What is the MPE (maximum permissible exposure) requirement for a 5G FR2 device and how is it tested?
5G FR2 MPE Testing
MPE compliance for 5G FR2 devices is one of the most complex regulatory challenges because: mmWave beamforming creates highly directional beams that can concentrate energy, the evaluation distance is very close to the device (2 mm), and the time-averaging for beamforming is still being standardized.
- Performance verification: confirm specifications against the application requirements before finalizing the design
- Environmental factors: temperature range, humidity, and vibration affect long-term reliability and parameter drift
- Cost vs. performance: evaluate whether the application demands premium components or standard commercial grades
- Interface compatibility: verify impedance, connector type, and mechanical form factor match the system architecture
Frequently Asked Questions
How is beamforming accounted for?
Beamforming and MPE: a 5G FR2 device with beamforming can create narrow beams with high EIRP, but: the beam direction changes rapidly as the device communicates with the base station. At any given moment: the beam may or may not be pointed toward the body. FCC KDB 447498 allows time-averaging of the beam exposure: the time-averaged power density accounts for the beam scanning pattern, traffic loading, and the probability that the beam points toward the body. Methods: beam peak search (measure the maximum PD from the worst-case beam), then apply a time-averaging reduction factor based on: the number of beams, the beam scan pattern, and the traffic duty cycle. The reduction factor can be 3-10× (i.e., the time-averaged PD is 3-10× lower than the peak beam PD).
What about SAR vs. PD for 6-10 GHz?
For frequencies between 6 and 10 GHz: both SAR and power density limits may apply (this is a transition region). FCC: requires power density evaluation above 6 GHz (SAR not required). ICNIRP 2020: requires power density above 6 GHz, with a transition at 6 GHz from SAR to PD. The transition is not perfectly clean, and manufacturers of devices operating in the 6-10 GHz range (e.g., Wi-Fi 6E at 5.925-7.125 GHz) must: evaluate both SAR (for the portion below 6 GHz) and power density (for the portion above 6 GHz). Or: if the entire band is above 6 GHz (Wi-Fi 6E upper channels), only PD is required.
What equipment is needed?
MPE measurement equipment for FR2: near-field probe scanner: a robotic arm that scans a mmWave probe (E-field or PD probe) across the evaluation surface near the device. Speag cDASY6 with mmWave probe modules (supports 10-110 GHz). Cost: $200,000-500,000+. Phantom: a flat dielectric phantom simulating the body surface. For mmWave: the phantom properties are standardized (skin-equivalent dielectric at the test frequency). Test chamber: an anechoic environment to prevent reflections from contaminating the measurement. Alternatively: computational methods using validated 3D EM simulation (HFSS, CST) with a calibrated device model can substitute for some measurements. The FCC accepts computational methods with measurement validation.