Media access control procedures for beam index indications
Methods, systems, and devices for wireless communications are described. A user equipment (UE) may select a beam from a set of beams and transmit an indication of the selected beam in a media access control (MAC) control element to establish a communication link with a base station. In some cases, the UE may select the beam and indicate the selection based on identifying a beam failure and/or determining to perform a random access procedure. When transmitting the indication of the selected beam, the UE may obtain an uplink resource for the transmission based on an availability of the uplink resource. For example, if the uplink resource is available, the UE may transmit the indication of the selected beam multiplexed with an uplink transmission. Alternatively, if the uplink resource is unavailable, the UE may request uplink resources for transmitting the indication of the selected beam.
1 . A method for wireless communications at a user equipment (UE), comprising:
determining to establish a communication link between the UE and a serving cell of a base station, wherein the serving cell comprises a secondary cell;
selecting, by the UE to establish the communication link, a first beam of a plurality of candidate beams of the serving cell;
transmitting, by the UE in accordance with a beam failure recovery procedure for the secondary cell associated with the base station, a scheduling request for one or more first uplink resources that the UE is to use to transmit a beam index indication of the selected first beam to the base station based at least in part on an unavailability of one or more second uplink resources for transmission of the beam index indication, the unavailability based at least in part on a number of slots between a current time and the one or more second uplink resources being greater than a threshold number of slots;
receiving, in response to the scheduling request, an indication of the one or more first uplink resources for the UE to use to transmit the beam index indication; and
transmitting, by the UE to the base station, the beam index indication of the selected first beam in an uplink media access control (MAC) control element on the one or more first uplink resources.
2 . The method of claim 1 , further comprising:
transmitting the scheduling request on uplink control resources that indicate, to the base station, a beam of the serving cell that has failed; and
receiving, from the base station in response to the scheduling request, the indication of the one or more first uplink resources in a second serving cell for the UE to use to transmit the beam index indication of the selected first beam.
3 . The method of claim 1 , wherein the scheduling request comprises a configuration of uplink control resources that are associated with the beam failure recovery procedure, and wherein:
the scheduling request is transmitted on dedicated uplink control resources configured to indicate, to the base station, a beam or a plurality of beams configured for the serving cell that have failed, and
the indication of the one or more first uplink resources is received from a second serving cell of the base station in response to the scheduling request and via a downlink control information indicating the one or more first uplink resources of the second serving cell for the UE to use to transmit the beam index indication of the selected first beam.
4 . The method of claim 1 , further comprising:
transmitting the scheduling request on uplink control resources of a transmission occasion, the uplink control resources indicating, to the base station, that a beam of the serving cell has failed, and the transmission occasion of the uplink control resources associated with a set of serving cells to indicate the serving cell on which the beam has failed; and
receiving, from the base station in response to the scheduling request, the indication of the one or more first uplink resources in a second serving cell different than the serving cell for the UE to use to transmit the beam index indication of the selected first beam.
5 . The method of claim 1 , wherein the scheduling request comprises a random access message of a random access procedure to establish the communication link between the UE and the serving cell of the base station.
6 . The method of claim 1 , wherein determining to establish the communication link comprises:
communicating with the base station via the communication link;
identifying a beam failure for the communication link between the UE the serving cell; and
determining to establish the communication link between the UE and the serving cell based at least in part on the identified beam failure.
7 . The method of claim 1 , further comprising:
receiving a configuration for beam failure recovery, the scheduling request for the one or more first uplink resources transmitted based at least in part on the received configuration.
8 . The method of claim 1 , wherein transmitting the scheduling request for the one or more first uplink resources to the base station comprises:
transmitting a scheduling request sequence indicating the scheduling request to the base station.
9 . The method of claim 8 , wherein the scheduling request sequence is transmitted on uplink resources corresponding to a highest priority logical channel configured for the UE.
10 . The method of claim 1 , wherein transmitting the beam index indication of the selected first beam comprises:
transmitting, to the base station, the uplink MAC control element in a random access message of a random access procedure.
11 . The method of claim 1 , further comprising:
performing a logical channel prioritization procedure for a MAC protocol data unit comprising the uplink MAC control element, wherein the logical channel prioritization procedure provides a priority for the uplink MAC control element greater than each other uplink MAC control element of the MAC protocol data unit, greater than data of the MAC protocol data unit, and less than information of a common control channel message of the MAC protocol data unit.
12 . The method of claim 1 , further comprising:
initiating a timer and a counter based at least in part on determining to establish the communication link;
incrementing the counter based at least in part on transmitting the beam index indication of the selected first beam; and
retransmitting the beam index indication of the selected first beam in the uplink MAC control element based at least in part on the timer expiring before a message is received on the selected first beam and the counter being below a counter threshold.
13 . The method of claim 1 , wherein the beam index indication of the selected first beam is transmitted based at least in part on a comparison of a timing of an availability of the one or more first uplink resources to a threshold value.
14 . A user equipment (UE) for wireless communications, comprising:
memory, and
one or more processors coupled with the memory and configured to cause the UE to:
determine to establish a communication link between the UE and a serving cell of a base station, wherein the serving cell comprises a secondary cell;
select, by the UE to establish the communication link, a first beam of a plurality of candidate beams of the serving cell;
transmit, by the UE in accordance with a beam failure recovery procedure for the secondary cell associated with the base station, a scheduling request for one or more first uplink resources that the UE is to use to transmit a beam index indication of the selected first beam to the base station based at least in part on an unavailability of one or more second uplink resources for transmission of the beam index indication, the unavailability based at least in part on a number of slots between a current time and the one or more second uplink resources being greater than a threshold number of slots;
receive, in response to the scheduling request, an indication of the one or more first uplink resources for the UE to use to transmit the beam index indication; and
transmit, by the UE to the base station, the beam index indication of the selected first beam in an uplink media access control (MAC) control element on the one or more first uplink resources.
15 . The UE of claim 14 , wherein the scheduling request comprises a configuration of uplink control resources that are associated with the beam failure recovery procedure, and wherein:
the scheduling request is transmitted on dedicated uplink control resources configured to indicate, to the base station, a beam or a plurality of beams configured for the serving cell that have failed, and
the indication of the one or more first uplink resources is received from a second serving cell of the base station in response to the scheduling request and via a downlink control information indicating the one or more first uplink resources of the second serving cell for the UE to use to transmit the beam index indication of the selected first beam.
16 . The UE of claim 14 , wherein the one or more processors are configured to cause the UE to:
perform a logical channel prioritization procedure for a MAC protocol data unit comprising the uplink MAC control element, wherein the logical channel prioritization procedure provides a priority for the uplink MAC control element greater than each other uplink MAC control element of the MAC protocol data unit, greater than data of the MAC protocol data unit, and less than information of a common control channel message of the MAC protocol data unit.
17 . The UE of claim 14 , wherein the beam index indication of the selected first beam is transmitted based at least in part on a comparison of a timing of an availability of the one or more first uplink resources to a threshold value.
18 . The UE of claim 14 , wherein the one or more processors are configured to cause the UE to:
transmit the scheduling request on uplink control resources that indicate, to the base station, a beam of the serving cell that has failed; and
receive, from the base station in response to the scheduling request, the indication of the one or more first uplink resources in a second serving cell for the UE to use to transmit the beam index indication of the selected first beam.
19 . The UE of claim 14 , wherein the one or more processors are configured to cause the UE to:
transmit the scheduling request on uplink control resources of a transmission occasion, the uplink control resources indicating, to the base station, that a beam of the serving cell has failed, and the transmission occasion of the uplink control resources associated with a set of serving cells to indicate the serving cell on which the beam has failed; and
receive, from the base station in response to the scheduling request, the indication of the one or more first uplink resources in a second serving cell different than the serving cell for the UE to use to transmit the beam index indication of the selected first beam.
20 . The UE of claim 14 , wherein the scheduling request comprises a random access message of a random access procedure to establish the communication link between the UE and the serving cell of the base station.
21 . The UE of claim 14 , wherein, to determine to establish the communication link, the one or more processors are configured to cause the UE to:
communicate with the base station via the communication link;
identify a beam failure for the communication link between the UE the serving cell; and
determine to establish the communication link between the UE and the serving cell based at least in part on the identified beam failure.
22 . The UE of claim 14 , wherein the one or more processors are configured to cause the UE to:
receive a configuration for beam failure recovery, the scheduling request for the one or more first uplink resources transmitted based at least in part on the received configuration.
23 . The UE of claim 14 , wherein, to transmit the scheduling request for the one or more first uplink resources to the base station, the one or more processors are configured to cause the UE to:
transmit a scheduling request sequence indicating the scheduling request to the base station.
24 . The UE of claim 23 , wherein the scheduling request sequence is transmitted on uplink resources corresponding to a highest priority logical channel configured for the UE.
25 . The UE of claim 14 , wherein, to transmit the beam index indication of the selected first beam, the one or more processors are configured to cause the UE to:
transmit, to the base station, the uplink MAC control element in a random access message of a random access procedure.
26 . The UE of claim 14 , wherein the one or more processors are configured to cause the UE to:
initiate a timer and a counter based at least in part on determining to establish the communication link;
increment the counter based at least in part on transmitting the beam index indication of the selected first beam; and
retransmit the beam index indication of the selected first beam in the uplink MAC control element based at least in part on the timer expiring before a message is received on the selected first beam and the counter being below a counter threshold.
27 . An apparatus for wireless communications at a user equipment (UE), comprising:
means for determining to establish a communication link between the UE and a serving cell of a base station, wherein the serving cell comprises a secondary cell;
means for selecting, by the UE to establish the communication link, a first beam of a plurality of candidate beams of the serving cell;
means for transmitting, by the UE in accordance with a beam failure recovery procedure for the secondary cell associated with the base station, a scheduling request for one or more first uplink resources that the UE is to use to transmit a beam index indication of the selected first beam to the base station based at least in part on an unavailability of one or more second uplink resources for transmission of the beam index indication, the unavailability based at least in part on a number of slots between a current time and the one or more second uplink resources being greater than a threshold number of slots;
means for receiving, in response to the scheduling request, an indication of the one or more first uplink resources for the UE to use to transmit the beam index indication; and
means for transmitting, by the UE to the base station, the beam index indication of the selected first beam in an uplink media access control (MAC) control element on the one or more first uplink resources.
28 . The apparatus of claim 27 , wherein the scheduling request comprises a configuration of uplink control resources that are associated with the beam failure recovery procedure, and wherein:
the scheduling request is transmitted on dedicated uplink control resources configured to indicate, to the base station, a beam or a plurality of beams configured for the serving cell that have failed, and
the indication of the one or more first uplink resources is received from a second serving cell of the base station in response to the scheduling request and via a downlink control information indicating the one or more first uplink resources of the second serving cell for the UE to use to transmit the beam index indication of the selected first beam.
29 . A non-transitory computer-readable medium storing code for wireless communications at a user equipment (UE), the code comprising instructions executable by one or more processors to:
determine to establish a communication link between the UE and a serving cell of a base station, wherein the serving cell comprises a secondary cell;
select, by the UE to establish the communication link, a first beam of a plurality of candidate beams of the serving cell;
transmit, by the UE in accordance with a beam failure recovery procedure for the secondary cell associated with the base station, a scheduling request for one or more first uplink resources that the UE is to use to transmit a beam index indication of the selected first beam to the base station based at least in part on an unavailability of one or more second uplink resources for transmission of the beam index indication, the unavailability based at least in part on a number of slots between a current time and the one or more second uplink resources being greater than a threshold number of slots;
receive, in response to the scheduling request, an indication of the one or more first uplink resources for the UE to use to transmit the beam index indication; and
transmit, by the UE to the base station, the beam index indication of the selected first beam in an uplink media access control (MAC) control element on the one or more first uplink resources.
30 . The non-transitory computer-readable medium of claim 29 , wherein the scheduling request comprises a configuration of uplink control resources that are associated with the beam failure recovery procedure, and wherein:
the scheduling request is transmitted on dedicated uplink control resources configured to indicate, to the base station, a beam or a plurality of beams configured for the serving cell that have failed, and
the indication of the one or more first uplink resources is received from a second serving cell of the base station in response to the scheduling request and via a downlink control information indicating the one or more first uplink resources of the second serving cell for the UE to use to transmit the beam index indication of the selected first beam.