## 5 Conclusions and Futures Extensions
### Concluding Remarks  
Future mobile systems over satellite may suffer a bottleneck in the feeder link when they have to provide connectivity to OTA applications on a global scale, such as software and firmware updates for autonomous driving. To tackle this issue, the use of optical wireless technologies has been seriously considered to implement the feeder link and satisfy the high data rate demand that will be required. In this regard, this paper presented closed-form formulas that estimate the impairments that are introduced in the optical uplink transmission in terms of the wind speed, wavelength of the optical signal, beam width and telescope aperture, refractive index structure of the atmosphere, and azimuth angle of transmission, among others. Furthermore, the use of time-packing has been also considered to increase the spectral efficiency of the optical feeder link even further. Three different relaying strategies have been analyzed for the GEO satellite, namely: (1) Detect-and-Forward with NB-IoT; (2) Decode-and-Forward with NB-IoT; and (3) Decode-and-Forward  
Figure 19: End-to-end BLER versus \(E_{\rm b}/N_{0}\) in the access link for a NB-IoT frame encapsulated in a DVB-S2(X) regenerative satellite (SNR\({}_{\rm fl}=15\,\)dB). NB-IoT code rates: \(1/3\) (red), \(2/3\) (blue) and \(0.96\) (magenta). DVB-S2(X) code rates: \(2/3\) (squares), \(0.8\) (circles) and \(0.9\) (diamonds). Solid (dashed-) lines with (un)filled markers: \({\rm BLER}\leq 10^{-2}\) requirement (not) fulfilled. Wind speed: \(v=21\,\)m/s. Diameter of telescope aperture: \(W_{0}=10\,\)cm. with NB-IoT/DVB-S2(X). From the simulation results that were obtained, it was possible to conclude that the wind speed has little effect on the end-to-end for mean SNR values larger than 20 dB in the optical feeder link. For this reason, the performance of the different relaying strategies has been studied in further detail for an optical feeder link at 15 dB (i.e., up to 10 dB atmospheric loss in the optical link budget). For fair comparisons among all the cases under study, a similar code rate has been included. For Detect-and-Forward and Decode-and-Forward relaying architectures with NB-IoT, these code rates correspond to the ones defined in the NB-IoT standard. In contrast, for Decode-and-Forward with NB-IoT/DVB-S2(X), the equivalent code rate is equal to the product between the code rates of the DVB-S2(X) and NB-IoT frames, which can be obtained by using two MCS with high code rates. After evaluating these relaying configurations, it was possible to conclude that the Detect-and-Forward with NB-IoT achieved its larger throughput when using the lowest NB-IoT code rate and the largest overlapping factor. In contrast, the relaying architecture based on Decode-and-Forward with NB-IoT obtained its largest throughput when using intermediate code rates and intermediate overlapping factors. Finally, for the Decode-and-Forward with NB-IoT/DVB-S2(X) relaying architecture, the largest throughput was obtained when using an intermediate code rate for DVB-S2(X), with a variable code rate for NB-IoT that was adjusted according to the \(E_{\text{b}}/N_{0}\) in the radio access links. In this latter case, the larger the \(E_{\text{b}}/N_{0}\), the higher the code rate that NB-IoT should be used. Regarding the most convenient overlapping factor, the highest throughput was observed when using the largest overlapping factors. All the proposed relaying architectures were evaluated assuming time-packed signalling in the optical feeder link. The ISI introduced by time-packing was mitigated using an adaptive MMSE equalizer. From simulations, it was concluded that the use of time-packing increases the throughput in all proposed regenerative strategies under study, when compared to the cases in which time-packing was not used. However, this gain was depended on the \(E_{\text{b}}/N_{0}\) of the access link. For Detect-and-Forward with NB-IoT, this gain varied from 10% to 50-65% for low and medium \(E_{\text{b}}/N_{0}\) in the access link, respectively.