  This letter considers a two-panel massive multiple-input multiple-output
(MIMO) communication system, where the base station (BS) is equipped with two
antenna panels that may use different frequency bands for communication. By
exploiting the geometric relationships between antenna panels, efficient
channel inference methods across antenna panels are proposed to reduce the
overhead of real-time channel estimation. Four scenarios are considered, namely
far-field free-space, near-field free-space, multi-path sharing far-field
scatterers, and multi-path sharing near-field scatterers. For both far-field
and near-field free-space scenarios, we show that the channel of one panel can
be fully inferred from that of the other panel, as long as the multi-path
components (MPCs) composing the channel can be resolved. On the other hand, for
the multi-path scenarios sharing far-field or near-field scatterers, only the
angles or range of angles of the MPCs can be inferred, respectively. Simulation
results based on commercial 3D ray-tracing software are presented to validate
our developed channel inference techniques.
