LTE: What does the use of SR-periodicity while handing Scheduling request ?

Question

UE can send SR at any time and CQI at configured time, for these two request how many RB's are allocated from ENB?

Answer 1

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.

Answer 2

The eNB has to strike a balance between the optimizing the limited PUCCH resource and the delay in performing RACH to obtain a PUCCH. Thus the SR is done in a periodic manner by the UE as defined by the sr-periodicity timer given by the eNB. The value of sr-periodicity should strike such a balance in order to expedite the resource allocation and manage the PUCCH load.