  In this work, we study underlay radar-massive MIMO cellular coexistence in
LoS/near-LoS channels, where both systems have 3D beamforming capabilities.
Using mathematical tools from stochastic geometry, we derive an upper bound on
the average interference power at the radar due to the 3D massive MIMO cellular
downlink under the worst-case `cell-edge beamforming' conditions. To overcome
the technical challenges imposed by asymmetric and arbitrarily large cells, we
devise a novel construction in which each Poisson Voronoi (PV) cell is bounded
by its circumcircle to bound the effect of the random cell shapes on average
interference. Since this model is intractable for further analysis due to the
correlation between adjacent PV cells' shapes and sizes, we propose a tractable
nominal interference model, where we model each PV cell as a circular disk with
an area equal to the average area of the typical cell. We quantify the gap in
the average interference power between these two models and show that the upper
bound is tight for realistic deployment parameters. We also compare them with a
more practical but intractable MU-MIMO scheduling model to show that our
worst-case interference models show the same trends and do not deviate
significantly from realistic scheduler models. Under the nominal interference
model, we characterize the interference distribution using the dominant
interferer approximation by deriving the equi-interference contour expression
when the typical receiver uses 3D beamforming. Finally, we use tractable
expressions for the interference distribution to characterize radar's spatial
probability of false alarm/detection in a quasi-static target tracking
scenario. Our results reveal useful trends in the average interference as a
function of the deployment parameters (BS density, exclusion zone radius,
antenna height, transmit power of each BS, etc.).
