WiFi 7 Transmitter Modulation Accuracy (IEEE 802.11be)

In Wi-Fi 7 (IEEE 802.11be), transmitter modulation accuracy is a critical parameter that measures how accurately the transmitted signal conforms to the ideal modulation scheme. This accuracy is vital for ensuring reliable data transmission, especially given the advanced modulation techniques used in WiFi 7, such as 4096-QAM, which require high precision.

Modulation accuracy is typically quantified using Error Vector Magnitude (EVM). EVM is a measure of the difference between the ideal (reference) signal and the actual transmitted signal, as shown in the graph below. It is expressed in EVM (dB) or EVM (%)rms. It provides an indication of how much distortion or noise affects the transmitted signal.

Key Points of Modulation Accuracy in Wi-Fi 7

• EVM is the primary metric used to quantify modulation accuracy. It measures the root-mean-square (RMS) deviation between the ideal and the actual transmitted symbols in the constellation diagram.

  The following formula is useful to convert EVM(%) to EVM (dB) and vice versa.

• A lower EVM indicates better modulation accuracy, which is crucial for higher-order modulation schemes like 4096-QAM used in Wi-Fi 7. High EVM can lead to symbol errors, reduced data rates, and higher packet error rates.

EVM Requirements for Wi-Fi 7

The EVM requirements become stricter with higher-order modulations. The table mentions EVM requirements for various modulation-code rates. For example:

• BPSK (Binary Phase Shift Keying): EVM must be ≤ -5 dB (31.6%).
• QPSK-3/4 (Quadrature Phase Shift Keying): EVM must be ≤ -13 dB (22.4%).
• 1024 QAM: EVM must be ≤ -35 dB (1.8%).
• 4096 QAM: EVM must be ≤ -38 dB (1.2%).

The following factors affect modulation accuracy:

1. Phase Noise: Variations in the phase of the transmitted signal can degrade modulation accuracy.
2. Nonlinearities: Imperfections in the transmitter’s amplifiers and mixers can introduce distortions.
3. IQ Imbalance: Mismatches in the in-phase (I) and quadrature (Q) components can affect EVM.
4. Frequency Offset: A frequency mismatch between the transmitter and receiver can degrade EVM.

Measurement and Testing

• Transmitters are tested for EVM compliance by comparing the transmitted signal against a reference signal using signal analyzers. The EVM must stay within specified limits across the supported bandwidths and modulation schemes.

• To achieve the required EVM levels, WiFi 7 devices often employ calibration techniques and compensations for imperfections in the hardware.

Impact of EVM on Performance

• Accurate modulation (low EVM) is essential for achieving higher data rates. As modulation order increases, symbols become closer together in the constellation, requiring more precise transmission.

• Low EVM improves the reliability of the connection by reducing the probability of symbol errors, which is especially important in challenging environments with interference and noise.

Example of EVM Tolerances

For 4096-QAM, the constellation points are very close together, making the signal highly susceptible to inaccuracies. WiFi 7 specifies that the EVM must be ≤ -38 dB, equating to an EVM percentage of about 1.2%. This high precision ensures that the transmitted symbols are as close as possible to their ideal positions, allowing for reliable high-speed data transmission.

In summary, transmitter modulation accuracy as defined by EVM is crucial for the performance of Wi-Fi 7 devices, ensuring that high data rates are maintained with minimal errors, even under complex modulation schemes like 4096-QAM.