Technical Tip: Low RSSI observed on the 6 GHz band with FortiAP devices G and K Series

Wi‑Fi 6E transmission context and regulatory framework.

In the 5 GHz band, the transmission method defined by international regulatory bodies such as the FCC in the Americas and ETSI in Europe is based on Equivalent Isotropically Radiated Power (EIRP).
This method assumes an ideal omnidirectional antenna that radiates energy equally across the entire channel bandwidth and in all directions.

With EIRP, narrower channels concentrate more energy per unit of spectrum.

As a result, a 20 MHz channel typically provides a larger cell size compared to wider channels such as 80 MHz, which can increase co‑channel interference and make high‑density Wi‑Fi designs more complex.

For Wi‑Fi 6E operation on the 6 GHz band, regulatory authorities such as the FCC have adopted Power Spectral Density (PSD) limits instead of EIRP.

Under PSD, the transmitted power is limited per MHz (5 dBm/MHz), ensuring that the cell size remains consistent regardless of channel width.

This approach enables the use of wider channels such as 160 MHz and 320 MHz without increasing interference, which is a fundamental design objective of Wi‑Fi 6E.

6 GHz operational modes for the USA.

To allow coexistence with incumbent services in the 6 GHz band, the FCC defines multiple operational power modes depending on the device type, deployment scenario, and installation environment:

• Standard Power (SP).

• Low Power Indoor (LPI).

• Very Low Power (VLP) for mobile devices.

To ensure compliance with regional power limits, devices operating in Standard Power mode must use an Automated Frequency Coordination (AFC) mechanism.

Otherwise, devices must operate under LPI or VLP restrictions.

For operation in other countries, validate with local authorities. Most FortiAP devices will fall back to LPI and VLPI modes only if a specific country does not consider an AFC mechanism for outdoor operation.

Fortinet validation and regulatory behavior by software release.

• FortiAP‑G and K version 7.4.
  Transmission power remains constant across all channel widths, following an EIRP‑based behavior.

• FortiAP‑G and k version 7.6.
  Transmission power dynamically adjusts based on channel width, following a PSD‑based behavior compliant with FCC regulations.

For example, higher transmission power is observed when using 160 MHz channels.

• Overall behavior aligns with applicable FCC and ETSI regulatory requirements.

Lower RSSI values on the 6 GHz band are therefore an expected result of PSD‑based operation and do not indicate a fault in FortiAP hardware or software.

Recommendations 

Based on Fortinet TAC testing and expertise for most Wi-Fi 6E AX 160 MHz client adapters, the following recommendations should be followed.

1. FortiAP firmware version.

Use the latest FortiAP‑G, K 7.6 software version.
Those versions comply with most current regulatory frameworks applied around the world; new countries or updated definitions will be available as many countries are still adapting and adjusting to 6 GHz operation.

2. Channel selection: PSC channels.

Use Preferred Scanning Channels (PSC) on the 6 GHz band.

Wireless stations rely on Reduced Neighbor Report (RNR) information from 5 GHz beacons or actively scan PSC channels in the 6 GHz band to identify available SSIDs.

If the FortiAP operates on a non‑PSC channel, some stations may consistently prefer 5 GHz over 6 GHz.

PSC channels include:
5, 21, 37, 53, 69, 85, 101, 117, 133, 149, 165, 181, 197, 213, 229.

3. Channel width.

Use a 160 MHz channel width when possible.
PSD operation allows higher aggregate transmission power on wider channels, resulting in measured transmission power of approximately 14 dBm, subject to regulatory domain restrictions.

4. Client configuration and SSID design.

Most Wi‑Fi clients follow a default band preference order, defined by the device manufacturer, for the desired behaviour for the use intended for those, i.e., 2.4 GHz → 5 GHz → 6 GHz, or vice versa. Check manufacturer documentation.

Even when a client exposes an option to prefer 6 GHz, this behavior is advisory and not strictly enforced.

Use separate SSIDs for 5 GHz and 6 GHz bands to simplify manual band selection and ensure consistent 6 GHz connectivity.

5. Wireless controller considerations.

Use FortiGate with the latest FortiOS v7.6 release for Wi‑Fi 6E deployments.

FortiOS v7.6 introduces enhanced hardware compatibility, additional Wi‑Fi features, and improved stability compared to FortiOS v7.4.

FortiAP v7.6 firmware also integrates the latest chipset radio code, providing improved performance, stability, and defect fixes relevant to Wi‑Fi 6E operation.