Synchronization Signal Block (SSB)

An SSB is a tightly packed time-frequency block consisting of three core components and their associated physical channels:

• Primary Synchronization Signal (PSS): A 127-length m-sequence in the time domain used for initial symbol timing and coarse frequency offset estimation.
• Secondary Synchronization Signal (SSS): A 127-length gold sequence used to determine the Physical Cell Identity (PCI) group and achieve radio frame synchronization.
• PBCH and PBCH DMRS: The Physical Broadcast Channel carries the Master Information Block (MIB), while its associated Demodulation Reference Signal (DMRS) is used for channel estimation and carries the SSB index for beam identification.

The SSB occupies a fixed resource grid in the time-frequency domain, designed for rapid detection:

• Bandwidth: 20 Resource Blocks (240 subcarriers) in the frequency domain.
• Duration: 4 OFDM symbols in the time domain.
• Symbol Mapping: PSS occupies symbol 0. SSS occupies symbol 2. PBCH and its DMRS are mapped to symbols 1, 2, and 3, interleaved around the SSS.
• Subcarrier Spacing (SCS): The SCS is case-dependent, using 15 kHz or 30 kHz for FR1 (sub-6 GHz) and 120 kHz or 240 kHz for FR2 (millimeter wave).

To provide cell coverage, SSBs are not transmitted in isolation but in a periodic burst set:

• SSB Burst Set: A group of up to L_max SSB candidates transmitted within a 5 ms half-frame window.
• Beam Sweeping: Each SSB candidate in the burst is transmitted on a different spatial beam, sequentially sweeping the cell sector. This is fundamental to 5G NR's beam-based initial access.
• Maximum Beams (L_max): Up to 4 or 8 beams for FR1, and up to 64 beams for FR2, allowing for highly directional millimeter-wave communication.

The network configures the periodicity of the SSB burst set, balancing access latency with overhead:

• Default Periodicity: 20 ms, which is the UE's initial assumption during a cell search.
• Configurable Range: The period can be set to 5, 10, 20, 40, 80, or 160 ms.
• Signaling: The actual periodicity is signaled in System Information Block 1 (SIB1) after the UE decodes the MIB from the PBCH. A longer periodicity reduces overhead for always-on synchronization signals, improving network energy efficiency.

The PSS and SSS together encode the Physical Cell Identity (PCI), a crucial identifier for the cell:

• PCI Formula: PCI = (3 × N_ID1) + N_ID2, yielding 1008 unique values.
• N_ID2 (0-2): Derived from the PSS sequence index, representing the physical-layer identity within a group.
• N_ID1 (0-335): Derived from the SSS sequence index, representing the physical-layer cell identity group.
• Purpose: The PCI allows the UE to distinguish between neighboring cells and is essential for scrambling other physical channels and reference signals.

The exact position of an SSB within a burst and the overall frame structure is conveyed through multiple layers:

• PBCH DMRS Sequence: The least significant bits (LSBs) of the SSB index are carried by the initialization seed of the DMRS sequence during each SSB transmission.
• PBCH Payload: The most significant bits (MSBs) of the SSB index and the System Frame Number (SFN) are carried as explicit bits in the Master Information Block (MIB) payload.
• Half-Frame Indication: A bit in the MIB payload tells the UE whether the detected SSB is in the first or second 5 ms half of a 10 ms radio frame.