  We investigate the coexistence of massive and critical Internet of Things
(IoT) services in the context of the unsourced multiple access (UMA) framework
introduced by Polyanskiy (2017), where all users employ a common codebook and
the receiver returns an unordered list of decoded codewords. This setup is
suitably modified to introduce heterogeneous traffic. Specifically, to model
the massive IoT service, a standard message originates independently from each
IoT device as in the standard UMA setup. To model the critical IoT service, we
assume the generation of alarm messages that are common for all devices. This
setup requires a significant redefinition of the error events, i.e.,
misdetections and false positives. We further assume that the number of active
users in each transmission attempt is random and unknown. We derive a
random-coding achievability bound on the misdetection and false positive
probabilities of both standard and alarm messages on the Gaussian multiple
access channel. Using our bound, we demonstrate that orthogonal network slicing
enables massive and critical IoT to coexist under the requirement of high
energy efficiency. On the contrary, we show that nonorthogonal network slicing
is energy inefficient due to the residual interference from the alarm signal
when decoding the standard messages.
