicast physical downlink shared channel in accordance with the indication received on the physical downlink control channel, the broadcast or multicast physical downlink shared channel being configured to provide downlink responses for multiple UEs, the downlink responses for multiple UEs multiplexed at a media access control layer and comprise a downlink response intended for the UE; and
receive the downlink response intended for the UE on the broadcast or multicast physical downlink shared channel, the downlink response intended for the UE being a second message of the two-step RACH procedure;
identify an uplink grant in the downlink response intended for the UE, the uplink grant indicating resources for transmission by the UE of a confirmation message that confirms the UE received the downlink response intended for the UE on the broadcast or multicast physical downlink shared channel; and
transmit the confirmation message based at least in part on the uplink grant.
9 . The apparatus of claim 8 , wherein the downlink response intended for the UE includes one or more of a random access preamble index, a cell radio network temporary identifier, or a UE identifier.
10 . The apparatus of claim 8 , wherein the instructions stored in the memory are executable by the processor to cause the apparatus to:
refrain from transmitting any hybrid automatic repeat request, HARQ, feedback with respect to decoding the physical downlink control channel or decoding the broadcast or multicast physical downlink shared channel.
11 . The apparatus of claim 8 , wherein the downlink response intended for the UE includes one or more of a random access preamble index, a cell radio network temporary identifier, or a UE identifier.
12 . The apparatus of claim 8 , wherein the confirmation message comprises an acknowledgement.
13 . The apparatus of claim 8 , wherein the instructions stored in the memory are executable by the processor to cause the apparatus to:
transmit the confirmation message using resources identified by a resource indicator of the downlink response intended for the UE or of the physical downlink control channel.
14 . The apparatus of claim 8 , wherein the downlink response intended for the UE comprises a random access preamble index (RAPID).
15 . An apparatus for wireless communication at a network entity:
a processor,
memory coupled with the processor; and
instructions stored in the memory and executable by the processor to cause the apparatus to:
receive an uplink request message from a user equipment, UE;
identify that the uplink request message is a first message of a two-step random access channel, RACH, procedure;
send a physical downlink control channel comprising an indication of resources of a broadcast or a multicast physical downlink shared channel on which a downlink response intended for the UE to the uplink request message is to be received, wherein the downlink response intended for the UE includes an uplink grant indicating resources for transmission by the UE of a confirmation message that confirms the UE received the downlink response intended for the UE, wherein the broadcast or multicast physical downlink shared channel is configured to provide a plurality of downlink responses to a corresponding plurality of UEs that includes the UE;
send, via the broadcast or multicast physical downlink shared channel, the downlink response intended for the UE, the downlink response intended for the UE being a second message of the two-step RACH procedure and comprising an uplink grant in the physical downlink control channel, the uplink grant indicating resources for transmission by the UE of a confirmation message that confirms the UE received the downlink response intended for the UE; and
confirm the UE receives the second message based at least in part on detecting a transmission on the resources indicated by the uplink grant.
16 . The apparatus of claim 15 , wherein the instructions stored in the memory are executable by the processor to cause the apparatus to:
multiplex at a media access control layer of the broadcast or multicast physical downlink shared channel the downlink responses for the plurality of UEs, wherein the downlink response intended for the UE is one of the multiplexed downlink responses.
17 . The apparatus of claim 15 , wherein the instructions stored in the memory are executable by the processor to cause the apparatus to:
include in each of the plurality of downlink responses at least one of a random access preamble index, RAPID, a timing advance, TA, a cell radio network temporary identifier, a UE identifier, or any combination thereof.
18 . The apparatus of claim 15 , wherein the instructions stored in the memory are executable by the processor to cause the apparatus to:
include in the broadcast or multicast physical downlink shared channel an acknowledgement/negative acknowledgement, ACK/NACK, resource indicator, ARI, for at least one of the plurality of downlink responses to indicate ACK/NACK feedback resources.
19 . The apparatus of claim 15 , wherein the instructions stored in the memory are executable by the processor to cause the apparatus to:
confirm the UE receives the second message based at least in part on detecting a transmission on the resources indicated by the uplink grant.