  3-dimensional Multiple-Input Multiple-Output (3D MIMO) systems have received
great interest recently because of the spatial diversity advantage and
capability for full-dimensional beamforming, making them promising candidates
for practical realization of massive MIMO. In this paper, we present a low-cost
test equipment (channel sounder) and post-processing algorithms suitable for
investigating 3D MIMO channels, as well as the results from a measurement
campaign for obtaining elevation and azimuth characteristics in an
outdoor-to-indoor (O2I) environment. Due to limitations in available antenna
switches, our channel sounder consists of a hybrid switched/virtual cylindrical
array with effectively 480 antenna elements at the base station (BS). The
virtual setup increased the overall MIMO measurement duration, thereby
introducing phase drift errors in the measurements. Using a reference antenna
measurements, we estimate and correct for the phase errors during
post-processing. We provide the elevation and azimuth angular spreads, for the
measurements done in an urban macro-cellular (UMa) and urban micro-cellular
(UMi) environments, and study their dependence on the UE height.
  Based on the measurements done with UE placed on different floors, we study
the feasibility of separating users in the elevation domain. The measured
channel impulse responses are also used to study the channel hardening aspects
of Massive MIMO and the optimality of Maximum Ratio Combining (MRC) receiver.
