I am interested to conceptually understand the layer mapping stage in #LTE downlink processing.

While all the literature clearly describe how the layer mapping is to be carried out for various use-cases, not one (the ones I have gone through) really explain the need for this stage in the pipeline. Without understanding the big picture, I am having trouble appreciating layer mapping very well.

The hunch I get is that layer mapping is done in order to create streams that can be transmitted per antenna ports. But what is not clear to me,

1) is the number of layers generally equal to no of antenna ports?

2) if no of layers < no of antenna ports then what happens to the extra antenna ports?

3) layer mapping also talks about mapping 2 code words into 3 layers. Does it mean that there are 3 antenna ports?.

Further, in the above case, the first code word is mapped as it is on layer 1 and the other code word is divided equally between layer 2 and 3. So, effectively, data transfer on layer 1 is going to be the slowest one and is going to determine the overall rate. Is this correct?

4) Probably, another related question is, whether transmit diversity is always carried out on 2 antenna ports or more than 2 could be involved?

Thanks a lot ...

1) No. If you are familiar with MIMO concepts like Spatial Multiplexing(SM), then layer refers to the number of layers in SM. For example in 2x2 MIMO system (2 antenna at Tx and 2 at Rx), the layers can be 1 or 2 but it can never exceed 2 - why because rank of the 2x2 matrix is 2. If you take 3x2 system, layers is 2 but number of Tx antennas is 3. So there will be some scheme to map 2 layers to 3 antennas.

2) Either they can switch off extra antennas or use beamforming techniques to map layers to Tx antennas as explained above.

3) If you map 2 codeword to 3 layers basically you are increasing the speed at which you are transmitting the information. In this case, yes it is determined by the slowest output.

4) It can be carried out on more than 2 antennas. Read about alamouti scheme, Space-Time Block Code (or Space Frequency Block Codes since LTE is OFDM system).

Reference : http://www.sharetechnote.com/html/LTE_Advanced_Lay... 

THanks. Can you explain, how in case of 3x2 system, layers = 2 is arrived at?

There 3 Tx antennas, 2 Rx antennas. $$y = Hx+n $$. $$ \hat{x} = H^{-1}y $$ is how you equalize for H and decode x. As you can see, you have to apply pseudo inverse of H to decode x. This will only be possible if rank of H*pinv(H) = 2. It can never be 3. You can read more about pseudo inverse matrices to understand this.  

Fist of all it is necessary that you clarify how is organized the LTE DL signal in the time-sub carrier framework.

Visit this page http://niviuk.free.fr/lte_resource_grid.htmland verify what happen if you select 1,2 or 4 antennas. (Look at RSRP signals)

BR

Daniele