Reciprocity — Why an uplink measurement answers a downlink question

5G Systems Notes · Concept map · Reciprocity Route · Hierarchy · Index · All concepts · Hub

Space — layers, ports, reporting

Where it sits

Sits atLevel 6 of the hierarchy · Space — layers, ports, reporting · explained
Learn firstTDD and FDD — 6 concepts in the full chain, see the paths
UnlocksSRS
Primary clauseTS 38.214 §6.2.1.2
Used innr-srs 9 · nr-pdsch 1 · ref-mimo 1
Scanned from the notes at page load and joined with terms.json; nothing on this card is typed by hand.

What it is

Reciprocity is why an uplink measurement answers a downlink question. The propagation between two antennas is the same in both directions at the same frequency and moment.

It holds for the path through the air, not for the electronics at the ends. The device's transmit and receive chains are different silicon with different gains and phase offsets, and closing that gap is a calibration problem.

And only at the same frequency. In TDD the directions share a frequency and take turns, so reciprocity applies; in FDD they use different frequencies and the fine structure simply differs. This is the honest reason SRS-based sounding is a TDD story.

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 Reciprocity (level 6) you first need 6 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 Slot
4 Frame and subframe
5 TDD and FDD
Immediately before Reciprocity: TDD and FDD.
Keep going — where this sits on the route
◀Precodingstep 66 · level 6
Step 67 of 118level 6 · Space
MIMOstep 68 · level 6▶
Level 6 → 7 · this unlocks
SRS
↳ all 13 concepts at level 7 · level 6 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.
5G Systems Notes · Concept map · Reciprocity Top · Concept map · Hub