Power Output Rules
The FCC enforces certain rules regarding the power radiated by the antenna element, depending on whether the implementation is a point-to-multipoint or a point-to-point implementation. The term used for the power radiated by the antenna is Equivalent Isotropically Radiated Power (EIRP).
PtMP links have a central point of connection and two or more noncentral connection points. PtMP links are typically configured in a star topology. The central connection point may or may not have an omnidirectional antenna (an omnidirectional antenna produces a 360 degree horizontal beam). It is important to note that when an omnidirectional antenna is used, the FCC automatically considers the link a PtMP link. Regarding the setup of a PtMP link, the FCC limits the EIRP to 4 watts in both the 2.4-GHz ISM band and upper 5-GHz U-NII band. Furthermore, the power limit set for the intentional radiator (the device transmitting the RF signal) in each of these bands is 1 watt. If the transmitting wireless LAN devices are adjustable with respect to their output power, the system can be customized to the needs of the user. The 4-watt EIRP limit for a PtMP link is based on a 1-watt maximum at the intentional radiator and a 6-dBi gain at the antenna.
Suppose a radio transmitting at 1 watt (+30 dBm) is connected directly to a 12-dBi omnidirectional antenna. The total output power at the antenna is about 16 watts, which is well above the 4-watt limit. The FCC stipulates that for each 3 dBi above the antenna's initial 6 dBi of gain, the power at the intentional radiator must be reduced by 3 dB below the initial +30 dBm. For our example, because the antenna gain is 12 dBi, the power at the intentional radiator must be reduced by 6 dB. This reduction will result in an intentional radiator power of +24 dBm (30 dBm minus 6 dB) or 250 mW, and an EIRP of 36 dBm (24 dBm plus 12 dBi), or 4 watts. Clearly, this rule can become confusing, but the end result must be that the power at the intentional radiator never be more than 1 watt, as shown on the Point-to-Multipoint Power Output Limits Table, and the EIRP must never be above 4 watts for a PtMP connection.
When using an omnidirectional antenna, the rules for point-to-multipoint links must be followed, regardless of whether the actual implementation is point-to-point or point-to-multipoint.
PtP links include a single directional transmitting antenna and a single directional receiving antenna. These connections will typically include building-to-building or similar links and must abide by special rules. When installing a PtP link, the 4-watt power limit all but disappears in favor of a sliding power limit. Regarding a PtP link, the FCC mandates that for every 3 dBi above the initial 6 dBi of antenna gain, the power at the intentional radiator must be reduced by 1 dB, starting at the initial +30 dBm.
Consider our previous example, using the same values: 1 watt (+30 dBm) at the intentional radiator and a 12 dBi antenna (in this case the antenna will be a directional antenna). The total output power is still 16 watts. In this example, because the antenna gain is 12 dBi, the power at the intentional radiator must be reduced by 2 dB, as opposed to a 6 dB reduction in the previous example. This reduction will result in an intentional radiator power of 28 dBm (30 dBm minus 2 dB), or about 630 mW and an EIRP of 40 dBm (28 dBm plus 12 dBi), or 10 watts. In the case of PtP links, the power at the intentional radiator is still limited to 1 watt, but the limit of the EIRP increases with the antenna gain, as shown on the Point-to-Point Power Output Limits Table. It is very important to clearly distinguish between the rules that govern PtP and PtMP wireless links.
The FCC has a different set of rules for PtP links in the upper U-NII band. Fixed point-to-point U-NII devices operating in the 5.725 to 5.825 GHz band may employ transmitting antennas with directional gain up to 23 dBi, without any corresponding reduction in the transmitter peak output power. For fixed, point-to-point U-NII transmitters that employ a directional antenna gain greater than 23 dBi, a 1 dB reduction in peak transmitter power for each 1 dBi of antenna gain in excess of 23 dBi is required. Notice that an output power maximum of +30 dBm at the intentional radiator, and a maximum of 23 dBi of antenna gain before any reduction in transmitter output power is required, allows these 5-GHz U-NII systems to have an output of 200 Watts EIRP.