  28 GHz outdoor-indoor coverage into modern office buildings with high thermal
efficiency windows is found to be severely limited due to 46 dB median
penetration loss at normal incidence and additional 15 dB median oblique
incidence loss. The study is based on measurements of path gain over 280
outdoor-indoor links, at ranges up to 100 m. A simple theoretical path gain
model is extended to include building penetration through multiple sides of the
building as well as a reflection from another building. The theoretical model
accounts for the building orientation relative to the source, resulting in 4.9
dB RMSE relative to data, as compared to 5.7 dB RMSE from a linear fit and 14.7
dB RMSE for the 3GPP recommended model. Only coarse description of the
buildings is required: building orientation and exterior wall composition,
without any interior details. Coverage range for SNR>-8 dB from an outdoor base
to a terminal just inside a high-efficiency building is under 35 m
