Wi-Fi 8 Physical Layer Testing: 802.11bn Compliance Guide

Introduction : As the industry prepares for the deployment of Wi-Fi 8 based on IEEE 802.11bn, the testing landscape is undergoing a significant transformation. Unlike previous generations that focused primarily on throughput, Wi-Fi 8 is built for Ultra-High Reliability (UHR). To achieve this, the physical layer (PHY) introduces complex features like Distributed tone Resource Units (DRU), Enhanced Long Range (ELR) PPDUs, and new intermediate Modulation and Coding Schemes (MCS). Consequently, transmitter and receiver testing must be more precise than ever to ensure cross vendor compatibility and network stability.

A. Transmitter Requirements: Ensuring Signal Integrity

Transmitter testing for Wi-Fi 8 focuses on signal quality, frequency precision and the impact of new tone mapping techniques on spectral purity.

1. Transmitter Constellation Error (EVM)

Error Vector Magnitude (EVM) remains the gold standard for transmitter quality. Wi-Fi 8 keeps the strict limits of Wi-Fi 7 for legacy MCS levels but introduces four new levels (17, 19, 20, 23) to fill the gaps between existing modulations.

• Standard UHR Limits: For 4096-QAM (MCS 13), the limit remains a challenging -38 dB. For the new Wi-Fi 8 levels, the targets are scaled proportionally.
• ELR Relaxation: Because Enhanced Long Range (ELR) is designed for “reach” rather than “density,” its EVM limits are relaxed to -4 dB (BPSK) and -5 dB (QPSK) to allow for more robust and long distance transmission.

2. Unused Tone Error for DRU

The introduction of Distributed tone Resource Units (DRU) creates a new challenge i.e. interference management. Since tones are now scattered, the transmitter must ensure it does not leak energy into unused tones within or outside its Distribution Bandwidth (DBW).

• Compliance Target: The unused tone error must generally remain below -38 dB or a calculated value relative to the used tone EVM minus the spreading gain of the DRU.

3. Pre-correction and Frequency Offset

For Trigger based (TB) and ELR uplink transmissions, the station (STA) must pre correct its frequency and timing based on the Access Point’s trigger.

• CFO Limit: For standard TB PPDUs, the Carrier Frequency Offset (CFO) error must be less than 350 Hz.
• ELR CFO Limit: For ELR, the requirement is less than 15 kHz at the 10 % point of the CCDF curve.
• Timing: For timing requirements, the STA needs to transmit within +/- 0.4 μs + 16 μs from the end of the last OFDM symbol of the triggering PPDU sent by the AP to trigger the UL transmission.

B. Receiver Requirements: Stability Under Pressure

For most of the part, IEEE 802.11bn re-uses the receiver requirements specified in the IEEE 802.11be (Wi-Fi 7 specifications). These requirements have been updated to include new MCS levels and ELR PPDU. Wi-Fi 8 receiver tests evaluate the device’s ability to decode signals in weak signal areas and ignore interference from “loud neighboring networks”.

1. Minimum Input Sensitivity

This test measures the weakest signal a receiver can decode with a Packet Error Rate (PER) of less than 10 %.

• UHR Limits: For a standard 20 MHz channel at MCS 0 (BPSK 1/2), the sensitivity limit is -82 dBm. As bandwidth increases to 320 MHz, the required sensitivity drops to -70 dBm due to the higher noise floor of wider channels.
• ELR Sensitivity: ELR modes are specifically tested at -82 dBm to ensure the “long range” promise is met.

2. Adjacent and Nonadjacent Channel Rejection

This measures how well a receiver handles a “wanted” signal when a strong interfering signal is nearby.

• Compliance: For BPSK 1/2, the receiver must be able to ignore an adjacent interferer that is 16 dB stronger than the wanted signal and a nonadjacent interferer that is 32 dB stronger.

3. Receiver Maximum Input Level

To ensure devices do not “choke” when they are very close to an Access Point, they are tested at high power levels.

Limits: Receivers must handle -30 dBm in the 5/6 GHz bands and -20 dBm in the 2.4 GHz band while maintaining a PER below 10%.

4. RCPI Accuracy

For the first time, Wi-Fi 8 introduces a mandatory PHY layer requirement for Received Channel Power Indicator (RCPI) accuracy.

Limit: The reported power value must be accurate within +/- 5 dB. This is crucial for Multi-AP coordination, where APs must know the exact power levels of nearby devices to manage interference effectively.

Wi-Fi 8 test tool providers

Conclusion

Achieving Ultra High Reliability requires rigorous validation of both the transmitter’s spectral purity and the receiver’s resilience. Tools like the Rohde & Schwarz CMP180 or CMX500 are essential for developers to verify these strict 802.11bn limits, ensuring that the “Reliability First” promise of Wi-Fi 8 becomes a reality for end users.

References for further study

1. IEEE P802.11bn Draft Amendment
2. Rohde & Schwarz White Paper - Setting New Performance Standards with IEEE 802.11bn Version 2.00
3. IEEE 802.11-2020 Standard