LTE-based satellite systems in LEO constellations are a promising solution for extending broadband coverage to areas not connected to a terrestrial infrastructure. However, the large delays and Doppler shifts over the satellite channel pose severe technical challenges to a traditional LTE system. In this paper, two architectures are proposed for a LEO megaconstellation realizing a satellite-enabled LTE system, in which the on-ground LTE entity is either an eNB (Sat-eNB) or a Relay Node (Sat-RN). Focusing on the latter, the impact of large delays and Doppler shifts on LTE PHY/MAC procedures is discussed and assessed. It will be shown that, while carrier spacings, Random Access, and RN attach procedures do not pose specific issues, HARQ requires substantial modifications. Moreover, advanced handover procedures will be also required due to the satellites' movement.

Summary The integration of satellite and terrestrial networks is a promising solution for extending broadband coverage to areas not connected to a terrestrial infrastructure, as also demonstrated by recent commercial and standardisation endeavours. However, the large delays and Doppler shifts over the satellite channel pose severe technical challenges to traditional terrestrial systems, as long‐term evolution (LTE) or 5G. In this paper, 2 architectures are proposed for a low Earth orbit mega‐constellation realising a satellite‐enabled LTE system, in which the on‐ground LTE entity is either an eNB (Sat‐eNB) or a relay node (Sat‐RN). The impact of satellite channel impairments as large delays and Doppler shifts on LTE PHY/MAC procedures is discussed and assessed. The proposed analysis shows that, while carrier spacings, random access and RN attach procedures do not pose specific issues and hybrid automatic repeat request requires substantial modifications. Moreover, advanced handover procedures will be also required due to the satellites' movement.

Next generation satellite payload technology is expected to provide digital processing capabilities. This will pave the way to regard satellites as flying base stations. However, the initial access procedure must be improved. In this work we focus on non-geostationary earth orbit (NGEO) satellite networks. In this scenario, the random access preamble signal and the detection must be robust to large carrier frequency offsets (CFOs). Towards this end, we investigate the adoption of the pruned discrete Fourier transform spread filter bank multicarrier waveform. The proposed design is suitable for the access scheme of forthcoming 5G-based NGEO satellite communications. The reason is twofold. First, it improves the spectral confinement with respect to the standard single-carrier frequency-division multiplexing (SC-FDM) waveform. Second, it achieves a high level of commonality with 5G new radio, by keeping unchanged the subcarrier spacing, the slot duration and the preamble sequence. Remarkably, the new design allows the straightforward application of non-coherent post detection integration (NCPDI) techniques, which divide the correlation in blocks. Numerical results show that the proposed solution reduces out-of-band emissions and the missed detection probability in presence of CFO, with respect to the conventional approach based on SC-FDM and preamble detection with full-length correlation.

This paper carries out a theoretical study of the data rate that a High Throughput Satellite (HTS) system with fullyregenerative payload may achieve when using an intensity modulation/direct detection optical feeder link. A low-order M -ary Pulse Amplitude Modulation (M-PAM) with time-packing is used to modulate the intensity of the laser diode beam, making use of an external Mach-Zehnder modulator. These M -PAM symbols are recovered in the satellite with the aid of a photodetector, and are then encapsulated into the 5G radio frame of the access link. The modulation order and the overlapping factor of the uncoded transmission are jointly selected to tackle the impact that slowly-varying weather conditions introduce. Moreover, the intersymbol interference that time-packing introduces is mitigated in reception using a Viterbi equalizer. As expected, time-packing enables a finer link adaptation granularity than the one that adaptive modulation can provide on an optical feeder link without coding, enabling to adjust the spectral efficiency according to slowly-varying attenuation that thin cloud layers introduce.

This paper focuses on the return link of a GEO satellite system that collects information from a large number of IoT devices that are sparsely distributed in a large geographical area. Narrow-Band (NB) IoT transmissions, with suitable Modulation and Coding Scheme (MCS), are Detected-and-Forwarded onboard the satellite, mapping each QAM symbol of the radio access link (uplink) into another equivalent PAM symbol that is suitable to modulate the intensity of the optical feeder link (downlink). Given the very large number of IoT devices that is expected to be served by the GEO satellite system, the feeder link (downlink) of the return channel is expected to be bottleneck. To tackle this limitation, apart from using an optical feeder link, time-packing is proposed to reduce the transmission time of the feeder link (downlink) without increasing the signal bandwidth and augment the number of IoT devices that could be simultaneously served in the radio access links (uplink). The Inter-Symbol Interference (ISI) that the time-packed feeder link generates is mitigated in part at the satellite gateway, using for this purpose an adaptive linear equalizer. After optical-to-electrical conversion, the NB-IoT codewords that are received in the gateway are decoded, correcting simultaneously errors introduced in both radio access and optical feeder links. The aim of this paper is to evaluate the error correction capability that the MCS of NB-IoT has when used to protect the hybrid radio/optical end-to-end return link, particularly when using a large overlapping factor to increase the optical feeder link data rate.

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The security in satellite communications is a key issue due to the large footprint of the beams. This it is specially critical in IoT devices that transmit data directly to satellite. Take into account that IoT devices are characterized by transmitting packets of short length. Consequently, it means that it is not feasible to augment the security level of the IoT packets via complex cryptographic algorithms. Otherwise, their packed length may be increased in a non-negligible way which could augment their collision probability, latency and energy consumption. For this reason, this paper proposes to take advantage of the time-packing technique. By doing so, it is possible to use the overlapping degree among the pulse-shapes to boost the secrecy-capacity. In particular, the overlapping degree between the pulse-shapes introduces an artificial interference that degrades the eavesdropper's channel. In this regard, it is necessary to highlight that there is a residual co-channel interference in the satellite beams. So, it means that these two sources of impairments make difficult to estimate the legitimate user's transmission parameters by the eavesdropper.