Description
The Global Synchronization Channel Number (GSCN) is a critical identifier in the 5G New Radio (NR) physical layer, introduced in 3GPP Release 15. It serves as a global index that points to the absolute radio frequency channel number (ARFCN) of the center frequency for a Synchronization Signal Block (SSB). The SSB carries the Primary Synchronization Signal (PSS), Secondary Synchronization Signal (SSS), and Physical Broadcast Channel (PBCH), which are the essential signals a User Equipment (UE) uses to discover, synchronize with, and decode basic system information from a 5G cell. The GSCN provides a simplified and efficient method for the network to signal and for the UE to search for these SSBs across the vast and complex 5G frequency range.
Architecturally, the GSCN is defined within the NR radio interface specifications (e.g., TS 38.104, TS 38.101). It works by establishing a mapping between the GSCN integer value and a specific SSB center frequency (in kHz). This mapping is defined differently for Frequency Range 1 (FR1: sub-6 GHz) and Frequency Range 2 (FR2: mmWave, 24.25 GHz and above) due to their different channel raster characteristics. For FR1, the GSCN step size corresponds to a frequency step (e.g., 1.2 MHz or 1.44 MHz depending on the band). For FR2, the step is larger, aligning with the wider bandwidths and different synchronization raster. The UE uses the GSCN, provided in system information or measurement configurations, to directly tune its receiver to the expected SSB frequency without needing to perform a blind search over a wide range of possible frequencies.
Key components involving GSCN include the synchronization raster, the SSB, and higher-layer signaling. The synchronization raster defines the set of allowed frequencies on which an SSB can be placed. The GSCN essentially numbers these raster points globally. In operation, the network broadcasts a list of GSCNs in the System Information Block 1 (SIB1) via the PBCH, indicating where in frequency the UE can find neighboring cells' SSBs for measurements (e.g., for cell reselection or handover). The gNodeB (gNB) also uses GSCN in measurement object configuration for connected-mode UEs via RRC signaling.
Its role is paramount for network discovery and mobility. It drastically reduces the time and power the UE spends on initial cell search, especially in mmWave bands where beams are used. By knowing the GSCN, the UE knows precisely where to look for the SSB, enabling faster beam sweeping and association. This efficiency is vital for supporting high mobility, energy saving, and reliable connectivity in 5G's diverse deployment scenarios, from wide-area coverage in low bands to hotspot capacity in high bands.
Purpose & Motivation
The GSCN was created to solve the significant cell search and measurement challenges introduced by 5G NR's extremely wide and flexible spectrum usage. Previous generations like LTE used a concept of EARFCN (E-UTRA Absolute Radio Frequency Channel Number) which was tied to the carrier center frequency. However, 5G introduced the SSB, which is not necessarily centered on the carrier and can be placed on a different raster (the synchronization raster). Furthermore, 5G supports a massive range of frequencies from below 1 GHz to 100 GHz, with fragmented spectrum allocations and bandwidths up to 400 MHz. A simple, contiguous numbering scheme like EARFCN was insufficient.
The primary problem GSCN addresses is the inefficiency of blind search. Without GSCN, a UE would have to scan every possible frequency point on the synchronization raster across multiple bands, a process that would be prohibitively time-consuming and power-intensive, particularly in mmWave bands where searching across many beams is already complex. GSCN provides a concise, globally unambiguous 'address' for the SSB, allowing the network to tell the UE exactly where to look. This enables fast initial access, efficient neighbor cell measurements, and reliable mobility.
The motivation stemmed from the need for scalable and efficient operation across 5G's heterogeneous landscape. The design allows for a unified method to signal SSB locations regardless of the frequency band or bandwidth part configuration. It abstracts the complex underlying frequency calculations into a simple integer, simplifying UE implementation and network configuration. This was a necessary evolution from LTE's approach to handle the new paradigm of decoupled synchronization and data channel rasters in NR, directly supporting features like wide bandwidth carriers and flexible SSB placement for beamforming.
Classification
Detected Changes Across Releases
from 3GPP Change RequestsSpecific changes extracted from the „Change history“ tables of 3GPP specifications (109 CRs across 5 releases). Complements the general historical overview above with the evidence-based evolution of this function.
In Release 15, the GSCN (Global Synchronization Channel Number) function was newly introduced to provide a global frequency raster for NR synchronization signal blocks, enabling efficient cell search and measurement procedures. This mechanism allows the UE to determine the frequency location of SS/PBCH blocks by converting a GSCN value into a corresponding RF reference frequency. The introduction of GSCN standardized the channel numbering across the wide range of NR operating bands and spectrum types supported in the release.
- Support maximum 8 SS/PBCH blocks for unpaired spectrum beyond 2.4GHz TS 38.213CR0006
- CR to TS 37.145-2: OTA Adjacent Channel Leakage Ratio (6.7.3) and OTA Operating band unwanted emissions (6.7.5) - corrections to text and tables TS 37.145CR0055
- CR to TS 38.104 on Combined updates from RAN4 #90 This document combines the proposed changes in the following Draft CRs from RAN4 #90: - R4-1900284, "Draft CR on NR PUCCH format2 performance requirements for TS 38.104" - R4-1900763, "Draft CR to TS 38.104: Update of performance requirement numbers for DFT-s-OFDM based PUSCH" - R4-1900876, "Draft CR to TS 38.104: On RX spurious emissions requirement" - R4-1900968, "Draft CR for 38.104: Performance requirements for NR PUCCH format 1" - R4-1901329, "Draft CR to 38.104: Annex C.6 correction" - R4-1901330, "Draft CR to 38.104: Abbreviations addition" - R4-1901387, "Draft CR to TS 38.104 BS demodulation PUCCH format 0 requirements" - R4-1901474, "Draft CR to TS 38.104: Corrections on transmitter co-existence and co-location requirements" - R4-1901483, "Draft CR to TS 38.104: Corrections on general intermodulation requirement" - R4-1902239, "Draft CR to TS 38.104: Addition of missing EIRP/EIS definitions in terminology in clause 3.1" - R4-1902241, "Draft CR to 38.104; clarification of BS power limits" - R4-1902245, "Draft CR to 38.104: Correction to FR2 OTA Interfering signal mean power units" - R4-1902246, "Draft CR to 38.104; Correction to definition of OTA reference sensitivity" - R4-1902260, "draft CR to TS 38.104 - update emissions scaling" - R4-1902338, "Draft CR: Update on FR1 range extension for TS38.104" - R4-1902389, "draftCR for 38.104 on PUSCH requirements with CP-OFDM and FR1" - R4-1902394, "Draft CR to TS 38.104 – PUSCH requirements with CP-OFDM for FR2" - R4-1902396, "CR: Updates to PUCCH formats 3 and 4 performance requirements in TS 38.104" - R4-1902444, "Draft CR to TS 38.104: Editorial CR for BS demodulation requirements" - R4-1902561, "Draft CR for updating PRACH performance requirements in TS38.104" - R4-1902571, "Corrections to 38.104 Delay profile calculation" - R4-1902642, "Draft CR to TS 38.104: Correction on multi-band operation related requirements" TS 38.104CR0019
- Correction on physical downlink control channel TS 38.213CR0020
- CR on the determination of the minimum number of PRBs for PUCCH transmission TS 38.213CR0036
In Release 16, the main update for the GSCN function was to accommodate the introduction of new and wider channel bandwidths for several NR operating bands, as detailed in multiple Change Requests for TS 38.104. These updates included new channel bandwidths such as 30MHz for n50 and 40MHz for n38, alongside wider bandwidths for bands including n1, n7, n28, n66, n77, and n78. This required ensuring the GSCN raster could support the channel arrangements for these expanded bandwidths within the specified operating bands.
- Addition channel bandwidth of 30MHz for n50 in TS 38.104 TS 38.104CR0031
- CR for TS 38.104: adding wider channel bandwidths in Band n7 TS 38.104CR0037
- CR for TS 38.104: adding wider channel bandwidths in Band n77/n78 TS 38.104CR0105
- CR for TS 38.104: Addition channel bandwidth of 40MHz for n38 TS 38.104CR0106
- Introducing new channel bandwidth for band n28 TS 38.104CR0131
- CR for TS 38.104: adding wider channel bandwidths for n66 TS 38.104CR0139
+ 29 more changes
In Release 17, the GSCN (Global Synchronization Channel Number) function was updated to accommodate new channel bandwidths, specifically the introduction of 35MHz and 45MHz options, as indicated by multiple Change Requests to TS 38.104. This expansion required corresponding corrections to the associated channel raster definitions and frequency offset symbols for spectrum emission masks. Additionally, the release included specific corrections to the channel raster for band n259 and addressed the aggregated channel bandwidth calculation to support these new bandwidths.
- CR for TS 37.145-2: introduction of channel bandwidths 35MHz and 45MHz TS 37.145CR0314
- Big CR to 38.104 - Additional Channel BW TS 38.104CR0258
- Big CR to 38.104 - Additional Channel BW TS 38.104CR0291
- Big CR to TS 38.104: Adding channel BW support in existing NR bands TS 38.104CR0319
- Big CR to TS 38.104: Adding channel BW support in existing NR bands TS 38.104CR0343
- CR for TS 38.104: introduction of channel bandwidths 35MHz and 45MHz NOTE The CR is not implemented TS 38.104CR0341
+ 17 more changes
In Release 18, the GSCN (Global Synchronization Channel Number) function was updated to support an enhanced channel raster, which was introduced to accommodate new channel bandwidths like 3 MHz and to add specific raster points such as for the 5925-5945 MHz range. This enhancement involved corrections and clarifications to the NR-ARFCN and GSCN raster points to align with these new bandwidths and band support definitions. The changes ensure the synchronization channel numbering remains consistent with the expanded and more flexible channel arrangements defined in the release.
- Big CR to TS 38.104: Adding channel BW support in existing NR bands TS 38.104CR0450
- Big CR to TS 38.104 on introduction of 3 MHz channel bandwidth TS 38.104CR0500
- Big CR to TS 38.104: Adding channel BW support in existing NR bands TS 38.104CR0512
- Big CR to TS 38.104: Adding channel BW support in existing NR bands TS 38.104CR0538
- CR to TS38.104: Introduction of an enhanced channel raster TS 38.104CR0536
- CR to TS 38.108: Introduction of an enhanced channel raster TS 38.108CR0048
+ 22 more changes
In Release 19, the primary update to the GSCN function was the introduction of reserved GSCN and ARFCN values to specifically support Non-Terrestrial Networks (NTN) operating with channel bandwidths less than 5MHz. This change, detailed in a CR to TS 38.108, facilitated the deployment of narrowband NR channels for NTN scenarios. The release also included broader channel bandwidth adjustments, such as the introduction of 3 MHz and 7 MHz channel bandwidths across various specifications, which necessitated corresponding corrections to measurement channels and requirements.
- CR to TS 37.145-2: 7MHz channel bandwidth introduction TS 37.145CR0398
- Big CR to TS 38.104: Adding channel BW support in existing NR bands TS 38.104CR0685
- CR to 38.104 on adding new UL channel bandwidth to band n48 TS 38.104CR0698
- Running CR to TS 38.104 - Introduction of 7 MHz channel BW TS 38.104CR0725
- CR to TS 38.141-2: 7MHz channel bandwidth introduction TS 38.141CR0666
- Addition of the 3MHz channel to TR 38.741 TS 38.741CR0010
+ 12 more changes
Explore further
Broader topics and technologies where GSCN plays a role.
Defining Specifications
3GPP specifications that define or reference GSCN, with the latest known release. Sourced from the 3GPP document catalog — see methodology.
| Specification | Title | Release |
|---|---|---|
| TS 37.145 vj10 | AAS Base Station Conducted Conformance Testing | Rel-19 |
| TS 37.862 vj00 | Adding channel bandwidth in existing NR bands | Rel-19 |
| TS 38.101 vj31 | NR User Equipment Radio Transmissions | Rel-19 |
| TS 38.104 vj20 | NR Base Station RF Requirements | Rel-19 |
| TS 38.108 vj20 | NTN NR Satellite Access Node RF Requirements | Rel-19 |
| TS 38.141 vj20 | NR Base Station RF Conformance Testing Part 1 | Rel-19 |
| TS 38.181 vj10 | NR Satellite Access Node RF Testing | Rel-19 |
| TS 38.213 vj10 | NR Physical Layer Control Procedures | Rel-19 |
| TS 38.521 vj20 | NR Physical Layer UE Conformance Testing | Rel-19 |
| TS 38.741 vj00 | NTN L-/S-band for NR Technical Specification | Rel-19 |
| TR 38.815 vf10 | NR Frequency Range 24.25-29.5 GHz Study | Rel-15 |
| TR 38.847 vh20 | NR 47.2-48.2 GHz Frequency Range | Rel-17 |
| TR 38.849 vi50 | Technical Report | Rel-18 |
| TR 38.852 vh50 | 1900MHz NR band for European Rail Mobile Radio | Rel-17 |
| TR 38.853 vh50 | 900MHz NR Band for European Rail Mobile Radio | Rel-17 |
| TS 38.863 vj10 | NR NTN RF and Co-existence Spec | Rel-19 |
| TS 38.873 vg00 | NR Band n48 Technical Report | Rel-16 |
| TS 38.887 vg00 | NR Band n259 Specification (39.5-43.5 GHz) | Rel-16 |
| TR 38.889 vg00 | NR-based access to unlicensed spectrum study | Rel-16 |