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5G NR Synchronization Signal Block (SSB) - Time/Frequency location

  • Writer: Venkateshu
    Venkateshu
  • Apr 16
  • 8 min read

Introduction

The Synchronization Signal Block (SSB) is a fundamental component of 5G New Radio (NR) systems, enabling user equipment (UE) to perform initial cell search, acquire time and frequency synchronization, and decode essential system information. Defined in 3GPP specifications, particularly TS 38.211, TS 38.213, and TS 38.331, the SSB integrates the Primary Synchronization Signal (PSS), Secondary Synchronization Signal (SSS), and Physical Broadcast Channel (PBCH) into a single block. This article explores the SSB’s structure, its critical role in 5G NR, differences from LTE synchronization mechanisms, and the detailed process of calculating its location in the resource grid, including the parameters involved.

Importance of SSB in 5G NR

The SSB is pivotal for several reasons:

  1. Cell Search and Synchronization: The SSB allows UEs to acquire time and frequency synchronization with a cell and detect the Physical Cell Identity (PCI). The PSS and SSS provide the PCI, which consists of 1008 possible identities (3 PSS × 336 SSS), enabling robust cell identification. The PBCH carries the Master Information Block (MIB), which includes critical parameters for accessing the cell, such as the System Frame Number (SFN) and configurations for System Information Block 1 (SIB1).

  2. Beam Management: In 5G NR, particularly in Frequency Range 2 (FR2, mmWave), beamforming is essential due to high path loss. SSBs are transmitted in bursts (SS Burst Sets) across multiple beams, allowing UEs to identify the optimal beam for communication. Each SSB within a burst set is associated with a unique SSB index, facilitating beam selection.

  3. Measurements: SSBs are used for Reference Signal Received Power (RSRP), Reference Signal Received Quality (RSRQ), and Signal-to-Interference-plus-Noise Ratio (SINR) measurements, supporting mobility, handover, and cell reselection.

  4. Energy Efficiency: Unlike LTE’s always-on reference signals, 5G NR minimizes always-on signals, with the SSB being the primary periodic signal. Configurable periodicities (5 ms to 160 ms) reduce network energy consumption, especially in low-traffic scenarios.

 

Differences Between 5G NR SSB and LTE Synchronization Signals

While both 5G NR and LTE rely on synchronization signals for cell search, their implementations differ significantly:

Aspect

LTE

5G NR

Structure

PSS, SSS, and PBCH are separate signals, not tightly integrated.

PSS, SSS, and PBCH are combined into a single SSB, occupying 4 OFDM symbols and 240 subcarriers.

Frequency Position

PSS/SSS occupy the central 6 PRBs (72 subcarriers) of the carrier.

SSB position is flexible, not fixed to the carrier center, defined by synchronization raster and parameters like offsetToPointA.

Subcarrier Spacing (SCS)

Fixed at 15 kHz for PSS/SSS/PBCH.

Variable SCS (15, 30 kHz for FR1; 120, 240 kHz for FR2), defined per frequency band (TS 38.104, Table 5.4.3.3-1).

Periodicity

PSS/SSS transmitted every 5 ms, PBCH every 10 ms.

SSB periodicity is configurable (5, 10, 20, 40, 80, 160 ms), with a default of 20 ms for initial cell search.

Beamforming

Limited beamforming; single transmission pattern.

Supports beam-sweeping; multiple SSBs in an SS Burst Set (up to 64 in FR2) for different beams.

PCI

504 PCIs (3 PSS × 168 SSS).

1008 PCIs (3 PSS × 336 SSS), supporting denser deployments.

Coding

PBCH uses tail-biting convolutional coding.

PBCH uses polar coding for improved error correction.

Transmission Pattern

Fixed time-domain pattern.

Multiple patterns (Cases A–E) based on SCS and frequency range, defined in TS 38.213, Section 4.1.

These differences reflect 5G NR’s design goals of flexibility, scalability, and support for diverse use cases, including mmWave and beamforming.


SSB Structure

An SSB spans 4 OFDM symbols in the time domain and 240 subcarriers (20 Resource Blocks, RBs) in the frequency domain. Its components are:

  • PSS: Occupies the first OFDM symbol (symbol 0), subcarriers 56–182 (127 subcarriers), using BPSK-modulated m-sequences for robust frequency offset resilience.

  • SSS: Occupies the third OFDM symbol (symbol 2), subcarriers 56–182, using BPSK-modulated gold sequences.

  • PBCH: Spans symbols 1, 3, and parts of symbol 2 (subcarriers 0–47 and 192–239), carrying the MIB and additional payload bits (e.g., SSB index, half-frame indicator).

  • PBCH DM-RS: Demodulation Reference Signals interleaved with PBCH, with positions determined by the PCI.


The SSB is transmitted in SS Burst Sets, confined to a 5 ms window, with the number of SSBs (Lmax) depending on the frequency band:

  • Lmax = 4: Below 3 GHz (FR1, low frequencies).

  • Lmax = 8: 3–6 GHz (FR1, mid frequencies).

  • Lmax = 64: Above 6 GHz (FR2, mmWave).

 

Calculating SSB Location in the 5G Resource Grid

The SSB’s position in the frequency and time domains is not fixed, unlike LTE. Its location is determined by a combination of parameters and the synchronization raster, ensuring UEs can locate it during cell search. Below, we detail the calculation process, referencing 3GPP TS 38.211 and TS 38.213.

Frequency Domain Location

The SSB’s frequency position is defined relative to a reference point, Point A, which is the lowest subcarrier of Common Resource Block (CRB) 0. The key parameters are:

  1. absoluteFrequencyPointA: The absolute frequency of Point A, expressed as an Absolute Radio Frequency Channel Number (ARFCN). It defines the reference frequency for the carrier’s resource grid.

  2. offsetToPointA: The frequency offset from Point A to the lowest subcarrier of the RB overlapping with the SSB, expressed in units of 15 kHz RBs for FR1 or 60 kHz RBs for FR2.

  3. ssb-SubcarrierOffset (kssb): The subcarrier offset from subcarrier 0 of the SSB’s CRB to subcarrier 0 of the SSB, with the 4 least significant bits provided by the MIB parameter ssb-SubcarrierOffset. If not provided, it’s derived from the frequency difference between the SSB and Point A.

  4. absoluteFrequencySSB: The center frequency of the SSB, expressed as an ARFCN, corresponding to the Global Synchronization Channel Number (GSCN) on the synchronization raster.

The above parameters will appear in MIB & SIB1 broadcast messages from gNB as shown in below example,

5G NR MIB:

  sfn : 652

  block_index : 0

  half_number : 0

  intra_freq_reselection : ALLOWED(0)

  cell_barred : NOT_BARRED(1)

  pdcch_config_sib1 : 96

    controlResourceSetZero : 6

    searchSpaceZero : 0

  dmrs_typea_position : POS2(2)

  ssb_subcarrier_offset : 10 -> This is called kSSB

  msb_for_dssb : 0

  subcarrier_spacing_common : SCS15(0)

  Spare_for_padding : 37

 

5G NR SIB1:

 

BCCH_DL_SCH_Message 

   message 

      c1 

         systemInformationBlockType1 

            cellSelectionInfo 

               q_RxLevMin = -62

            cellAccessRelatedInfo 

               plmn_IdentityInfoList[0] 

                  plmn_IdentityList[0] 

              ……….

              ………

              ………

              downlinkConfigCommon

  frequencyInfoDL 

                     frequencyBandList[0] 

 

                     offsetToPointA = 14

                     scs_SpecificCarrierList[0] 

                        offsetToCarrier = 0

                        subcarrierSpacing = kHz15

                        carrierBandwidth = 106

                  initialDownlinkBWP 

                     genericParameters 

                        locationAndBandwidth = 28875

                        subcarrierSpacing = kHz15

              …………….

              …………….

              …………….

              uplinkConfigCommon 

                  frequencyInfoUL 

                     frequencyBandList[0] 

                     absoluteFrequencyPointA = XXXXXX

                     scs_SpecificCarrierList[0] 

                        offsetToCarrier = 0

                        subcarrierSpacing = kHz15

                        carrierBandwidth = 106

 

Calculation Steps

  1. Determine Point A Frequency:

    • Convert absoluteFrequencyPointA (ARFCN) to frequency using the ARFCN formula (TS 38.104):

FREF = FREF-Offs + ΔFGlobal (NREF – NREF-Offs)

where (FREF-Offs), (ΔFGlobal l), and (NREF-Offs) are band-specific (e.g., for 0–100 GHz, (ΔFGlobal = 5 kHz)) shown below.


  1. Calculate SSB CRB Start:

    • The SSB’s CRB starts at:

CRBSSB = Point A + (offsetToPointA * RB size)

where RB size is 180 kHz (12 × 15 kHz) for FR1 with 15 kHz SCS or 720 kHz (12 × 60 kHz) for FR2 with 60 kHz SCS.

  1. Adjust for Subcarrier Offset:

    • The SSB’s first subcarrier is offset by (kSSB) subcarriers:

SSB Start Frequency = CRBSSB + (kSSB * SCS)

where SCS is the subcarrier spacing (e.g., 15 kHz or 30 kHz for FR1).

  1. Verify with Synchronization Raster:

    • The SSB’s center frequency aligns with a GSCN on the synchronization raster (TS 38.104, Section 5.4.3). The GSCN is converted to frequency using:

FSSB = 2400 * GSCN + Offsetband-specific (in kHz)

This ensures the SSB is at a predefined frequency for initial access.


Example

For band n78 (FR1, 3300–3800 MHz), assume:

  • absoluteFrequencyPointA = 643008 (ARFCN, ~3640.83 MHz).

  • offsetToPointA = 690 RBs (15 kHz SCS).

  • kssb = 0.

  • SCS = 30 kHz.

  • Step 1: Point A = 3640.83 MHz.

  • Step 2: SSB CRB start = (3640.83 + (690 * 180 kHz) = 3640.83 + 124.2 = 3765.03 MHz ).

  • Step 3: SSB start = 3765.03 MHz (since ( kSSB = 0 )).

  • Step 4: SSB bandwidth = (240 * 30 kHz = 7.2 MHz ), centered at ~3765.03 MHz, aligning with a GSCN.

 

I have created a web tool that visualizes the SSB block on a frequency resource grid as output image/PDF based on the input parameters. Please take a look.


Time Domain Location

The SSB’s time-domain position is defined by its starting OFDM symbol within a 5 ms half-frame, determined by the SCS and frequency band (TS 38.213, Section 4.1). Five patterns (Cases A–E) are defined:

Case

SCS (kHz)

Frequency Range

Lmax

Starting Symbol Indices

A

15

FR1 (<3 GHz)

4

{2, 8, 16, 22}

A

15

FR1 (3–6 GHz)

8

{2, 8, 16, 22, 30, 36, 44, 50}

B

30

FR1 (<3 GHz)

4

{4, 8, 16, 20}

B

30

FR1 (3–6 GHz)

8

{4, 8, 16, 20, 32, 36, 44, 48

C

30

FR1 (TDD)

4/8

Same as Case A

D

120

FR2 (>6 GHz)

64

{4,8,16,20 … 508,512,520,524}

E

240

FR2 (>6 GHz)

64

{8,12,16,20 … 480,484,488,492}

Parameters

  1. ssb-PeriodicityServingCell: Configures the SSB periodicity (5, 10, 20, 40, 80, 160 ms) via RRC signaling. Default is 20 ms for initial cell search.

  2. SSB Index: Identifies the SSB within the SS Burst Set (0 to Lmax–1). For Lmax = 4/8, the index is derived from PBCH DM-RS; for Lmax = 64, the 3 MSB bits come from PBCH payload, and 3 LSB bits from DM-RS.

  3. Half-Frame Flag: A bit in the PBCH payload indicates whether the SSB is in the first or second 5 ms of a 10 ms frame.

Calculation Steps

  1. Determine Case: Based on SCS and frequency band, select the appropriate case (e.g., Case B for 30 kHz SCS in FR1, 3–6 GHz).

  2. Identify Starting Symbol:

    • Use the symbol indices for the selected case. For example, in Case B (Lmax = 8), SSBs start at symbols {4, 8, 16, 20, 32, 36, 44, 48.}

    • The SSB index maps to these symbols in ascending order.

  3. Map to Slot:

    • For 30 kHz SCS, each slot is 0.5 ms (14 symbols). Symbol 4 in a half-frame corresponds to slot 0, symbol 4.

    • The SSB spans 4 symbols, so SSB at symbol 4 occupies symbols 4–7.

Example

For band n78 (30 kHz SCS, 3–6 GHz, Case B, Lmax = 8):

  • SSB periodicity = 20 ms.

  • SSB index = 2.

  • Starting symbol = 16 (third SSB in {4, 8, 16, 20, 32, 36, 44, 48).

  • Slot = (floor (16 / 14) = 1 ), symbol 2 (since ( 16 mod 14 = 2 )).

  • SSB occupies symbols 2–5 in slot 1 of the half-frame.


Conclusion

The 5G NR SSB is a cornerstone of the NR physical layer, enabling efficient cell search, synchronization, and beam management. Its flexible design, compared to LTE’s fixed synchronization signals, supports diverse frequency bands, numerologies, and beamforming requirements. Calculating the SSB’s location involves determining its frequency position using absoluteFrequencyPointA, offsetToPointA, and kssb, and its time position using SCS-specific patterns and SSB indices. By leveraging 3GPP specifications (TS 38.211, TS 38.213, TS 38.104), network engineers can precisely configure and optimize SSB transmission, ensuring robust UE connectivity in 5G networks.


References

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