SRS — Sounding reference signal — the uplink probe

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Reference signals

Where it sits

Sits atLevel 7 of the hierarchy · Reference signals · explained
Learn firstResource element · Antenna port · Reciprocity — 9 concepts in the full chain, see the paths
UnlocksNothing else depends on it directly.
Primary clauseTS 38.211 §6.4.1.4, TS 38.214 §6.2.1
Used innr-srs 55 · index 5 · nr-frame-structure 5 · nr-pdsch 2 · ref-mimo 2 · ref-signals 1 · ref-ue-state 1
Scanned from the notes at page load and joined with terms.json; nothing on this card is typed by hand.

What it is

The sounding reference signal is the uplink probe — the device transmits a known signal so the base station can measure the channel directly, rather than asking for a report.

It works because of reciprocity: in TDD the two directions share a frequency, so an uplink measurement is a downlink measurement.

The catch is that it only works for antennas the device transmits from, and a phone has more receive antennas than transmit chains. So it sounds them in turn, through a switch — antenna switching, TS 38.214 clause 6.2.1.2, whose title is the whole explanation: UE sounding procedure for DL CSI acquisition.

Forty-eight devices can sound one OFDM symbol, separated by comb offsets and cyclic shifts.

Read on

The full treatment is in nr-srs — this page is the definition and the map reference; that note is where the subject is actually developed.

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Before this concept, the hierarchy says to learn the following — the full chain, in order:

To understand SRS (level 7) you first need 9 other concepts. Read them in this order — everything on one line can be read in any order, but no line before the one above it:
Level Learn these
0 Subcarrier
1 OFDM
2 OFDM symbol
3 Resource element · Slot
4 Antenna port · Frame and subframe
5 TDD and FDD
6 Reciprocity
Immediately before SRS: Resource element, Antenna port, Reciprocity.
Keep going — where this sits on the route
◀RLCstep 72 · level 7
Step 73 of 118level 7 · Reference signals
Code blockstep 74 · level 7▶
Nothing depends on this one directly
It is a leaf of the graph — follow the route rather than the branch.
↳ all 8 concepts at level 8 · level 7 in full · the whole route
The route is every concept in the folder ordered by level, so nothing here needs anything after it. Computed at page load from terms.json; the same numbering as the route page.
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