In 5G NR, the Synchronization Signal Block (SSB) is the first downlink signal a device looks for when it powers on or searches for a cell. It occupies a small block of time and frequency within every 5 ms half frame and carries the primary and secondary synchronization signals (PSS and SSS) plus the physical broadcast channel (PBCH) and its reference symbols. This page visualizes where the SSB sits in the resource grid, how its components are arranged, and how the pattern changes with subcarrier spacing and channel bandwidth according to 3GPP specifications.

SSB (Synchronization Signal Block) — 3GPP TS 38.211 §7.4.3

The SSB (also called SS/PBCH Block) is the fundamental downlink signal a UE detects during initial cell search. It carries:

• PSS (Primary Synchronization Signal) — length-127 m-sequence (BPSK), used for symbol-level timing and N_ID⁽²⁾ (∈ {0,1,2}).
• SSS (Secondary Synchronization Signal) — length-127 Gold sequence (BPSK), determines N_ID⁽¹⁾ (0–335). Together with PSS, identifies the Physical Cell ID: N_cell = 3·N_ID⁽¹⁾ + N_ID⁽²⁾ (0–1007).
• PBCH (Physical Broadcast Channel) — carries the MIB (Master Information Block), QPSK modulated, 432 REs per SSB burst.
• PBCH DMRS — QPSK reference signal on every 4th subcarrier within PBCH symbols.

The Physical Cell ID combines the two synchronization-signal indices:

SSB Structure (4 symbols × 240 subcarriers = 20 RBs)

• Symbol 0: PSS occupies 127 subcarriers centered within the 240 SC block (SC 56–182).
• Symbol 1: Full 240 SC of PBCH + PBCH DMRS (every 4th SC).
• Symbol 2: SSS (127 SC, centered) flanked by PBCH on the sides (SC 0–47 and SC 192–239) with DMRS. SC 48–55 and SC 183–191 are unused gaps.
• Symbol 3: Full 240 SC of PBCH + PBCH DMRS.

SSB Cases (3GPP TS 38.213 §4.1)

The SSB periodicity and time-domain positions depend on the SSB Case, which is tied to subcarrier spacing and frequency range:

• Case A: 15 kHz SCS, FR1 — first symbols {2, 8} + 14n, L_max = 4 or 8.
• Case B: 30 kHz SCS, FR1 — first symbols {4, 8, 16, 20} + 28n, L_max = 4 or 8.
• Case C: 30 kHz SCS, FR1 (paired) — first symbols {2, 8} + 14n, L_max = 4 or 8.
• Case D: 120 kHz SCS, FR2 — first symbols {4, 8, 16, 20} + 28n, L_max = 64.
• Case E: 240 kHz SCS, FR2 — first symbols {8, 12, 16, 20, 32, 36, 40, 44} + 56n, L_max = 64.

Frequency Position

The SSB’s location in the channel bandwidth is defined by offsetToPointA (in RBs on the 15 kHz common raster) and k_SSB (subcarrier offset). In this visualizer the SSB is centered in the configured channel bandwidth.

Simulation

The interactive simulator is below. Use the controls to explore the concepts described above.

Usage

Use the controls to explore the SSB resource grid:

1. SSB Case: Selects the subcarrier spacing and time-domain pattern (A/B/C/D/E). The SCS and first-symbol positions update automatically.
2. L_max: Maximum SS/PBCH blocks per 5 ms half frame. FR1 supports 4 or 8; FR2 supports 64.
3. BW: Channel bandwidth in MHz (5–100). Combined with SCS, this determines the total number of RBs in the grid. Invalid SCS/BW pairs are filtered.
4. TX Bitmap: A string of ‘1’ and ‘0’ of length L_max. Each bit controls whether the corresponding SSB burst is transmitted. For example, ‘10101010’ transmits every other burst.
5. Cell ID: Physical Cell ID (0–1007). Changes PSS/SSS sequences and PBCH DMRS scrambling.
6. Preset: Quick-select common configurations to see how the grid changes for different cases and bandwidths.

Tabs

• Full Resource Grid: The complete 5 ms half frame. Horizontal axis = OFDM symbol index; vertical axis = subcarrier (RB 0 at bottom). SSB bursts are outlined in magenta. Slot boundaries shown as white lines.
• SSB Structure: Zoomed view of a single SSB block (4 symbols × 240 subcarriers) showing the exact layout of PSS, SSS, PBCH, and DMRS REs. Dashed lines mark the PSS/SSS region.
• SSB Time Pattern: Shows when SSB bursts appear in the 5 ms half frame. Top row: time axis with slot boundaries. Bottom row: per-symbol view.
• Constellation: I/Q scatter plots for each SSB component (PSS – BPSK, SSS – BPSK, PBCH – QPSK, DMRS – QPSK) plus a combined view.

Key 3GPP References

• TS 38.211 §7.4.2: PSS and SSS sequence definitions and m-sequence generators.
• TS 38.211 §7.4.3: SS/PBCH block structure, RE mapping, and PBCH DMRS.
• TS 38.213 §4.1: SSB time-domain positions (Cases A–E) and L_max.
• TS 38.101: Maximum RBs per channel bandwidth and subcarrier spacing.

Limitations

• RE-mapping visualizer, not a receiver. The tool shows where PSS/SSS/PBCH/DMRS sit in the grid and their ideal constellations; it does not run a real cell-search detector, decode the MIB, or recover timing from a received waveform.
• SSB centered in the band. The block is drawn centered in the configured bandwidth; the real offsetToPointA and k_SSB raster placement (and the GSCN sync raster) are described but not positioned arbitrarily.
• No channel impairments. Constellations are ideal — no noise, fading, frequency/timing offset, or phase rotation — so they always show perfect BPSK/QPSK points rather than a received scatter.
• Idealized cases and bandwidths. SSB Cases A–E and the SCS/BW table follow the spec, but uncommon or operator-specific configurations and beam-sweep details beyond the TX bitmap are not modeled.
• Single SSB structure shown. One SS/PBCH block layout is rendered; multi-beam spatial behavior, actual PBCH payload bits, and scrambling outputs are not generated.
• Teaching tool. Intended to build intuition for the 5G NR SSB grid and timing pattern, not to validate a gNodeB implementation.