How does the gNB determine the grant size after a UE sends a Scheduling Request (SR)?

[susbose10]

Hello Experts,

I have a basic question:

When the UE sends a Scheduling Request (SR), how does the gNB determine the size of the grant to allocate?

[RFSpecialist]

Answer is here, based on buffer statuts report

LTE: Scheduling Request Procedure

[susbose10]

But to send a BSR (Buffer Status Report), the UE needs an uplink grant, right?

This grant is provided after the gNB receives the UE’s Scheduling Request (SR).

[RFSpecialist]

BSR (Buffer Status Report) is sent periodically by the UE.

The Scheduling Request (SR) is triggered only when the UE does not already have resources allocated based on its BSR reporting.

[susbose10]

@sadanandk2

SR (Scheduling request) is used to notify the eNB that there is data to transmit at the UE RLC/PDCP. So eNB grant some minimum resources to UE.

With these resources UE start transmitting UL data, where MAC header consist of MAC CE , indicating BSR. By reading BSR then eNB keep giving grant to UE as long as there is BSR.

So in short, BSR cannot be sent in uplink unless UE have some UL grant. And to get UE UL grant, UE has to send SR

This is the actual source of my query

[pappu_kumar_Chaurash]

The MAC Control Element (MAC CE) of the BSR (Buffer Status Report) is included at the MAC layer during uplink transmission. It provides an index to indicate how much data is in the UE’s uplink buffer, allowing the network to allocate further uplink grants. This process continues until the BSR indicates the buffer is empty.

And SR, as already defined, it is used by UE to notify the network about some new data in UL buffer.

At scheduling request attempt I think very minimal resources is provide in UL grant and once UL transmission started, the information about UL buffer is indicated by BSR… now no need to send SR.

[susbose10]

@pappu_kumar_Chaurash

I think very minimal resources is provide in UL grant

Thanks, I want to understand “I think very minimal resources is provide in UL grant”.

Here’s my question: How does the gNB determine the minimum uplink grant to provide?

Is it based on predefined configurations, or are there other factors involved in this decision?

[pappu_kumar_Chaurash]

No, I haven’t come across any configuration directly or indirectly related to the Scheduling Request that specifies the grant size included in the request.

It might be dependent on the OEM, but I’m not entirely sure.

[sadanandk2]

@susbose10

Here’s my question: How does the gNB determine the minimum uplink grant to provide?

Is it based on predefined configurations, or are there other factors involved in this decision?

So your question is “After SR is received, how much grant is given to UE and on what basis”, right?

[susbose10]

Yes, exactly.

[sydanjm]

It depends on the vendor implementation.
gNB generally allocates the SR grant for a fixed bytes where it can receive some of the required MAC CE (like BSR, PHR) and also if it can accommodate smaller upper layer packet in first grant itself.
I have seen it to be not configurable (e.g. fixed 180 bytes) which makes to 2/3 PRBs. But if UE is running at poor radio condition, it could go low also to keep lower number of PRBs.