  Although the idea of using wireless links for covering large areas is not
new, the advent of LPWANs has recently started changing the game. Simple,
robust, narrowband modulation schemes permit the implementation of low-cost
radio devices offering high receiver sensitivity, thus improving the overall
link budget. The several technologies belonging to the LPWAN family, including
the well-known LoRaWAN solution, provide a cost-effective answer to many
Internet-of-things (IoT) applications, requiring wireless communication capable
of supporting large networks of many devices (e.g., smart metering). Generally,
the adopted MAC strategy is based on pure ALOHA, which, among other things,
allows to minimize the traffic overhead under constrained duty cycle
limitations of the unlicensed bands. Unfortunately, ALOHA suffers from poor
scalability, rapidly collapsing in dense networks. This work investigates the
design of an improved LoRaWAN MAC scheme based on slotted ALOHA. In particular,
the required time dissemination is provided by out-of-band communications
leveraging on FM-RDS broadcasting. An experimental setup based on low-cost
hardware is used to characterize the obtainable synchronization performance and
derive a timing error model. Consequently, improvements in success probability
and energy efficiency have been validated by means of simulations in very large
networks with up to 10000 nodes. It is shown that the advantage of the proposed
scheme over conventional LoRaWAN communication is up to 100% when short update
time and large payload are required. Similar results are obtained regarding the
energy efficiency improvement, which is close to 100% for relatively short
transmission intervals and long message duration; however, due to the
additional overhead for listening to the time dissemination messages,
efficiency gain can be negative for short-duration messages fastly repeating.
