Synchronization Raster GSCN in 5G-NR

The GSCN is similar to the concept of the channel raster in LTE, where the channel raster is 100 kHz. However, in 5G NR, the frequency interval of the channel raster is not fixed at 100 kHz like LTE. Due to the narrower frequency range of 5G, scanning every 100 kHz interval would be time-consuming and impact UE performance.

  • To address this issue, 5G NR adopts a different approach. The SSB’s position is not fixed due to different subcarrier spacings, but there is a limited set of possible locations in each band, known as synchronization raster or GSCN positions. To efficiently search for the SSB, the UE performs sparse and specific searches at these possible locations, similar to the concept of the channel raster in LTE.
  • The GSCN represents the center frequency point of the SSB, and the UE scans the entire bandwidth using this synchronization raster. The UE performs a narrow-width scan at each step, aiming to reach the central frequency point of the SSB within this synchronization raster. This process helps to quickly locate the SSB without having to scan the full bandwidth at a narrow interval.
  • The width of the synchronization raster, or the scanning step, is wider than the 100 kHz channel raster in LTE. This wider scanning step optimizes the search for the SSB, reducing the time it takes for the UE to identify the SSB location.

In conclusion, the concept of GSCN or synchronization raster is employed in 5G NR to efficiently search for the SSB, reducing search time and improving UE performance compared to LTE’s fixed 100 kHz channel raster.

For 5G NR, a wider frequency width is used to scan the entire frequency spectrum. In contrast to LTE, where the positions of the Primary Synchronization Signal (PSS) and Secondary Synchronization Signal (SSS) are fixed at the carrier’s center frequency, the position of the Synchronization Signal Block (SSB) in 5G NR is not fixed. The SSB can be placed at various locations within the carrier’s frequency range.

To locate the SSB, the UE employs a small scanning width to try and reach the central frequency point of the SSB, known as SSREF or GSCN. The GSCN represents the multiple of the synchronization cluster, and it helps in locating the SSB in both frequency and time domains.

To understand the relationship between SSB center frequency point and GSCN, there is a conversion formula from Ssref to GSCN and vice versa.

The formula to calculate SSREF is Ssref = N * 1.2 MHz + M * 15 kHz, where N and M are integer values.

Additionally, the GSCN can be calculated as GSCN = 3 * N + (M – 3)/2 for a specific frequency range (0-3000 MHz).

UE scans GSCN as indicated in the below table

Table Applicable SS raster entries per operating band

  • In conclusion, the positioning of the SSB is variable, and the UE uses GSCN or synchronization raster to efficiently search for the SSB location in 5G NR. By knowing the values of N and M, the UE can calculate the GSCN and SSREF, facilitating the search process for the SSB.
  • SSB follows the GSCN raster in standalone mode, where the UE scans the entire bandwidth with special granularity to locate the SSB’s central frequency point in both time and frequency domains.
  • To find the SSB location, the UE performs a frequency scan across the entire band, using a special granularity called the synchronization raster.
  • The synchronization raster divides the carrier bandwidth into finite locations where SSBs can potentially be deployed. By scanning the band with the synchronization raster, the UE tries to reach the central frequency point of the SSB.

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