  This paper proposes a long range-frequency hopping spread spectrum (LR-FHSS)
transceiver design for the Direct-to-Satellite Internet of Things (DtS-IoT)
communication system. The DtS-IoT system has recently attracted attention as a
promising nonterrestrial network (NTN) solution to provide high-traffic and
low-latency data transfer services to IoT devices in global coverage. In
particular, this study provides guidelines for the overall DtS-IoT system
architecture and design details that conform to the Long Range Wide-Area
Network (LoRaWAN). Furthermore, we also detail various DtS-IoT use cases.
Considering the multiple low-Earth orbit (LEO) satellites, we developed the
LR-FHSS transceiver to improve system efficiency, which is the first attempt in
real satellite communication systems using LR-FHSS. Moreover, as an extension
of our previous work with perfect synchronization, we applied a robust
synchronization scheme against the Doppler effect and co-channel interference
(CCI) caused by LEO satellite channel environments, including signal detection
for the simultaneous reception of numerous frequency hopping signals and an
enhanced soft-output-Viterbi-algorithm (SOVA) for the header and payload
receptions. Lastly, we present proof-of-concept implementation and testbeds
using an application-specific integrated circuit (ASIC) chipset and a
field-programmable gate array (FPGA) that verify the performance of the
proposed LR-FHSS transceiver design of DtS-IoT communication systems. The
laboratory test results reveal that the proposed LR-FHSS-based framework with
the robust synchronization technique can provide wide coverage, seamless
connectivity, and high throughput communication links for the realization of
future sixth-generation (6G) networks.
