A.7 Interference

This device complies with part 15 of the FCC Rules. Operation is subject to the following two conditions: (1) This device may not cause harmful interference, and (2) this device must accept any interference received, including interference that may cause undesired operation.

[Labeling requirement in Part 15.19]

A.7.1 Description

Of course, interference is typically the state of the signal you are interested in while it's being destructively overpowered by a signal you are not interested in.

The FCC has a specific definition of "harmful interference":

Part 2.1(c) Harmful interference - Interference which endangers the functioning of a radio-navigation service or of other safety services or seriously degrades, obstructs, or repeatedly interrupts a radio-communication service operating in accordance with these [International Radio] Regulations.

In Part 15 it is repeated as:

Part 15.3(m) Harmful interference.

Any emission, radiation or induction that endangers the functioning of a radio navigation service or of other safety services or seriously degrades, obstructs or repeatedly interrupts a radio communications service operating in accordance with this chapter.

Interference will be a factor in your deployment. The 2.4GHz band is a bit more congested than the 5.8GHz band, but both have co-users that you must consider (see Table A-1).

Table A-1. Spectrum allocations for 802.11b and co-users

Part / Use

Start GHz

End GHz

Part 87



Part 97



Part 15



RF lighting



Part 18



Part 80



ISM - 802.11b



Part 74



Part 101



Part 90



Part 25






U-NII Middle



Part 97



U-NII High






Part 18



The following subsections describe users that you may encounter while deploying 802.11 devices and detail what interference mitigation may be possible for each.

A.7.2 Devices that Fall into Part 15 of the ISM Band (2400 to 2483 MHz)

This includes unlicensed telecommunications devices such as cordless phones, home spy cameras, and Frequency Hopping (FHSS) or Direct Sequence (DSSS) Spread Spectrum LAN transceivers.

You have neither priority over nor parity with any of these users. Any device that falls into Part 15 must not cause harmful interference to all other licensed and legally operating Part 15 users, but it must accept interference from all licensed and legally operating Part 15 users. A friend of mine who used to work in the enforcement division of the FCC said, "You have as much right to the band as a garage door opener does."

This is explicitly defined in 15.5:

15.5(b) Operation of an intentional, unintentional, or incidental radiator is subject to the conditions that no harmful interference is caused and that interference must be accepted that may be caused by the operation of an authorized radio station, by another intentional or unintentional radiator, by industrial, scientific and medical (ISM) equipment, or by an incidental radiator.

Or basically, everything.

15.5(c) The operator of a radio frequency device shall be required to cease operating the device upon notification by a Commission representative that the device is causing harmful interference. Operation shall not resume until the condition causing the harmful interference has been corrected.

Interference objections don't necessarily have to come from a "Commission representative." Operators of other licensed and non-licensed devices can inform you of interference and require that you terminate operation.

Users of 802.11b can interfere with each other even if they are on different channels, as the channels are 22MHz wide and spaced only 5MHz apart. Channels 1, 6, and 11 are the only channels that don't interfere with each other (see Table A-2).

Table A-2. United States 802.11b channel allocations


Bottom (GHz)

Center (GHz)

Top (GHz)













































A.7.3 Devices That Fall into the U-NII Band

Unlike the 2.4GHz band, this band does not have overlapping channels. In the lower 200MHz of the U-NII band, there are eight 20MHz-wide channels. You can use any of the channels without interfering with other radios on other channels that are within earshot. Ideally, it's good to know what other Part 15 users are out there. Looking into groups under the banner of "FreeNetworks" is a good place to start.

A.7.4 Industrial, Scientific, and Medical (ISM) Devices: Part 18

This is also an unlicensed service. Typical ISM applications include the production of physical, biological, or chemical effects such as heating, ionization of gases, mechanical vibrations, hair removal, and the acceleration of charged particles.

Users of this band include ultrasonic devices such as jewelry cleaners, ultrasonic humidifiers, and microwave ovens. Medical devices, such as diathermy equipment and magnetic resonance imaging equipment (MRI) also use ISM, along with some industrial devices such as paint dryers (18.107). RF should be contained within the devices, but other users must accept interference from them.

Part 18 frequencies that could affect 802.11 devices are 2.400 to 2.500GHz and 5.725 to 5.875GHz.

It is difficult to coordinate with the users of Part 18 devices because they are unlicensed and may not realize the impact their equipment has on 802.11 devices.

A.7.5 Satellite Communications: Part 25

This part of the FCC's rules is applicable to the uplink or downlink of data to and from satellites in Earth orbit. One band that overlaps the U-NII band is reserved for Earth-to-space communications at 5.091 to 5.25GHz. Within this spectrum, 5.091 to 5.150GHz is also allocated to the fixed-satellite service (Earth-to-space), for non-geostationary satellites on a primary basis. The FCC is trying to decommission this band for "feeder" use to satellites. See Section A.7.8 for details.

Because satellite transmissions involve very narrow aperture antennas pointing into the sky and relatively high power, you are not likely to interfere with them. If you are near one of these installations, there is a very slight chance they could interfere with you.

A note in Part 2.106 [S5.446] allocates 5.150 through 5.216GHz for a similar use: space-to-Earth communications. You have a higher chance of interfering with these installations, because Earth stations deal with very low-level signals from distant satellites.

A.7.6 Broadcast Auxiliary: Part 74

The traffic on this part of the spectrum normally comes from electronic news gathering (ENG) video links traveling back to studios or television transmitters. Remote news vehicles (such as helicopters and trucks) must be licensed, and only Part 74-eligible entities (usually TV stations) can hold these licenses (74.600).

These transmitters are typically scattered all around an area, as TV remote trucks can go anywhere. Their broadcasts can cause interference to your 802.11 gear, particularly if you're using APs deployed with omni-directional antennas to service an area.

The "receive" points for ENG are often mountain tops and towers. Depending on how 802.11 transmitters are deployed at these same locations, they could cause interference to ENG links. Wireless providers should consider contacting a local frequency coordinator for any Part 74 frequencies that might be affected. The Society of Broadcast Engineers web site (http://www.sbe.org/) can provide you with a listing of coordinators for your area.

There have been reports of FHSS devices interfering with these transmissions, because the dwell time for this FHSS tends to punch holes in video links. DSSS is less likely to cause interference to ENG users, but their links can cause problems for your 802.11 deployment.

ENG frequencies that overlap 802.11 devices are 2.450 to 2.467GHz (channel A08) and 2.467 to 2.4835GHz (channel A09) (Part 74.602).

A.7.7 Stations in the Maritime Services: Part 80

2.4 to 9.6GHz is used for "Radiodetermination," such as RADAR. As with other RADAR users, it is unlikely you will interfere with them. They can interfere with you.

A.7.8 Aviation Services: Part 87

The frequencies used by this part are for "radio navigation stations" or RADAR. They span the frequencies from 470MHz to 2.450GHz, which overlaps the channels used by 802.11b. They also span 2.450 to 10.500GHz, which overlaps the channels used by 802.11a. It's unlikely that you will ever cause any problems for them. It is far more likely that they will be a nuisance to you.

A.7.9 Land Mobile Radio Services: Part 90

Users on subpart C of this part can be anyone engaged in a commercial activity. They can use from 2.450 to 2.835GHz, but can license only from 2.450 to 2.483GHz (90.35(a)(3)).

Local government uses subpart B. This includes organizations such as law enforcement, fire departments, etc. Some other uses may include video downlinks for flying platforms such as helicopters, also known as terrestrial surveillance.

"Even if you are in the right, never argue with someone with a badge and a gun." ?Bill Ruck

Even if they are not licensed, official users of this band can put you in jail for interfering with a peace officer in the performance of his duties.

Depending on the commercial or government agency, coordination of this band is performed by different groups, such as the Association of Public Safety Communications Officials (APCO). Consider going to their conferences. You can also try to network with engineering companies that the government uses to outsource frequency coordination.

A.7.10 Amateur Radio: Part 97

Amateur radio frequencies that overlap 802.11b are found from 2.390 to 2.450GHz. 802.11a is overlapped at 5.650 to 5.925GHz. Amateur users are primary from 2.402 to 2.417GHz and secondary at 2.400 to 2.402GHz.

Amateurs are very protective of their spectrum. The American Radio Relay League (ARRL) is a powerful lobbying force in Washington. They are concerned about all unlicensed devices, and they believe that the FCC doesn't have the right to hand out any part of the spectrum to users of 802.11 devices. You may find that the local groups of amateurs agree and are active with the ARRL's efforts. Getting involved with these local groups and establishing a dialog with them can help minimize interference and avoid conflicts.

A.7.11 Fixed Microwave Services: Part 101

Users of this band are Local Television Transmission Service (LTTS) and Private Operational Fixed Point-to-Point Microwave Service (POFS). This band is used to transport video. The allocation is from 2.450 to 2.500GHz.

Engineering companies (like CSI Telecommunications) use frequency search companies such as ComSearch to coordinate this part of the spectrum.

A.7.12 Federal Usage: NTIA/IRAC

The Federal government uses this band for "radiolocation" or "radionavigation." Several warnings in the FCC's Rules and Regulations disclose this fact.

In the case of 802.11b, a note in the Rules warns:

15.247(h) Spread spectrum systems are sharing these bands on a noninterference basis with systems supporting critical Government requirements that have been allocated the usage of these bands, secondary only to ISM equipment operated under the provisions of Part 18 of this chapter. Many of these Government systems are airborne radiolocation systems that emit a high EIRP which can cause interference to other users.

The FCC addresses 802.11 with a note in Part 15.407:

Commission strongly recommends that parties employing U-NII devices to provide critical communications services should determine if there are any nearby Government radar systems that could affect their operation.

Of course, they may not even tell you where such systems are! Coordination is not available, as this band is managed by the NTIA/IRAC. You will need to sniff around using non-802.11 equipment (such a spectrum analyzer) to see what the conditions are.