Power Limits
Ideally, a
well-engineered path has just the amount of power required to get
from point A to point B with decent reliability. Good engineering
limits the signal to the area being served, which results in reduced
interference and a more efficient use of the spectrum. Using too much
power will cover more area than is needed and may interfere with
other users of the band.
Because 802.11 is designed for short-range use in offices and homes,
it is limited to very low power.
A.5.1 FCC 15.247 and 80211.b
Section 15.247 of the FCC regulations covers the operation of 802.11b
devices.
A.5.1.1 Point-to-multipoint communication
You are allowed up to 30dBm or 1 watt of Transmitter Power Output
(TPO) with a 6dBi antenna, or 36dBm or 4 watts Effective Isotropic
Radiated Power (EIRP). The TPO must be reduced 1dB for every dB of
antenna gain over 6dBi.
A.5.1.2 Point-to-point communication
The FCC encourages directional antennas to minimize interference with
other users. The FCC is more lenient with point-to-point links: the
TPO must be reduced by one-third of a dB, instead of the full dB for
point-to-multipoint communication.
More specifically, for every 3dB of antenna gain over a 6dBi antenna,
you must reduce the TPO 1dB below one watt. For example, a 24dBi
antenna is 18dB over a 6dBi antenna. You would have to lower a
one-watt (30dBm) transmitter 6dB (one-third of 18) to 24dBm or
one-quarter watt.
A.5.2 FCC 15.407 and 802.11a
Section 15.407 of the FCC regulations covers the operation of 802.11a
devices.
A.5.2.1 Point-to-multipoint communication
As described previously, the U-NII band is chopped into three
sections. The "low" band runs from
5.15GHz to 5.25GHz and has a maximum power of 50mW (TPO). This band
is meant for in-building use only, as defined by sections 15.407 (d)
and (e):
(d) Any U-NII device that operates in the 5.15-5.25 GHz band shall
use a transmitting antenna that is an integral part of the device.
(e) Within the 5.15-5.25 GHz band, U-NII devices will be restricted
to indoor operations to reduce any potential for harmful interference
to co-channel MSS operations.
The "middle" band runs from 5.25GHz
to 5.35GHz, with a maximum power limit of 250mW. Finally, the
"high" band runs from 5.725GHz to
5.825GHz, and has a maximum transmitter power of 1 watt and an
antenna gain of 6dBi/36dBm/4 watts EIRP.
A.5.2.2 Point-to-point communication
As with 802.11b, the FCC does give some latitude to point-to-point
links in 15.407(a)(3). For the 5.725GHz to 5.825GHz band, the FCC
allows a TPO of 1 watt and up to a 23dBi gain antenna without
requiring reduction of the TPO 1dB for every 1dB of gain over 23dBi.
15.247(b)(3)(ii) does allow the use of any gain antenna for
point-to-point operations without reduction of the TPO for the
5.725GHz to 5.825GHz band. Look at the Part your equipment is
certified under to see which EIRP restrictions apply.
Sections
