  The pilot reuse issue in massive multi-input multi-output (MIMO) antenna
systems with interfering cells is closely examined. This paper considers
scenarios where the ratio of the channel coherence time to the number of users
in a cell may be sufficiently large. One such practical scenario arises when
the number of users per unit coverage area cannot grow freely while user
mobility is low, as in indoor networks. Another important scenario is when the
service provider is interested in maximizing the sum rate over a fixed,
selected number of users rather than the sum rate over all users in the cell. A
sum-rate comparison analysis shows that in such scenarios less aggressive reuse
of pilots involving allocation of additional pilots for interfering users
yields significant performance advantage relative to the case where all cells
reuse the same pilot set. For a given ratio of the normalized coherence time
interval to the number of users per cell, the optimal pilot assignment strategy
is revealed via a closed-form solution and the resulting net sum-rate is
compared with that of the full pilot reuse.
