  Internet of Things applications have gained widespread recognition for their
efficacy in typical scenarios, such as smart cities and smart healthcare.
Nonetheless, there exist numerous unconventional situations where IoT
technologies have not yet been massively applied, though they can be extremely
useful. One of such domains is the underground mining sector, where enhancing
automation monitoring through wireless communications is of essential
significance. In this paper, we focus on the development, implementation, and
evaluation of a LoRa-based multi-hop network tailored specifically for
monitoring underground mining environments, where data traffic is sporadic, but
energy efficiency is of paramount importance. We hence define a synchronization
framework that makes it possible for the nodes to sleep for most of the time,
waking up only when they need to exchange traffic. Notably, our network
achieves a sub 40us proven synchronization accuracy between parent-child pairs
with minimum overhead for diverse topologies, rendering it highly viable for
subterranean operations. Furthermore, for proper network dimensioning, we model
the interplay between network's throughput, frame size, and sampling periods of
potential applications. Moreover, we propose a model to estimate devices' duty
cycle based on their position within the multi-hop network, along with
empirical observations for its validation. The proposed models make it possible
to optimize the network's performance to meet the specific demands that can
arise from the different subterranean use cases, in which robustness, low power
operation, and compliance with radio-frequency regulations are key requirements
that must be met.
