Hello Experts,
I have a very basic doubt on beam sweeping.
For an 8 bit SsbBurst, if I set all 8 bit to 1s; Is the gNB transmitting all beams simultaneously or switching between them?
It will transmit all 8 beams in different momentts in time.
8 ssb in different moments each and different locations each.
SSBs are separated by time withing SSB burst.
So not all SSBs can be transmitted simultaneously.
Every SSB is aplied with different spatial filtering co-efficient.
So they will transmit in different direction each.
Lets say at t=0, two UEs at opposite direction from the GNB, one can only see beam#0 and other beam#7, would both of them be seeing their respective beams at t=0?
Ideally, NO.
Basically I am thinking at a particular time instant, all the UEs shud be able to do DL sync, which would require all beams to be active simultaneously.
Then doesn’t this actually result in more sync time requirement?
There are different beams in a cell. SSB Beam, CSI-RS Beam, PDSCH beam, etc.
All these type of beam can have different number, in a cell.
For example, a cell can have 8 SSB beams, and 32 CSI-RS beams.
So a any instantof time, UE must be deconding/reading something.
So there should not be any Sync problem
This is my understanding so far.
PDSCH beam equal to ssb or CSI-RS as data carry on layers only.
1 beam equal 2 layers or 2 streams or 2 narrow beams.
What happens is that t0 UE1 will measure RSRP of beam0 wit the highest RSRP value meaning it will select beam0.
Similarly at t1 UE2 will measure RSRP of beam 7 with highest RSRP so it will select beam 7 to send RACH.
This is how each UE “camps” on a different SSB beam number.
Idea is that 2 SSB beams are never transmiited in the same time but in different moment in time.
What each UE has to do is to find the moment in time when RSRP is the strongest and will use this time to send RAHC (not exactly this time but a coresponding time).
Got it.
But then don’t UEs loose time since at t0 UE2 cannot find beam 7?
Ssbthresholdrsrp parameter used for same configured at gNB end.
It’s for idle mode only.
Beam sweeping.
UE is always searching for strongest RSRP.
Even in idle mode still looking aalways for strongest RSRP.
This point is clear. Only that at a particular time instant, only one beam has best RSRP which means other UE cannot see their beam at that time instant.
I think @Nitin is asking what happen when there is no beam serving UE at particular instant of time.
In my opinion, such situation may not arrive.
Because if not SSB then CSI-RS beam UE able to read through.
You got my point @sadanandk2.
But let’s say both UEs are trying to sync, so there is no csi available at this time, right?
At t0 UE1 will measure RSRP_beam0 with -85 dBm and UE2 will measure RSRP_beam0 with -110 dBm.
So UE1 will definitely select RSRP_beam0 whereas UE2 will not because it will measure another beam stronger at a different moment in time
Yes.
This situation comes in idle mode only.
And in idle mode there in NO Sync established in DL or UL.
UE just have to keep reading strongest beam.
When i say Sync, it means Radio link monitoring.
RLM function is not performed in idle mode.
I agree.
So when UE1 is seeing beam0 and Synchronising, UE2 won’t be able to sync at that time.instant as gNB can only transmit 1 beam at a time, right?
Right.
But by word syncronization, do you mean initial synchronization by PSS-SSS?
PSS-SSS content are same in every SSB beam transmitted in the cell.
So by reading PSS-SS in any of the beam, UE can get time synchronization and decode PCI of the cell.
Particular strongest beam will be required to perform the RACH.