CSI-RS — Channel state information reference signal
Reference signals
Where it sits
What it is
What it is. A known signal the network transmits so the device can measure the downlink and report back. TS 38.211 clause 7.4.1.5, reception procedure in TS 38.214 clause 5.1.6.1. It is sent on antenna ports numbered from 3000 (TS 38.211 clause 7.2).
It is the other half of the argument the SRS note makes. SRS §1.2 sets out the two ways a base station can learn the downlink — ask the device or measure the device — and says the first is CSI-RS. This is that path:
Ask (CSI-RS). The network transmits, the device measures and reports a compressed opinion back on PUCCH or PUSCH. Works in FDD as well as TDD, because nothing depends on reciprocity. Costs downlink resource and uplink reporting overhead, and the report is quantised and stale.
Measure (SRS). The device transmits, the network measures it directly and gets the raw channel. Cheaper and better, but only in TDD, and only for antennas the device can transmit from — which is the entire subject of the sounding note.
A CSI-RS resource is configured, not fixed: a number of ports (1 to 32), a density, a time-domain behaviour (periodic, semi-persistent, aperiodic) and a position in the slot. The port count is what matters most — a report about a 32-port array is what makes the eight-layer transmission of PDSCH §6.3 schedulable at all.
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: