  In wireless networks, interferences between trans- missions are modelled
either in time or frequency domain. In this article, we jointly analyze
interferences in the time- frequency domain using a stochastic geometry model
assuming the total time-frequency resources to be a two-dimensional plane and
transmissions from Internet of Things (IoT) devices time- frequency patterns on
this plane. To evaluate the interference, we quantify the overlap between the
information packets: provided that the overlap is not too strong, the packets
are not necessarily lost due to capture effect. This flexible model can be used
for multiple medium access scenarios and is especially adapted to the random
time-frequency access schemes used in Low-Power Wide-Area Networks (LPWANs). By
characterizing the outage probability and throughput, our approach permits to
evaluate the performance of two representative LPWA technologies
Sigfox{\textsuperscript \textregistered} and LoRaWA{\textsuperscript
\textregistered}.
