  Over the last decade, the growing amount of UL and DL mobile data traffic has
been characterized by substantial asymmetry and time variations. Dynamic
time-division duplex (TDD) has the capability to accommodate to the traffic
asymmetry by adapting the UL/DL configuration to the current traffic demands.
In this work, we study a two-tier heterogeneous cellular network (HCN) where
the macro tier and small cell tier operate according to a dynamic TDD scheme on
orthogonal frequency bands. To offload the network infrastructure, mobile users
in proximity can engage in D2D communications, whose activity is determined by
a carrier sensing multiple access (CSMA) scheme to protect the ongoing
infrastructure-based and D2D transmissions. We present an analytical framework
to evaluate the network performance in terms of load-aware coverage probability
and network throughput. The proposed framework allows to quantify the effect on
the coverage probability of the most important TDD system parameters, such as
the UL/DL configuration, the base station density, and the bias factor. In
addition, we evaluate how the bandwidth partition and the D2D network access
scheme affect the total network throughput. Through the study of the tradeoff
between coverage probability and D2D user activity, we provide guidelines for
the optimal design of D2D network access.
