# Spectral Coexistence of QoS-Constrained and IoT Traffic in Satellite Systems
## 6 Conclusions and Outlook  
In this paper, we investigated the potential of letting two services, a QoS-constrained (\(\mathcal{F}_{\text{a}}\)) and a mMTC (\(\mathcal{F}_{\text{b}}\)), share a common spectrum by overlaying their transmissions in an AWGN scenario modeling the uplink of a satellite communication system. The receiver attempts decoding of \(\mathcal{F}_{\text{a}}\) and, if successful, removes its contribution by means of IC, possibly allowing to retrieve data units of the mMTC traffic transmitted with a slotted ALOHA policy. Leveraging analytical tools, we have shown that an overlay allocation is beneficial in most situations, compared to a more traditional orthogonal allocation among the two services. Starting with an asymptotic scenario, where both the codewords and the number of time slots are very large, we delved into a non-ergodic setting (finite number of time slots) and the more practical finite-length regime (both time slots and codewords are finite). Achievable rate regions and expected gains for the IoT aggregate rate when the overlay allocation is adopted are presented for all scenarios. Furthermore, a rate tuple \((\text{R}_{\text{a}},\text{R}_{\text{b}})\) maximizing the improvement on the aggregate rate for the mMTC service is identified, showcasing a possible optimal operating point for the overlay system. The presented work aims at stimulating further research in the context of the spectral coexistence between mMTC and QoS-constrained traffic in satellite scenarios. The possibility to upgrade the medium access policy of the mMTC service to more advanced solutions and exploring the benefits of _modern_ random access schemes is an interesting direction. For example, the use of repetition-based solutions, e.g., [37], may unleash further benefits of the overlay allocation for low enough rates of the QoS-constrained service. In particular, when strong forward error correction is adopted on service \(\mathcal{S}_{\text{a}}\), the mMTC service is not able to fully exploit it since high channel load values are detrimental in slotted ALOHA (cf. Figures 2 and 4, for example). A repetition-based scheme instead, can reap the rewards of a stronger interference rejection in \(\mathcal{S}_{\text{a}}\) (lower rate) by heavily loading the physical layer with packet copies. Furthermore, we focused in this paper on perfect power control, i.e., all terminals of service \(\mathcal{F}_{\text{b}}\) are received with the same power. In practical scenarios, in turn, fading and topology trigger variability in the received power levels so that the _capture_ of packets is viable at the receiver. The impact of this aspect is also worth exploring, as it may trigger other relevant benefits and trade-offs, all the more so when coupled with IC, becoming especially relevant in the context of modern random access policies for IoT. The decoding order of services--first \(\mathcal{S}_{\text{a}}\), then IC and subsequently \(\mathcal{F}_{\text{b}}\)--can also be further investigated when power variability is present, along the lines of [28]. Finally, the appli  
Figure 5: Ratio of the maximum rate achievable by service \(\mathcal{S}_{\text{b}}\) with an overlay allocation (\(\text{R}_{\text{b}}\) overlay) over the one with an orthogonal allocation (\(\text{R}_{\text{b}}\) orth.) as a function of \(\text{R}_{\text{a}}\). Both the ergodic asymptotic setting and the finite length scenarios are presented. In the latter, we set the target average codeword error probability of service \(\mathcal{S}_{\text{a}}\) to \(10^{-3}\) for both SNR setups. \(\text{M}=25\) and \(\text{M}=100\) slots together with \(n_{s}=100\) and \(n_{s}=300\) channel uses are investigated for \(\rho_{\text{a}}/\sigma^{2}=\rho_{\text{b}}/\sigma^{2}=0\) dB, while only \(n_{s}=100\) channel uses for \(\rho_{\text{a}}/\sigma^{2}=10\) dB, \(\rho_{\text{b}}/\sigma^{2}=0\) dB are shown.