Frequency Raster

In 5G NR, the Frequency Raster defines the valid center frequencies for radio transmission. Unlike LTE, which typically used a fixed 100 kHz raster, 5G uses a more flexible system divided into two concepts:

Global Frequency Raster: The fundamental high-resolution grid that maps frequencies to unique numbers (NR-ARFCN).
Channel Raster: The specific subset of that grid defining the allowed center frequencies for a particular band.

Global Frequency Raster

To assign a unique number to every possible frequency, 3GPP defines the NR-ARFCN (New Radio Absolute Radio Frequency Channel Number).

The calculation depends on the frequency range. 3GPP TS 38.104 specifies the global frequency raster parameters used to calculate the NR-ARFCN as follows:

\text{NR-ARFCN} = N_{\text{REF-OFFS}} + \frac{\text{Center Frequency} - F_{\text{REF-OFFS}}}{\Delta F_{\text{Global}}}

Frequency Range	$\Delta F_{\text{Global}}$	$F_{\text{REF-OFFS}}$ (MHz)	$N_{\text{REF-OFFS}}$	Range of NR-ARFCN
0 — 3 GHz	5 kHz	0	0	0 — 599000
3 — 24.25 GHz	15 kHz	3000	600000	600000 — 2016666
24.25 — 100 GHz	60 kHz	24250.08	2016667	2016667 — 3279165

Channel Raster

While the global raster defines every possible NR-ARFCN, specific bands (like n28 or n78) do not support every single ARFCN. They enforce a coarser grid known as the Channel Raster.

Valid Raster

A frequency is considered a valid raster if it aligns perfectly with the granularity ( $\Delta F_{\text{Raster}}$ ) defined for that specific band.

(\text{NR-ARFCN} - \text{Start}_{\text{Grid}}) \pmod {\text{Step}_{\text{Band}}} = 0

Off-Raster

A frequency is off-raster if it corresponds to a valid NR-ARFCN (it exists on the global grid) but does not align with the step required by the band.

Example: If band n1 requires a 100 kHz step:

1920.00 MHz is valid.

1920.05 MHz is off-raster. Although physically possible on the global grid, you cannot center a carrier there for band n1.