Reference signals — everything sent that carries no message

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Frame structure §9 draws the line: a channel carries information from a higher layer; a signal carries none and exists for the radio itself. The downlink has eight kinds of signal and the uplink three. It reads them as one argument; each also has a page of its own.

In one line. Every reference signal answers one question the receiver cannot answer alone — what is the channel doing? (DM-RS, CSI-RS), is my oscillator drifting? (PT-RS), where am I? (PRS), which cell is this and when does it start? (SSB). The design question is always the same: how often must it be sent, and what does that cost.

Concepts defined here. CSI-RS · Zero-power CSI-RS · TRS · PT-RS · SS/PBCH block · PRS · CRS · The overhead table

Already explained elsewhere. DM-RS in PDSCH §7.1; SRS in the sounding note, which is about nothing else.

The concepts this note covers

Each concept now has a page of its own. This note is the narrative — the group read as one argument, with the connective tissue that a page-per-concept cannot carry. The pages are the definitions, and each one shows what to learn before it.

Concept	What it is	Needs	Primary clause
CRS	LTE's cell-specific reference signal — the always-on NR abolished	1	TS 36.211 §6.10.1
CSI-RS	Channel state information reference signal	2	TS 38.211 §7.4.1.5, TS 38.214 §5.1.6.1
PRS	Positioning reference signal	2	TS 38.211 §7.4.1.7, TS 38.214 §5.1.6.5
PT-RS	Phase tracking reference signal	2	TS 38.211 §7.4.1.2
SS/PBCH block	The object a device with no knowledge finds first	2	TS 38.211 §7.4.3
TRS	CSI-RS configured for tracking	1	TS 38.214 §5.1.6.1.1
Zero-power CSI-RS	A declared hole — a signal that is not transmitted	1	TS 38.211 §7.4.1.5, TS 38.214 §5.1.4

Which to read. Come here to see how these ideas hang together; go to a term page when you want one definition, its prerequisites, and everything that depends on it. The concept map indexes all 118, and its hierarchy puts them in learning order.

What the signals cost

Every reference element is an element PDSCH does not get. Collecting the costs makes the design rule visible.

Signal	Answers	Clause	Typical cost	Sent when nobody is scheduled?
DM-RS	What is the channel, right here?	38.211 §7.4.1.1	12 REs of 168 — one symbol per PRB per slot	no
CSI-RS	What will the channel be, for scheduling?	38.211 §7.4.1.5	Configured; periodic, semi-persistent or aperiodic	if configured periodic
TRS	Has my timing or frequency drifted?	38.214 §5.1.6.1.1	A dense CSI-RS set, hence its rate-matching exception	usually periodic
PT-RS	Has the constellation rotated?	38.211 §7.4.1.2	Sparse in frequency, dense in time; only at high MCS	no
SS/PBCH block	Which cell, and when does it start?	38.211 §7.4.3	4 symbols × 20 PRBs, per burst	always — the one exception
PRS	Where am I?	38.211 §7.4.1.7	Wide and occasional; muting patterns coordinate cells	when positioning is active
CRS (LTE)	Everything, for anybody	36.211 §6.10.1	Whole carrier, every subframe	always — which is why it went

Table 1: The downlink reference signals, what each answers, and what it costs. The DM-RS figure is the worked case from PDSCH section 9.2. The right-hand column is the one that explains NR's whole approach: only DM-RS and the SS/PBCH block are unavoidable, and only the SS/PBCH block is transmitted when nobody is listening.

Read the last column downwards. NR's rule is that nothing is transmitted unless it is for somebody, with one unavoidable exception — the object a device with no knowledge has to find. That single principle explains the deletion of CRS, the invention of DM-RS-inside-the-allocation, the existence of zero-power CSI-RS, and the fact that an idle NR cell is nearly silent.

Sources

Document	Clause	What it gives
TS 38.211	7.4.1.1	PDSCH DM-RS — explained in PDSCH §7.1
	7.4.1.2	PT-RS
	7.4.1.5	CSI-RS, zero-power and non-zero-power
	7.4.1.7	PRS
	7.4.2, 7.4.3	PSS and SSS; the SS/PBCH block and its burst set
TS 38.214	5.1.6.1, 5.1.6.1.1	CSI-RS reception; CSI-RS for tracking (the TRS)
	5.1.6.3, 5.1.6.5	PT-RS and PRS reception, including Release 19 carrier phase positioning
	5.1.4	Rate matching around SSB, CSI-RS and the LTE CRS patterns
TS 36.211	6.10.1	The LTE cell-specific reference signal — what NR removed and still avoids

Table 2: The clauses this reference note stands on, all in the archive folder Courses (RAW)/5G Topics.

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