  In this paper, by means of simulations, we evaluate the uplink (UL)
performance of an Internet of Things (IoT) capable ultra-dense network (UDN) in
terms of the coverage probability and the density of reliably working user
equipments (UEs). From our study, we show the benefits and challenges that UL
IoT UDNs will bring about in the future. In more detail, for a low-reliability
criterion, such as achieving a UL signal-to-interference-plus-noise ratio
(SINR) above 0 dB, the density of reliably working UEs grows quickly with the
network densification, showing the potential of UL IoT UDNs. In contrast, for a
high-reliability criterion, such as achieving a UL SINR above 10 dB, the
density of reliably working UEs remains to be low in UDNs due to excessive
inter-cell interference, which should be considered when operating UL IoT UDNs.
Moreover, considering the existence of a non-zero antenna height difference
between base stations (BSs) and UEs, the density of reliably working UEs could
even decrease as we deploy more BSs. This calls for the usage of sophisticated
interference management schemes and/or beam steering/shaping technologies in UL
IoT UDNs.
