To increase edge rates by as much as 500%, a simple,
apparently highly suboptimal association method centered
on aggressive but static biasing towards small cells and
blanking about half of the macrocell transmissions has been
shown in [71]. The combined problem of user association and
resource allocation in two tier heterogeneous networks, with
adaptive tuning of the biasing and blanking in each cell, is
considered in [69], [70], and [72]–[77]. A model of hotspot
traffic shows that the optimal cell association is done by rate
ratio bias, instead of power level bias [73]–[75]. An active
model of cell range extension as shown in [79], the traffic
arrives as a Poisson process in time and at the possible arrival
rates, for which a steadying scheduling policy subsists. With
massive MIMO at the base stations, user association and
load balancing in a heterogeneous networks, is considered
in [79]. An exciting game theoretic approach is used in [80]
for the problem of radio access technology selection, in which
union to Nash equilibria and the Pareto-efficiency of these
equilibria are deliberated