  This paper devises a general modeling and analyzing framework for a
heterogeneous wireless network (HetNet) in which several wireless subnetworks
coexist and use multiple radio access technologies (multi-RATs). The coexisting
coverage and network capacity in such a multi-RAT HetNet are hardly
investigated in prior works. To characterize the coexisting interactions in a
multi-RAT HetNet, in this paper we consider a HetNet consisting of K-tier APs
and two different RATs, RAT-L and RAT-U, are adopted in the HetNet. RAT-L is
adopted by the access points (APs) in the first K-1 tiers and APs in the Kth
tier only use RAT-U. Both noncrossing-RAT and crossing-RAT user association
scenarios are considered. In each scenario, the void probability and channel
access probability of the APs in each tier are first found and then the tight
lower bounds and their lowest limits on the proposed coexisting coverage and
network capacity are derived. We show that multi-RAT networks in general can
achieve higher link coverage and capacity by using opportunistic CSMA/CA that
avoids/alleviates severe interfering between all coexisting APs. Also,
crossing-RAT user association is shown to achieve much higher coexisting
coverage and network capacity than noncrossing-RAT user association. Finally,
numerical simulations for the LTE-U and WiFi networks coexisting in the HetNet
validate our findings.
