5G NR MIMO 4x4 in DL

RFSpecialist · June 7, 2020, 3:00am

Hi All,
I have a question related to 5G NR MIMO 4x4 in DL.
I am trying to understand the phrase highlighted in yellow in attached picture from 5G in bullets book.
I can’t understand how gnodeB is able to generate 4 streams, using all antenna elements, using same PRBs, using same time and point them towards 2 different directions.
This would be impossible in my opinion.

It is possible to do 2 streams with half of antenna elements and the other 2streams with the other half of antenna elements.

ran_core_consultant · July 17, 2020, 4:54am

This note is bit confusing as different concepts are mixed together.
Here is my understanding:
Antenna elements are actual physical antenna hardware, radiating signal.
Mostly LTE has 2 columns of atennas elements, +45/-45 and can be 4 or 8 of these.
5G uses massive antenna arrays .
4x4 requires 4 logical antenna ports (because of 4 distinct CRSs).
These 4 logical ports can still be mapped to above 4 or 8 Antenna elements in beamforming, sub-arrays are used to direct strong beam in certain direct.
Number of beams depend on on whole Antenna system (more number antenna elements), more number of strong beams.
And yes, what you described is 2 beams x 2-layers each. so you can have 2 MU-MIMO users such that each will be using same time-freq resource block in DL and each user will 2x2 SU-MIMO.

Also check this link:

RFSpecialist · July 17, 2020, 4:57am

This is why I asked: how can we deliver 4 streams of MIMO to an UE using all antenna elements and point into 2 different directions like explained in the book.

ran_core_consultant · July 17, 2020, 4:58am

Hello @RFSpecialist, please check the link i sent.
That explains these concepts very well.
But if you are system architect working on beamforming then this might not be enough for you.
MIMO (SU/MU), Beamforming are related concepts.
Depending on AAS type you can have 4 MU-MIMO users (each 4x4 SU-MIMO, so total 16 Layers), using same time-frequency resource.

RFSpecialist · July 17, 2020, 4:59am

Yes I 've read the info in the link.
I am not a system architect just interested to know. My discussion is for single user MIMO, not for Mu-MIMO.
Info in the link is too general.
Question is how can gNodeB point the beams into 2 distinct directions using all antenna elements?

So once again query is: how can gnodeB generate 4 streams of DL MIMO for single user (2 in one beam 2 in another beam) in the same time using all antenna elements and pointing them into 2 different directions (to have uncorrelated paths)?

samohiuddin · July 17, 2020, 5:02am

It’s not a simple concept to explain without Equation and Matrix.

There is a Course on IIT rooorke on YouTube where the professor explains these concepts in details with calculation.

cmarkoz · July 17, 2020, 5:03am

Follow this file, it can help to clarify the flow:

layers and codings v6-1.xlsx (325.6 KB)

ran_core_consultant · July 17, 2020, 5:04am

For SU-MIMO, beamforming means nothing.
4 layers are transmitted.
Please note these 4 layers are logical…physically there can be 4 or 8 antenna elements transmitting these 4 layers.

See this link for antenna port to physical port concept:

ran_core_consultant · July 17, 2020, 5:06am

How beamforming is done is separate concept.
It can be digital or analogue.
See:

RFSpecialist · July 17, 2020, 5:08am

My question is very simple: how gNodeB use all antenna elements and still point the beam in 2 directions, not one?

samohiuddin · July 17, 2020, 5:08am

Please see the below course, It has excellent basics about Wireless communications and how MIMO works.

RFSpecialist · July 17, 2020, 5:09am

Thanks I will watch the videos although I am sure there’s too much math inside.

To point 2 beams in 2 directions is very easy if you use half of the elements to steer one beam in one direction and the other half of elements to steer the other beam. But using all antenna elements to steer 2 beams in 2 distinct directions in the same time ( as explained in the book)sounds impossible to me.

samohiuddin · July 17, 2020, 5:10am

There is a Concept called Antenna Polarization that is used in Beamforming, please check that.

RFSpecialist · July 17, 2020, 5:11am

I am aware about +45 and -45 polarization.
Yet to do 4x4 DL MIMO one need 2 beams, each beam having a +45 and -45 polzarization stream.

RFSpecialist · July 17, 2020, 5:12am

I saw this post on LinkedIn, hope this will give you some idea.

RFSpecialist · July 17, 2020, 5:13am

@samohiuddin, this is my post and it is one case of MIMO.
In the book it is explained another case of MIMO that I am asking questions about.

samohiuddin · July 17, 2020, 5:13am

It was a good post.

samohiuddin · July 17, 2020, 5:14am

Yes there is a lot of math involved, I did complete most of the course took me some time though

samohiuddin · July 17, 2020, 5:15am

Ahhh, I see I got this question now.
The Author is talking about MU-MIMO with each UE receiving 2x2 MIMO.
This is absolutely possible, I have worked on this feature.

RFSpecialist · July 17, 2020, 5:16am

Nope, in highlighted phrase in yellow it is about 4x4 DL MIMO.
4x4 DL MIMO for single UE, using all antenna elements yet 2 beams need to point in 2 distinct directions.