Spectral Flatness Measurement in Wi-Fi 7 (IEEE 802.11be)
In Wi-Fi 7, defined by the IEEE 802.11be standard, spectral flatness is an important parameter that ensures the uniformity of signal power distribution across the channel bandwidth. This is crucial for maintaining signal integrity, especially in high data rate transmissions that WiFi 7 supports.
Spectral flatness refers to the requirement that the power across the entire bandwidth of the transmitted signal remains relatively uniform. This characteristic is vital for minimizing distortion and maintaining the integrity of modulated data, particularly with the advanced modulation schemes used in WiFi 7, such as 4096-QAM.
Key Points of Spectral Flatness in Wi-Fi 7
1. Uniform Power Distribution: Spectral flatness ensures that the power
levels across the frequency bins or subcarriers within a channel are within a specified
range relative to each other. This uniformity is necessary for accurate data demodulation
and decoding at the receiver.
2. Flatness Criteria: For Wi-Fi 7, the standard specifies that the power levels of the subcarriers
within the occupied bandwidth should not deviate significantly from the average power level.
This is often measured in decibels (dB).
3. Deviation Limits: Typically, spectral flatness is defined by an acceptable
range of deviation, such as follows.
• Subcarriers must stay within +/- 2 dB relative to the average power level of the subcarriers within
the occupied bandwidth. A few subcarriers (often a small percentage) may be allowed to deviate further, up to +/- 4 dB,
but these are strictly limited.
4. Frequency Ranges: The flatness requirement applies across the entire channel bandwidth,
which can be 20 MHz, 40 MHz, 80 MHz, 160 MHz, or 320 MHz in WiFi 7. Maintaining flatness
over wider bandwidths like 160 MHz and 320 MHz is more challenging but necessary for optimal
performance.
5. Importance of Flatness:
• Consistency in Modulation: High spectral flatness ensures that all subcarriers carry
data at consistent power levels, which is essential for higher modulation schemes like
4096-QAM that are sensitive to power variations.
• Improved Performance: A flat spectrum minimizes the potential for bit errors, improves
overall signal quality and enhances data throughput.
• Reduced Signal Distortion: By maintaining spectral flatness, signal distortion due to
uneven amplification across the channel is minimized, leading to a clearer signal at the receiver.
6. Testing and Compliance: Devices are tested to ensure compliance with spectral flatness
specifications. This involves transmitting test signals and measuring the power across the subcarriers
using specialized equipment.
The measurements must fall within the acceptable deviation limits set by the standard.
Image Courtesy : Rohde & Schwarz
The figure depicts spectral flatness for 160 MHz bandwidth for EHT non-punctured frame. The table shows maximum deviation (dB) allowed with and without preamble puncturing for difference bandwidths such as 20 MHz, 40 MHz, 80 MHz, 160 MHz and 320 MHz.
These flatness specifications ensure that the transmitted signal maintains a consistent power profile across its entire bandwidth, which is essential for achieving the high data rates and low error rates that Wi-Fi 7 promises.
WiFi 7 Resources as per IEEE 802.11be
RF and Baseband Measurement Links
WiFi 6 Resources as per IEEE 802.11ax
Useful Links to Legacy WLAN Standards
IoT Wireless Technologies
RF Wireless Test and Measurement Solutions
BER Testing equipments
Agilent T & M Solutions
Anritsu T & M Solutions
Rohde & Schwarz T & M solutions
wireless channel emulator
Vector signal generator
Vector signal analyzer
Power Analyzer Equipments
Protocol Analyzer