If there is no RAR response from eNB/gNB will UE keep sending msg1 till preambleTransMax?

Hello Experts.

During RACH, if there is no RAR response from eNB/gNB, then will UE keep sending msg1 till preambleTransMax?

If preambleTransMax has reached, then random access failure is reported to higher layer. If this is keep on happning many times, what is behaviour of UE/higher layers?

Will UE stay on that cell only and keep attempting RACH or can it select/reselect new cell for RACH?


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It will send SCG failure with cause RACH failure.

No, in this case I am talking about initial access. Not EN-DC.

It depends on reselection conditions.
Will return back to idle and UE will follow reselection conditions & criteria.
But NAS layer will trigger traffic soon and so a new RACH process will occur.

For RACH failure I think UE will apply a back-off time what got eNB to delay a transmission of a subsequent random access preamble.

The eNB can signal the UEs that they have to wait a certain time before they try to connect again.

Yes what after many RACH failure?

Yes after RACH failure, if bfi is not there then UE will do wastage of battery.

Ok. I was just exploring whether UE perform cell selection or reselection again after many rach failures.

Yes it will do reselection, because this is what UE does in idle mode.

After rlf I think process will happen what you saying.

For rach failure Cases ue will wait as per bfi & then resend rach request.

After RLF it’s cell selection always.

Sends while T300 is running.

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Nice explanation from @Jaeku_Ryu in Sharetechnote:

Sharetechnote → Backoff Indicator

Do you mean how many rach failure there can be before UE goes to idle?

It performs reselection (not selection) after rach failure.

This part I want to explore.
Any spec which says this?

Cell selection happens only at power on and coming from no coverage (PLMN search).

Some details here: LTE UE cell selection | LTE UE cell reselection

Quote:What happens when lower layers sends RACH problem indication?

When upper layers receives RACH problem indication, UE will re-try to camp on to the same cell, while parallel doing PLMN search to find any other alternative cells.

Alternative cells can be of same PLMN also?

I think that’s more hypothetical.
If Continuous RACH fails and it will try other EARFCN of that cell I believe.

When there is no msg2 response, UE can send 5 RACH attampts each seperated by a specific NAS timer. Each RACH attempt contains a cluster of MSG1 requests allowable by RACH parameters.

After the failure counter reaches 5, NAS layer will start T3402 and block that PLMN.

From the network if back off indicator timer configure then UE will re for Rach else UE will check for second priority cell from initial search table.