5G NR Downlink Channel Mappings: The Map You Need Before Anything Else

Why start here

Every time you read about PDSCH, DCI, MIB or SIB1, you are reading about one layer of a three-layer map. If the map is not clear in your head, the rest of this chapter feels like a pile of acronyms. So before any of the "difficult" material, let's build the map.

The mental model: post office, trucks, roads

Think of the downlink as a delivery system with three views of the same shipment:

The logical channel is the label on the parcel ("fragile", "bulk mail"). The transport channel is the delivery service chosen for that parcel (a fixed broadcast run, or the flexible scheduled truck). The physical channel is the actual road — the resource elements in the time–frequency grid.

The key thing to notice: the number of options shrinks as you go down. Many logical channels → few transport channels → very few physical channels. Almost everything ends up on one road: the PDSCH.

The three layers, one by one

Logical channels (the "what")

Downlink logical channels in NR (TS 38.321):

• BCCH — Broadcast Control Channel. Carries the MIB and the SIBs. Every UE in the cell can read it.
• PCCH — Paging Control Channel. Carries paging messages for UEs in RRC_IDLE and RRC_INACTIVE.
• CCCH — Common Control Channel. Used before a dedicated connection exists (e.g. RRC Setup). Uses SRB0.
• DCCH — Dedicated Control Channel. Point-to-point RRC and NAS signalling once connected (SRB1, SRB2, SRB3).
• DTCH — Dedicated Traffic Channel. Your user-plane data (DRBs).

Control channels vs traffic channels is the only real split here. The prefix tells you the scope: B = broadcast to everyone, P = paging, C = common (before identity is known), D = dedicated to one UE.

Transport channels (the "how")

NR downlink has only three transport channels (TS 38.300, TS 38.321):

• BCH — Broadcast Channel. Fixed format, fixed timing, no HARQ, carries only the MIB. It is deliberately dumb so that a UE that knows nothing about the cell can still decode it.
• PCH — Paging Channel. Supports DRX so idle UEs only wake up at their paging occasions.
• DL-SCH — Downlink Shared Channel. The flexible workhorse: HARQ, dynamic link adaptation (MCS), dynamic resource allocation, beamforming support. Everything except the MIB uses it.

Why so few? Because the transport channel encodes a delivery strategy, and there are only three genuinely different strategies: "blind broadcast for the very first bits", "wake-up on schedule", and "scheduled and adaptive for everything else".

Physical channels (the "where")

Three downlink physical channels (TS 38.211):

• PBCH — Physical Broadcast Channel. Carries the BCH (MIB) inside the SS/PBCH block.
• PDCCH — Physical Downlink Control Channel. Carries DCI (scheduling grants, slot format, power control). It has no transport channel above it; DCI is generated by the PHY/MAC scheduler, not by RLC.
• PDSCH — Physical Downlink Shared Channel. Carries DL-SCH and PCH. This is where SIBs, paging, RRC messages and user data all land.

Plus the physical signals — things that carry no bits from upper layers but are needed to make the channels decodable:

• PSS / SSS — Primary and Secondary Synchronisation Signals (find the cell, get timing and PCI).
• DM-RS — Demodulation Reference Signal (channel estimation; one per channel: PBCH DM-RS, PDCCH DM-RS, PDSCH DM-RS).
• CSI-RS — Channel State Information Reference Signal (link adaptation, beam management, tracking, mobility).
• PT-RS — Phase Tracking Reference Signal (phase-noise compensation, mostly FR2).

The mapping diagram

LOGICAL          TRANSPORT         PHYSICAL
(what)           (how)             (where)

![Logical channels map to transport channels which map to physical channels.](/images/articles/dl-01-channel-mapping.svg)

BCCH ──(MIB)───▶ BCH ────────────▶ PBCH
BCCH ──(SIBs)──▶ DL-SCH ────┐
PCCH ──────────▶ PCH ───────┼────▶ PDSCH
CCCH ──────────▶ DL-SCH ────┤
DCCH ──────────▶ DL-SCH ────┤
DTCH ──────────▶ DL-SCH ────┘

(no logical / transport channel) ▶ PDCCH   (carries DCI)

Physical signals (no upper-layer bits):
PSS, SSS, DM-RS, CSI-RS, PT-RS

Two lines deserve a second look:

1. BCCH splits. The MIB goes BCCH → BCH → PBCH. The SIBs go BCCH → DL-SCH → PDSCH. Same logical channel, two very different deliveries. That is why the MIB is tiny and rigid, while SIB1 can be hundreds of bytes and is scheduled by a PDCCH like any other data.
2. PDCCH stands alone. It is the only physical channel with nothing above it. Its content (DCI) is about the other channels — where the PDSCH is, which MCS, which HARQ process.

What changed from LTE (and why it matters)

If you know LTE, the NR downlink looks stripped down:

The theme is the same everywhere in NR: nothing is transmitted "always on" unless it is strictly necessary. LTE had signals that lit up every subframe even in an empty cell. NR transmits the SSB periodically, and everything else only when scheduled. This is what people mean by NR being an "ultra-lean" design.

How to remember it

• Three layers: what / how / where.
• Three transport channels: BCH, PCH, DL-SCH — dumb broadcast, scheduled wake-up, flexible shared.
• Three physical channels: PBCH, PDCCH, PDSCH — MIB, control, everything else.
• The MIB is the only thing that avoids the PDSCH.
• The PDCCH is the only channel with nothing above it.

Common confusions

Spec pointers

Recap

Interview questions

The questions below are drawn from this topic and phrased the way they tend to come up. Try each one out loud before revealing the answer.