Free tool · 3GPP TS 38.211 §7.4.1.5

CSI-RS, row by row

Choose a row of the CSI-RS locations table, place it with k₀ and l₀, and watch the exact resource elements light up — CDM groups colored, port count and density explained, periodicity on a slot timeline.

CSI-RS resource

Rows 1–8 of TS 38.211 Table 7.4.1.5.3-1 (of 18 — higher rows extend the same CDM logic to 12–32 ports)

One PRB × one slot — CDM groups colored

Under the hood

How 32 ports fit in a few REs

CDM is the trick. A CDM group is a small block of REs (2 in frequency, optionally × 2 in time) where multiple ports transmit simultaneously, separated by orthogonal cover codes — fd-CDM2 gives 2 ports per group, CDM4 gives 4, CDM8 gives 8. Row number ⇒ how many groups and which CDM type, so 32 ports need only 32 REs per PRB even though they sound enormous.

Density ρ scales frequency presence: ρ=1 means every PRB, ρ=0.5 every other PRB (halving overhead for wideband averaging), and row 1's ρ=3 puts three single-port REs in every PRB for fine TRS-style tracking.

Periodicity trades freshness for overhead: 20 slots (10 ms at 30 kHz) is a common beam-management cadence; 320–640 suits slow-moving FWA. Aperiodic CSI-RS can be triggered on demand for a one-shot sounding.

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FAQ

Common questions

What is CSI-RS used for in 5G?
The channel-state-information reference signal lets the UE measure the downlink channel per antenna port and report CQI/PMI/RI — the feedback the scheduler and beamformer run on. It also serves beam management (L1-RSRP per beam), mobility measurements and fine time/frequency tracking (TRS).
What does the row in the CSI-RS table mean?
TS 38.211 Table 7.4.1.5.3-1 has 18 rows; each row fixes the number of ports (1–32), density, CDM type and the (k̄, l̄) RE pattern. RRC signals the row plus bitmap-coded frequency/time offsets, and the UE knows exactly which REs carry which port.
What is the difference between NZP and ZP CSI-RS?
Non-zero-power CSI-RS actually transmits the sequence for measurement. Zero-power CSI-RS reserves the same REs with nothing in them — used to mute interference for neighbour-cell measurements and to rate-match PDSCH around another cell’s CSI-RS.
How should CSI-RS periodicity be chosen?
Fast enough that the channel report is still valid when the scheduler uses it, slow enough not to waste REs: 10–20 ms is typical for mobile users, longer for static FWA. The visualizer’s timeline shows exactly which slots carry the resource for your periodicity and offset.
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CSI-RS is how massive MIMO sees: the masterclass animates beam management, CQI/PMI/RI feedback and how 32 ports squeeze into a handful of REs — then ties it back to real scheduler behaviour.

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