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FCC Testing and Certification of Helium Compatible Gateways

This page has not been fully updated to represent the latest state of the Helium Network following the migration to Solana on April 18, 2023.

Introduction​

Helium hotspots (miners) operate in the US915, also known as the US902-928 MHz unlicensed band, as modified LoRaWAN compatible gateways. Currently the Helium system operates on 8 channels in sub-band 2 of the US915 band as defined by LoRaWAN Regional Parameters document.

Standard LoRaWAN gateways downstream transmissions are in the upper frequency range (923.3 – 927.5 MHz) using 500 kHz modulation in digital DTS mode. In addition, Helium compatible gateways transmit in the lower frequency range (903.9 to 905.3 MHz) using 125 kHz channel modulation to support proof-of-coverage (PoC) functions. The PoC transmissions are similar in format and requirements to LoRaWAN endpoint devices. This PoC operation falls under a different FCC classification known as a DSS Hybrid system and specific testing is required for certification.

Using FCC pre-certified LoRaWAN gateway modules based on Semtech Corcell LoRaWAN concentrator card designs from marketplace suppliers are not likely to have been tested and certified to support Helium’s PoC 125 kHz hybrid transmissions. In this case, the FCC requires full recertification with a new FCC product identification (FCC ID) number. It is the responsibility of manufacturers providing Helium compatible network products to ensure that proper testing and certification or recertification is obtained.

This application note helps the reader understand the FCC certification testing needed to support a Helium gateway with standard LoRaWAN DTS functions and additional DSS Hybrid mode requirements to support PoC.

Glossary​

  • ANSI: American National Standards Institute
  • BW: Bandwidth
  • CFR: Code of Federal Regulations (US)
  • DSS: Digital Spread Spectrum
  • DTS: Digital Modulation Mode
  • FCC: Federal Communications Commission
  • FHSS: Frequency Hopping Spread Spectrum
  • Hybrid: System employing both frequency hopping and digital modulation
  • KDB: FCC Knowledge Database
  • LoRa: Long Range radio modulation technique
  • LoRaWAN™: LoRa low power Wide Area Network protocol
  • PoC: Helium Proof of Coverage function
  • RX: Receive / Receiver
  • SF: Spreading Factor
  • TX: Transmit / Transmitter

References​

FCC Certification Requirements​

As stated in the introduction, a Helium gateway must support 2 different modes of operation and therefore requires multiple (DTS and DSS Hybrid) FCC certification testing. The test descriptions below are grouped by these modes, DTS digital modulation and DSS Hybrid operation which employs a combination of frequency hopping and digital modulation techniques.

According to FCC KDB 550874 section 1, the 2 modes of operation in the same gateway require two, Form 731 applications and the unit is considered a composite system. However, only a single product identifier (FCC ID) is needed.

Frequency charts and modulation types for Helium network operation are shown below for reference. Normal LoRa gateway uplink frequencies (endpoint transmissions received by the gateway) occur in the 902 to 915 portion of the US915 band and downlink frequencies (transmissions from the gateway) occur in the 923 to 928 MHz portion of the band. The uplink channels can be 125 kHz (64 total channels) and 500 kHz (8 total channels) wide bandwidth. The operation for Helium gateway uplink PoC transmissions is in sub-band 2 as defined by LoRa Alliance RP002-1.0.3 Regional Parameters specification. These are 8 channels, 200 kHz spaced, from 903.9 to 905.3 MHz. While sub-band 2 is the minimum requirement for Helium network gateway support, developers may want to test and certify their gateways for all possible PoC channels in the entire range of 902.3 to 914.9 MHz. Future network enhancements may include additional sub-bands support.

Frequency (MHZ)Spreading Factor
903.9SF7BW125 to SF10BW125
904.1SF7BW125 to SF10BW125
904.3SF7BW125 to SF10BW125
904.5SF7BW125 to SF10BW125
904.7SF7BW125 to SF10BW125
904.9SF7BW125 to SF10BW125
905.1SF7BW125 to SF10BW125
905.3SF7BW125 to SF10BW125
904.6SF8BW500
Frequency (MHZ)Spreading Factor
923.3SF7BW500 to SF12BW500 (RX1)
923.9SF7BW500 to SF12BW500 (RX1)
924.5SF7BW500 to SF12BW500 (RX1)
925.1SF7BW500 to SF12BW500 (RX1)
925.7SF7BW500 to SF12BW500 (RX1)
926.3SF7BW500 to SF12BW500 (RX1)
926.9SF7BW500 to SF12BW500 (RX1)
927.5SF7BW500 to SF12BW500 (RX1)
923.3SF12BW500 (RX2)

LoRaWAN Digital Modulation DTS Operation​

Standard LoRaWAN digital operation mode referred to as “DTS” mode is defined by the FCC as having a minimum 6 dB bandwidth of 500 kHz and a maximum spectral power density of +8 dBm in any 3 kHz segment and a +30 dBm (1 Watt) maximum output. Spurious emissions are also regulated according to different frequency ranges which are considered restricted and non-restricted. Helium only gateways only transmit in the 500 kHz DTS mode in the upper frequency range of 923.3 to 927.5 MHz. The following are the descriptions of the tests required for FCC certification.

To demonstrate compliance, the device under test is normally measured at maximum power output on 3 frequencies that are at the low, middle, and high end of the frequency range.

FCC CFR 15.247 (a)(2) 6 dB Transmit Bandwidth​

The FCC requirement has a 6 dB minimum bandwidth of 500 kHz to be considered as DTS operation. The measurement guidance for certification testing is contained in the FCC Guidance for Compliance document KDB 558074. The KDB 558074 also refers to the acceptable test methodologies contained in ANSI document C63.10 section 11.8. Different test methods for measuring the 6 dB bandwidth are given in C63.10 depending on the capabilities of the test equipment.

FCC CFR 15.247 (b)(3) Output Power​

The maximum permissible output power for DTS operation is 30 dBm (1 Watt). While this is a straightforward specification the FCC does permit 2 different measurements of measuring power: 1) Maximum peak conducted output power method or 2) Maximum conducted (average) output power. The key to understanding this measurement guidance is given in KDB 558074 section 8.3. This also refers the reader to ANSI C63.10 sections 11.9.1 and 11.9.2 for specific test procedures. The recommended test method is to use the average power output procedure. The reason for this is the next test requirement listed below for power spectral density requires the same test method be used for both measurements. There is a power advantage realized by using an average measurement for power spectral density due to the nature of the chirp spread spectrum modulation on LoRa designs.

FCC CFR 15.247 (e) Power Spectral Density​

The FCC also places a power spectral density limit on all transmissions of 8 dBm maximum in any 3 kHz segment of the transmission band. KDB 558074 8.4 lists this requirement and refers to ANSI C63.10 section 10.11. Note that the test setups for this parameter is identical to the Output Power (15.247 (b)(3)) above and 15.247 (e) mandates that the same method for determining the output power shall be used to determine the power spectral density. As stated above for the output power requirement, the average power test method is recommended.

FCC CFR 15.247 (d) Out of Band Emissions​

Out of band emissions, also known as spurious emissions, are transmitter power that is present out of the current operating band. There are several parameters to note from 15.247 (d). First, the limit does depend on how the transmitter power was measured in 15.247 (b)(3). Second, spurious emissions will fall into “restricted” and “non-restricted” frequency bands that are defined in FCC CFR 15.205 (a). Emissions that fall into restricted frequency bands must have radiated levels lower than limits defined in FCC CFR 15.209 (a).

FCC KBD 558074 section 8.5 refers to the test methodology given in ANSI C63.10 section 11.11 for non-restricted measurements. FCC KBD 558074 section 8.6 points to ANSI C63.10 section 11.12 for testing in restricted frequency bands.

Helium PoC DSS Hybrid operation​

Operation in the lower portion of the US915 frequency band using 125 kHz modulation bandwidth for Helium’s PoC transmissions falls under the FCC classification of a Hybrid system and test requirements for certification differ from standard digital modulation modes. A Hybrid system is defined by the FCC as employing both frequency hopping and digital modulation techniques at the same time on the same carrier. A Hybrid system must also meet the definition of true frequency hopping systems. See FCC KDB 558074 section 1 and 10 for the definition of a Hybrid system. The Helium network in the US currently uses 8 channels in sub-band 2 in the lower end of the US915 frequency band. The channel bandwidth for PoC transmissions is 125 kHz and a LoRa coding rate of 4/5. The packet transmission time is approximately 329 ms with spreading factors of 7 to 10. The channel hopping is random. Power output in hybrid mode is also limited by regulations compared to the DTS mode. Below are listed the specific requirements for certification and how these apply to the Hybrid transmission mode.

FCC CFR 15.247 (a)(1) Channel Separation, Random Channels​

This paragraph of the FCC regulations defines the minimum channel separation dependent on modulation bandwidth and the randomness of the frequency hopping. FCC KDB 558074 10 (b)(6)(iii) points to ANSI C63.10 section 7.8.2 for testing carrier separation. A description from the applicant of the pseudorandom hopping sequence and equal hopping frequency use is also needed for the filing. See FCC KDB 558074 section 9 (c)(1) for the requirement.

FCC 15.247 (d) Out of Band Emissions​

This requirement for Hybrid mode is identical to the DTS mode operation that is described in section 4.1.4 above.

FCC 15.247 (f) Dwell Time and Power Spectral Density​

This paragraph describes the maximum time spent on any single channel (0.4 seconds) when frequency hopping and also the maximum power output permitted. The maximum power is specified as +8 dBm in any 3 kHz band of the channel. For LoRaWAN 125 kHz modulation, and by using the test methods in the ANSI C63.10 document, this results in approximately +21 dBm maximum power output on any channel, also see LoRa Alliance RP002-1.0.3 2.5.2. Note that frequency hopping is typically turned off with special test code for this test. FCC KDB 558074 refers to ANSI C63.10 sections 7.8 and 11 for test methods.

FCC 15.247 (g) Channel Distribution​

The language of this paragraph ensures that the equipment under test is capable of operation as a true frequency hopping system. The device must demonstrate pseudorandom hopping and equal channel usage with short data bursts or continuous (long) data streams. See FCC KDB 558047 9 (c)(2) for a description of this requirement.

Antenna Requirements​

There are several important factors to consider with the antenna that will be included with a gateway. The antenna parameters are included in the FCC filing and directly affects certification.

Fixed or Integrated Antenna​

A fixed antenna or one that is integrated into the design and cannot be removed from the device requires radiative test methods since there are no conductive test capabilities. The FCC does allow certification if there is no antenna test port available by only using radiation measurements. This is described in FCC KDB 558074 section 3. An integrated antenna more frequently applies to LoRaWAN endpoint devices while gateways usually have a removable antenna or a test port.

FCC CFR 15.247 (b)(4) Antenna Gain​

FCC regulations allows normal output power for devices up to and including an antenna with 6 dBi (over an isotropic radiator) gain. If the antenna included with the gateway is over 6 dBi gain, the power output must be reduced in an equal amount that the antenna is over 6 dBi. Also see FCC KDB 558074 section 7 for further clarification of antenna gain requirements.

FCC CFR 15.203 Antenna Connection​

Another requirement specifies that other antennas not specified by the manufacturer cannot be attached to the unit. Permanently attached or the use of unique coupling (connector) for the antenna will meet this requirement. Note that using industry standard antenna jacks and electrical connectors is prohibited. The intent of this regulation is to prevent the end user from changing the antenna and modifying the peak radiated power from the device.

AC Power Line Conducted Emissions​

The assumption is the majority of Helium gateway designs will be powered from the AC power line. FCC CFR 15.207 specifies the maximum RF voltage in the 150 kHz to 30 MHz range that can be conducted back to the utility power line. The test facility will normally perform this test during the certification process.

Frequency Stability​

While FCC CFR 15.247 does not explicitly specify frequency stability of the transmitter there are other CFR sections which are applicable to ensure operation within the frequency band over temperature and voltage variations. See FCC CFR 15.215 (c) for keeping transmissions within designated frequency bands and FCC CFR 2.1055 (d) for the requirements of voltage variation.

Summary of Certification Testing Requirements​

DTS Digital Modulation Requirements​

FCC RuleTestsLimitsResults
15.247 (a)(2)6 dB Bandwidth500 kHz min
15.257 (b)(3)Output Power+30 dBm max
15.247 (e)Spectral Density+8 dBm / 3 kHz max
15.247 (d)Radiated EmissionsSee rules for limits
15.209 (a)Radiated EmissionsSee rules for limits
15247 (b)(4)Antenna Requirements6 dBi antenna gain Connection requirements
15.203Antenna Requirements6 dBi antenna gain Connection requirements
15.207Conducted Emissions150 kHz – 30 MHz see rules for limits
15.215 (c)Frequency StabilityTX within band

DSS Hybrid Modulation Requirements​

FCC RuleTestsLimitsResults
15.247 (a)(1)Channel RequirementsSee rules
15.247 (d)Radiated EmissionsSee rules for limits
15.209 (a)Radiated EmissionsSee rules for limits
15.247 (f)Spectral Density+8 dBm / 3 kHz max
15.247 (g)Channel DistributionSee rules
15247 (b)(4)Antenna Requirements6 dBi antenna gain Connection requirements
15.203Antenna Requirements6 dBi antenna gain Connection requirements
15.207Conducted Emissions150 kHz – 30 MHz see rules for limits
15.215 (c)Frequency StabilityTX within bands

Helium Foundation Expected Minimum Performance​

While all Helium gateways for operation in the US must meet FCC standards as presented in the sections above, additional performance parameters not regulated by federal standards are expected to ensure the intended operation of the Helium network. Maximum output power for the operational modes is regulated by FCC rules, however, the FCC does not specify minimum power output. Likewise, there are no US federally regulated receiver minimum sensitivity levels.

For the proper operation of the Helium network, minimum power outputs and receiver sensitivity are expected for Helium’s hotspot approval process (see HIP19). The minimum performance levels are based on Semtech Corecell reference design using the SX1302 baseband processor and SX1250 RF transceiver. The Semtech performance report (link in the references) shows typical power output in section 6 and receiver performance for various spreading factors in section 8. Helium expects power output to be within 1 dB of the Corcell reference design and receiver sensitivities to be no worse than 2 dB above the reference design levels.