What does the eNodeB reply to UE when the requested radio resource are not available?

The UE needs to request an uplink resource for transmitting uplink data via the scheduling request procedure. I wonder what happens if the requested uplink radio resource can’t suffice. For example, the UE requested to transfer n-KB data which needs 25PRBs, however, the eNodeB only has 15 PRBs at this moment. Does the eNodeB just ignore this SR, ignore this SR with a replay which indicates there are not enough radio resources, or assign UE as many PRB as the eNodeB has? Which one is correct?

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It Just Schedules the UE in next TTI

Thank you @Adeeba.
Does that mean this scheduling is failed and nothing needs to send back to UE? UE will launch a new scheduling request at the next properly TTI.
And do you know which part of 3GPP specification introduces this procedure? I’ve looked the 36.321, which describes the SR procedure on UE side in Sec 5.4.4, however, I do not find the description on eNode side.

The eNB should give the 15 PRBs available and, at the time of the UL transmission, the UE will send a buffer status report (BSR) along with PUSCH. The BSR will tell the eNB that the UE still has data to be transmitted in its buffer. The BSR will serve as an SR for the remaining data.

Thank you @gzilli for this helpful information.

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Consider that in real scenario, probably there are multiple users in need of uplink resources so even if 25 PRBs are available, all will not be assigned to single UE.
If UE still has data to send after first allocation in uplink, based on BSR, UE will be scheduled required PRBs for remaining data in buffer in next TTIs untill buffer is empty.

One point to consider regarding uplink grant is that BSR is mandatory for UE for each UL Grant, this is one of reasons for increase of UL interference once Preallocation is activated. When network send UL Grant even when UE does not have any data to transmit, UE transmit all 00 data or some garbage data and long padding 0s. In this situation, UE would put BSR MAC CE as part of MAC PDU and set BSR index value all 0.

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@WingPig99
eNB does need to inform UE. UE will transmit SR and wait based on SR prohibit timer when it expires UE transmits SR again and restarts the prohibit timer and so on. If the number of times UE sends SR without receiving UL grant reaches dsrtransmax, ue will release Pucch resources and use Random access instead