  We propose a novel scheme for estimating the large-scale gains of the
channels between user terminals (UTs) and base stations (BSs) in a cellular
system. The scheme leverages TDD operation, uplink (UL) training by means of
properly designed non-orthogonal pilot codes, and massive antenna arrays at the
BSs. Subject to Q resource elements allocated for UL training and using the new
scheme, a BS is able to estimate the large-scale channel gains of K users
transmitting UL pilots in its cell and in nearby cells, provided K<=Q^2. Such
knowledge of the large-scale channel gains of nearby out-of-cells users can be
exploited at the BS to mitigate interference to the out-of-cell users that
experience the highest levels of interference from the BS. We investigate the
large-scale gain estimation performance provided by a variety of non-orthogonal
pilot codebook designs. Our simulations suggest that among all the code designs
considered, Grassmannian line-packing type codes yield the best large-scale
channel gain estimation performance.
