SSS

Secondary Synchronization Signal

Physical Layer →
Introduced in Rel-8

SSS is the Secondary Synchronization Signal, a physical layer signal in LTE and NR that works with the PSS to enable a UE to identify the physical cell ID and achieve symbol timing during cell search.

Category
Physical Layer
Introduced
Rel-8
Where
Radio Access Network › NG-RAN (5G)
Specifications
31 specs
SSS Description Purpose Related Detected Changes Specifications

Description

The Secondary Synchronization Signal (SSS) is a critical downlink physical signal transmitted by the base station (eNodeB in LTE, gNB in NR). Its primary function is to facilitate the cell search procedure, where a User Equipment (UE) detects and synchronizes to a cell. The SSS is always transmitted in conjunction with the Primary Synchronization Signal (PSS). While the PSS provides coarse symbol timing and indicates one part of the physical cell identity (PCI), the SSS provides the remaining, and larger, part of the PCI. Specifically, in LTE, the 504 possible PCIs are grouped into 168 unique cell identity groups, each containing 3 unique identities. The SSS conveys the group identity (0-167), while the PSS conveys the within-group identity (0-2). In NR, the concept is similar but adapted for more flexible numerology and wider bandwidths; the 1008 possible PCIs are derived from combinations of sequences carried on the PSS and SSS.

The SSS is constructed using a specific sequence, such as an M-sequence in LTE or a Gold sequence in NR, which is mapped to specific resource elements within the synchronization signal block (SSB). In LTE, the SSS is transmitted in the central 62 subcarriers (excluding the DC carrier) of the last OFDM symbol of slots 0 and 10 within a radio frame for FDD, and in specific subframes for TDD. In NR, the SSS is located within the SS/PBCH block (SSB), occupying 127 subcarriers. The exact time-frequency position relative to the PSS allows the UE to determine the system frame timing (i.e., the 10ms radio frame boundary) after detecting both signals.

Upon powering on or during handover, the UE performs a blind search for the PSS first, achieving 5ms timing and a candidate PCI subset. It then searches for the SSS within the expected time window. By successfully detecting the SSS sequence, the UE decodes the full PCI and achieves frame synchronization. This process is robust to high Doppler shifts and initial frequency offsets. The SSS design, including its sequence properties and mapping, is optimized for reliable detection under low signal-to-noise ratio (SNR) conditions, which is crucial for cell-edge performance. Furthermore, the SSS aids in distinguishing between cells using the same PSS sequence, thereby preventing ambiguity in dense network deployments.

Purpose & Motivation

The SSS was created to solve the fundamental problem of initial cell acquisition and synchronization in cellular networks. Before a UE can decode any system information or establish a connection, it must first find a cell, determine its identity, and align its receiver in time and frequency with the cell's transmissions. The PSS alone is insufficient as it only provides partial cell identity and timing information. The SSS completes the cell identification process and delivers critical frame timing.

Historically, synchronization signals existed in earlier standards like UMTS, but with the introduction of OFDMA in LTE, a new synchronization scheme was required. The paired design of PSS and SSS in LTE and NR provides a fast, reliable, and computationally efficient two-step detection process. This design addresses limitations of single-signal approaches by distributing the detection complexity and improving robustness against interference and fading. It enables quick cell search, which is essential for reducing connection setup time and improving handover performance, directly impacting user experience in terms of call setup delay and mobility reliability.

In NR, the purpose extends to support a wider range of frequencies (including mmWave) and flexible numerologies. The SSS, as part of the SSB, is beamformed in higher frequencies. Its design ensures reliable detection across diverse deployment scenarios, from wide-area coverage below 6 GHz to targeted beam-based coverage in millimeter-wave bands, which was a key motivation for its evolution from LTE.

Detected Changes Across Releases

from 3GPP Change Requests

Specific changes extracted from the „Change history“ tables of 3GPP specifications (23 CRs across 5 releases). Complements the general historical overview above with the evidence-based evolution of this function.

Studied in Rel-8, normative work from Rel-15.

Rel-15 1 change

In Release 15, no specific new feature or procedural change for the Secondary Synchronization Signal (SSS) function itself is described in the provided materials. The grounding context exclusively details new minimum requirements and performance tests for various network-signalled values (e.g., NS_29, CA_NS_04) and carrier aggregation combinations, but these are measurement and demodulation requirements, not SSS-specific enhancements. The listed Change Request titles also refer to RRC state transition signalling, which is unrelated to the physical layer SSS.

  • Capture signalling flows where the last serving gNB moves the UE to RRC_IDLE TS 38.300CR0087
Rel-16 7 changes

In Release 16, there were no specific new features introduced for the Secondary Synchronization Signal (SSS) function itself. The provided Change Requests and grounding context focus on corrections and enhancements for other areas, including sidelink synchronization procedures, reference signals, and new minimum requirements for various network-signalled values (NS_xx and CA_NS_xx). Therefore, the release did not define novel SSS capabilities but refined related systems like sidelink.

  • Correction on OFDM signal generation and PSSCH DM-RS time-domain OCC in TS 38.211 TS 38.211CR0072
  • 38.213 CR Correction on HARQ-ACK codebook for secondary PUCCH group TS 38.213CR0167
  • CR on Timing for secondary cell activation / deactivation with sub-slot PUCCH TS 38.213CR0197
  • Corrections for the reference signal used for sidelink power control TS 38.213CR0212
  • Correction on synchronization procedure for sidelink transmission TS 38.213CR0249
  • Corrections for transmitting sidelink reference signals in TS 38.214 TS 38.214CR0197

+ 1 more changes

Rel-17 1 change

In Release 17, there were no specific changes introduced for the Secondary Synchronization Signal (SSS) function itself. The provided grounding context and Change Request titles detail updates to minimum requirements and capabilities for various network-signalled values (NS_xx and CA_NS_xx) and other areas like UE security signaling, but none of these are directly related to modifying the SSS. The listed modifications focus on performance requirements, carrier aggregation, and other UE capabilities rather than the synchronization signal structure or procedure.

  • UE Security Capabilities signaling in NG-RAN [UE_Sec_Caps] TS 38.300CR0427
Rel-18 5 changes

In Release 18, the changes to the SSS function were not defined by new signal structures or sequences but were refined through corrections to its operation during specific network states. Specifically, the release included a correction for the behavior of physical channels and signals, including the SSS, during cell DTX/DRX operation to ensure reliable synchronization. This ensures the SSS remains correctly detectable and functional during these discontinuous transmission and reception periods.

  • Correction of physical channels and signals during cell DTX/DRX operation TS 38.214CR0566
  • Correction on CSI processing criteria for new NES capability signaling TS 38.214CR0584
  • Correction of network timing synchronization status monitoring TS 38.300CR0817
  • Correction on coexistence between CHO and satellite switching with re-synchronization TS 38.300CR0903
  • CR on default pathloss reference signal for SRS and PUCCH TS 38.213CR0548
Rel-19 9 changes

In Release 19, the changes to the Secondary Synchronization Signal (SSS) function are not detailed in the provided grounding context or the listed CR titles. The CR titles primarily focus on introducing a low-power wake-up signal and receiver, secondary cell measurement skipping for NR ATG, and corrections to other signal types, with no specific mention of modifications to the SSS. The grounding context exclusively lists various minimum requirement tables for network-signalled values and other performance tests, none of which directly reference the SSS.

  • Introduction of low-power wake-up signal TS 38.211CR0152
  • Introduction of low-power wake-up signal and receiver for NR TS 38.213CR0708
  • Introduction of Low-Power Wake-Up Signal and Receiver for NR TS 38.300CR1015
  • Introduction of Secondary Cell Measurement Skipping for NR ATG TS 38.300CR1027
  • Corrections on low-power wake-up signal and receiver for NR TS 38.213CR0720
  • Corrections on low-power wake-up signal and receiver for NR TS 38.213CR0740

+ 3 more changes

Explore further

Broader topics and technologies where SSS plays a role.

Topics
LTE / LTE-Advanced5G NR (New Radio)Lawful InterceptUMTS / WCDMARadio Access Network
Technologies
LTE

Defining Specifications

3GPP specifications that define or reference SSS, with the latest known release. Sourced from the 3GPP document catalog — see methodology.

SpecificationTitleRelease
TS 36.101 vj30 LTE UE Radio Transmission & Reception Requirements Rel-19
TS 36.116 vj00 E-UTRA Relay RF Requirements Rel-19
TS 36.117 vj00 E-UTRA Relay RF Test Methods & Requirements Rel-19
TS 36.785 ve00 LTE Sidelink V2V Services Study Rel-14
TS 36.786 ve00 TR on V2X Services based on LTE sidelink Rel-14
TS 36.787 vf00 V2X New Band Combinations for LTE Rel-15
TS 36.788 vf00 V2X Phase 2 Technical Report for LTE Rel-15
TS 36.825 vd00 Study on Additional LTE TDD Configurations Rel-13
TS 36.863 vc00 CRS Interference Mitigation for Homogeneous Networks Rel-12
TS 36.877 vc00 LTE Device to Device Proximity Services Rel-12
TS 36.878 vd00 LTE Performance Enhancements for High Speed Scenarios Rel-13
TS 36.894 vd00 Study on LTE Measurement Gap Enhancement Rel-13
TR 36.976 vj00 LTE-based 5G Terrestrial Broadcast Overview Rel-19
TR 37.910 vj00 5G SRIT and NR RIT Self-Evaluation Report Rel-19
TS 38.106 vj20 NR Repeater Radio Transmission and Reception Rel-19
TS 38.133 vj20 5G UE Radio Requirements for RRC_IDLE Mobility Rel-19
TS 38.151 vj00 NR UE MIMO OTA Performance Requirements Rel-19
TS 38.174 vj10 NR Integrated Access and Backhaul Radio Spec Rel-19
TS 38.176 vj20 IAB Conformance Testing Specification Rel-19
TS 38.211 vj10 NR Physical Channels and Modulation Rel-19
TS 38.213 vj10 NR Physical Layer Control Procedures Rel-19
TS 38.214 vj10 NR Physical Layer Procedures for Data Rel-19
TS 38.300 vj00 NG-RAN Overall Description Rel-19
TS 38.523 vj20 5G NR UE Conformance Testing: Idle/Inactive Rel-19
TS 38.551 vi30 User Equipment (UE) Multiple Input Multiple Output (MIMO) Over-the-Air (OTA) performance Rel-18
TS 38.761 vj00 MIMO OTA Performance Measurements for UE Rel-19
TS 38.762 vj00 Dynamic MIMO OTA Test Methodology for NR FR1 Rel-19
TS 38.811 vf40 Study on NR Support for Non-Terrestrial Networks Rel-15
TR 38.833 vh00 NR Demodulation Performance Enhancement Rel-17
TR 38.864 vi10 Technical Report on Network Energy Savings for NR Rel-18
TR 38.878 vi40 Technical Report on Advanced Receiver for MU-MIMO Rel-18