Mist AP Transmit Power Notation

Transmit power throughout the Mist GUI and API is represented per Tx Chain.  That is, the maximum transmit power allowed for any data rate.

Here is how to convert to other common industry notations:

      • Total Power Out (TPO) = per chain tx power + 10log(num. Tx chains)
        • Rule of thumb, add 6 dB for AP41 or AP43 or AP 45 and 3 dB for AP34 for MIMO gain
          • 4ss AP = 6 dB of MIMO Gain
          • 3ss AP = 4.7 dB of MIMO Gain
          • 2ss AP = 3 dB of MIMO Gain
        • Example: 17 dBm per chain + 6 dB MIMO gain = 23 dBm total power out
      • EIRP = total power out + antenna gain – antenna losses

6 GHz Band

With 6 GHz, in some regulatory domains, such as the FCC transmit power is limited by power spectral density (PSD), rather than EIRP.  With PSD, the density of power decreases as you increase channel bandwidth.  So an 80 MHz channel would allow for higher max EIRP than a 20 or 40 MHz channel.

  • In the US, the FCC allows up to 5 dBm/MHz up to 30 dBm EIRP for low power indoor (LPI) operations.
  • EU allows 10 dBm/MHz up to 23 dBm EIRP for LPI.

Converting between EIRP and PSD

We can convert between PSD and EIRP with this formula:

      • EIRP = PSD + 10log(channel width)
      • 21 dBm EIRP = 5 dBm/MHz PSD + 10log(40 MHz)

Reference of PSD and EIRP for Low Power Operation

This is for the US, where PSD limit is 5 dBm/MHz.  For EU, the limit is 10 dBm/MHz.

Channel Width PSD EIRP Noise Floor Net EIRP Available Channels
20 MHz 5 dBm/MHz 18 dBm 18 dBm 59
40 MHz 5 dBm/MHz 21 dBm +3 dBm 18 dBm 29
80 MHz 5 dBm/MHz 24 dBm +6 dBm 18 dBm 14
160 MHz 5 dBm/MHz 27 dBm +9 dBm 18 dBm 7
320 MHz 5 dBm/MHz 30 dBm +12 dBm 18 dBm 3

Here is a diagram to compare EIRP and PSD across channel bandwdiths.  Go here more information about PSD: https://www.mist.com/power-spectral-density/

Considerations for Ekahau

When performing a predictive design in Ekahau, here is how to convert the transmit power in Ekahau to transmit power in Mist.  Ekahau represents the total transmit power of the access point (total power out), that is the combination of all the transmitters.  Which means to convert Ekahau power, you must subtract MIMO gain to get to Mist transmit power.

For example, the transmit power of the 5 GHz radio on this simulated AP43 in Ekahau is 14 dBm.  That would equate to 8 dBm in Mist, using the formula from the previous section (6 dBm of MIMO gain).

To provide a little more detail- Here are two simulated APs in Ekahau, both configured for 14 dBm.  The AP on the left is 1×1:1, the AP on the right is 4×4:4 to illustrate the number of transmitters in Ekahau does not impact the predicted coverage of the AP.