CRS — LTE's cell-specific reference signal — the always-on NR abolished
Reference signals
Where it sits
What it is
What it is. LTE's cell-specific reference signal. TS 36.211 clause 6.10.1. It is in this note not because NR has one — NR does not — but because NR still has to transmit around it.
What it was. A reference signal spread across the entire carrier, in every subframe, on every antenna port, whether or not anybody was being scheduled. Any device could measure it at any time without being told anything, which made it simple. It was also the single largest fixed overhead LTE carried.
Why NR removed it. Three reasons, and PDSCH §3.2 makes the argument: a cell radiating CRS constantly cannot sleep, so it costs energy in an empty cell; it fixes the carrier's shape, so the band cannot be reconfigured; and it is precoded independently of the data, which blocks the transparent precoding NR relies on (PDSCH §6.4).
What replaced it. DM-RS inside the allocation — sent only where there is data, and precoded with it.
Why it still matters. Wherever an NR carrier shares spectrum with a live LTE one — dynamic
spectrum sharing — the LTE CRS is still on the air, and NR must map PDSCH around it. That is what
lte-CRS-ToMatchAround and lte-CRS-PatternList1-r16 do
(TS 38.214 clause 5.1.4), and the Release-16 and Release-18 entries in
PDSCH §13 are the standard adding more of these patterns as the
deployments grew. A deleted channel that still shapes allocations a decade later.
Read on
This concept was first written up in ref-signals, which reads the whole group as one argument.
Before this concept, the hierarchy says to learn the following — the full chain, in order: