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The FCC's Part15 Rules and Regulation and 802.11b emissions in the ISM 2.4GHz
Band
By Tim Pozar - pozar@lns.com
for the Bay Area Wireless User Group
Background
Introduction
With the unlicensed use of 802.11b radio Ethernet devices in the Industrial,
Scientific and Medical band that has been set aside for such use, there is
confusion of what is allowed or limited by the Federal Communication Commissions
Rules and Regulations. This paper is meant to help guide folks through the
cryptic nature of these rules.
What is the FCC involvement in this mess?
The FCC is a regulation body whose purpose was defined in the Communications
Act of 1934 as:
"For the purpose of regulating interstate and foreign commerce in communication
by wire and radio so as to make available, so far as possible, to all the
people of the United States a rapid, efficient, Nation-wide, and world-wide
wire and radio communication service with adequate facilities at reasonable
charges, for the purpose of the national defense, for the purpose of promoting
safety of life and property through the use of wire and radio communications,
and for the purpose of securing a more effective execution of this policy
by centralizing authority heretofore granted by law to several agencies and
by granting additional authority with respect to interstate and foreign commerce
in wire and radio communication, there is created a commission to be known
as the "Federal Communications Commission", which shall be constituted as
hereinafter provided, and which shall execute and enforce the provisions
of this chapter."
The FCC, with the Act of 1934, was empowered to regulate wire and wireless
communications. Wired communications regulation was needed to monitor and
regulate monopolies. Wireless regulation is needed as the spectrum is finite.
The FCC is the "traffic cop" to ensure that communications is not interfered
with.
Part 15 of the Rules and Regulations
Almost every bit of spectrum is regulated by the FCC with the exception of
extreamly high or low frequency spectrum and bands managed by the Intergovernmental
Radio Advisory Committee (IRAC) for the military and other goverment orginizations,
by licensing operators of radio equipment. The part of the FCC's rules that
cover the operation of equipment that does not need a license is Part 15
and has this role and is defined below:
"Part 15.1 -
This part sets out the regulations under which an intentional, unintentional,
or incidental radiator may be operated without an individual license."
A "radiator" is a device that emits radio frequency energy. An unintentional
or incidental radiators are devices that have the secondary effect that they
create radio emissions. The primary purpose of the device would have nothing
to do with emiting radiation to work. A computer would fall into that category
as the primary purpose is to "calculate" but it would not need to emit radio
waves to complete its job. We are interested in what is known as a intentional
radiator. This category covers devices such as low power FM transmitters
or wireless microphones like a "Mr. Microphone" or cord-less phones and 802.11b
cards.
The Rules and where to find it...
The Rules and Regulations are the "bible" for the FCC. It is the text that
defines all that the FCC regulates. Interpretation of the Rules are up to
FCC employees or the courts. They are written in half "legalese" and half
technical jargon. It is no wonder that most folks have a hard time wading
through, let alone finding them.
Until recently the rules were only available by going down to your local
Government Printing Office Bookstore, ordering them from Washington DC, or
subscribing to third-party vendors of the rules like Pike and Fisher for
a rather high fee. Fortunately, the Web came along and a number of folks
have worked hard to get the Rules on the net. Harold Hallikainen's site has
a nice interface to the rules.
Just the facts... aka Part 15.247
Part 15.247 covers intentional Radiators in the ISM bands that are the frequencies
902-928 MHz, 2400-2483.5 MHz, and 5725-5850 MHz. Besides covering the modulation
schemes this part also covers the various power restrictions that the FCC
has for devices like 802.11b. The critical section is 15.247(b)(1) through
15.247(b)(3)(i) quoted below:
"(b)The maximum peak output power of the intentional radiator shall not exceed
the following:
(1) For frequency hopping systems operating in the 2400-2483.5 MHz or 5725-5850
MHz band and for all direct sequence systems: 1 watt.
(2) For frequency hopping systems operating in the 902-928 MHz band: 1 watt
for systems employing at least 50 hopping channels; and, 0.25 watts for systems
employing less than 50 hopping channels, but at least 25 hopping channels,
as permitted under paragraph (a)(1)(i) of this section.
(3) Except as shown in paragraphs (b)(3) (i), (ii) and (iii) of this section,
if transmitting antennas of directional gain greater than 6 dBi are used
the peak output power from the intentional radiator shall be reduced below
the stated values in paragraphs (b)(1) or (b)(2) of this section, as appropriate,
by the amount in dB that the directional gain of the antenna exceeds 6 dBi.
(i) Systems operating in the 2400-2483.5 MHz band that are used exclusively
for fixed, point-to-point operations may employ transmitting antennas with
directional gain greater than 6 dBi provided the maximum peak output power
of the intentional radiator is reduced by 1 dB for every 3 dB that the directional
gain of the antenna exceeds 6 dBi."
Lets dissect this section...
Part 15.247(b)(1) defines the maximum power that an intentional radiator
can put out as 1 watt.
Part 15.247(b)(2) doesn't apply as it is covering devices in the 902-928
MHz band and 802.11b devices are in the 2400-2483.5 MHz band.
Part 15.247(b)(3) covers the need for limiting the amount of radiation the
"intentional radiator" can emit with "directional gain" antennas. It says
that in general (with an exception coming up) that if the gain of the antenna
system is more than 6 dBi, the intentional radiator needs to be turned down
to keep the emission at the 1 watt maximum plus 6 dBi (36 dBm or 4 watts
EIRP). The FCC encorages the use of directional antennas. With that they
give you 6 dBi more power for not poluting the rest of your space with radiation
that is not needed to do what you need to do.
Part 15.247(b)(3)(i) covers the need for limiting the amount of radiation
the "intentional radiator" can emit running "fixed, point-to-point" with
"directional gain" antennas. This means that the transmitter is mounted not
on a moblie device and is talking to one other transmitter.
Do we need to turn down the transmitter?
Omni-directional or Point to Multi-point paths...
15.247(b)(3) makes the assumption that you are running a point to multi-point
network much like an Apple Airport or Cisco/Aironet AP box with a number
of computers connecting to the network. They may be randomly surrounding
the access point so you are not using a directional antenna.
But what does the FCC mean when they limit the "intentional radiator" to
one watt?
This is a critical sticking point in understanding what the FCC is talking
about. There is some question of what an "intentional radiator" consists
of and what and where exactly is 1 watt measured. Unfortunatly if you just
look at these poorly written rules you will not understand what the FCC means
here. One has to look a bit deeper to the "Report and Order" and Notice of
Proposed Rulemaking" that generated this section of the rules.
Things get a little clearer when we read this sentence in paragraph 4 of
the Report and Order...
"The current regulations limit spread spectrum systems to a maximum peak
transmitter output power of one watt. When operating at that power level,
the maximum directional gain of the associated antenna may not exceed 6 dBi,
resulting in a maximum equivalent isotropically radiated power (EIRP) of
four watts, i.e., 6 dBW."
With the old rules they are refering to the "intentional radiator" as a whole
with a directional antenna can't exceed 6 dBw or 36 dBm and the antenna gain
can't be more than 6 dBI. The transmitter can be up to one watt.
In order to know if we are legal or if we need to turn down the transmitter
we need to know the gain of your "intentional radiator". Let's say your access
point actually puts out 1 watt of power and you want to put an omni-directiona
antenna on it that has a gain of 5 dBi such as the ORiNOCO Range Extender
Antenna".
We know the gain of the antenna, the transmitter but we also need to know
the loss of the transmission line going to the antenna as this attenuats
the transmitter output power going into the antenna. Looking up the attenuation
of a common coax cable such as RG-8 on an coax attenuation table we find
that at 2.4 GHz we have 16 dB of loss with 100 feet of cable. With a 10 foot
cable your loss is about 1.6 dB. So your new "intentional radiator" will
be radiating transmitter power output plus antenna gain minus coax loss or
(30dBm + 5 dBi - 1.6 db) or 33.4 dBm or 2.2 watts EIRP.
Since this is a non-directional antenna you are limited to 1 watt EIRP or
30 dBm. The transmitter will need to be turned down 3.4 dB to 26.6 dBm or
about 0.45 watts (450 mW) to get you back to 30 dBm or 1 watt EIRP.
If you think about this you may ask, "why add an omni-directional gain antenna
it if I already was at 30 dBm?" You would be correct that it would be a waste
of time. But if you had something like an Apple Airport that will only put
out 15 dBm or 30 mW then you can add an omni-directional gain antenna and
it will extend your "roaming" area. In fact you can add up to 15 dB of gain
with an omni-directional antenna before you need to attenuate the output
of the Wavelan card in the Airport.
Use a directional antenna and get more power - or - this is where the Rules
get even more hard to follow...
Part 15.247(b)(3) actually gives you a free 6 dBi if you use a directional
antenna your "intentional radiator". How do the do this? Only if the gain
of the antenna is over 6 dBi will the Feds want you to roll back the EIRP
of your "intentional radiator". You don't have to do it right at 1 watt EIRP.
When would you do this? Say if you have an access point in the corner of
a building and it needs to aim back into the work area. You don't want an
omni-directional antenna as about 75% of the power would be going out the
windows. Why not use a directional to keep the signal in the building and
penetrate through the walls better? If we have antenna gain of about 12 dBi
and in this case the antenna is a directional antenna. With the transmitter
putting out 30 dBm and the coax has 1.6 dB of loss we have an "intentional
radiator" that is putting out (30 dBm + 12 dBi - 1.6 dB) or 40.4 dBm or just
over 10 watts EIRP. Since the antenna gain is 12 dBi and we have to reduce
the power of this "intentional radiator" 1 db for every db we go over 6 dBi
of the antenna we would have to roll the power back to 34.4 dBm or 2.2 watts
EIRP (40.4 dBm - (12 dBi - 6 dBi)). Well, it is slightly better than 30 dBm
or 1 watt EIRP.
Fixed, point-to-point paths and get even more power...
There is another exception to this section of the FCC rules. Part 15.247(b)(3)(i)
covers systems that are "fixed, point-to-point". That means this path only
has two transmitters involved and they are bolted down by never moving their
locations. Automobiles may not apply. An example would be if you have an
access point and a user that is a couple blocks or even tens of miles away
that you want to connect to.
This exception is more lenient as you only need to turn down the "intentional
radiator" 1 dB for every 3 dB of signal over the 6 dBi of the antenna system.
The FCC does this as it knows that these paths will not likely not be omni
directional on each end and will have less of a chance to interfere with
others as well as the need to span some long distances.
Lets look at an example using the same antenna, transmission line and transmitter
as above. Without turning anything down we had an "intentional radiator"
that was producing 40 dBm or 10 watts EIRP. Since the antenna gain is 12
dBi and we have to reduce the power of this "intentional radiator" 1 db for
every 3 db we go over 6 dBi of antenna gain we would have to roll the power
back to 38.4 dBm or 7 watts EIRP (40.4 - (12 dBi - 6 dBi) / 3).
Real world examples...
Omnidirectional Point-to-Multi-point...
Directional Point-to-Multi-point...
Directional Fixed, Point-to-Point...
Recently I put up a short path between myself and a neighbor about 2 blocks
away (.2 miles). I have an Apple Airport that uses the Lucent Wavelan Silver
card that puts out 30 mW or about 15 dBm. The antennas have a gain of 24
dBi with a transmission line loss of about 6 db. This gives me an "intentional
radiator" power of 48 dBi. Since the antenna gain is 18 dBi over the 6 dBi
that the FCC gives you and since it is a fixed, point-to-point link I would
have to limit my
[...]
Since the little Wavelan card only puts out 15 dBm I am legal.
Quicky Definitions...
deciBels - dB
dB, or one tenth of a Bel, is a unit of mesurment that looks at the ratio
of one value to another. Gain or loss can be measured in dB. The dB scale
is an exponential scale using the formula log(ratio)*10. This means that
3 dB is about twice the power, 10 dB is 10 times the power, 13 dB is about
20 times the power and 20 dB is 100 times the power.
dBm
dBm is deciBels referenced to a value of 1 miliWatt of power. Power over
or under 1mW would be plus or minus dBm respectively.
If you have a transmitter that produces 1 watt of power that would be 1000
times more than 1 mW so that converts to 30 dBm.
dBW
dBW is deciBels referenced to a value of 1 Watt of power. Power over or under
1 Watt would be plus or minus dBW respectively.
Effective Isotropic Radiated Power - EIRP
Effective Isotropic Radiated Power defines the gain of an antenna over an
"isotropic antenna" that would radiate equally in all directions.
If you have an antenna that radiates better in one direction than another,
it would have gain in this direction. The amount of gain would be shown as
"dBi" or dB gain (or loss) over an "isotropic antenna".
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