LTE Scheduling Request

Scheduling Request is triggered when UE is in uplink sync with eNodeB and doesn’t have any PUSCH/PUCCH resource allocated for transmission of the new control plane or user plane data. 

UE uses the SR-Configuration received in RRC connection setup or RRC reconfiguration request to determine the next Scheduling Request (SR) available opportunity (computes the frame and sub-frame).

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}

}

}

Based on the ConfigIndex, UE-specific periodicity and subframe offset is determined as below.

SR configIndex (I)	SRperiodicity (ms)	SR subframe offset
0 – 4	5	I
5 – 14	10	I-5
15 – 34	20	I-15
35 – 74	40	I-35
75 – 154	80	I-75
155 – 156	2	I-155
157	1	I-157

SR indication is transmitted based on the configured sr-PUCCHResourceIndex in the SFN and sub-frame number which satisfies the below condition.

((SFN*10+subframe)-SRoffset) mod SRperiodicity = 0

Nevertheless SR will NOT be transmitted if:

1. SR opportunity falls in measurement gap or
2. SR-Prohibit status timer is running or
3. DL PDCCH with UL resources is received

If eNB doesn't allocate any DCI's till when UE needs to retry SR?

UE is configured with SR-Prohibit timer & max SR retries. Whenever a SR is transmitted, SR Prohibit timer is started and UE is not supposed to re-transmit (when no valid PUSCH resource is received) SR  till SR-Prohibit timer is expired. When UE has retried max SR times, UE is considers to have lost uplink synchronization with eNB and will release PUCCH/SRS resources and initiate RACH procedure. 

References:

1. 3GPP 36.321
2. 3GPP 36.213