  Both the use of very large arrays of antennas and flexible time division
duplexing (TDD) designs have become prominent features of next generation 5G
cellular systems. However, both enabling technologies suffer from severe
interference effects, respectively known as pilot contamination and
base-station-to-base-station (B2B) interference. In this paper, a practical
novel TDD design principle is proposed for massive multiple-input
multiple-output (MIMO) heterogeneous networks (HetNets) that leverages the
inherent features of a flexible TDD design to mitigate both the beamformed
interference caused by the pilot contamination effect and B2B interference. The
design is based on the key observation that the transmission path chosen for
training by the non-massive MIMO base stations plays an important role in the
interference behavior of the network, and the data slots need to be configured
accordingly. We propose TDFLEX, a low-complexity heuristic solution that
follows these design guidelines. Performance evaluation results show
significant gains when our design is compared to the standard TD-LTE.
