  Many new narrowband low-power wide-area networks (LPWANs) (e.g., LoRaWAN,
Sigfox) have opted to use pure ALOHA-like access for its reduced control
overhead and asynchronous transmissions. Although asynchronous access reduces
the energy consumption of IoT devices, the network performance suffers from
high intra-network interference in dense deployments. Contrarily, synchronous
access can improve throughput and fairness, but it requires time
synchronization. Unfortunately, maintaining synchronization over the narrowband
LPWANs wastes channel time and transmission opportunities. In this paper, we
propose the use of out-of-band time-dissemination to relatively synchronize
LoRa devices and thereby facilitate resource-efficient slotted uplink
communication. To this end, we conceptualize and analyze a co-designed
synchronization and random access mechanism that can effectively exploit
technologies providing limited time accuracy, such as FM radio data system
(FM-RDS). While considering the LoRa-specific parameters, we derive the
throughput of the proposed mechanism, compare it to a generic synchronous
random access using in-band synchronization, and design the communication
parameters under time uncertainty. We scrutinize the transmission time
uncertainty of a device by introducing a clock error model that accounts for
the errors in the synchronization source, local clock, propagation delay, and
transceiver's transmission time uncertainty. We characterize the time
uncertainty of FM-RDS with hardware measurements and perform simulations to
evaluate the proposed solution. The results, presented in terms of success
probability, throughput, and fairness for a single-cell scenario, suggest that
FM-RDS, despite its poor absolute synchronization, can be used effectively to
realize slotted LoRa communication with performance similar to that of more
accurate time-dissemination technologies.
