Interference

A.7.1 Description

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.

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)

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.

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

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

A.7.6 Broadcast Auxiliary: Part 74

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.

A.7.7 Stations in the Maritime Services: Part 80

A.7.8 Aviation Services: Part 87

A.7.9 Land Mobile Radio Services: Part 90

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.

A.7.10 Amateur Radio: Part 97

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

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.