Power Limits

Power Limits
Ideally, a well-engineered path has just the amount of power required to get from point A to point B with
good reliability. Good engineering limits the signal to only the area being served. This has the effect of
reducing interference and enabling more efficient use of the spectrum. Using too much power covers
more area than is needed and could potentially interfere with other users of the band. Because 802.11 is
designed for short-range use, such as offices and homes, it is limited to very low power.
802.11b—Its Relationship to FCC Part 15, Section 247 Regulatory aspects of 802.11 per Part 15 are
oriented around the applications of the wireless technologies. The distinction is point-to-multi-point and
point-to-point.
Point to Multipoint 802.11 service providers are allowed up to 30 dBm or 1 watt of Transmitter Power
Output (TPO) with a 6 dBi antenna or 36 dBm or 4 watts effective isotropic radiated power (EIRP). The
TPO needs to be reduced 1 dB for every decibel of antenna gain over 6 dBi.
Point to Point The FCC encourages the use of directional antennas to minimize interference to other
users. In fact, the FCC is more lenient with point-to-point links by requiring only the TPO to be reduced by
one-third of a decibel instead of a full decibel for point to multipoint. More specifically, for every 3 dB of
antenna gain over a 6 dBi antenna, a wireless Internet service provider (WISP) needs to reduce the TPO
1 dB below 1 watt. For example, say a 24 dBi antenna is 18 dB over a 6 dBi antenna. This requires
lowering a 1 watt (30 dBm) transmitter 18/3 or 6 dB to 24 dBm or 1/4 watt.
802.11a—FCC Part 15, Section 407 Regulatory aspects of 802.11 per Part 15 are oriented around the
applications of the wireless technologies. The distinction is point-to-multi-point and point-to-point.
Point to Multipoint As described before, the Unlicensed National Information Infrastructure (U-NII) band is
chopped into three sections. The low band runs from 5.15 to 5.25 GHz, with a maximum power of 50 mW
(TPO). This band is meant to be in-building only as defined by the FCC's Rules and Regulations Part
15.407(d) and (e):
l (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.
l (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 5.25 to 5.35 GHz, with a maximum power limit of 250 mW. Finally, the high
band runs from 5.725 to 5.825 GHz, with a maximum transmitter power of 1 watt and antenna gain of 6
dBi or 36 dBm or 4 watts EIRP.
Point to Point As with 802.11b, the FCC does give some latitude to point-to-point links in Part 15.407(a)
(3). For the 5.725–5.825 GHz band, the FCC allows a TPO of 1 watt and up to a 23 dBi gain antenna
without reducing the TPO 1 dB for every 1 dB of gain over 23 dBi.
Part 15.247(b)(3)(ii) does allow the use of any gain antenna for point-to-point operations without having to
reduce the TPO for the 5.725–5.825 GHz band.