FCC Rules for 902–928 MHz — and Where LoRa Chirp Spread Spectrum Fits
We comply with the law because we're good citizens, not because the cops exist.
TL;DR
The FCC regulates 902–928 MHz through two coexisting sections: 47 CFR §15.247 (spread spectrum / digital, up to 1 W) and 47 CFR §15.249 (narrowband, ~0.75 mW EIRP). Both are current and in force. §15.247(a)(2) requires a 6 dB bandwidth of at least 500 kHz for any digital-modulation device. A 62.5 kHz LoRa channel falls below that floor and is outside §15.247 entirely — but it is not prohibited: it is lawful under §15.249, which imposes no bandwidth minimum, at a much lower power ceiling. The FCC chose 500 kHz as a deliberate eligibility threshold to keep narrowband systems out of §15.247's higher power limits; it is not derived from the 8 dBm/3 kHz power-density cap. This was settled in DA 12-839 (Starkey, 2012), where the Commission dismissed a petition to lower the floor and denied a waiver, stating the purpose explicitly.
This page is written for two readers: someone who's never read an FCC regulation and wants to understand what's going on, and someone who needs the exact section numbers to check a design. The summary below skips the legal style; the rest is the cited detail. Every figure is from the current U.S. Code of Federal Regulations (eCFR, Part 15) as of September 2026, and every citation links to its section.
Summary: what the rules say, in plain English
The setup. 902–928 MHz is "license-free" (the FCC calls it an ISM band, Industrial, Scientific, and Medical). That does not mean "use it however you want." It means the FCC doesn't issue you a personal license, but it does set strict limits on how much power you can put out and how wide you must spread it, so that a pile of unlicensed radios don't drown out the licensed services that share or sit next to this band — and don't break into each other. The FCC's operating assumption is: anyone's transmitter can land on the same frequency as anyone else's, at any time, with no coordination. Every limit below is built to survive that assumption.
The one number that matters: bandwidth. For a digital transmitter like LoRa, §15.247(a)(2) says your signal's 6 dB bandwidth — the width where the signal drops 6 dB from its peak — must be at least 500 kHz. A LoRa radio is a single chirp on one channel, so this bandwidth test is essentially its entire compliance question.
What 62.5 kHz is and isn't
Read the rule straight: §15.247(a)(2) sets a hard floor of 500 kHz on the 6 dB bandwidth of any digitally modulated transmission in 902–928 MHz. A 62.5 kHz LoRa channel does not meet that floor, so it is outside §15.247 — the higher-power spread-spectrum / digital section. That is not the same as "illegal." §15.249 is a separate, current rule for 902–928 MHz that sets no bandwidth minimum. It caps you at 50 mV/m fundamental field strength at 3 m (~0.75 mW EIRP). A 62.5 kHz LoRa radio can operate lawfully under §15.249 — just at much lower power. The FCC confirmed this exact situation in DA 12-839: Starkey's narrowband hearing-aid devices were already authorized under §15.249, and it petitioned to move them under §15.247's higher power. The Commission denied both the petition and the waiver.
Why 500 kHz is the floor — the actual reason
The 500 kHz floor is not derived from the power-density limit; the causality runs the other way. The floor dates to the original 1985 Spread Spectrum Report & Order (Gen. Docket 81-413), where the Commission adopted 500 kHz as the minimum 6 dB bandwidth that a device must spread across in order to earn §15.247's higher power. The 8 dBm/3 kHz power-density cap was added later, in 1990, to ensure the energy is actually spread uniformly across that 500 kHz floor — so a device can't nominally occupy 500 kHz while concentrating all its power into a narrow spike. In the FCC's own words from DA 12-839: the 500 kHz minimum was chosen "to ensure that wideband digital devices … operating with higher power than normally allowed under the Part 15 general emission limits would operate compatibly with other authorized radio systems," and specifically "to exclude narrowband systems from using the higher power limits of Section 15.247." It is an eligibility rule, not an interference-math outcome.
The counter-intuitive part — and why the PSD math isn't the reason
It feels backwards that "wider = safer," and the natural objection is that a wider signal must spill into more of the spectrum. The rule inverts that: total power is capped (1 W) no matter how wide you are, so widening divides the same watt across more hertz and lowers the power spectral density. A narrow slice is where narrowband licensed receivers live, so spreading thin protects them. But that argument, while true, is not why the FCC set the floor — and it does not carry the rule. The proof is that the argument fails in both directions at the boundary: a 62.5 kHz signal at 20 dBm (0.1 W) sits at ~7 dBm/3 kHz, under the PSD cap, yet is still outside §15.247; and a 500 kHz signal can still violate the PSD cap if its spectrum is peaky rather than flat. Starkey made exactly this "lower PSD = lower interference" argument in DA 12-839 — that 100 kHz signals at the same 8 dBm/3 kHz density would have less total power and interference potential — and the Commission rejected it, reaffirming that 500 kHz is an eligibility line, not a PSD threshold. Don't quote the arithmetic as the rationale; quote the eligibility purpose.
Bottom line for LoRa/Meshtastic in the US: the LoRaWAN US915 channel plan uses 64 × 125 kHz uplink channels and 8 × 500 kHz downlink channels. The 125 kHz uplink channels fall below the §15.247(a)(2) digital-modulation floor and are operated under §15.247's frequency-hopping provision (≥ 50 hopping frequencies for 1 W). The 500 kHz downlink channels meet the digital path directly. A device certified under §15.249 (no bandwidth floor, ~0.75 mW EIRP) can also transmit at 62.5 kHz in this band, but at far lower power.
1. The governing sections (902–928 MHz)
The sections that do the work, all linked to the eCFR:
| Citation | Title | What it covers |
|---|---|---|
| 47 CFR 15.247 | Operation within 902–928 MHz, 2400–2483.5 MHz, 5725–5850 MHz | Primary rule — the bandwidth, power, and spectral limits for unlicensed 902–928 MHz. |
| 47 CFR 15.249 | Operation within 902–928 MHz, 2400–2483.5 MHz, 5725–5875 MHz, 24.0–24.25 GHz | Older/alternative provision; 50 mV/m fundamental field strength at 3 m. |
| 47 CFR 15.245 | Operation within 902–928 MHz, 2435–2465 MHz, etc. | Field disturbance sensors (radar) only in 902–928 MHz. |
| 47 CFR 15.205 | Restricted bands of operation | Bands where only spurious emissions are permitted; restricted-band compliance. |
| 47 CFR 15.209 | Radiated emission limits; general requirements | Out-of-band / spurious field-strength limits at 3 m. |
| 47 CFR 15.3 | Definitions | Intentional radiator, unintentional radiator, harmful interference. |
| 47 CFR 2.1 | Definitions (Part 2) | Spread spectrum, direct sequence, frequency hopping. |
| 47 CFR 15.35 | Measurement provisions | Peak emission limiting, detector selection, measurement frequency range. |
| 47 CFR 1.1307 / 1.1310 | RF radiation exposure (MPE) | Mandatory for all Part 15 intentional radiators. |
| 47 CFR 2.1091 / 2.1093 | MPE limits and measurement methods | Applies to mobile/portable devices. |
2. What the FCC means by "spread spectrum"
These definitions live in 47 CFR §2.1 (Part 2), not Part 15. The classic processing-gain-based definitions that used to sit in §15.3 have been removed from current Part 15. What remains:
Spread Spectrum Systems
A spread spectrum system is an information-bearing communications system in which: (1) information is conveyed by modulation of a carrier by some conventional means; (2) the bandwidth is deliberately widened by means of a spreading function over that which would be needed to transmit the information alone. (In some spread spectrum systems, a portion of the information being conveyed may be contained in the spreading function.)
Direct Sequence Systems
A spread spectrum system in which the carrier has been modulated by a high-speed spreading code and an information data stream. The high-speed code sequence dominates the "modulating function" and is the direct cause of the wide spreading of the transmitted signal. LoRa chirp is not direct sequence in this sense: it has no high-speed spreading code. The FCC has never formally classified chirp as direct sequence, frequency hopping, or any specific spread-spectrum category. This is precisely why §15.247(a)(2) uses a technology-neutral bandwidth test: it governs whatever modulation the signal uses, so long as it meets the 500 kHz 6 dB bandwidth floor.
Also in §2.1: Frequency Hopping Systems (carrier changes frequency at fixed intervals under a coded sequence) and Hybrid Spread Spectrum Systems. The hopping family is the other legal path into this band; the next section explains why it doesn't apply to LoRa.
3. How LoRa chirp spread spectrum fits in
LoRa is a chirp spread spectrum (CSS) physical layer. It encodes data as chirps — linear sweeps in frequency whose slope and duration carry the symbol. LoRaWAN is the network protocol on top; "LoRa" is the modulation. The FCC rules do not recognize "chirp" as a category, so a LoRa signal is classified by the same paths that classify any unlicensed 902–928 MHz transmitter:
- Digital modulation path — §15.247(a)(2): the 6 dB bandwidth must be at least 500 kHz.
- Frequency hopping path — §15.247(a)(1): a set of hopping channels.
A standard LoRa transmission is a single chirp on a single channel. That makes it a "digitally modulated intentional radiator," not a hopping system. So it lands on the digital path, and the single binding constraint is the 500 kHz minimum 6 dB bandwidth. That is the whole compliance question for LoRa and Meshtastic in 902–928 MHz. A 62.5 kHz channel is simply below that floor — a narrowband emission that doesn't qualify on this path and isn't a hopping system either.
4. Power limits, by bandwidth (902–928 MHz)
All limits in §15.247(b) are peak conducted output power at the transmitter. For LoRa/Meshtastic (single-channel, digital), the relevant picture is:
| Bandwidth (6 dB BW) | Legal as digital? | Power limit | Why |
|---|---|---|---|
| ≥ 500 kHz | Yes — §15.247(a)(2) | 1 W (30 dBm) | Clears the 500 kHz floor; PSD stays under 8 dBm/3 kHz |
| 500 kHz → up | Yes | 1 W (30 dBm) | Wider than the floor only lowers PSD further |
| 125 kHz, 62.5 kHz, etc. | No — below the floor | Not permitted as a single channel | PSD exceeds 8 dBm/3 kHz at 1 W; only legal as a hopping system (≥ 50 channels), which LoRa is not |
The 500 kHz line is where 1 W and the 8 dBm/3 kHz power-density cap meet. Anything narrower can't carry a full watt without blowing the density limit — which is why the rule makes 500 kHz the floor rather than an arbitrary number.
Alternative digital measurement — §15.247(b)(3)
Compliance with the 1 W digital limit may instead be shown using Maximum Conducted Output Power — total transmit power delivered to all antennas/elements, averaged across all symbols in the signaling alphabet at maximum power control, summed across all antennas, excluding intervals where the transmitter is off or at reduced power. If you meet the power limit this way (RMS averaging), the out-of-band attenuation requirement in §15.247(d) tightens from 20 dB to 30 dB.
5. Antenna gain in 902–928 MHz
The power limits assume antennas with directional gain ≤ 6 dBi (§15.247(b)(4)). If you go higher, reduce conducted power by the amount in dB that the gain exceeds 6 dBi — one-for-one. Unlike the 2.4 GHz and 5.8 GHz bands, 902–928 MHz has no fixed point-to-point antenna-gain relaxation. Directional gain in the 900 MHz band is simply a 1:1 power trade.
6. Out-of-band and spectral limits
Out-of-band attenuation — §15.247(d): in any 100 kHz bandwidth outside the operating band, RF power shall be at least 20 dB below the in-band level in the 100 kHz containing the highest power. For digital systems compliant via RMS averaging, it's 30 dB. Emissions landing in restricted bands (§15.205) must also meet the §15.209 limits.
Power spectral density (digital) — §15.247(e): conducted PSD to the antenna shall not exceed 8 dBm in any 3 kHz band during continuous transmission. This is the limit that makes the 500 kHz floor what it is.
General radiated emission limits — §15.209: out-of-band / spurious field strength at 3 m:
| Frequency | Field strength | Distance |
|---|---|---|
| 216–960 MHz | 200 µV/m (quasi-peak) | 3 m |
| Above 960 MHz | 500 µV/m (average) | 3 m |
Unwanted emissions shall not exceed the fundamental emission level.
7. Bandwidth summary (902–928 MHz, digital/CSS)
| Parameter | Requirement |
|---|---|
| Min 6 dB bandwidth | ≥ 500 kHz |
| Peak conducted power | 1 W |
| Power spectral density | ≤ 8 dBm in any 3 kHz band |
| Out-of-band attenuation | ≥ 20 dB (30 dB if using RMS-averaged power) |
| Max antenna gain (no derate) | 6 dBi (reduce power 1:1 dB above this) |
8. Other mandatory compliance
- §15.247(i) — RF radiation exposure per §1.1307(b), §1.1310, §2.1091, §2.1093. Mobile/portable devices need an MPE compliance statement in the equipment authorization application.
- §15.205(c) — all intentional radiators in 902–928 MHz are subject to restricted-band rules for any out-of-band emissions landing in restricted bands.
- §15.247(h) — 902–928 MHz is shared on a noninterference basis with critical Government systems, many airborne radars. The FCC notes it may reduce power limits in the future.
- §15.21 — the user manual must warn that unapproved modifications void the user's authority to operate.
9. §15.249 (legacy / alternative)
§15.249 still permits 902–928 MHz operation with a fundamental field strength of 50 mV/m at 3 m and harmonics ≤ 500 µV/m, without the digital-modulation structure of §15.247. Most modern unlicensed 900 MHz devices — including LoRa radios — are certified under §15.247, not §15.249. If you're designing new hardware, treat §15.247 as the governing path.
Glossary
Key terms, each linked to where it's defined or applied in the eCFR:
| Term | Meaning | Source |
|---|---|---|
| Intentional radiator | A device that intentionally generates and emits RF energy by radiation or induction. | §15.3(o) |
| Unintentional radiator | A device that generates RF energy for internal use or by conduction to associated equipment, not intended to emit by radiation. | §15.3(z) |
| Spread spectrum system | A communications system where bandwidth is deliberately widened by a spreading function beyond what the information alone requires. | §2.1 |
| Direct sequence system | A spread spectrum system where a high-speed code dominates the modulating function and spreads the signal. | §2.1 |
| Digital modulation | The §15.247(a)(2) operating path; requires 6 dB bandwidth ≥ 500 kHz in 902–928 MHz. LoRa chirps fall here. | §15.247(a)(2) |
| 6 dB bandwidth | The width of a signal measured between the points 6 dB below its peak. The digital-modulation floor is set at 500 kHz. | §15.247(a)(2) |
| Peak conducted output power | The maximum conducted power at the transmitter during the peak of a transmission — the basis for the 1 W limit. | §15.247(b) |
| Maximum Conducted Output Power | Total transmit power averaged across all symbols at max power control; an alternative way to show 1 W digital compliance. | §15.247(b)(3) |
| Power spectral density (PSD) limit | For digital systems, conducted PSD to the antenna ≤ 8 dBm in any 3 kHz band. | §15.247(e) |
| Out-of-band emission | Emission outside the band; must be 20 dB (30 dB digital) below in-band over any 100 kHz bandwidth. | §15.247(d) |
| Restricted band | A frequency band where only spurious emissions are permitted from an intentional radiator. | §15.205 |
| Spurious emission | Emission on frequencies outside the necessary bandwidth that can be reduced without affecting the transmitted information. | §15.3 / §2.1 |
| Harmful interference | Emission that endangers a safety/navigation service or seriously degrades a radiocommunications service. | §15.3(m) |
| MPE (Maximum Permissible Exposure) | RF radiation exposure limits; all Part 15 intentional radiators must comply, with mobile/portable requiring a compliance statement. | §1.1307, §1.1310, §2.1091, §2.1093 |