Layer mapping concept and purpose

Specs & Protocol Testing
1

Hello Everyone.
I have two questions need to be asked.

  1. As layer mapping is distribute symbols across different layers, why we using layer mapping as concept and what is the purpose of that?
  2. When layer mapping goes to precoding in order to let the layered symbols maps set of antennas ports, we using this precoding because for a good diversity and array gain as concept?
2
  1. Purpose is to spread the data among different multipaths evenly so not to be much affected by fading.
  2. We use precoding to maximize the air interface matrix and obtain low interference between MIMO streams.

Please read the SVD process:

http://www.sharetechnote.com/html/Communication_ChannelModel_SVD.html

3

Purpose is to spread the data among different multipaths evenly so not to be much affected by fading. → Nice explanation.

Just a thought. If there are no multipaths present, then even if NW supports 4x4, it will have to bring down the number of layers implemented (case with rural areas as opposed to urban areas)?
Is my understanding right?

4

If no multipaths presents UE will report rank indicator = 1 and gNodeB will use 1 layer only.

5

Yes MIMO will not work is LOS in rural areas.

6

Yes, without reflection, refraction… it’s tough to rank4.

7

In cluster drivetest rank4 is in the range of 5-10%. In dense urban a little higher.

8

I still have some queries related to layer mapping and precoding.

Since we know that layer mapping is break the data symbols corresponding to CodeWord into multiple layer parts and transmit different parts over different antennas. The word of layer in general what is a layer? because most people confused with word layer because they think it is OSI layers.

For precoding if we use 1 layer and assume 4x4 MIMO is configured in 5G, signal data symbol is emitted from each of the transmit antennas with appropriate gain like signal power is maximized at the reception, right? If that so, this means that at reception the signal data symbol after precoding cannot simultaneously maximize the signal level at all of the receive antennas.

To maximize the signal level, we need to increase number of layers?

9

“CodeWord into multiple layer parts and transmit different parts over different antennas.”
This statement is wrong. You don’t transmit different parts. In very simple terms , you sum all the parts and create phase shifted versions or streams ( this process is codebook based precoding in 3gpp lingo), then send the phase shifted versions out from different antennas. Therefore each transmitted waveform is summation of all layers , but with different phase shift. Purpose of sending these different versions is not combining at Rx side for gain, but to subtract and recover original phase shifted versions from received signal. This is only possible when primary propagation path is different for each transmitted stream.

When you talk about gain, it is beamforming (and again it is precoding - any signal processing on transmit side can be attributed as precoding)

1 Like