Satellite Communication systems represent a solid solution to extend and complement terrestrial networks in un-/under-served areas. The recent standardisation endeavours within 3GPP led to the Non-Terrestrial Networks (NTN) study item for New Radio, aimed at deploying satellite systems either stand-alone or as integration to terrestrial networks in mobile broadband and machine-type communication scenarios. In this context, the main focus is currently related to: i) evaluating the system level performance; and ii) understanding the impact at link level of typical satellite channel impairments on the PHY/MAC procedures. In this paper, we focus on the former; in particular, we provide: i) a detailed discussion on the current system architecture and assumptions; ii) we thoroughly describe the methodology to perform link budget computations; and iii) we show the numerical results for GEO and LEO systems.

This paper studies the End-to-End (E2E) performance of a High Throughput Satellite (HTS) system that utilizes a transparent (non-regenerative) optical feeder link to transport the DVB-S2 signals associated with the large number of spot-beams in the radio access link. Two main sources of non-linearity are considered to characterize the effect that the satellite forward link has in the DVB-S2 signal, namely the optical Mach-Zehnder Modulator (MZM) in the gateway and the High Power Amplifier (HPA) in the satellite. Digital Pre-Distortion (DPD) and linear equalization are also implemented to mitigate the impact of the non-linear distortion and the inter-symbol interference on the Packet Error Rate (PER) of the received DVB-S2 signal. The performance analysis is carried out using the Total Degradation (TD) as performance indicator. As expected, the Input Back-Off (IBO) of the HPA and the modulation index of the MZM should be jointly optimized according to the DVB-S2 MODCOD that is selected for transmission. Therefore, the stronger is the Forward Error Correction (FEC) code that is used, the lower is the IBO and the higher is the modulation index that can be utilized to minimize the impact of non-linearities in the E2E performance.

In this paper, we review the key figures of merit to assess the performance of advanced random access (RA) schemes exploiting physical layer coding, repetitions, and collision resolution techniques. We then investigate RA modeling aspects and their impact on the figures of merit for the exemplary advanced RA schemes: contention resolution diversity slotted ALOHA (CRDSA), irregular repetition slotted ALOHA (IRSA), coded slotted ALOHA (CSA) and enhanced spread-spectrum ALOHA (E-SSA). We show that the typical simplifications of the reception model when used to optimize RA schemes lead to inaccurate findings, both in terms of parameter optimization and figures of merit, such as the packet loss ratio (PLR) and throughput. We also derive a generic RA energy efficiency model able to compare the schemes in terms of the energy required to transmit a packet. The combination of achievable RA throughput at the target PLR and energy efficiency, for the same average user power investment per frame and occupied bandwidth, shows that E-SSA, which is an unslotted scheme, provides the best overall performance, while, in terms of the slotted schemes, CRDSA outperforms the more elaborated IRSA and CSA. This surprising results are due to the fact that the IRSA and CSA optimization has so far been performed using RA channel models that are not accurately reflecting the physical layer receiver behavior. We conclude by providing insights on how to include more accurate reception models in the IRSA and CSA design and optimization.

This paper investigates the implications of employing multiple transmissions as a diversity mechanism to counter the turbulence induced fading in satellite systems with optical feeder link and RF user links. Such a technique necessitates optimal estimation of the transmitted signal using the different streams on-board the satellite. Unlike traditional combining techniques like Maximal Ratio Combining (MRC) which assume signal independent noise, the aforementioned estimation is made interesting by the signal dependent noise characteristic of the optical processing in such systems. This paper investigates various estimators in the presence of signal dependent noise and assesses them based on performance and complexity. Towards this, the paper devises a signal model to illustrate the effect of various signal dependent and independent noise sources and derives estimators based on (a) averaging, (b) Minimum Mean Square Error (MMSE), and (c) Maximum Likelihood (ML). The bias and the Mean Squared Error (MSE) of these estimators are analyzed and the impact of signal dependent noise characterized. Based on these results and complexity considerations, the paper presents recommendations for the receiver to be used in future generation of satellite systems with optical feeder links.

There is an urgent need for substantially enhancing random access schemes to be able to support native massive machine-to-machine communications in the next generation wireless systems. Lately, contention resolution diversity slotted ALOHA (CRDSA) and its variants have contributed to a drastic improvement in throughput compared with the traditional slotted ALOHA protocols. In this paper, we further enhance the CRDSA flexibility and performance along different physical layer dimensions. Based on simulation results, we show that there is still room for improvement in terms of throughput and, using heuristic methods, we find an optimized distribution of the packets power that is entering the demodulator for an additive white Gaussian noise channel. Applying this power distribution, which is easy to implement at the terminal in open loop and in a decentralized manner, is shown to achieve significantly enhanced performance. By introducing multi-frequency CRDSA, a drastic reduction in the terminal's peak power requirement is attained. Furthermore, we investigate spread spectrum CRDSA, which improves the robustness of the scheme against irresolvable packet collisions allowing for enhanced energy efficiency. To support the work presented herein, we have developed a computationally efficient yet accurate methodology for estimating CRDSA performance, a novel approximate centrally coordinated CRDSA achievable sum rate analysis accounting for modulation and forward error correction constraints, as well as a more accurate loop probability analysis.

Precoding for multibeam satellites with aggressive frequency reuse has attracted interest of late towards enhancing system capacity. Most of the works on precoding mitigate the linear co-channel interference between the beams caused by frequency reuse. However, the high power amplifier (HPA), an integral part of the satellite payload, is inherently non linear. Non-linear amplification combined with the linear co-channel interference introduces non-linear co-channel distortions at the receiver. Further, signals with very high peak to average power ratios (PAPR), typical of spectrally efficient modulations, are sensitive to the non-linear characteristic of the HPA and necessitate large back-off to have manageable distortion levels. In this work, a novel architecture comprising multistream Crest Factor Reduction (signal pre-clipping) and Signal Predistortion (SPD) in cascade, is devised to counter the non-linearities and co-channel interference in multibeam satellite systems. An iterative algorithm to optimize the parameters of the signal clipping and predistortion is devised taking recourse to analytical derivations. The proposed joint estimation paradigm is shown to compare favorably with state-of-art and provides a framework to combine predistortion and precoding.