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Maximum Transmitter Power Levels

When defining rules and regulations surrounding transmitter power, several methods can be used to define the limits. First, there is straight transmitter output power, which is the amount of RF energy sent from the transmitter power amplifier to the antenna connector. As discussed in Chapter 2, "Understanding RF Fundamentals," these power specifications are typically rated in decibels per milliwatt (dBm) for the WLAN industry. Second, various limitations pertain to the antenna gain that may be used.

EIRP

Regulatory requirements for power output levels are sometimes rated in actual transmitter power, or in many cases effective power based on both the transmitter and antennas values. This method is known as the Effective Isotropic Radiated Power (EIRP). This value is a calculated value, using not only the transmitter power, but as you might have guessed from the name, the isotropic gain of an antenna (dBi ratings). This value also includes any losses for cable, lightning arrestors, or any other devices placed between the antenna and the transmitter connector. This is the effective power that is radiated from the antenna.

To obtain an EIRP rating, you can just take the transmitter power (in dBm), add the gain of the antenna (in dBi), subtract the losses of the cable or other inserted devices (in dB), and you will end up with an EIRP value. An example follows:

Transmitter with 100-mW output power (+20 dBm)

Yagi antenna with a 13.5-dBi gain rating

50 foot of cable with a loss of 2.2 dB

TX power + Antenna gain Cable loss = EIRP

+20 dBm + 13.5 dBi 2.2 dB = 31.3-dBm EIRP

North American Regulatory Power Levels

One parameter that is tightly restricted is the transmitter power that is permitted from a WLAN radio transmitter. Because the frequencies used for WLAN are unlicensed, the regulatory bodies thought it necessary to impose limitations as a way of reducing interference.

2.4-GHz Power Levels for the North American Regulatory Domain

The NA regulatory domain sets limits for both maximum transmitter power and EIRP. The maximum power level for transmitters is regulated to 1 watt, or +30 dBm. This is true for both the 900-MHz and the 2.4-GHz WLAN bands. For the 5-GHz bands, however, these maximum power levels are different.

The NA regulations also specify both a maximum antenna gain and an EIRP limit. The regulations limit the antenna gain to 6 dBi if you are using the full +30-dBm transmitter power. This provides a maximum EIRP value of +36 dBm. The NA regulations do permit high antenna gains, however, if the transmitter power is reduced according to the rules and topology of the radio network. This can be confusing to understand, so the following sidebar explains the topologies available.

5-GHz Power Levels (NA)

The NA regulatory domain identifies the power levels for 5 GHz based on the three different UNII bands. UNII1 band was intended for indoor use, and provides only 50-mW transmitter power. The UNII2 band is intended for both indoor and outdoor usage, and the maximum power is increased to 250 mW. The UNII3 band is intended for use in outdoor systems and has a power limit of 1 W.

Note

In the 5-GHz band in the NA regulatory domain, power may not exceed the lesser of the following:

UNII1 50 mW or 4 dBm + 10logB, where B is the 26-dB emission bandwidth in MHz.

UNII2 250 mW or 11 dBm + 10logB, where B is the 26-dB emission bandwidth in MHz.

UNII3 1 watt or 17 dBm + 10logB, where B is the 26-dB emission bandwidth in MHz.

Antenna gain is again limited, as with the 2.4-GHz band for NA. It is 6 dBi maximum in a PTMP mode. To increase the antenna gain, the transmitter must be reduced by the same amount.

In the UNII3 band, however, fixed point-to-point application antennas of up to 23-dBi gain may be used, without reductions in power. For antennas with higher gain than 23 dBi, a reduction of 1-dB transmitter power is required for every 1-dB increase the antenna has above 23 dBi.

NEMA enclosure. Because you could not separate the antenna from the radio device, you had to mount the AP where you need the antenna. UNII2 and UNII3 bands permitted external antennas, with a maximum EIRP limit of 250 mW for UNII2 and 1 W for UNII3.

This restriction has been removed for UNII1, and products may now use external antennas for all channels within the 5-GHz band.

If a device combines operation of the UNII1 band with other bands, the device must comply with the UNII1 regulation requiring a permanently attached antenna.

ETSI Regulatory Power Levels

Similar to the NA domain regulations, the 2.4-GHz and 5-GHz bands have varying power limitations. For the 2.4-GHz band, ETSI regulations are quite a bit more restrictive than the corresponding NA regulations.

2.4-GHz Power Levels (ETSI)

Under the ETSI regulations, the power output and EIRP regulations are much different from what they are in the NA regulatory domain. The ETSI regulations specify maximum EIRP as +20 dBm. Because this includes antenna gain, this limits the antennas that can be used with a transmitter. To use a larger antenna, the transmitter power must be reduced, so the overall gain of the transmitter plus the antenna gain (less any losses in coax) are equal to or less than +20 dBm EIRP. This drastically reduces the overall distance an outdoor link can operate when compared to an NA type system. It also reduces the gain of the antennas that can be used with many indoor WLAN systems, unless they can reduce transmitter power. (Many APs do not have that option.)

ETSI has developed standards that have been adopted by many European countries as well as many others outside of Europe. In some cases, the standards limit the power to +20-dBm EIRP limits, and in others they may set different transmit power, antenna gain, or EIRP limits.

5-GHz Power Levels (ETSI)

The power levels for some countries using the ETSI regulatory domain vary quite widely. Table 3-3 depicts some of the power levels.

Japan Domain Power Levels

Japan uses a different method for specifying power. Instead of using a peak power method, they measure power in relationship to bandwidth. The measured value is rated in megawatts/megahertz.

2.4 GHz (Japan)

The power level for the Japanese 2.4-GHz band is rated at 10 mW/MHz. This is also an EIRP rating, which as you know by now requires the gain of the antenna to be added into the equation. This compares to approximately 19 dBm on a typical 802.11b transmitter.

Japan also requires that any antenna gain be offset by a reduction in transmitter power, keeping the EIRP level equal to or below 10 mW/MHz.

Also note that you can only use antennas that have been certified by the TELEC with the transmitter.

5 GHz (Japan)

The use of 5-GHz WLAN in Japan today is limited to indoor use only. As for power limits, it also has a maximum of 10 mW/MHz EIRP, as well as the requirement to reduce transmitter power to offset antenna gain, keeping the EIRP under 10 mW/MHz.

World Mode (802.11d)

Because of the various regulations around the globe that vary based on power levels and channel use, it becomes difficult to provide a single product that can be used in all locations of the world. Therefore, a device that is set up for use in the United States (using the NA regulatory domain) may not be permitted in the United Kingdom (where ETSI regulations are in place) because of the power-level differences. A U.K. device may not be used in the United States because of the extra two channels that are available in the product (but that are prohibited by the NA regulatory domain). This fact hinders mobility and portability (two key benefits of WLANs). Globetrotters moving from country to country need to carry various cards based on the specific regulatory domains. Even more problematic, a global company has to order, stock, and ship to its users different products based on the locations in which they will be working.

But what happens when the radio device is embedded inside your computer? How do you physically change the radio from one domain to another? One option is to require users to set the frequency and power parameters for the location where they are working. Unfortunately, most regulatory agencies fear that users would not do this properly, and therefore this is not permitted in most locations.

A second choice is to set the radio parameters to the lowest common denominator for the majority of the regulatory domains. Consider, for example, a 2.4-GHz implementation. You could set the maximum power to 30 mW (15 dBm). With a 2.2-dBi dipole, this keeps the EIRP to less than 20 dBm, the EIRP limit of ETSI, and meets the 10-mW/MHz Japanese limit; it is also well below the NA limits. For the channel selection, permit only the U.S. channels where all 11 are usable (which is most ETSI countries and Japan). This then excludes only a few select countries that have limited channel operation. However, it also limits the flexibility of the WLAN systems, reducing channel capabilities in ETSI and Japan countries, as well as range (because of lower power) in NA domains.

There is still a third choice. Have the WLAN devices select the domains automatically. But how do the devices know what domain they are in? Because APs are typically installed in a single location and not moved from one location to another, they can be set to the proper domain. In most cases, this means ordering the proper domain from the manufacturer. Then the AP could send out information as to what domain it is set for, and the roaming client devices could listen and adjust accordingly, all with no user interaction.

Permitting world-mode roaming with a single device is exactly what the IEEE 802.11d specification set out to define. However, not all devices today support world-mode roaming. If this is something you are interested in, you need to be certain that the AP as well as the intended clients provide this support.

Figure 3-10 shows one example of the ability to select 802.1d world roaming.