  The growing complexity of heterogeneous cellular networks (HetNets) has
necessitated the need to consider variety of user and base station (BS)
configurations for realistic performance evaluation and system design. This is
directly reflected in the HetNet simulation models considered by
standardization bodies, such as the third generation partnership project
(3GPP). Complementary to these simulation models, stochastic geometry based
approach modeling the user and BS locations as independent and homogeneous
Poisson point processes (PPPs) has gained prominence in the past few years.
Despite its success in revealing useful insights, this PPP-based model is not
rich enough to capture all the spatial configurations that appear in real world
HetNet deployments (on which 3GPP simulation models are based). In this paper,
we bridge the gap between the 3GPP simulation models and the popular PPP-based
analytical model by developing a new unified HetNet model in which a fraction
of users and some BS tiers are modeled as Poisson cluster processes (PCPs).
This model captures both non-uniformity and coupling in the BS and user
locations. For this setup, we derive exact expression for downlink coverage
probability under maximum signal-to-interference ratio (SIR) cell association
model. As intermediate results, we define and evaluate sum-product functionals
for PPP and PCP. Special instances of the proposed model are shown to closely
resemble different configurations considered in 3GPP HetNet models. Our results
concretely demonstrate that the performance trends are highly sensitive to the
assumptions made on the user and SBS configurations.
