  Deploying Sub-6GHz networks together with millimeter wave (mmWave) is a
promising solution to achieve high data rates in traffic hotspots while
guaranteeing sufficient coverage, where mmWave small cells are densely deployed
to provide high quality of service. In this paper, we propose an analytical
framework to investigate the integrated Sub-6GHz-mmWave cellular networks, in
which the Sub-6GHz base stations (BSs) are modeled as a Poisson point process,
and the mmWave BSs are clustered following a Poisson cluster process in traffic
hotspots. We conduct stochastic geometry-based analysis and derive the
performance metrics including the association probability,
signal-to-interference-plus-noise ratio coverage probability and average
achievable rate, which are validated to be accurate by Monte Carlo simulations.
We analyze the impact of various deployment parameters on the network
performance to give insights on the network design. In particular, it is shown
that deploying mmWave small cells in traffic hotspots will outperform both
traditional Sub-6GHz heterogeneous network and isolated mmWave system in terms
of the coverage probability. It can also be shown that extremely high and
extremely small association weight for mmWave BSs will deteriorate the
performance for cell edge users and cell interior users, respectively.
Moreover, there exists an optimal pre-decided dispersion parameter of mmWave
BSs that contributes to the maximum coverage probability.
