When a UE is powered on or moves to a new area, it must quickly find and synchronize with a cell. For this purpose, each NR cell periodically transmits downlink synchronization signals. These signals include the Primary Synchronization Signal (PSS) and Secondary Synchronization Signal (SSS). By detecting the PSS and SSS and decoding the Physical Broadcast Channel (PBCH), the UE achieves downlink synchronization and obtains basic cell information.
In NR, the PSS, SSS, and PBCH together form an SSB. One SSB occupies four consecutive OFDM symbols in time and 20 resource blocks (240 subcarriers) in frequency. The SSB structure and its frequency position are predefined so that the UE can efficiently detect it.
In LTE, the PSS and SSS are always transmitted at the center frequency of the carrier. Once an LTE UE finds them, it immediately knows the carrier center. If the UE does not know the carrier frequency in advance, it must search across all possible frequency positions using a fixed 100 kHz channel raster, which is manageable because LTE bandwidths are limited.
NR works differently. NR supports much wider bandwidths, and its channel raster granularity can be as small as 15 kHz. If a UE searched all possible channel raster positions like LTE, cell search would take too long.
To solve this, NR introduces the concept of synchronization rasters. The SSB is not fixed at the carrier center. Instead, it can be transmitted at one of a limited number of predefined frequency positions. The UE only needs to search these sparse synchronization rasters rather than the entire channel raster, which greatly reduces cell search time.
SSB Frequency-Domain Position Configuration Modes
The frequency-domain position of an SSB can be configured in two ways: based on NR-ARFCN or based on GSCN. In Huawei live networks, this is controlled by the NRDUCell.SsbDescMethod parameter. When it is set to SSB_DESC_TYPE_NARFCN, the SSB position is configured using NR-ARFCN. When it is set to SSB_DESC_TYPE_GSCN, the SSB position is configured using GSCN.
These two configuration modes exist to support different planning and deployment needs. NR-ARFCN-based configuration gives more flexibility and precise control of the SSB frequency position, which is useful in customized or complex spectrum planning scenarios. GSCN-based configuration follows standardized synchronization raster rules, making it simpler and more suitable for typical commercial deployments and easier UE cell search.
To better understand when to use each mode, we first need to understand how the NR-ARFCN mode and GSCN mode work individually and what scenarios they are designed for.
What is the NR-ARFCN mode?
NR-ARFCN mode is a way to configure the SSB frequency position using an absolute frequency number.
ARFCN stands for Absolute Radio Frequency Channel Number. It was first used in GSM to represent an exact RF frequency. The same idea continues in later technologies: UARFCN in UMTS, EARFCN in LTE, and NR-ARFCN in 5G NR. In all cases, the purpose is the same: to map a physical frequency to a unique number.
In NR, the supported frequency range is very wide, from 0 to 100 GHz. To manage this wide range, NR defines a global frequency raster. The entire 0–100 GHz band is divided into 3,279,166 small frequency steps, numbered from 0 to 3,279,165. Each number corresponds to one exact frequency position, and this number is called an NR-ARFCN.
During cell configuration, the network first decides the center frequency of the cell based on the deployed frequency band. From this center frequency, the corresponding cell NR-ARFCN is calculated. For SSB configuration, the same logic is applied: the center frequency of the SSB is determined first, and then the NR-ARFCN of the SSB is calculated. How the SSB center frequency is chosen is a key point and will be explained in the next steps.
What is the GSCN mode?
A GSCN is a global synchronization channel number. Each GSCN corresponds to an SSB frequency-domain position. GSCNs are allocated in ascending order of frequency. The maximum number is 26639. GSCN is a new concept used in 5G. Why is GSCN introduced while NR-ARFCN has been defined in protocols? As mentioned above, NR features a very large bandwidth and a small channel raster granularity. If the UE blindly searches channel rasters for the SSB, the search speed is very slow. To address this issue, synchronization raster is defined. The synchronization raster granularity is larger than the channel raster granularity. Accordingly, the quantity of GSCNs of synchronization rasters is much less than that of frequency numbers of channel rasters. Furthermore, channel rasters are a subset of frequencies designated by NR-ARFCNs (which will be described later). Therefore, the quantity of GSCNs is much less than that of NR-ARFCNs, which increases the search speed of the UE.
Application Scenarios
Configuration based on NR-ARFCN is mainly used in NSA networking, while configuration based on GSCN can be used in SA, NSA, and SA+NSA networking scenarios.
In SA networking, a UE must blindly search for the SSB and then decode the MIB to obtain basic cell information such as NR bandwidth. Using GSCN-based configuration limits the number of frequency positions that the UE needs to search, which significantly improves SSB detection speed.
In NSA networking, the UE does not need to blindly search for the SSB. The LTE cell provides NR system information in advance through the LTE RRC connection reconfiguration message, including the NR SSB frequency number, NR band, and NR bandwidth. Therefore, both GSCN and NR-ARFCN configurations are supported in NSA scenarios.
Vendor normally recommends using the GSCN mode in all current scenarios because it offers faster cell search and better UE experience. The NR-ARFCN mode can be kept if it is already in use, but it is not the preferred option for new deployments.
SSB Frequency-Domain Position Calculation Modes
First, the center frequency number of the cell is determined. Based on this, the SSB frequency-domain position is calculated. The frequency offset between the SSB position and the first RB of the cell must be an integer multiple of the subcarrier spacing to ensure proper alignment.
There are two calculation modes for determining the SSB frequency-domain position: NR-ARFCN–based calculation and GSCN–based calculation. These methods are described separately as NR-ARFCN Calculation and GSCN Calculation.
Although GSCN is used to define the SSB position, its calculation still relies on frequency-to-number conversions, which use the same formulas defined for NR-ARFCN calculation. This means NR-ARFCN formulas are the foundation for both calculation modes.