Interference

 
Interference
Of course, interference is typically the state of the signal you are interested in while it is being
destructively overpowered by a signal you are not interested in. The FCC provides a specific definition of
harmful interference in Part 15.3(m):
l (m) 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
radiocommunications service operating in accordance with this chapter.
Because this band has other users, interference will be a factor in 802.11 deployments. The 2.4 GHz
band is a bit more congested than the 5.8 GHz band, but both have their co-users. Table 9-1 describes
the other users of this spectrum and what interference mitigation may be possible for each.
Table 9-1: Spectrum allocation for 802.11 and co-users
Part/User Start
GHz
End
GHz
Part 87 0.4700 10.5000
Part 97 2.3900 2.4500
Part 15 2.4000 2.4830
Fusion lighting 2.4000 2.4835
Part 18 2.4000 2.5000
Part 80 2.4000 9.6000
Industrial, Scientific, and Medical (ISM)—802.11b 2.4010 2.4730
Part 74 2.4500 2.4835
Part 101 2.4500 2.5000
Part 90 2.4500 2.8350
Part 25 5.0910 5.2500
U-NII low 5.1500 5.2500
U-NII middle 5.2500 5.3500
Part 97 5.6500 5.9250
U-NII high 5.7250 5.8250
ISM 5.7250 5.8500
Part 18 5.7250 5.8750
Source: Tim Pozar, Bay Area Wireless Users Group from FCC sources
Devices that Fall into Part 15 This band includes unlicensed telecommunications devices like cordless
phones, home spy cameras, and frequency-hopping spread spectrum (FHSS) and direct sequence
spread spectrum (DSSS) local area network (LAN) transceivers. Operators have no priority over or parity
with any of these users, and any device that falls into Part 15 must not cause harmful interference to all
licensed and legally operating Part 15 users and must accept interference from all licensed and legally
operating Part 15 users. This is stated in Part 15.5(b) and (c):
l (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 ISM
equipment, or by an incidental radiator.
l (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.
Operators of other licensed and nonlicensed devices can inform you of interference and require that you
terminate operation. It doesn't have to be a Commission representative.
Using 802.11b, you can interfere even if you are on different channels, as the channels are 22 MHz wide
and only spaced 5 MHz apart. Channels 1, 6, and 11 are the only channels that do not interfere with each
other. (See Table 9-2.)
Table 9-2: Spectrum bands of 802.11b
Channel Bottom (GHz) Center (GHz) Top (GHz)
1 2.401 2.412 2.423
2 2.406 2.417 2.428
3 2.411 2.422 2.433
4 2.416 2.427 2.438
5 2.421 2.432 2.443
6 2.426 2.437 2.448
7 2.431 2.442 2.453
8 2.436 2.447 2.458
9 2.441 2.452 2.463
10 2.446 2.457 2.468
11 2.451 2.462 2.473
Source: Tim Pozar, Bay Area Wireless Users Group from FCC sources
Devices that Fall into the U-NII Band Unlike the 2.4 GHz band, this band does not have overlapping
channels. The lower U-NII band has eight 20 MHz wide channels. You can use any of the channels
without interfering with other radios on other channels that are within earshot. Ideally, it would be 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.
ISM—Part 18 This band is also an unlicensed service. Typical ISM applications are the production of
physical, biological, or chemical effects such as heating, the ionization of gases, mechanical vibrations,
hair removal, and the acceleration of charged particles. This band carries ultrasonic devices such as
jewelry cleaners and ultrasonic humidifiers, microwave ovens, medical devices such as diathermy
equipment and magnetic resonance imaging (MRI) equipment, and industrial uses such as paint dryers
(Part 18.107). Radio frequency (RF) should be contained within the devices, but other users must accept
interference from these devices. Part 18 frequencies that could affect 802.11 devices are in the 2.400 to
2.500 GHz and 5.725 to 5.875 GHz ranges. As Part 18 devices are unlicensed and operators are likely
clueless of the impact, it will be difficult to coordinate with them. Fusion lighting is also covered by Part 18.
Satellite Communications—Part 25 This part of the FCC's rules is used for the uplink or downlink of data,
video, and so on to/from satellites in Earth's orbit. One band that overlaps the U-NII band is reserved for
Earth-to-space communications at 5.091 to 5.25 GHz. Within this spectrum, 5.091 to 5.150 GHz is also
allocated to the fixed-satellite service (Earth to space) for nongeostationary satellites on a primary basis.
The FCC is trying to decommission this band for feeder use to satellites as "after 01 January 2010, the
fixed-satellite service will become secondary to the aeronautical radio-navigation service." A note in Part
2.106, §5.446 also allocates 5.150 to 5.216 GHz for a similar use, except it is for space-to-Earth
communications. There is a higher chance of interfering with these installations, as Earth stations deal
with very low signal levels from distance satellites.
Broadcast Auxiliary—Part 74 Normally, the traffic on this band is Electronic News Gathering (ENG) video
links going back to studios or television transmitters. These remote vehicles such as helicopters and
trucks need to be licensed. Only Part 74 eligibles such as TV stations, networks, and so on can hold
these licenses (Part 74.600). Typically, these transmitters are scattered all around an area, as TV remote
trucks can go anywhere. This can cause interference to 802.11 gear such as access points (APs)
deployed with omnidirectional antennas servicing an area. Also the receive points for ENG are often
mountain tops and towers. Depending how 802.11 transmitters are deployed at these same locations,
they could cause interference to these links. Wireless providers should consider contacting a local
frequency coordinator for Part 74 frequencies that would be affected. There have been reports of FHSS
devices interfering with these transmissions as the dwell time for this FHSS tends to punch holes in the
video links. DHSS is less likely to cause interference to ENG users, but their links can cause problems
with another person's 802.11 deployment. ENG frequencies that overlap 802.11 devices are 2.450 to
2.467 GHz (channel A08) and 2.467 to 2.4835 GHz (channel A09) (Part 74.602).
Land Mobile Radio Services—Part 90 For subpart C of this part, users can be anyone engaged in a
commercial activity. They can use 2.450 to 2.835 GHz, but can only license 2.450 to 2.483 GHz. Users in
subpart B would be local government. This would include organizations such as law enforcement, fire
departments, and so on. Some uses may be video downlinks for flying platforms such as helicopters, also
known as terrestrial surveillance. Depending on the commercial or government agency, coordination
goes through different groups like 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 out-sources to for their frequency coordination.
Amateur Radio—Part 97 Amateur radio frequencies that overlap 802.11b are 2.390 to 2.450 GHz and
5.650 to 5.925 GHz for 802.11a. They are primary from 2.402 to 2.417 GHz and secondary from 2.400 to
2.402 GHz. There is a Notice of Proposed Rule Making (NPRM) in with the FCC to change the 2.400 to
2.402 GHz range to primary. Amateurs are very protective about their spectrum.
Fixed Microwave Services—Part 101 Users of this band are known as Local Television Transmission
Service (LTTS) and Private Operational Fixed Point-to-Point Microwave Service (POFS). This band is
used to transport video. The channels are allocated from 2.450 to 2.500 GHz.
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 Part 15.247(h) gives the following warning:
l (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.
In the case of 802.11a, the FCC has a note in Part 15.407(a)(3) stating the following:
l The 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.