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As we have multiple antenna ports, the input stream of data or signal need to be "split" or "mapped" to these antenna ports before precoding.
Each mapped stream is called "a layer".
The process is called "layer mapping".
If you notice, the number of d signals or antenna ports is equal to the number of layers.
Also, as already mentioned in earlier section, the number of layers cannot be more than the number of transmit antenna ports.
In practice, instead of stream or continuous data, processing happen on chunk of data at certain time interval (aka slot or TTI).
This chunk of data is known as "a codeword".
A codeword is generated after "modulating" "coded" "transport block".
Below figure shows above processing blocks.
Fig "Layer mapping"
Coding refer to processing of transport block data (bits) by error detection and recover algorithms.
Codeword is actually modulated signal, represented as complex frequency domain values.
Refer this article for top level view of PHY DL chain processing.
Based on the number of layers that we intend to transmit, size of transport block differ.
More the number of layers, bigger will be the transport block, and more will be the throughput.
Two layers will provide twice throughput (assuming MCS value remain the same).
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