Site taking traffic only after modifying PRACH config index

Dear Experts

Modifying PRACH config index from 3 to 35 site is taking traffic.

But not for 3.

Any rca for it?

4G Site

This might be because in 3 RACH congestion might be happening there.

Since in 3 UE can send RACH in any system frame number.

Yes logically correct but while checking in logs.

  • Unable to find anything in PHY and MAC logs.

  • In interface logs MME is seen sending attach reject with PLMN not allowed and eps service not available when index chnaged to 3.

But Why this behaviour? Would be there from Core?

Maybe Core people can explain.

Whose CN is it? NOKIA?

Yes :grin:.

Only from Nokia Core such rejections there.

With PRACHConfiguration Index = 3, the preamble can be only sent in odd and even frame SFN subframe number 1. Refer to below pic for more understanding.
Please check your configuration onAccess side also related to PRACH. eNodeB calculates PUSCH resources needed for UE uplink transfer based on the preamble sequence. Maybe some config mismatch is not allowing enodeb to allocate PUSCH resources properly. which may be working properly for PRACHConfiguration Index = 35.

1 Like

Hi Mohit,

I think you are noticing two different things. RACH configuration can not have any correlation with the Core releases / attach failures as Core has no idea of the RAN side RACH access and physical layer.
My theory is that potentially your cell has very large cell range. With preamble format 0 (configindex = 3), the RACH are reaching the enodeb at a time where it is discarded due to distance. But when you change prach config index to 35, your preamble format will also switch to 3 which has larger SEQ and guard period both. Now either this cell has large physical range (like fiber optics connected for indoor where length of fiber was large) OR you seriously have an interference problem (two same PCI on the same site different sectors) as this preamble format has slightly better RACH decoding performance.
I still think it is related to the preamble format change and not related to core though as I cannot think of any correlation,.