Module 2 — Cell Search

SSB —
Synchronization Signal Block

The SSB is the fundamental unit of 5G cell identification. It is a compact block of 4 OFDM symbols that carries everything a UE needs to synchronize with a cell and start reading system information — PSS, SSS, and PBCH packed into 240 subcarriers.

TS 38.211 §7.4.3 TS 38.213 §4.1

What is in an SSB?

The SSB contains exactly three signals, arranged across 4 OFDM symbols and 240 subcarriers (20 resource blocks). Each signal has a distinct job, and they must be processed in order.

An SS/PBCH block consists of PSS, SSS and PBCH, including the PBCH DM-RS, and shall be contained in 4 consecutive OFDM symbols. The SS/PBCH block occupies 20 consecutive resource blocks.

3GPP TS 38.211, Section 7.4.3.1

PSS

Symbol 0

127 subcarriers. Gives timing + N²_ID

PBCH

Symbols 1, 2 sides, 3

Carries MIB (Polar coded, 32 bits)

SSS

Symbol 2 center

127 subcarriers. Gives N¹_ID → PCI

DMRS

Within PBCH

Every 4th subcarrier. Enables channel estimation

SSB time-frequency structure

The exact mapping of each signal to subcarriers is defined precisely in the spec. PSS and SSS each occupy 127 subcarriers in the center of the SSB, with zeros on either side. PBCH fills the remaining subcarriers in symbols 1, 2 (sides only), and 3.

SSB subcarrier mapping — 4 symbols × 240 subcarriers TS 38.211 §7.4.3.1

■ PSS (127 subcarriers) ■ SSS (127 subcarriers) ■ PBCH ■ PBCH DMRS ■ Zeros (unused)

SSB periodicity and beam sweeping

The gNB transmits SSBs periodically. The default period assumed by UEs during initial cell search is 20 ms, though the actual periodicity (5, 10, 20, 40, 80, or 160 ms) is configured by the network and signalled in SIB1.

In 5G NR, beamforming is fundamental — especially in FR2 (mmWave). The gNB transmits multiple SSBs in each burst, each in a different beam direction. A UE measures them all and identifies which beam gives the best signal. This SSB index is embedded in the PBCH and used later to link the UE's RACH transmission to the correct beam.

For FR1, the maximum number of SS/PBCH blocks within a half frame is 4 for f ≤ 3 GHz, and 8 for 3 GHz < f ≤ 6 GHz.
For FR2, the maximum number of SS/PBCH blocks within a half frame is 64.

3GPP TS 38.213, Section 4.1

max SSBs per half-frame (FR1, n78)

Each in a different beam direction

max SSBs per half-frame (FR2)

mmWave requires narrow pencil beams

20 ms

default assumed period

UE assumes this during initial search

SSB timing — where in the half-frame?

Within each 5 ms half-frame, the positions of the SSB symbols are fixed by the spec. For 30 kHz SCS (our n78 example), the first SSB starts at OFDM symbol 4 of slot 0. Each SSB occupies 4 consecutive symbols.

SSB positions within half-frame — SCS 30 kHz, n78 (up to 8 SSBs) TS 38.213 §4.1, Table 4.1-2

Symbol durations — our example

For 30 kHz SCS, each OFDM symbol has a useful duration of 33.33 μs (= 1/30000 Hz) plus a cyclic prefix of 2.34 μs, giving a total symbol duration of 35.68 μs. The entire SSB therefore takes 4 × 35.68 μs = 142.7 μs.

SSB timing calculations — SCS = 30 kHzTS 38.211 §4.3

Useful symbol duration  = 1 / SCS = 1 / 30,000 Hz = 33.33 μs
Cyclic prefix (normal) = 144 / 30.72×10⁶ = 2.34 μs   (first symbol: 160/30.72MHz)
Total symbol duration  = 33.33 + 2.34 = 35.68 μs

In samples (sampling rate = 61.44 Msps for 100 MHz channel):
  Useful part = 61.44×10⁶ × 33.33×10⁻⁶ = 2048 samples
  CP          = 61.44×10⁶ × 2.34×10⁻⁶  = 144 samples
  Total       = 2192 samples per symbol

Full SSB = 4 symbols = 4 × 2192 = 8768 samples = 142.7 μs

SSB — key facts for our n78 example

SSB center frequency → 3498.24 MHz (GSCN 7845)

SSB bandwidth → 7.2 MHz (240 subcarriers × 30 kHz)

Symbol duration → 35.68 μs (2192 samples at 61.44 Msps)

SSB duration → 142.7 μs (4 symbols)

Max beams per half-frame → 8 (FR1, f > 3 GHz)

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