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Maximizing the Link Throughput of Spectrum Sharing IoT-based Systems through Retransmissions

This paper analyzes the throughput of an IoT-based system in a spectrum sharing scenario where messages decoded in outage may be retransmitted. We assume the IoT devices are the secondary users that communicate using the primary uplink channel. In this case, the primary users that interfere in the secondary transmissions are mobile devices whose spatial distribution are assumed to follow a Poisson point process with respect to a reference secondary link. We investigate how the number of allowed retransmissions and the spectrum efficiency jointly affect the throughput in [bits/s/Hz] of a reference secondary link for different primary network densities, constrained by a given required error rate. The optimal throughput is derived for this case as a function of the network density. We also prove that the optimal constrained throughput can always reach the unconstrained optimal value. Our numerical results corroborate those analytical findings, also illustrating how the number of allowed retransmissions that leads to the optimal throughput changes with the error rate requirements.