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SSB

Summary

  • SSB (Synchronization Signal Block) is the first “beacon” transmitted by a 5G gNB (base station).
  • It allows a UE (device) to:
    • Detect and identify a cell
    • Achieve time and frequency synchronization
    • Obtain initial system information (via MIB)
    • Start the random access procedure

Think of SSB as:

"Hello, I am a 5G cell. Here is how you synchronize with me and begin access."

SSB Structure

Time-Frequency Structure SSB

Figure 1. Time Frequency Structure SSB1

If you are confused...

The picture above depicts the overall SSB structure, but it introduces a lot of terminology at once — subcarrier, OFDM symbol, and so on. Don't worry if you don't recognize these yet; we'll gradually explain them throughout these docs. For now, just focus on the middle section, which shows what the SSB structure looks like: four OFDM symbols in time, carrying PSS, SSS, and PBCH.

Synchronization Signals (SS)

These signals allow the UE to detect the cell, align timing, and determine the physical cell identity.

Primary Synchronization Signal (PSS)

  • Provides initial time and frequency synchronization
  • Carries \( N_{ID}^{(2)} \in \{0, 1, 2\} \)
  • Helps the UE locate the start of the SSB
Purpose

"Where am I in time? Let’s get rough synchronization first."

SSS (Secondary Synchronization Signal)

  • Completes the physical cell identity
  • Provides \( N_{ID}^{(1)} \in \{0, 1, \ldots, 335\} \)
  • Combined with PSS to derive full PCI

Relationship

\[ N_{ID}^{Cell} = 3 \times N_{ID}^{(1)} + N_{ID}^{(2)} \]
\[ with\; N_{ID}^{(1)} \in \{0, \ldots, 335\} \;and\; N_{ID}^{(2)} \in \{0, 1, 2\}, \;so: \]
\[ N_{ID}^{Cell} \in \{0, 1, \ldots, 1007\} \]
Purpose

"Which specific cell is this?"

PBCH (Physical Broadcast Channel)

Carries essential system information required for initial access.

Purpose

"Now that you are synchronized and know the cell, here is how to decode system information (SIB1).”

PBCH Payload (MIB – Master Information Block)

The MIB includes:

  • Partial System Frame Number (SFN) — the 6 MSBs; the remaining 4 LSBs plus a half-frame flag are carried in the PBCH payload itself rather than the MIB IE
  • SSB index (beam index information) — see SSB Index Encoding below
  • Subcarrier spacing configuration for SIB1
  • CORESET#0 configuration (used to decode PDCCH for SIB1)
PBCH Payload Composition

The 24 bits from the MIB IE (23 bits) plus the BCCH-BCH choice bit aren't the whole PBCH payload — the physical layer adds 8 more bits that never appear in the RRC-visible MIB IE itself:

  • 4 bits — the remaining LSBs of the 10-bit SFN
  • 1 bit — half-frame indicator (which of the two 5 ms half-frames within the 10 ms frame)
  • 3 bits — either extra MSBs of the SSB index (only meaningful when \( L_{max} = 64 \)) or otherwise reserved, depending on \( L_{max} \)

This brings the total PBCH transport-block payload to 32 bits — the actual bit sequence that gets channel-coded and transmitted.

Tip

If you're new to 5G, the sections below go a bit deeper into physical-layer details. It may be helpful to revisit them once you're comfortable with the overall system flow — this is a good stopping point for a first read-through.

SSB Physical Structure

NR PBCH DM-RS

Figure 2. NR PBCH DM-RS2

This picture, focused on the PBCH DM-RS resource elements, is adapted from ShareTechnote — it's a good reference for understanding what the SSB actually "looks like" at the resource-element level.

An SSB spans 4 consecutive OFDM symbols in time and 240 subcarriers (20 resource blocks) in frequency:

OFDM Symbol Content
0 PSS
1 PBCH + PBCH DM-RS
2 SSS + PBCH (edges) + PBCH DM-RS
3 PBCH + PBCH DM-RS

Table 1. OFDM Symbols

PBCH DM-RS

The PBCH DM-RS (Demodulation Reference Signal) is interleaved with the PBCH data across OFDM symbols 1–3, occupying 1 out of every 4 subcarriers. It serves two purposes:

  • Channel estimation — it gives the UE a known reference signal to coherently demodulate the PBCH payload.
  • Partial SSB index signalling — the specific DM-RS sequence used (one of up to 8 possibilities) encodes the 2 or 3 LSBs of the SSB index (see SSB Index Encoding), so the UE can determine part of the index by correlation alone, before even decoding the PBCH payload.

The DM-RS scrambling sequence is initialized using the PCI (\( N_{ID}^{Cell} \)), so it also implicitly confirms the cell identity already derived from PSS/SSS.2

Physical-layer Cell Identity (PCI)

The Physical Cell Identity (PCI), denoted \( N_{ID}^{Cell} \), is a local, physical-layer identifier used in 5G NR to distinguish between neighboring cells. Unlike global identifiers, the PCI is designed for efficiency at the radio interface. It is derived from \( N_{ID}^{(1)} \) and \( N_{ID}^{(2)} \), carried by the SSS and PSS respectively, and takes a value in \( \{0, 1, \ldots, 1007\} \) — giving 1008 unique physical cell identities.

SSB Burst Set

SSB Start Symbol Positions, 15 kHz SCS (Case A)

Figure 3. SSB Start Symbol Positions, 15 kHz SCS (Case A)1

  • All the beam-swept SSBs (see Beam Sweeping below) belong to one burst set, confined to a 5 ms half-frame window.
  • The maximum number of SSBs in a burst set, \( L_{max} \), depends on the carrier frequency:
    • \( L_{max} = 4 \) for frequencies up to 3 GHz
    • \( L_{max} = 8 \) for 3–6 GHz
    • \( L_{max} = 64 \) for 6–52.6 GHz (FR2)
  • The burst set repeats periodically. The RRC parameter ssb-periodicityServingCell configures this period as 5, 10, 20, 40, 80, or 160 ms, with 20 ms being the most common choice in deployments — if the field is absent, the UE assumes 5 ms.

SSB Candidate Positions by Subcarrier Spacing (Case A–E)

The exact OFDM symbols an SSB can start on within the half-frame depend on the SSB subcarrier spacing, defined in 3GPP TS 38.213 §4.1 as Cases A through E:

Case SCS Candidate first-symbol indices Typical \(L_{max}\) Frequency range
A 15 kHz \(\{2, 8\} + 14n\) 4 (≤3 GHz) / 8 (3–6 GHz) FR1
B 30 kHz \(\{4, 8, 16, 20\} + 28n\) 4 (≤3 GHz) / 8 (3–6 GHz) FR1
C 30 kHz \(\{2, 8\} + 14n\) 4 (≤3 GHz) / 8 (3–6 GHz) FR1
D 120 kHz \(\{4, 8, 16, 20\} + 28n\) 64 FR2
E 240 kHz \(\{8, 12, 16, 20, 32, 36, 40, 44\} + 56n\) 64 FR2

Table 2. Cell Search SSB SCS Cases A-E 5

The valid range of \(n\) (which candidate positions actually exist) varies by case and frequency band — see 3GPP TS 38.213 §4.1 for the complete set of rules.

More Cases

NR SSB SCS Cases A-E

Figure 4: NR SSB SCS Cases A-E3

This picture from ShareTechnote lays out the remaining SCS cases (B through E) visually, which is a helpful complement to the table above.

Beam Sweeping

SSB Beam Sweeping

Figure 5: SSB Beam Sweeping4

At higher frequencies (especially FR2/mmWave), a single wide beam can't reliably cover an entire cell — signal range and quality drop off sharply outside a narrow beam's footprint. To solve this, a gNB transmits each SSB in a burst set on a different, narrow beam, sweeping sequentially through all directions until the whole cell area has been covered.

  • The UE measures each SSB it detects and reports the one with the best signal quality (RSRP).
  • The SSB index of that best SSB effectively tells the network which beam direction reaches the UE.
  • That same beam association is then reused for the downlink control channel and, on the UE side, to select the matching RACH preamble/occasion for initial access — so the network doesn't need a separate beam-discovery step later.
Which SSB in the picture above has the best signal quality as the UE moves left to right?
  • Left position: SSB #1 — the UE sits inside SSB #1's beam footprint, so it gives the strongest RSRP there.
  • Middle position: SSB #4 — as the UE moves further right, SSB #1's beam weakens while SSB #4's beam becomes dominant.
  • Right position: SSB #8 — by the time the UE reaches the right edge of the cell, SSB #8 gives the best signal.

This is exactly why beam sweeping matters: no single beam covers the whole cell well, so the "best" SSB — and therefore the best serving beam — depends entirely on where the UE physically is. The UE continuously re-measures all detectable SSBs and reports whichever one currently gives the strongest RSRP.

SSB Index Encoding

How do we determine the index, like SSB #1–#8 above?

Each SSB in the burst set is assigned a unique index (0 to \( L_{max} - 1 \)) so both the UE and the network know exactly which beam/position a given SSB corresponds to. The two mechanisms below explain how that index is actually encoded and transmitted.

Per 3GPP TS 38.213 §4.1, the SSB index isn't signalled as one clean field — it's split across two physical-layer mechanisms depending on \( L_{max} \):

  • For \( L_{max} = 4 \): the 2 LSBs of the index are derived from the PBCH DM-RS sequence.
  • For \( L_{max} > 4 \) (i.e. 8): the 3 LSBs of the index are derived from the PBCH DM-RS sequence.
  • For \( L_{max} = 64 \): the DM-RS still supplies the 3 LSBs, and the remaining 3 MSBs are carried explicitly in the PBCH payload — 6 bits total are needed to address 64 SSB positions.

SSB Transmission Pattern (ssb-PositionsInBurst)

Not every candidate SSB position up to \( L_{max} \) has to be transmitted — a network can selectively skip some to save resources. Which SSBs are actually sent is signalled via the RRC bitmap ssb-PositionsInBurst, where each bit (left to right) maps to SSB index 0, 1, 2, ...; 1 means transmitted, 0 means skipped.

The bitmap structure differs slightly by deployment:

  • Standalone (SA), signalled in SIB1 (ServingCellConfigCommonSIB): an 8-bit inOneGroup field plus an optional 8-bit groupPresence field for \( L_{max} = 64 \), which together address all 64 positions.
  • Non-Standalone (NSA), signalled via the LTE anchor's RRC reconfiguration (ServingCellConfigCommon): a CHOICE of shortBitmap (4 bits), mediumBitmap (8 bits), or longBitmap (64 bits), matching whichever \( L_{max} \) applies.
Example

Take the burst set shown in Figure 3 above (Case A, \( L_{max} = 4 \)). The gNB isn't required to transmit all 4 candidate SSBs — for instance, a deployment that only needs to cover a narrow area (like a corridor cell) might not need every beam direction. In that case the network could signal shortBitmap = 1010, meaning only SSB #0 and SSB #2 are actually transmitted, while SSB #1 and #3 are skipped to save transmission resources.

Useful Resources

  1. KP. (2019, October). 5G NR: Synchronization signal/PBCH block (SSB) (Blog post). How LTE Stuff Works. https://howltestuffworks.blogspot.com/2019/10/5g-nr-synchronization-signalpbch-block.html ↩↩

  2. Ryu, J. (n.d.). 5G/NR – PBCH DM-RS. ShareTechnote. https://www.sharetechnote.com/html/5G/5G_PBCH_DMRS.html ↩↩

  3. Ryu, J. (n.d.). 5G/NR – SS block. ShareTechnote. https://www.sharetechnote.com/html/5G/5G_SS_Block.html ↩

  4. Eladawi, A. (n.d.). Illustration of 5G SSB beam sweeping: Freq/time domain methods (LinkedIn post). LinkedIn. https://www.linkedin.com/pulse/illustration-5g-ssb-beam-sweeping-freqtime-domain-methods-eladawi/ ↩

  5. 3GPP. (n.d.). NR; Physical layer procedures for control (Technical Specification TS 38.213). 3rd Generation Partnership Project. https://portal.3gpp.org/desktopmodules/Specifications/SpecificationDetails.aspx?specificationId=3215 ↩