UE MAC layer sends scheduling request when data is arrived for one of the logical channel at UE and there is no PUSCH resource available to transmit data towards eNodeB. Here, it is assumed that the UE has PUCCH resource to send scheduling request to eNodeB.
UE receives scheduling request related configuration from eNodeB either through rrc connection setup or rrc connection reconfiguration message. There is an IE "PhysicalConfigdedicated" presents within the radioResourceConfigDedicated IE. The "PhysicalConfigDedicated" IE carries the "SchedulingRequestConfig" which contains all the necessary configuration information need to be known by UE to send Scheduling Request on PUCCH channel.
SchedulingRequestConfig ::= CHOICE {
release NULL,
setup SEQUENCE {
sr-PUCCH-ResourceIndex INTEGER (0..2047),
sr-ConfigIndex INTEGER (0..157),
dsr-TransMax ENUMERATED {
n4, n8, n16, n32, n64, spare3, spare2, spare1}
}
}
sr-ConfigIndex informs to UE about SR periodicity and SR sub frame offset.
Please check the table for detailed description from the physical layer specificaiton 36.213.
Hope, I answered your query.