  Massive co-located devices require new paradigms to allow proper network
connectivity. Internet of things (IoT) is the paradigm that offers a solution
for the inter-connectivity of devices, but in dense IoT networks time
synchronization is a critical aspect. Further, the scalability is another
crucial aspect. This paper focuses on synchronization for uncoordinated dense
networks without any external timing reference. Two synchronization methods are
proposed and compared: i) conventional synchronization that copes with the high
density of nodes by frame collision-avoidance methods (e.g., CSMA/CA) to avoid
the superimposition (or collision) of synchronization signals; and ii)
distributed synchronization that exploits the frames' collision to drive the
network to a global synchronization. The distributed synchronization algorithm
allows the network to reach a timing synchronization status based on a common
beacon with the same signature broadcasted by every device. The superimposition
of beacons from all the other devices enables the network synchronization,
rather than preventing it. Numerical analysis evaluates the synchronization
performance based on the convergence time and synchronization dispersion, both
on collision and non-collision scenario, by investigating the scalability of
the network. Results prove that in dense network the ensemble of signatures
provides remarkable improvements of synchronization performance compared to
conventional master-slave reference.
