By Jim Geier
July 18, 2002
Understand the limitations of transmit power and antenna gain limitations so that you don’t mistakenly violate FCC regulations.
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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