  IoT networks with grant-free radio access, like SigFox and LoRa, offer
low-cost durable communications over unlicensed band. These networks are
becoming more and more popular due to the ever-increasing need for ultra
durable, in terms of battery lifetime, IoT networks. Most studies evaluate the
system performance assuming single radio access technology deployment. In this
paper, we study the impact of coexisting competing radio access technologies on
the system performance. Considering $\mathpzc K$ technologies, defined by time
and frequency activity factors, bandwidth, and power, which share a set of
radio resources, we derive closed-form expressions for the successful
transmission probability, expected battery lifetime, and experienced delay as a
function of distance to the serving access point. Our analytical model, which
is validated by simulation results, provides a tool to evaluate the coexistence
scenarios and analyze how introduction of a new coexisting technology may
degrade the system performance in terms of success probability and battery
lifetime. We further investigate solutions in which this destructive effect
could be compensated, e.g., by densifying the network to a certain extent and
utilizing joint reception.
