Layer — One spatial stream
Space — layers, ports, reporting
Where it sits
What it is
A layer is one spatial stream. Up to eight on the downlink, dealt out from one or two codewords by a round-robin mapping.
One to four layers use one codeword; five to eight use two, split 2+3, 3+3, 3+4 and 4+4 (TS 38.211 Table 7.3.1.3-1).
Two codewords exist because eight layers are not equally good. Spatial multiplexing produces streams of very different quality; one MCS for all of them wastes the good and fails the bad. Each codeword carries its own MCS, redundancy version and acknowledgement, and can be disabled independently.
They stop at two because feedback is expensive — eight would mean eight acknowledgements and eight MCS fields in every grant.
Read on
The full treatment is in nr-pdsch — this page is the definition and the map reference; that note is where the subject is actually developed.
Before this concept, the hierarchy says to learn the following — the full chain, in order: