Sunday, 22 September 2013

SR and SRmax in LTE



SR (Scheduling Request) is a request of scheduling radio resource for uplink transmission by the UE to the eNB. UE send SR on PUCCH when regular BSR (Buffer status report) is triggered and UE does not have radio resource to transmit this BSR to eNB. 

To send this SR, UE must be configured with SR periodicity. If PUCCH resource is not configured for SR, UE start RA (Random Access) procedure. Since PUCCH resource is limited, SR periodicity should not be very less (short periodicity leads to fast resource allocation). If we fix short periodicity for SR, UE may not utilize the PUCCH for SR transmission allocated each time (UE may not have data to transmit in uplink always).  There must be a tradeoff between PUCCH load and SR periodicity. 

UE always keep track of SR transmission on PUCCH. After reaching the maximum number of SR transmission (SR Max) and UE has not been allocated any radio resource for uplink transmission, it releases PUCCH resource for SR and start Random access procedure. This behavior could be because of bad transmission power, wrong configuration of SR, uplink coverage is very poor or UE may be in Handover situation. Radio resource for uplink transmission is granted only after 4ms of SR transmission.

How to configure SR periodicity and SR subframe offset for SR transmission (Refer 3GPP 36.213, section 10.1).
UE transmit SR on sub-frames which satisfy below formula:

Where,
nf = system frame number
ns = {0,1…19} slot index within the frame
NOFFSET,SR = SR subframe offset (Mentioned in below Table)
SRPeriodicity = SR Periodicity (Mentioned in below Table)


Table : UE-specific SR periodicity and subframe offset configuration
 

 Example: If Higher layer is configured with srconfigindex (ISR) =6  for an UE , then on each 1st sub-frame with 10ms periodicity , UE will transmit SR request to eNB and eNB will grant radio resource to UE after 4ms of SR transmission.

Questions are welcome.
 


3 comments:

  1. hi,
    Please if you could answer my query,how the subframe offsets should be calculated in different tdd configurations,because SR should be received in ul specific subframes only.please if you could help here a bit.

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  2. Hi Marshall,

    I think there should not be any differences between calculation of subframe offsets in TDD and FDD , also I have not seen any different table in 3GPP specs particularly for TDD UE-specific SR periodicity and subframe offset configuration , so I assume we have single table ( 3GPP 36.213, section 10.1) for both FDD and TDD. Differences between the two are very limited to the physical layer implementation and only minor ones in higher layers. If you have any specific SR configuration parameter values for TDD please let me know, I will look into that.

    The same spectrum will be used for DL and UL alternatively in TDD. Hence, when it will be used for UL transmission , SR will go on subframe according to what we have configured on higher layer and it will be calculated by same UE-specific SR periodicity and subframe offset configuration table (3GPP 36.213, section 10.1 ) .

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  3. Hi Marshal,
    in case of TDD sr-ConfigIndex should be chosen such a way that offset always result in a uplink sub-frame.

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