This chapter first makes a case about the importance of the metrology form MIMO when it comes to RF-exposure in order to provide meaningful data for the regulatory bodies so they can, in turn, make informed and sensible decisions on how to set exposure limits for 5G BS. It then describes some of the existing works on the topic and focus in more detail on experimental work. It then explains the way forward that is currently taken to define how to measure and regulate the RF-exposure of 5G BS, by mainly using statistical approaches to better reflect the beamforming nature of the mMIMO transmission. Statistical models based on simple theory and system level simulations have first been proposed, before data from real 5G BS have been acquired to confirm the results of these models. The main finding is that using statistical approaches for defining the exclusion zone of 5G BS seems reasonable in comparison with the traditional method that creates unrealistically large exclusion zones. However, more works need to be done to fully understand how the RF-exposure generated by mMIMO BS fluctuates over time and space; a metrology method based on a fully reconfigurable 5G mMIMO testbed is fully detailed to do so. The main findings of this method are: the RF exposure decays in a quadratic manner as a function of the distance, even when there is no main beam pointing directly into this direction; the peak of the RF exposure at a particular point in space is reached as an active beam is directly steered towards it; the overall RF-exposure at a given point increases with the number of users that are served simultaneously by the mMIMO BS and this effect is more significant close to the mMIMO BS than further away; and the RF-exposure of a mMIMO BS varies significantly as a function of the amount of data it transmits.

Chapter Contents:

• 21.1 Motivation for 5G RF-exposure new metrology and guidelines
• 21.2 Measuring 5G RF-exposure
• 21.3 Experimental assessment of the RF-exposure of massive MIMO base station via a reconfigurable testbed
• 21.3.1 mMIMO testbed set-up
• 21.3.2 Calibration of the mMIMO testbed equipment
• 21.3.3 Experimental set-up and test scenarios
• 21.3.4 First experiment
• 21.3.5 Second experiment
• 21.4 Summary
• References

Inspec keywords: array signal processing; MIMO communication; 5G mobile communication; statistical analysis

Other keywords: beamforming; statistical approaches; 5G BS; fully reconfigurable 5G mMIMO testbed; RF-exposure metrology; mMIMO transmission; massive MIMO system

Subjects: Mobile radio systems; Signal processing and detection; Other topics in statistics