LTE: Scheduling Request Procedure


The Scheduling Request (SR) is used for requesting UL-SCH resources for new transmission.
UE’s MAC triggers scheduling request when a regular BSR is triggered and UE doesn’t have uplink resources for transmission of at least the regular BSR. Regular BSR is triggered when data becomes available for transmission in the uplink.
eNodeB needs to configure the UE with SR configuration via RRC signalling in order for the UE to transmit SR on PUCCH. The SR configuration structure is as given below

sr-PUCCH-ResourceIndex indicates the UE with the frequency domain resources whereas sr-ConfigIndex determines the time domain resources of PUCCH which carriers SR. eNodeB controls the maximum number SR transmissions from each UE on PUCCH using the parameter dsr-TransMax
If the UE has no valid PUCCH resources (SR is either not configured or released), then the UE initiates Random Access procedure
Once SR is triggered, the UE calculates the SR periodicity and offset (explained at the end of this post) which is based on sr-ConfigIndex IE. After transmitting the first SR on PUCCH, if the UE doesn’t receive uplink resources from the eNodeB, then based on the periodicity, the UE re-sends SR on PUCCH. This process continues till UE transmits SR for dsr-TransMax number of times on PUCCH if the UE doesn’t receive uplink resources from the eNodeB. After transmitting SR for maximum (dsr-TransMax) number of times, the UE releases SR resources (frequency as well as time), initiates random access procedure and cancels all pending (triggered) SRs
SR periodicities of 5, 10, 20, 40, and 80 ms are initially proposed in release-8. In release-9, short SR periodicities of 1 and 2 ms are introduced which reduces the latency.
When a short SR period is configured or when running VoIP traffic, the SR can be retransmitted unnecessarily. To avoid unnecessary SR transmissions, in release-9, an SR prohibit timer (sr-ProhibitTimer-r9) is introduced to reduce the load on PUCCH.
sr-ProhibitTimer-r9 IE is under mac-MainConfig and it can take values from 0 to 7. SR prohibit timer value is in number of SR period(s). Value 0 means no timer for SR transmission on PUCCH is configured. Value 1 corresponds to one SR period, Value 2 corresponds to 2*SR periods and so on. The UE starts this timer after transmitting an SR. When this timer is running, the UE is not supposed to be transmitting SR on PUCCH.
UE uses PUCCH Format 1 for transmitting SR alone. When SR and HARQ feedback in uplink happens to coincide in the same subframe, the UE shall transmit HARQ feedback on SR's frequency resource using PUCCH Format 1a/1b. Since PUCCH received is on SR resource, the eNodeB can easily understand that HARQ feedback and SR are present in the same subframe
If the UE is not configured for simultaneous PUSCH and PUCCH transmission or, if the UE is configured for simultaneous PUSCH and PUCCH transmission and not transmitting PUSCH, in case of collision between CSI and SR in a same subframe, CSI is dropped
UE release SR resources in the following cases
  • When UE has transmitted SR for maximum amount times (dsr-TransMax)
  • After timeAlignmentTimer expiry
  • During MAC reset procedure
  • During Handover (during Handover, MAC reset is performed)
During Handover, the source eNodeB has to provide the UE with a new SR configuration to be used in target eNodeB as the UE releases the existing SR resources in source eNodeB
SR configuration table is given below (Table 10.1.5-1 from 3GPP TS 36.213). SR transmission instances are the uplink subframes satisfying (10*SFN + subframe ‒ Noffset,SR) mod SRperiodicity = 0. 
SR configuration Index ISR
SR Periodicity (ms)
SRperiodicity
SR subframe offset Noffset,SR
0 ‒ 4
5
ISR
5 ‒ 14
10
ISR ‒ 5
15 ‒ 34
20
ISR ‒ 15
35 ‒ 74
40
ISR ‒ 35
75 ‒ 154
80
ISR ‒ 75
155 ‒ 156
2
ISR ‒ 155
157
1
ISR ‒ 157

One can also calculate SR instances from the below 'SR Calculator'

sr-ConfigIndex (0-157):          TDD UL-DL Config:

                     Is TDD?

SR timing will be displayed here

Full details of Scheduling Request subframes within one full SFN cycle

Posted by KP
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39 comments:

  1. AnonymousAugust 29, 2014 at 11:47 AM

    Can you please tell me from which 3gpp standard all this written detail can be found. Any link would be really help full.

    ReplyDelete
  2. AnonymousJanuary 31, 2015 at 4:07 PM

    When max SR is reached, UE initiates rach procedure. In this case, MSG 3 will be Re-establishment msg or CRNTI msg or RRC connection Request?

    ReplyDelete
    Replies
    1. KPJanuary 31, 2015 at 9:58 PM

      C-RNTI MAC CE.

      Delete
    2. AnonymousMay 14, 2022 at 6:48 PM

      It's not actually MSG3, ENB provides the RAR after decode RACH from UE,so in the RAR UL grant is available to send BSR only.

      Delete
    3. AnonymousMay 14, 2022 at 6:49 PM

      So UE will use UL grant and Send BSR report then next allocation to be done

      Delete
  3. AnonymousMarch 21, 2015 at 5:01 PM

    great

    ReplyDelete
  4. vijyantMarch 21, 2015 at 5:04 PM

    reason of rach request is ul data arrival after max sr transmission

    ReplyDelete
  5. AnonymousSeptember 1, 2015 at 4:19 AM

    The only reason that MAC initial a RACH procedure is to send a BSR while SR resource is not available. If the target cell has SR configuration available, how does MAC trigger a RACH during HO procedure? Thanks.

    ReplyDelete
    Replies
    1. KPSeptember 1, 2015 at 9:08 AM

      The SR configuration is cleared during HO.

      36.332: During handover, the UE performs a MAC reset, which involves reverting to the default CQI/ SRS/ SR configuration in accordance with subclause 5.3.13 and TS 36.321 [6, 5.9 & 5.2]. Hence, for these parts of the dedicated radio resource configuration, the default configuration (rather than the configuration used in the source PCell) is used as the basis for the delta signalling that is included in the message used to perform handover...

      The target eNB has to provide SR configuration during RRC connection establishment/reconfiguration procedure. Prior to that UE's MAC can only use RACH...I hope it helps...

      Delete
    2. Amit KJune 26, 2019 at 8:10 PM

      I have question if sr is used for uplink grant then why rach procedure is required.

      Delete
    3. AnonymousJune 1, 2022 at 11:52 AM

      It Requires RACH for Uplink grant because the EnodeB fails to assign the resources for UE in SR.

      Delete
  6. satishOctober 28, 2015 at 10:37 PM

    Could you explain in which case Network will provide UL grants for "PUSCH without UL-SCH data"
    Is it same as empty UL grants?

    ReplyDelete
  7. McKeagueJanuary 27, 2016 at 5:00 AM

    hi Kumar, nice explanation. Can you comment on how the UE figures out which RB pair to use in the frequency domain?

    ReplyDelete
    Replies
    1. KPFebruary 2, 2016 at 10:17 AM

      The UE gets to know the RB allocation in DCI format 0/4 transmitted on PDCCH. For more information, please refer to http://howltestuffworks.blogspot.se/2012/07/resource-allocation-type-2.html

      Delete
    2. KPFebruary 2, 2016 at 10:22 AM

      This is for Uplink

      Delete
  8. UnknownJune 28, 2016 at 5:09 PM

    Hi Kumar,
    Can you help me with the mapping of "sr-PUCCH-ResourceIndex" with the frequency domain resources, similar to that of time domain you have given.

    Thanks

    ReplyDelete
    Replies
    1. KPJuly 11, 2016 at 11:38 AM

      Hi Dileep,

      Please check the following post for PUCCH resources.
      http://howltestuffworks.blogspot.se/2015/05/pucch-frequency-resource-calculator.html

      Delete
  9. Social JargonDecember 22, 2016 at 6:56 AM

    Thank you Kumar...It is nice...

    ReplyDelete
  10. eng ali ismailMarch 1, 2017 at 12:40 AM

    Hi Kumar , Thanks for this great explanation, Could you please explain how SR works?

    ReplyDelete
  11. AnonymousMarch 18, 2017 at 10:55 PM

    Hi Kumar,
    As you said, UE may choose PUCCH Format 1A/1B in case HARQ/CQI to be transmitted along with SR.
    How does eNB know the PUCCH format used by UE.

    Thanks

    ReplyDelete
    Replies
    1. KPSeptember 7, 2017 at 11:49 AM

      Hi,
      The eNB knows how many TBs it had sent to this UE and based on 1-bit or 2-bit HARQ Feedback needed from the UE, the eNB respectively expect PUCCH Format 1a/1b.

      Delete
  12. fawazApril 20, 2017 at 2:45 PM

    hi Sir,,
    can you please tell me which the best simulation tools can I used Group paging for pull based approach that used for MTC devices.. please

    ReplyDelete
    Replies
    1. KPMay 15, 2017 at 9:24 AM

      Hello,

      I am really sorry that I can't help you on this due to my unawareness of the topic.

      Delete
  13. AnonymousSeptember 26, 2017 at 3:26 PM

    Hello Kumar,
    That is a very nice explanation. Well, Can you please provide some details on why the range for sr-pucch-resource-index is between 0 -2047? What I understand, using this index cyclic shift, orthogonal cover and PRB on which UE can send a SR are derived. But why the range is kept as 0-2047 is what I am trying to understand.

    ReplyDelete
  14. AnonymousJune 19, 2018 at 5:04 AM

    Hi Kumar,

    Thank you very much for the explanation. However about possible values for prohibit timer should be ms1, ms2 ... ms128? At least spec 38.331 says so.

    ReplyDelete
  15. AnonymousMarch 23, 2019 at 11:18 AM

    I could not find this parameter in RRC Connection setup even it is configured in the Cell
    sr-ProhibitTimer-r9
    Please your help

    ReplyDelete
  16. UnknownSeptember 1, 2019 at 6:16 PM

    I have a question about scheduling request :

    *If scheduling request configuration coming in RRC CONNECTION SETUP MSG, so how to justify scheduling request before the RACH procedure*.

    ReplyDelete
    Replies
    1. AtulJune 26, 2021 at 10:58 PM

      As far as I know, before msg4 in fresh attach, UE can't sent SR,that's a another reason UE sents prach for ul grant,

      Delete
  17. AnonymousJuly 2, 2021 at 8:28 PM

    Hi Kumar
    How does the eNB know how much data UE wants to transfer? In other words how many transport blocks to allocate the the UE?

    will greatly appreciate any pointers.

    ReplyDelete
    Replies
    1. McKeagueJuly 7, 2021 at 10:07 PM

      its in the BSR - see https://www.sharetechnote.com/html/Handbook_LTE_BSR.html

      Delete
  18. MunzirAugust 7, 2021 at 2:36 PM

    What is the negative impact of reducing SR periodicity

    ReplyDelete
    Replies
    1. AnonymousOctober 24, 2021 at 10:26 PM

      See the following contribution by Huawei, HiSilicon: R2-1705625 3GPP TSG-RAN2 Meeting #98 Hangzhou, China, 15 – 19 May 2017 for the tradeoff.

      Delete
  19. AnonymousOctober 24, 2021 at 10:21 PM

    In LTE, how many SR configurations can a MAC entity be configured with?

    ReplyDelete
  20. AnonymousDecember 4, 2021 at 11:19 PM

    Hello
    For phr, does UE needs to send SR ?

    ReplyDelete
  21. UnknownJanuary 3, 2022 at 4:33 AM

    how enodeb get to know this rach for dsr max reached? what parameter and where we can know?

    ReplyDelete
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  23. AnonymousJune 21, 2022 at 2:01 PM

    Can anyone tell me the concept of negative SR in LTE and when it should come?

    ReplyDelete
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