5G NR SS/PBCH Blocks and Bursts: Beams, Timing and the 5 ms Window
How PSS, SSS and PBCH are packaged into one SSB, how multiple SSBs form a burst set for beam sweeping, and the Case A–E timing patterns.
The idea in one sentence
An SS/PBCH block (SSB) is the smallest self-contained "hello" a cell can send — PSS, SSS and PBCH bundled into 4 symbols — and an SS burst set is a group of those hellos, each sent on a different beam, packed into a 5 ms window so a UE can hear at least one of them no matter which direction it is in.
Why NR needed a "block" at all
In LTE the sync signals and PBCH were fixed at the carrier centre and broadcast omnidirectionally. That does not work at mmWave, where a single wide beam does not reach far enough. NR needed a way to send the same sync information in many directions, one after another. Packaging PSS + SSS + PBCH into one compact block that can be transmitted, then re-transmitted on another beam a few symbols later, is the solution. The block is the unit of beam sweeping.
Anatomy of one SSB
Four consecutive OFDM symbols, 240 subcarriers (20 resource blocks) wide (TS 38.211 §7.4.3):
◄────────── 240 subcarriers (20 RBs) ──────────►
Symbol 0 ........[========== PSS (127) ==========]........
Symbol 1 [==================== PBCH (240) ====================]
Symbol 2 [== PBCH 48 ==][===== SSS (127) =====][== PBCH 48 ==]
Symbol 3 [==================== PBCH (240) ====================]
- Symbols 0 and 2 carry the sync signals in the central 127 subcarriers (56–182).
- Symbols 1, 2 (edges) and 3 carry PBCH plus its DM-RS (every 4th subcarrier).
- The SSB has its own subcarrier spacing, which may differ from the data SCS of the carrier. This is one of the most confusing points in NR and is worth dwelling on below.
The actual bandwidth of an SSB depends on the SSB SCS: 240 × 15 kHz = 3.6 MHz, 240 × 30 kHz = 7.2 MHz, 240 × 120 kHz = 28.8 MHz, 240 × 240 kHz = 57.6 MHz.
Where the SSB sits in frequency
Two facts to hold together:
- The SSB centre is on the synchronisation raster (a GSCN value), which the UE scans blindly.
- The SSB does not have to be at the centre of the carrier, and its subcarrier grid does not have to line up with the carrier's common resource block (CRB) grid. The MIB field k_SSB tells the UE the offset (in subcarriers of the common SCS, 15 kHz in FR1 / 60 kHz in FR2) from the SSB's lowest subcarrier to the nearest CRB boundary, and offsetToPointA (in SIB1 / RRC) gives the number of RBs from Point A to the SSB.
The reason for this freedom: operators can place the SSB where the sync raster allows, and can even run an SSB with 15 kHz SCS on a carrier whose data uses 30 kHz. The UE simply needs the two offsets to translate between the two grids.
SS burst set: many SSBs, one 5 ms window
An SS burst set is the collection of all SSBs (one per beam) transmitted within a 5 ms half-frame. The burst set repeats with a configurable periodicity of 5, 10, 20, 40, 80 or 160 ms (ssb-periodicityServingCell). A UE doing initial cell search assumes 20 ms.
Each SSB in the set has an SSB index (0 to L_max − 1). In practice, SSB index = beam index: the gNB transmits SSB 0 on beam 0, SSB 1 on beam 1, and so on. A UE reports which SSB index it heard best, and the network now knows the best beam for that UE — all before any dedicated signalling.
How many SSBs per burst set (L_max)
| Frequency range | L_max (max SSBs per burst set) |
|---|---|
| FR1, carrier ≤ 3 GHz | 4 |
| FR1, carrier > 3 GHz | 8 |
| FR2 | 64 |
Lower bands have wider beams (or no beamforming), so fewer SSBs are needed. mmWave needs many narrow beams, hence 64. The network can transmit fewer than L_max: the bitmap ssb-PositionsInBurst in SIB1 (and in dedicated RRC) tells the UE which indices are actually transmitted, so the UE does not waste time measuring empty positions and the PDSCH can rate-match around the used positions only.
Timing patterns: Cases A to E
Where exactly the 4-symbol SSBs sit inside the 5 ms window depends on the SSB SCS and the frequency range. TS 38.213 §4.1 defines five cases. Each case lists the first symbol index of each candidate SSB, counted from the start of the half-frame.
| Case | SSB SCS | Applies to | First-symbol positions | n values (≤3 GHz / >3 GHz / FR2) |
|---|---|---|---|---|
| A | 15 kHz | FR1 | {2, 8} + 14·n | n = 0,1 (L=4) / n = 0,1,2,3 (L=8) |
| B | 30 kHz | FR1 | {4, 8, 16, 20} + 28·n | n = 0 (L=4) / n = 0,1 (L=8) |
| C | 30 kHz | FR1 (paired and unpaired spectrum, band-dependent) | {2, 8} + 14·n | n = 0,1 (L=4) / n = 0,1,2,3 (L=8) |
| D | 120 kHz | FR2 | {4, 8, 16, 20} + 28·n | n = 0,1,2,3,5,6,7,8,10,11,12,13,15,16,17,18 (L=64) |
| E | 240 kHz | FR2 | {8, 12, 16, 20, 32, 36, 40, 44} + 56·n | n = 0,1,2,3,5,6,7,8 (L=64) |
You do not need to memorise the numbers. What you should notice:
- Two SSBs per slot (Case A/C) or four per two slots (Case B/D/E). The positions avoid the first symbols of a slot (reserved for PDCCH) and the last symbols (reserved for uplink in TDD). That is why SSBs start at symbol 2 or 4, never 0.
- Gaps in the n sequence for FR2 (no n = 4, 9, 14, 19) — these leave room for uplink slots in a TDD pattern, so an FR2 cell can still have UL opportunities during the burst.
- Case B vs Case C, both 30 kHz. Case B is the pattern originally designed for LTE-NR coexistence with 15 kHz-aligned gaps; Case C is the "clean" 30 kHz pattern. Which one applies is fixed per band in TS 38.101-1, so the UE knows it in advance.
- Case E (240 kHz) exists so that 64 SSBs fit into a short time span; 240 kHz is only ever used for the SSB, never for data.
A worked example: 30 kHz, Case C, L_max = 8
A 30 kHz slot is 0.5 ms with 14 symbols. Case C puts SSBs at symbols {2, 8} + 14n, n = 0…3. So:
- Slot 0: SSB 0 at symbols 2–5, SSB 1 at symbols 8–11
- Slot 1: SSB 2 at symbols 16–19 (= slot 1 symbols 2–5), SSB 3 at 22–25
- Slot 2: SSB 4, SSB 5
- Slot 3: SSB 6, SSB 7
All eight SSBs are done within the first 2 ms of the half-frame. The remaining 3 ms of the 5 ms window are free for normal traffic. With periodicity 20 ms, the SSB overhead is tiny.
SSB SCS vs data SCS: the point people trip on
The SSB SCS is chosen from the sync-raster/band tables (15 or 30 kHz in FR1; 120 or 240 kHz in FR2). The data SCS is whatever the operator configures for the BWPs. These can differ — e.g. an n78 cell with SSB at 30 kHz and data at 30 kHz is common, but a low-band cell might use a 15 kHz SSB with 15 kHz data, and an FR2 cell typically uses a 120 kHz SSB with 120 kHz data (or a 240 kHz SSB with 120 kHz data). The MIB's subCarrierSpacingCommon tells the UE the SCS for SIB1, Msg2/Msg4 and paging, which need not equal the SSB SCS either. Three SCS values can be in play at once: SSB SCS, "common" SCS, and dedicated BWP SCS.
What the UE measures on the SSB
- SS-RSRP, SS-RSRQ, SS-SINR — per SSB index (i.e. per beam), measured on the SSS (and optionally PBCH DM-RS). These are the cell-level and beam-level measurements used for cell selection, reselection and handover.
- The best SSB index also determines which RACH occasion the UE uses, so the gNB learns the UE's best beam from the preamble alone.
Common confusions
- "SSB periodicity is 20 ms." The default assumption during initial search is 20 ms; the actual configured value can be 5–160 ms.
- "The SSB is at the carrier centre." Not necessarily; it is on the sync raster, and k_SSB / offsetToPointA relate it to the carrier.
- "64 SSBs means 64 cells." No — 64 SSBs, 64 beams, one cell, one PCI.
- "SSB index and beam are the same thing in the spec." The spec never says "beam"; it says SSB index. The beam mapping is the gNB's implementation choice, but in practice they are one-to-one.
- "An SSB must be transmitted in every candidate position." Only the positions flagged in ssb-PositionsInBurst are used.
Spec pointers
- TS 38.211 §7.4.3 — SS/PBCH block structure and RE mapping
- TS 38.213 §4.1 — Cases A–E, L_max, half-frame timing
- TS 38.331 — ssb-PositionsInBurst, ssb-periodicityServingCell, offsetToPointA
- TS 38.101-1 / 38.104 §5.4.3 — SSB SCS per band, sync raster
- TS 38.215 — SS-RSRP / SS-RSRQ / SS-SINR definitions
Recap
One SSB = 4 symbols × 240 subcarriers = PSS + SSS + PBCH. A burst set = up to 4 / 8 / 64 SSBs in a 5 ms half-frame, one per beam, repeating every 5–160 ms. Cases A–E just say where in the half-frame the blocks start for each SSB SCS. The SSB index is effectively the beam index, and it drives beam measurement, RACH occasion selection and paging — which is why this small block is the foundation of NR mobility and beam management.
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.
18 questions
- Four consecutive OFDM symbols by 240 subcarriers carrying PSS, SSS and PBCH together as one transmittable unit.
- So the whole set can be beamformed as a unit and swept. LTE transmitted them separately and omnidirectionally, which does not work when coverage depends on narrow beams.
- The collection of SSBs transmitted within one 5 ms half-frame window, each typically on a different beam.
- The maximum number of SSBs in a burst set: 4 or 8 in FR1 depending on frequency, and 64 in FR2.
- Beams are far narrower at mmWave, so many more are needed to cover the same sector. Below 3 GHz a handful of wide beams suffices.
- It bounds how long a UE must observe to see every beam, which bounds cell search time. The window is half a frame regardless of how many SSBs it holds.
- Five defined symbol-position patterns tied to subcarrier spacing and frequency range: A is 15 kHz FR1, B and C are 30 kHz FR1, D is 120 kHz FR2 and E is 240 kHz FR2.
- They place SSBs at different symbol positions, and which applies depends on the band and whether the spectrum is paired. It is a band-dependent choice, not a network preference.
- Yes, and this is the point people trip on. The SSB has its own numerology signalled in the MIB, independent of what the carrier uses for data.
- SS-RSRP, SS-RSRQ and SS-SINR, per SSB index — which is what makes beam-level measurement and reporting possible.
- By reporting the SSB index alongside the measurement. The network uses the index to select the beam for subsequent transmission.
- 20 ms. The configured value can be 5, 10, 20, 40, 80 or 160 ms, but a UE with no configuration assumes 20.
- Shorter periods speed up cell search and beam measurement but consume more resources and radiate more often, which matters for energy saving features.
- SSBs are transmitted whether or not there is traffic, so a lightly loaded cell still radiates. That is why longer periodicities and SSB-less operation are active areas in 5G-Advanced.
- From the PBCH DM-RS sequence, extended by payload bits in FR2 where 64 indices need more than the DM-RS alone can carry.
- One to one in practice: each index corresponds to a beam direction the cell sweeps, though the specification does not mandate that mapping.
- SSBs occupy the defined symbol positions within each 0.5 ms slot, repeating across the first slots of the 5 ms window until eight have been sent, then nothing until the next period.
- The SSB is the beamformable unit carrying sync and broadcast together, and the burst set is one sweep of up to 64 of them inside a 5 ms window, which is how a beamformed cell makes itself findable.
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