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Transmit Power Notation for Juniper APs

Radio Power Levels and Conversions

Radio resource management (RRM) provides sophisticated radio and antenna power management when enabled and set to auto, and we recommend that you use it. See Radio Management. However, if you need to configure the settings manually or just want to understand the power calculations and values, the following explanation will help.

  • In the Mist portal, the power values used are for the total AP transmit power of the entire transmit (Tx) chain.

  • Transmit power for 6-GHz is limited by the power spectral density (PSD) in the United States (and some other regulatory domains) rather than by Effective Isotropic Radiated Power (EIRP). EIRP is a calculated value used to represent transmitter output power, cable loss, and antenna gain.

  • For predictive power plan designs created in Ekahau, you need to subtract multiple-input multiple-output (MIMO) gains before using the values for transmit power in the Mist portal.

Figure 1: Power Levels in Current Radio Values Power Levels in Current Radio Values

Rule of Thumb for MIMO Gain Values

A simple rule of thumb for manual settings for AP41, AP43, and AP45 devices is to add 6 dB for MIMO gain. For AP34 devices, add 3 dB. In terms of radios, the rule of thumb looks like this:

  • 4 spatial streams (4x4): 6 dB of MIMO gain
  • 3 spatial streams (3x3): 4.7 dB of MIMO gain
  • 2 spatial streams (2x2): 3 dB of MIMO gain
Table 1: AP Radio Gains
AP Type 2.4-GHz 5-GHz
AP32E Directional 8 dBi 10 dBi
  Omni 4 dBi 6 dBi
AP41E Directional 8 dBi 8 dBi
  Omni No cert, use AP41 No cert, use AP41
AP43E Directional 8 dBi 10 dBi
  Omni 4 dBi 6 dBi
AP61E Directional 8 dBi 8 dBi
  Omni 4 dBi 6 dBi
AP63E Directional 8 dBi 10 dBi
  Omni 4 dBi 6 dBi

Calculating TPO and EIRP

The total power output (TPO) for Juniper APs is equal to the transmit power per radio chain, plus the log value of the total number of radio chains. Radio chains are comprised of the transceiver, antenna, and hardware needed for signal processing.

  • TPO = Tx power per chain + 10log (Tx chains)

So, for example, if you have a Juniper AP with 17 decibel-milliwatts (dBm) per chain, you add 6 dB MIMO gain for a total transmit power of 23 dBm.

Calculating the EIRP, which is a value for the estimated output power radiated by the antenna, is similar:

  • EIRP = TPO + antenna gain - antenna losses

EIRP (for 6-GHz band radios)

Some regulatory domains, including the United States, use PSD rather than EIRP for radio transmit power limits. With PSD, the power density decreases as channel bandwidth increases.

For a fuller understanding of PSD and an illustration comparing EIRP and PSD across channel bandwidths, see: https://blogs.juniper.net/en-us/industry-solutions-and-trends/power-spectral-density.

In addition:

  • Wide channels such as 80 MHz can yield higher EIRP than typical channel width (20 and 40-MHz).

  • In the United States, the FCC allows up to 5 dBm/MHz PSD, or up to 30 dBm EIRP for low power indoor (LPI) operations.

  • In the EU, regulators allow up to 10 dBm/MHz PSD, or up to 23 dBm EIRP for LPI.

Converting Between PSD and EIRP

EIRP is equal to PSD plus the log of the total channel width. You can use the formula shown here to convert between PSD and EIRP:

  • EIRP = PSD + 10log (channel width)

So, if, for example, you have a PSD of 5 dBm/MHz and 40-MHz channels, the EIRP would be 5 + the base 10 log of 40, which is 1.6, for a total dBm of 21.

Table 2: PSD and EIRP Reference for LPO
Channel Width PSD EIRP Noise Floor Net EIRP Available Channels
20-MHz 5 dBm/MHz 18 dBm na 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

Working with Ekahau

As noted earlier, Ekahau considers total transmit power to be the combination of all transmitters on the AP (the total power out), whereas in the Mist portal, the value does not include the cumulative MIMO gains. Thus, to convert Ekahau transmit power to Mist transmit power, you must subtract the MIMO gain.

For example, say that in Ekahau, you see a value of 14 dBm for the simulated transmit power of a Mist AP43. That same value would be 8 dBm as used in the Mist portal.

In another example, consider two simulated APs in Ekahau, where one is a 1×1:1 and the other is a 4×4:4 (one radio vs four). Transmit power for both APs is set at 14 dBm. In the Ekahau design simulation, because the software does not take into consideration the number of transmitters in the AP, the predicted transmit radius of both APs would be the same.