  Massive multiple-input multiple-output (MIMO) is a fundamental enabler to
provide high data throughput in next generation cellular networks. By equipping
the base stations (BSs) with tens or hundreds of antenna elements, narrow and
high gain beams can be used to spatially multiplex several user equipment (UE)
devices. While increasing the achievable performance, focusing the transmit
power into specific UE directions also poses new issues when performing the
radio frequency (RF) exposure assessment. In fact, the spatial distribution of
the actual BS transmit power strongly depends on the deployment scenario and on
the position of the UEs. Traditional methods for assessing the RF exposure
compliance boundaries around BS sites are generally based on maximum transmit
power and static beams. In massive MIMO systems, these approaches tend to be
very conservative, in particular when time averaging is properly considered. In
this work, we propose to leverage the three dimensional spatial channel model
standardized by the Third Generation Partnership Project in order to assess
reasonably foreseeable compliance boundaries of massive MIMO BSs. The analysis
is performed by considering BSs fully loaded and different configurations of
active UEs per cell. Numerical results show that the statistical approach
developed in this paper allows reducing to nearly half the compliance distance
when compared to the traditional method.
