A striking example of self-organization in nature occurs every evening in some parts of South-East Asia: thousands of fireflies gather on trees at dawn and start emitting flashes regularly; over time, synchronization emerges from a seemingly chaotic situation, which makes it seem as though the whole tree is flashing in perfect synchrony. This fascinating phenomenon is the inspiration for the topic treated in this thesis, which is concerned with slot synchronization in wireless networks.
Synchronization phenomena in nature are mathematically described by the theory of pulse-coupled oscillators (PCOs); each entity naturally oscillates and blinks periodically, and coupling is performed through the discrete emissions of light. Each node adjusts its internal reference when perceiving blinks from its neighbors, and following simple rules, synchronization always emerges after some time. Conditions for convergence under ideal assumptions were derived by Mirollo and Strogatz in their seminal work published in 1990, and provide a framework for the following slot synchronization studies.
The PCO synchronization rules are remarkably simple and robust, which makes their application to wireless networks very appealing. In particular, slot synchronization requires nodes in the network to agree on a common time reference for the start of a slot, in a similar way to fireflies that agree on a common blinking instant. Direct application of the PCO rules is not feasible, and an adaptation, termed Mobile Emergent Firefly Synchronization (MEMFIS), is proposed so that constraints of wireless networks are integrated with the PCO rules. With this modification, the simplicity and robustness of the PCO scheme is retained; nodes are able to synchronize starting from any random misalignment, and achieve an accuracy equal or lower to the direct propagation delay.
Application of the PCO model to cellular systems is investigated. The goal is to maintain base stations synchronized, even when there is no direct communication between them. Synchronization in Cellular Firefly Synchronization (CelFSync) is performed by letting some selected user terminals participate in the network synchronization process, and achieving an out-of-phase synchronization regime. Furthermore propagation delays, which are problematic in large-scale networks, are mitigated by combining the proposed adaptation with the timing advance procedure, so that an acceptable inter-base station accuracy is achieved.