Tools / nr-gscn-calculator

5G NR GSCN - SSB Frequency Calculator

Convert 5G NR GSCN to SSB reference frequency and NR-ARFCN, or convert SSB frequency back to GSCN. The calculator also validates applicable SS raster entries per band, including the Release 17 n79 bandwidth-dependent step size.

Band-Aware Filters

Use the filters to validate whether a GSCN or SSB frequency is allowed inside a selected band. You can also narrow results by SSB subcarrier spacing when a band supports multiple SSB cases. For n79, channel bandwidth below 40 MHz uses the Release 17 step size of 1.

GSCN to SSB Frequency

SSB Frequency to GSCN

Global GSCN formulas follow 3GPP TS 38.104 clause 5.4.3, including the special 3 MHz raster and n100 special entries.

Calculation Result

Enter a GSCN or SSB frequency and calculate.

Applicable SS Raster Entries

All GSCN Reference Entries

Method Notes

• Global conversion: uses 3GPP TS 38.104 clause 5.4.3.1 formulas for standard raster, 3 MHz raster, and the n100 special GSCN values.
• Band validation: uses the SS raster entries from TS 38.104 clause 5.4.3.3, including band-specific explicit GSCN lists.
• Release 17 n79: the calculator distinguishes 8480 - <16> - 8880 for bandwidths >= 40 MHz and 8475 - <1> - 8884 for bandwidths < 40 MHz.
• n263: the 120 kHz and 480 kHz cases are generated from the formula expressions defined in the spec, not approximated.
• NR-ARFCN: the output ARFCN is the nearest global NR-ARFCN to the computed SSB frequency.

How to Use This Tool

1. Choose an optional NR band and channel bandwidth if you want the result validated against a specific operating band.
2. Use SSB SCS when a band supports more than one SSB configuration, such as n258 with 120 kHz and 240 kHz options.
3. Keep Raster Mode on Auto unless you specifically want to test the standard raster or the special 3 MHz raster only.
4. To convert from GSCN, enter the GSCN value in GSCN to SSB Frequency and press Calculate.
5. To convert from SSB frequency, enter the frequency in MHz in SSB Frequency to GSCN and press Calculate.
6. Check Calculation Result for the final GSCN, SSB frequency, and nearest NR-ARFCN.
7. Review Applicable SS Raster Entries to confirm whether the result is allowed for the selected band and bandwidth.

5G NR GSCN and SSB Frequency Explained

When a 5G UE first tries to find a cell, it does not immediately know the exact carrier details. It first looks for the SS/PBCH block, usually called the SSB. To make that search practical, 3GPP defines a synchronization raster. The frequency positions on that raster are identified by GSCN, which stands for Global Synchronization Channel Number. In the spec wording, the synchronization raster gives the frequency positions of the synchronization block that a UE can use for system acquisition when the SSB position has not been explicitly signaled. [ETSI]

So the big idea is simple:

• GSCN is the index.
• SSB frequency is the actual frequency location.
• The spec connects the two through SSREF, the SS block reference frequency. [ETSI]

1) What exactly is the SSB?

The SSB is the block the UE uses for initial synchronization and broadcast information. In NR, an SS/PBCH block occupies 4 OFDM symbols in time and 240 contiguous subcarriers in frequency. [ETSI]

3GPP also defines how the reference frequency relates to that block: the synchronization raster maps to resource element index k = 120 inside the SS/PBCH block. In other words, the GSCN does not point to the left edge of the SSB. It points to a defined reference location inside the block. [ETSI]

2) Why do we need GSCN?

NR has a separate concept for carrier/channel positioning and for synchronization. The channel raster identifies RF channel positions using NR-ARFCN, while the synchronization raster identifies allowed SSB positions using GSCN. That is why GSCN and ARFCN are related to frequency planning, but they are not the same thing. [ETSI]

A practical way to remember it is:

• ARFCN tells you where the carrier channel sits.
• GSCN tells you where the UE should look for the SSB. [ETSI]

3) The global GSCN formulas

3GPP gives formulas that map GSCN to SSREF. For FR1 above 3 MHz channel bandwidth, the global synchronization raster is split into two frequency regions:

For 0 to 3000 MHz:

SSREF = N × 1200 kHz + M × 50 kHz
M ∈ 5
GSCN = 3N + (M - 3)/2

For 3000 to 24250 MHz:

SSREF = 3000 MHz + N × 1.44 MHz
GSCN = 7499 + N

For FR2:

SSREF = 24250.08 MHz + N × 17.28 MHz
GSCN = 22256 + N

3GPP also defines a special 3 MHz channel-bandwidth case in FR1, plus additional special entries for band n100. [ETSI]

4) The most important idea: not every GSCN is valid in every band

The formulas above define the global raster. But a real NR band only allows a subset of those GSCNs. 3GPP therefore gives a per-band table with:

• the allowed SSB subcarrier spacing
• the SSB pattern case
• the valid GSCN range
• and the step size between valid GSCNs. [ETSI]

This is why you should never take a random GSCN from the global formula and assume it works in any band. You always have to check the band-specific table. [ETSI]

5) What are Cases A, B, C, D, and E?

The SSB is not only defined in frequency. It is also defined in time. 3GPP TS 38.213 defines different SSB timing patterns called Case A, B, C, D, E and, in newer FR2-2 operation, also Case F and Case G. The cases are tied to the SSB subcarrier spacing:

• Case A → 15 kHz
• Case B → 30 kHz
• Case C → 30 kHz
• Case D → 120 kHz
• Case E → 240 kHz
• Case F → 480 kHz
• Case G → 960 kHz [ETSI]

For most everyday NR work, the usual shortcut is:

• FR1 mostly uses Cases A, B, or C
• FR2 mostly uses Cases D or E [ETSI]

6) Example: GSCN in band n78

For band n78, 3GPP lists:

n78   30 kHz   Case C   7711 – <1> – 8051

That means n78 uses 30 kHz SSB SCS, Case C, and valid GSCNs from 7711 to 8051 with a step size of 1. [ETSI]

Now take an example:

GSCN = 7811

Since this is in the 3000 to 24250 MHz FR1 region:

N = 7811 - 7499 = 312
SSREF = 3000 + 312 × 1.44
SSREF = 3449.28 MHz

So a GSCN of 7811 corresponds to an SSB reference frequency of 3449.28 MHz. That is the SSB reference frequency the UE uses for synchronization, not simply "the carrier center frequency." [ETSI]

7) Example: GSCN in band n258

For band n258, 3GPP lists:

n258   120 kHz   Case D   22257 – <1> – 22443
       240 kHz   Case E   22258 – <2> – 22442

So n258 supports two SSB configurations depending on deployment: 120 kHz / Case D or 240 kHz / Case E. [ETSI]

Take this example for Case D:

GSCN = 22300

For FR2:

N = 22300 - 22256 = 44
SSREF = 24250.08 + 44 × 17.28
SSREF = 25010.4 MHz

So GSCN 22300 maps to an SSB reference frequency of 25010.4 MHz. [ETSI]

8) Why SSB frequency can confuse beginners

A common beginner mistake is to assume that:

• GSCN = carrier center frequency, or
• SSB frequency = channel center frequency.

That is not how NR defines it. The specs separate channel raster and synchronization raster, and the SSB reference is mapped to k = 120 inside the SS/PBCH block. So GSCN is really about where the SSB sits on the sync raster. [ETSI]

9) A simple step-by-step method

1. Identify the band
   Example: n78 or n258. [ETSI]
2. Check the band table
   Find the allowed SSB SCS, case, GSCN range, and step size. [ETSI]
3. Choose the right global formula
   Use the FR1 or FR2 SSREF equation. [ETSI]
4. Convert GSCN to SSREF
   This gives the SSB reference frequency. [ETSI]
5. Remember that SSREF maps to k = 120 inside the SSB
   That tells you what point inside the SS/PBCH block the frequency refers to. [ETSI]

10) Final takeaway

You can think of GSCN as the NR "sync-channel index" for the SSB. It tells the UE where to search on the synchronization raster. The actual SSB frequency is expressed as SSREF, and 3GPP gives formulas to convert between the two. But the final valid values always depend on the band-specific SSB table, including the allowed SCS, case, GSCN range, and step size. [ETSI]

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• NR GSCN Calculator
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