The spectrum regulatory body of each country restricts signal power levels
of various frequencies to accommodate needs of users and avoid RF
interference. Most countries deem 802.11 wireless LANs as license free.
In order to qualify for license free operation, however, the radio devices must
limit power levels to relatively low values.
In many cases, installers would prefer to use comparatively high transmit power
to increase the range of access points. The problem, however, is that RF interference
with other nearby equipment would occur more often. The RF spectrum is limited,
so we must control the amount of power must we use.
The FCC makes the rules
In the U.S., the FCC (Federal Communications
Commission) defines power limitations for wireless LANs in FCC
Part 15.247. Manufacturers of 802.11 products must comply with Part 15
to qualify for selling their products within the U.S. Regulatory bodies in other
countries have similar rules.
Part 15.247 provides details on limitations of EIRP (equivalent isotropically
radiated power). EIRP represents the total effective transmit power of the radio,
including gains that the antenna provides and losses from the antenna cable.
You must take all of these into account when calculating the EIRP for a specific
radio.
The gain of an antenna represents how well it increases effective signal power
in a particular direction, with dBi (decibels relative to an isotropic radiator)
as the unit of measure. dBi represents the gain of an antenna as compared to
an isotropic radiator, which transmits RF signals in all directions equally.
More precisely, dBi equals 10 times the logarithm (base 10) of the electromagnet
field intensity of the antennas favored direction divided by the electromagnetic
field intensity of an isotropic antenna (with measurements taken at the same
distance).
Manufacturers determine the antenna’s dBi value, so it’s a relief we don’t have
to calculate it. What we do need to know, however, is that every three dBi
doubles the power of an RF signal. As a result, higher values of dBi extend
the range of a wireless LAN.
FCC tighter on mobile WLANs
A typical indoor WLAN consists of enough access points to cover the facility
to enable wireless mobility for users. Radio NICs in user devices and access
points generally have omni-directional antennas that propagate RF energy in
most directions, which maximizes connectivity for mobile applications. When
using omni-directional antennas having less than 6 dB gain in this scenario,
the FCC rules require EIRP to be 1 watt (1,000 milliwatts) or less.
In most cases, you’ll be within regulations using omni-directional antennas
supplied by the vendor of your radio NICs and access points. For example, you
can set the transmit power in an 802.11b access point or client to its highest
level (generally 100 milliwatts) and use a typical 3 dB omni-directional antenna.
This combination results in only 200 milliwatts EIRP, which is well within FCC
regulations.
FCC loosens up
The FCC eases EIRP limitations for fixed, point-to-point systems that use higher
gain directive antennas. If the antenna gain is at least 6 dBi, the FCC allows
operation up to 4 watts EIRP. This is 1 watt (the earlier limitation) plus 6
dB of gain.
The higher gain antennas have greater directivity, which propagate RF energy
more in one direction than others. This reduces the possibility of causing RF
interference with other nearby systems. Thus, the use of higher gain antennas,
even if they result in higher EIRP, is acceptable. The users benefit by having
greater range, and neighboring systems are much less likely to encounter RF
interference.
For antennas having gain greater than 6 dBi, the FCC requires you to reduce
the transmitter output power if the transmitter is already at the maximum of
1 watt. The reduction, however, is only 1 dB for every 3 dB of additional antenna
gain beyond the 6 dBi mentioned above. This means that as antenna gain goes
up, you decrease the transmitter power by a smaller amount. As a result, the
FCC allows EIRP greater than 4 watts for antennas having gains higher than 6
dBi.
As you can see, the deployment of a wireless LAN for typical mobile applications
using omni-directional antennas is fairly straightforward in terms of EIRP limitations.
The problems come into play when installing systems to connect buildings within
a metropolitan area. In this case, pay close attention to the FCC rules. You
could find yourself violating the rules if you don’t calculate the EIRP and
see if you’re within limitations.
Jim Geier provides independent consulting services to companies
developing and deploying wireless network solutions. He is the author of the
book, Wireless LANs
(SAMs, 2001), and regularly instructs workshops on wireless LANs.
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