Description
The New Radio-Secondary Synchronization Signal (NR-SSS) is the companion signal to the NR-PSS within the 5G NR SS/PBCH block. Following the detection of the PSS, the UE knows exactly where in time to look for the SSS, as their relative positions within the SSB are fixed by specification. The primary role of the NR-SSS is to convey the remaining portion of the cell's Physical Cell Identity (PCI). While the PSS indicates one of 3 possible values (N_ID^(2)), the SSS indicates one of 336 possible values (N_ID^(1)). Combining these (PCI = 3 * N_ID^(1) + N_ID^(2)) gives the full PCI range of 0 to 1007. Furthermore, the detection of the SSS allows the UE to determine the 10 ms radio frame boundary, resolving the 5 ms ambiguity left after PSS detection.
The NR-SSS is also based on a frequency-domain M-sequence, but it uses a longer and more complex generation method compared to the PSS. It is a length-127 sequence generated from two interleaved M-sequences, which are scrambled depending on the PSS sequence index (N_ID^(2)). This design ensures orthogonality and low cross-correlation between different SSS sequences, reducing the probability of false detection. Like the PSS, it is mapped to 127 contiguous subcarriers within the SSB, occupying the same set of subcarriers but at a different OFDM symbol time. The specific symbol location of the SSS within the SSB depends on the subcarrier spacing.
Architecturally, the NR-SSS is generated by the physical layer of the gNB for each SSB transmission. Its detection by the UE is a critical step in the cell search and selection process. After finding the PSS and SSS, the UE has acquired the cell's PCI, symbol timing, and frame timing. This information is essential for descrambling reference signals (like the DM-RS for PBCH) and for beginning to decode the NR-PBCH to obtain the NR-MIB. The SSS also aids in distinguishing between cells that may share the same PSS sequence. The combined PSS/SSS design is optimized for low latency cell search, supporting quick network discovery for mobile devices, IoT sensors, and other 5G endpoints across diverse deployment scenarios from macro cells to dense small cells.
Purpose & Motivation
The NR-SSS was created to complete the cell identification process started by the NR-PSS, addressing the need for a robust and unambiguous cell identity mechanism in 5G's diverse environments. The problem it solves is providing the UE with the exact frame timing and the full Physical Cell Identity, which is necessary for all subsequent physical layer procedures, including channel estimation, demodulation of broadcast channels, and interference identification. In LTE, the SSS also provided frame timing, but the 5G design had to accommodate a more flexible frame structure and the possibility of very dense networks with many overlapping cells.
The historical motivation for its design includes the need for a larger cell identity space (1008 PCIs) to support ultra-dense network deployments without PCI conflicts. The interleaved M-sequence design of the NR-SSS was chosen to provide good auto-correlation and cross-correlation properties, ensuring reliable detection even in the presence of high interference from neighboring cells—a common scenario in urban 5G deployments. Furthermore, its dependency on the PSS sequence (through scrambling) ties the two signals together, improving the overall robustness of the cell search procedure. This design ensures that a UE can quickly and accurately identify a cell, which is fundamental for mobility (handovers), network selection, and efficient radio resource management in the 5G ecosystem.
Key Features
- Based on a length-127 sequence generated from two interleaved M-sequences
- Provides 336 possible sequences (N_ID^(1) = 0...335) to complete the PCI (0-1007)
- Enables detection of the 10 ms radio frame boundary, resolving 5 ms timing ambiguity
- Sequence scrambling depends on the detected PSS index (N_ID^(2))
- Transmitted at a fixed symbol offset from the PSS within the same SSB
- Essential for UE to acquire full cell identity before decoding the PBCH/MIB
Evolution Across Releases
Defined alongside the NR-PSS in early 5G standardization studies. The interleaved M-sequence design was established to provide a large set of robust sequences for cell identification, supporting the dense network scenarios anticipated for NR. Its role in delivering frame timing and completing the PCI was solidified in this release.
Defining Specifications
| Specification | Title |
|---|---|
| TS 38.802 | 3GPP TR 38.802 |
| TS 38.912 | 3GPP TR 38.912 |