  Massive multiple-input multiple-output (MIMO) systems hold the potential to
be an enabling technology for 5G cellular. Uniform planar array (UPA) antenna
structures are a focus of much commercial discussion because of their ability
to enable a large number of antennas in a relatively small area. With UPA
antenna structures, the base station can control the beam direction in both the
horizontal and vertical domains simultaneously. However, channel conditions may
dictate that one dimension requires higher channel state information (CSI)
accuracy than the other. We propose the use of an additional one bit of
feedback information sent from the user to the base station to indicate the
preferred domain on top of the feedback overhead of CSI quantization in
frequency division duplexing (FDD) massive MIMO systems. Combined with
variable-rate CSI quantization schemes, the numerical studies show that the
additional one bit of feedback can increase the quality of CSI significantly
for UPA antenna structures.
