Resource element — One subcarrier for one symbol — the smallest addressable thing
The grid and its units
Where it sits
What it is
A resource element is one subcarrier during one OFDM symbol — the smallest addressable thing in 5G, and the atom every rate calculation counts.
It is written $(k, l)_{p,\mu}$ in the specification: subcarrier $k$, symbol $l$, on antenna port $p$ at numerology $\mu$ (TS 38.211 clause 4.4.2).
One resource element carries one modulation symbol — between 2 and 10 bits depending on the modulation order. Every throughput number in these notes is ultimately resource elements per second times bits per resource element.
Read on
The full treatment is in nr-frame-structure — 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: