  Coexistence of Human-Type Communications (HTCs) and Machine-Type
Communications (MTCs) is inevitable. Ultra-Dense Networks (UDNs) will be
efficacious in supporting both types of communications. In a UDN, a massive
number of low-power and low-cost Small Cells (SCs) are deployed with density
higher than that of the HTC users. In such a scenario, the backhaul capacities
constitute an intrinsic bottleneck for the system. Hence, we propose a multiple
association scheme where each HTC user associates to and activates multiple SCs
to overcome the backhaul capacity constraints. In addition, having more active
cells allows for more MTC devices to be supported by the network. Using tools
from stochastic geometry, we formulate a novel mathematical framework
investigating the performance of the limited-backhaul capacity UDN in terms of
Area Spectral Efficiency (ASE) for both HTC and MTC and the density of
supported MTC devices. Extensive simulations were conducted to verify the
accuracy of the mathematical analysis under different system parameters.
Results show the existence of an optimum number of SCs to which an HTC user may
connect under backhaul capacity constraints. Besides, the proposed multiple
association scheme significantly improves the performance of MTC in terms of
both ASE and density of supported devices.
