The Lighthouse 01 · Anatomy 02 · DM-RS Comb 03 · Beam Ballet 04 · The Heartbeat 05 · Identity 06 · Mastery
Beam sweeping live · TS 38.211 §7.4.3 · TS 38.213 §4.1

The lighthouse
that 5G never turns off.

Every 20 milliseconds, every gNB on Earth sweeps a fan of beams across its cell, each carrying the same tiny postage stamp of spectrum: 4 OFDM symbols × 240 subcarriers — the SS/PBCH Block. PSS, SSS, and the MIB, folded into 960 resource elements. This is its anatomy, choreography, and heartbeat.

Open the blueprint Watch the beam ballet
4 × 240symbols × subcarriers
960resource elements
L = 64beams max (FR2)
20 msdefault heartbeat
scroll to assemble
PROLOGUE · ONE BLOCK, THREE JOBS

Everything a stranger needs, in 960 squares

The SSB is the only thing a gNB transmits for devices it has never met. It must be found blind, decoded blind, and it must carry just enough to bootstrap everything else. Four symbols, three payloads, zero prior coordination.

PSS — the flare

Symbol 0. A 127-chip m-sequence the UE can detect with zero knowledge. Gives symbol timing, coarse frequency, and N₍ID₎² — one of 3. Full PSS chapter →

SSS — the surname

Symbol 2 center. A 127-chip Gold sequence carrying N₍ID₎¹ — one of 336. Together with the PSS it spells the full Physical Cell ID, and pins the SSB's frequency reference.

PBCH — the letter

Symbols 1, 3 and the wings of 2. 432 data RE of Polar-coded MIB + 144 DM-RS pilots: SFN, SSB index MSBs, kSSB, and the pointer to CORESET#0 — the door to SIB1.

The budget — every RE accounted for (TS 38.211 Table 7.4.3.1-1)960 RE total = 127 (PSS) + 127 (SSS) + 432 (PBCH data) + 144 (PBCH DM-RS) + 130 (guards & padding)
CHAPTER 01 · ANATOMY

The Blueprint — watch the block assemble itself

20 resource blocks wide, 4 symbols deep. Press assemble and watch each payload take its seat: PSS first (it must be findable alone), PBCH wrapped around the SSS like armor, and DM-RS pilots stitched through every PBCH symbol so the channel can be estimated before anything is decoded.

240 SC × 4 symbols · 20 RB
SS/PBCH block · TS 38.211 §7.4.3.1CafeTele PHY Lab
PSSSSSPBCH dataPBCH DM-RS (every 4th SC)guard / set to zero
127
PSS RE — sym 0, SC 56–182
127
SSS RE — sym 2, SC 56–182
432
PBCH data RE
144
DM-RS pilot RE
Reading order is survival order: PSS needs no channel knowledge → SSS is decoded coherently using the PSS as phase reference → PBCH needs full channel estimation, which is exactly why 144 DM-RS pilots are laced through it before a single MIB bit is trusted.
CHAPTER 02 · THE PILOT COMB

The DM-RS Comb — the cell ID is hiding in the pilots

PBCH pilots occupy every 4th subcarrier — but which 4th? The comb offset is v = N₍ID₎ᶜᵉˡˡ mod 4. Neighbouring cells get different offsets, so their pilots don't collide — and a UE that already knows the PCI knows exactly where to look before decoding a single bit. Drag the PCI:

pilots at SC 0, 4, 8, …
PBCH DM-RS mapping · v = PCI mod 4
v = 0
PCI 0, 4, 8, … pilots @ 0+4k
v = 1
PCI 1, 5, 9, … pilots @ 1+4k
v = 2
PCI 2, 6, 10, … pilots @ 2+4k
v = 3
PCI 3, 7, 11, … pilots @ 3+4k
Planner's rule of thumb: avoid giving neighbour cells PCIs with the same mod 4 (DM-RS collision) — and the same mod 3 (PSS collision). This is why PCI planning tools exist.
CHAPTER 03 · CHOREOGRAPHY

The Beam Ballet — one block, swept across the sky

At mmWave a single wide beam can't reach anyone. So the gNB transmits the SSB up to 64 times in a 5 ms half-frame — same block, different beam, each stamped with its SSB index. The pattern of which OFDM symbols carry SSBs is fixed by the numerology: Case A–E. Pick one and watch the sweep.

first symbols {2,8} + 14·n · n = 0,1
SS-burst set · 5 ms half-frame · TS 38.213 §4.1
L = 4
FR1 ≤ 3 GHz
L = 8
FR1 3–6 GHz
L = 64
FR2 mmWave — Case D/E
How the UE knows which beam hit it: for L=4/8 the SSB index rides entirely in the PBCH DM-RS sequence; for L=64 the DM-RS carries the 3 LSBs and the PBCH payload carries the 3 MSBs. The UE reports the strongest index back in Msg1 — that's how beam alignment is born before any connection exists.
CHAPTER 04 · RHYTHM

The 20 ms Heartbeat — rhythm of an idle world

The burst repeats with a configurable period: 5 to 160 ms. But there's a covenant baked into the spec — during initial cell search, every UE on Earth assumes 20 ms. Slower saves gNB energy but stretches search time; faster burns power to be found quicker. Feel the rhythm:

★ initial-search assumption (TS 38.213)
ssb-PeriodicityServingCell · 160 ms window shown
CHAPTER 05 · IDENTITY

Forging a Cell ID — 3 × 336 = 1008 names

The whole synchronization story compresses into one multiplication. PSS hands over N₍ID₎² (the "group member", 0–2). SSS hands over N₍ID₎¹ (the "group", 0–335). Build any of the 1008 possible identities:

168
N₍ID₎¹ · SSS · 336 values
× 3 +
1
N₍ID₎² · PSS · 3 values
=
505
N₍ID₎ᶜᵉˡˡ · PCI · 0–1007
mod 3 = 1
PSS group — avoid same-mod-3 neighbours
mod 4 = 1
DM-RS comb offset v
1008
total PCIs — reused across the planet
CHAPTER 06 · MASTERY

The numbers, the spec, the test

Engineer's reference card

PropertyValue
Block size4 OFDM symbols × 240 SC (20 RB)
PSSsym 0 · SC 56–182 · m-sequence ×3
SSSsym 2 · SC 56–182 · Gold ×336
PBCHsym 1, 3 full · sym 2 SC 0–47 & 192–239
PBCH DM-RSevery 4th SC · offset v = PCI mod 4
SSB SCS15/30 kHz (FR1) · 120/240 kHz (FR2)
Burst patternsCase A–E · Lmax = 4 / 8 / 64
Burst windowfirst 5 ms of a half-frame
Periodicity5–160 ms · UE assumes 20 ms in search
Cell IDN₍ID₎ᶜᵉˡˡ = 3·N₍ID₎¹ + N₍ID₎² (1008)
SpecTS 38.211 §7.4.3 · TS 38.213 §4.1

Prove it — 6 questions 0 / 6

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