How UE decide SSB beam to select in 5G SA?

Hi Experts,
For SA call flow, sib1 content is very important message.
But I find under RACH config common for SA parameter RSRP SSB threshold not there.
Now query is: how UE decide to select SSB beam for SA if there are 2 SSB beams configured on site?

For NSA I’m able to find it, under RACH config common which make sense as per logic, but for SA why is it missing?

If RSRP threshold is not defined, then UE will select strongest beam.

Is it not allowed parameter for SA?
In my view some bug here…

May be. Parameter can be there if configured.

Yes, I’m checking validity for parameter is it for SA or not, but point is that without it how UE will judge? Is it blind selection of SSB beam?

Why blind selection?
Strongest one will be selected.
Even if threshold is defined, and all the beams are weaker than threshold, then also strongest beam is selected.
NSA or SA this parameter can be there if configured.

But UE need some algorithm for it.
It’s not inbulit in UE to select strongest beam, it can select week one as well my view.

What algorithm UE have when no beam satisfy the criteria?
To select strongest beam right?

3GPP TS 38.321 5.1.2 Random Access Resource selection

1> else (i.e. for the contention-based Random Access preamble selection):
2> if at least one of the SSBs with SS-RSRP above rsrp-ThresholdSSB is available:
3> select an SSB with SS-RSRP above rsrp-ThresholdSSB.
2> else:
3> select any SSB.

1 Like

Hello ,

Would you mind sharing for NSA under RACH config common what parameter is there to hep UE select SSB beam?


It is the rsrp-ThresholdSSB


Hi NR Experts.

Please tell for mobility between 2 cells having multiple ssb depends on L3 measurement basically on cell level measurements.

After handover how UE decide to latch on particular SSB beam index (if cell having multiple SSB beams)

Cell 1 : ssb beam index : 0,1,2
Cell 2 : ssb beam index : 3,4,5

Ue need to handover from cell 1 to cell 2.
How will it decide SSB beam index?
After handover.

In the handover command, UE will be given RACH configuration of target cell.

UE just uses this info (RACH occasions, and in case if CFRA dedicated preamble associated with each SSB beam) to access target cell/beam.


So it’s on basis of SSB RSRP threshold as well like fresh attach on NR SA by RACH procedure?

While sending MR before handover, UE gives beam wise measurement also to gNB.

On that basis gNB can prepare handover.

Like if UE report beam id 1 from target cell, then PRACH preamble specific to beam id 1 will be given to UE in handover command.

In initial attach UE select proper PRACH occasion based on beam id and association period configuration.

Yes beam level result there, but we consider generally cell level results which is linear average of SSB beam exceeding SSB threshold defined under meas object.

So let’s say UE latched over SSB beam index 1 on cell 1 after handover move to which SSB beam index on cell 2.

It’s not defined

But as you mentioned, after HO UE will select new beam index on the basis of SSB RSRP threshold, so it’s ok.

+ in handover command, gNB can include beam specific premable number for strongest beam in mr, or for all beams of target cell.

Thank You!

Now clear, let me check.