  The evolution of mobile cellular networks has brought great changes of
network architecture. For example, heterogeneous cellular network (HetNet) and
Ultra dense network (UDN) have been proposed as promising techniques for 5G
systems. Dense deployment of base stations (BSs) allows a mobile user to be
able to access multiple BSs. Meanwhile the unbalance between UL and DL in
HetNets, such as different received SINR threshold and traffic load, etc.,
becomes increasingly obvious. All these factors naturally inspire us to
consider decoupling of uplink and downlink in radio access network. An
interesting question is that whether the decoupled uplink (UL) /downlink (DL)
access (DUDA) mode outperforms traditional coupled uplink (UL)/downlink (DL)
access (CUDA) mode or not, and how big is the performance difference in terms
of system rate, spectrum efficiency (SE) and energy efficiency (EE), etc. in
HetNets. In this paper, we aim at thoroughly comparing the performance of the
two modes based on stochastic geometry theory. In our analytical model, we take
into account dynamic transmit power control in UL communication. Specifically,
we employ fractional power control (FPC) to model a location-dependent channel
state. Numerical results reveals that DUDA mode significantly outperforms CUDA
mode in system rate, SE and EE in HetNets. In addition, DUDA mode improves load
balance and potential fairness for both different type BSs and associated UEs.
