5G NR MIMO 4x4 in DL

Oh, got it.
Then UE will need to be camped on two SSB beams?

I think those are dedicated user beams i.e PDSCH ones, not cell beams like SSB.

I think I understand the text in book now.
In its example, basically antenna array is 8 elements (4 +45 + 4 -45).
Author is suggesting that 1st UE is being served 4x4 layer using 4 +45 polarization and 2nd UE being served 4x4 using 4x -45 polarization.
I had long discussion on similar topic with a colleague long time ago and we sort of agreed that +45/-45 polarization of 1 antenna element actually points in same direction.
This book is suggesting otherwise.
I cannot confirm this.

I think i found the answer in Sharetechnote (@Jaeku_Ryu) page that each stream of MIMO is generated by a sub-array and not by all antenna elements.
Please see attached picture and the link here:


Hope this clarify all the doubts.
So if there are 8 streams for 8x8 DL MIMO there will be 4 subarrays generating them each subarray having a +45 and a -45.
Don’t know what was written in that book and I am not trying further to understand…

Hi @RFSpecialist.
If we have 128 AE (64 T64R) ie 64 (+45) and 64 (-45) then considering case of dual stream.
I think all 128 AE will use for steam generation ie 64 for one steam and 64 for second stream.
How to decide which AE to used out of 128?

I also think the same that for 2x2 DL MIMO layers will be 2 streams one +45 from 64 AE and another -45 from 64 AE.
I don’t know what priprietary algos decide which AE to be used for that but I think it has to do with N1, N2, O1and O2 numbers and number of CSI-RS ports as written on sharetechnote page.

Yes same apply for 5G NR active antenna as well.

UE will know type of antenna like explained here.

@RFSpecialist, above picture given for TM9 mode?
Or 5G NR you referring?

Is 5G from same page Sharetechnote.

As I know, ur give feedback about their condition, if the condition is bad, gnb try to change direction to another degree of beam for that UE…

Text written in book is correct. In the book , author is explaining about creating 2 beams that are individually steered. The 2 RF chain or antenna ports are connected to a physically separated set of AE’s , in this case at ±45 polarization. When he says the other 2 beams use same antenna ports , what that mean is the RF chain for these 2 beams are using same set of AEs. The term beam is a bit misleading when it comes to digital modulation. When the same set of AEs transmit signal from multiple RF chains , the radiation pattern is not unidirectional , but scattered with some directions more prominent then others. This decision is not arbitrary but antenna design choice.

The more prominent directions have to be spatially separate, that is the reason author talks about dependency on multipath for UE to recover these 2 different pair of beams.