5G NR Channel Bandwidths: From Licensed MHz to Usable Resource Blocks
The supported channel bandwidths in FR1 and FR2, the transmission bandwidth configuration tables, how guard bands work, and why NR reaches ~98% spectrum utilisation.
The mental model: the pipe and what fits through it
The channel bandwidth (BW_channel) is the width of the pipe the regulator licensed — 20 MHz, 100 MHz, and so on. You cannot fill it edge to edge, because the OFDM signal's spectrum leaks; you must leave a guard band at each edge to stay inside the emission mask. What is left is the transmission bandwidth configuration, N_RB — the number of resource blocks you may actually use. NR's achievement is making the guard bands small: about 98 % of the pipe carries data, versus 90 % in LTE.
Supported channel bandwidths
FR1 (410 MHz – 7.125 GHz): 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100 MHz (3 MHz added in Rel-17 for some narrow bands). Which values a band supports is listed per band in TS 38.101-1 Table 5.3.5-1.
FR2 (24.25 – 52.6 GHz): 50, 100, 200, 400 MHz.
Compare LTE: 1.4, 3, 5, 10, 15, 20 MHz. NR's single-carrier maximum is 5× (FR1) to 20× (FR2) wider. Beyond that, NR uses carrier aggregation (up to 16 component carriers).
Why the FR1 ceiling is 100 MHz: the baseband is designed around a 4096-point FFT. At 30 kHz, 4096 × 30 kHz = 122.9 MHz of sampled bandwidth — enough for 100 MHz plus guard. At 15 kHz you would need 8192 points, which is why 15 kHz stops at 50 MHz.
The N_RB tables
N_RB depends on both the channel bandwidth and the SCS: a narrower subcarrier means more, narrower RBs in the same MHz. TS 38.101-1 Table 5.3.2-1 (FR1):
| BW (MHz) | 5 | 10 | 15 | 20 | 25 | 30 | 40 | 50 | 60 | 70 | 80 | 90 | 100 |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 15 kHz | 25 | 52 | 79 | 106 | 133 | 160 | 216 | 270 | — | — | — | — | — |
| 30 kHz | 11 | 24 | 38 | 51 | 65 | 78 | 106 | 133 | 162 | 189 | 217 | 245 | 273 |
| 60 kHz | — | 11 | 18 | 24 | 31 | 38 | 51 | 65 | 79 | 93 | 107 | 121 | 135 |
TS 38.101-2 Table 5.3.2-1 (FR2):
| BW (MHz) | 50 | 100 | 200 | 400 |
|---|---|---|---|---|
| 60 kHz | 66 | 132 | 264 | — |
| 120 kHz | 32 | 66 | 132 | 264 |
Sanity check the headline number: 273 RB × 12 × 30 kHz = 98.28 MHz used out of 100 MHz → 98.3 %. LTE 20 MHz: 100 RB × 12 × 15 kHz = 18 MHz → 90 %.
Guard bands
The guard band is what remains on each side:
guard band (per side) = ( BW_channel − N_RB × 12 × SCS ) / 2
For 100 MHz at 30 kHz: (100 − 98.28)/2 ≈ 0.86 MHz; the spec's exact figure is 845 kHz, slightly less, because the RB grid sits half a subcarrier off the channel edge. The minimum guard band is defined per (bandwidth, SCS) in TS 38.101-1 Table 5.3.3-1 and grows with SCS, because a wider subcarrier has a wider sidelobe skirt. This is the concrete price of a high numerology in a narrow channel: at 5 MHz, 15 kHz gives 25 RBs (4.5 MHz used) while 30 kHz gives 11 RBs (3.96 MHz used).
Why NR does so much better than LTE: LTE's 10 % guard was a conservative choice for early filter technology. NR assumes better transmit filtering / windowing (implementation-specific, not standardised) and defines tighter emission requirements, so the standard could shrink the guard to roughly 2 %.
The gotcha worth remembering
5 MHz at 30 kHz gives 11 RBs — but the SSB needs 20 RBs. So a 5 MHz carrier cannot carry a 30 kHz SSB; the SSB would have to be 15 kHz. More generally, any band/SCS combination must satisfy N_RB ≥ 20 for the SSB and ≥ 24 for the smallest CORESET#0, which is why the CORESET#0 tables are indexed by "minimum channel bandwidth".
Asymmetric and multi-carrier cases
- A UE may support a narrower channel bandwidth than the gNB on the same carrier; the UE then operates in a BWP that fits its capability. This is the BWP mechanism doing its job.
- In carrier aggregation the guard bands between contiguous carriers can be reduced with an "extended" grid (nominal guard bands are defined per band in TS 38.101-1 §5.3A), but each component carrier still has its own N_RB.
- Rel-17 RedCap devices are limited to 20 MHz in FR1 and 100 MHz in FR2, again handled through BWPs on a wider carrier.
Common confusions
- "N_RB is fixed for a bandwidth." It depends on the SCS as well: 50 MHz is 270 / 133 / 65 RBs at 15 / 30 / 60 kHz.
- "Guard band is chosen by the vendor." The minimum is standardised; vendors may not go below it.
- "100 MHz means 100 MHz of data." 98.28 MHz of subcarriers carry data; the rest is guard.
- "A UE must support the cell's full bandwidth." Not in NR — that is why BWPs exist.
Spec pointers
- TS 38.101-1 §5.3 — FR1 UE channel bandwidths, Table 5.3.2-1 (N_RB), Table 5.3.3-1 (guard bands), Table 5.3.5-1 (per band)
- TS 38.101-2 §5.3 — FR2 equivalents
- TS 38.104 §5.3 — base-station side (same tables)
- TS 38.211 §4.4 — resource grid, 275 RB maximum
Recap
Channel bandwidth is the licensed pipe; N_RB is what fits after the guard bands; the guard grows with SCS. FR1 tops out at 100 MHz (273 RB at 30 kHz), FR2 at 400 MHz (264 RB at 120 kHz). NR uses ~98 % of the pipe versus LTE's 90 %. Remember the 5 MHz / 30 kHz / 11 RB trap and the 20-RB SSB minimum.
Interview questions
The questions below are drawn from this topic and phrased the way they tend to come up. Try each one out loud before revealing the answer.
18 questions
- Channel bandwidth is the licensed spectrum occupied, including guard bands. The transmission bandwidth configuration is the usable part, expressed as N_RB resource blocks.
- Guard bands at each edge are needed to meet the spectrum emission mask and adjacent channel leakage limits. They are regulatory necessity, not waste.
- Both the channel bandwidth and the numerology. A wider subcarrier spacing means fewer, wider resource blocks and a different guard band requirement.
- 5, 10, 15, 20, 25, 30, 40, 50, 60, 80, 90 and 100 MHz, though not every band supports every value.
- 50, 100, 200 and 400 MHz.
- The guard band does not scale linearly with subcarrier spacing, so proportionally less of the channel is spent on guard as the spacing rises — up to the point where the RB granularity becomes coarse.
- 275. It bounds the size of the resource allocation fields in DCI and the complexity of the FFT the UE must implement.
- Yes. In FDD they are separate carriers and may be configured differently, which is common where uplink spectrum is scarcer.
- The FFT must cover the occupied bandwidth at the chosen subcarrier spacing. Larger channel bandwidths at low spacing drive large FFTs, which is a real UE complexity cost.
- It is a practical limit on FFT size and RF bandwidth at sub-6 GHz frequencies, and matches the largest contiguous allocations typically licensed there.
- It is how a device exceeds the single-carrier limit. Two 100 MHz carriers aggregated give 200 MHz of usable spectrum without needing a 200 MHz FR1 carrier definition.
- The smallest permitted spacing between the channel edge and the first usable resource block, tabulated per band, bandwidth and numerology in TS 38.104.
- The UE operates within the configured carrier. Capability is an upper bound, not an instruction; the cell's configuration governs.
- Coexistence with an adjacent operator, refarming in stages, or matching the bandwidth to the traffic so that power is not spent transmitting reference signals across unused spectrum.
- Wider bandwidth spreads the same transmit power over more spectrum, lowering the power spectral density and therefore the edge SNR. Wider is faster in the centre and no better at the edge.
- The ratio of transmission bandwidth configuration to channel bandwidth. NR reaches around 98% in favourable configurations against roughly 90% for LTE.
- LTE fixed its guard band as a proportion of channel bandwidth. NR tabulates guard bands per configuration, allowing them to be tightened where the numerology permits.
- 273. It is worth memorising alongside 106 RBs for 40 MHz and 51 RBs for 20 MHz, all at 30 kHz.
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