Geostationary (GEO) Data Relay Systems (DRS) are expected to play an increasing role in space in view of the proliferation of non geostationary satellites (LEOs or MEOs) as well as unmanned-aircraft vehicle system (UAVS), remotely piloted aerial vehicle (RPAV), remotely piloted aircraft system (RPAS) expected in coming years. Existing GEO DRS constellations include NASA’s Tracking and data relay satellite (TDRS) and ESA’s European Data Relay System (EDRS). Nevertheless, there are further systems under development such as JAXA’s Japan’s Optical Data Relay System (JDRS and NASA’s Laser Communications Relay Demonstration (LCRD) [1].
EDRS, which is operational, since 2016, has been designed to provide data relaying (quasi real time) services using GEO/LEO, RF and optical satellite and Intersatellite links [1]. It is very important that DRS satisfy the service and link availability requirements and QoS specifications in order to guarantee also the targeted latency, which is one of the main benefit of such a system.
GEO DRS is expected to relay a large amount of data coming a multiplicity of (RF or Optical) ISLs back to ground. To do so, the system typically employs a high RF frequency band, Ka-band or even higher frequency bands, such as Q/V (40/50GHz) and W (75/85 GHz) bands that may be used in the next generation relays systems. This is considered an essential step in meeting the ever-increasing requirements for higher data rates and system capacity. The utilization of these bands provides greater bandwidth and alleviates the spectrum scarcity problems that used to barrier the adoption of emerging services. Despite such advantages, signal propagation in these bands is particularly prone to the various atmospheric phenomena (e.g. gases, clouds, rain and tropospheric turbulence) [2]. Unless addressed methodically by the system designer, the aforementioned impairments can become a limiting factor in terms of overall system performance and availability. It is therefore evident that tropospheric degradation has to be evaluated and quantified by means of concise and accurate propagation models and propagation measurements, aiding the development of Propagation Impairments Mitigation Techniques (PIMTS) [2]. The most efficient (highest gain) PIMT is Site diversity (SD) technique, taking advantage of the spatial structure of the troposphere and mainly the rainfall medium to statistically avoid the worse fading conditions. In the context of institutional GEO systems, recently SD has also been implemented in EUMETSAT Meteosat Third Generation system [3].
Contrary to multibeam High Throughput Satellite (HTS) telecom systems, GEO DRS architectures cover few large ground stations (gateways) with a limited number of beams (either spot or regional). Therefore, applying short scale SD (for example within one spot beam) may become an important enabler. NTUA has already installed and fully tested two identical Q-band receivers at the NTUA campus (Athens) and at the Lavrion Technological and Cultural Park, about 36.5 km apart from each other. The receivers are based mainly on high-grade off-the-shelf parts and build upon the SDR principle. Some results from this site diversity experiment will be presented in this paper. Additionally, in this paper, we will discuss about the possible application of site diversity in the next generation DRS that will take place in more rain climatic regions (such as Japan, Mediterranean) and employ instead of Ka band (26GHz) the Q band (40GHz) downlink. There are many interesting scenarios that will be evaluated: i) ground stations within the same beam with distance of about 10Km, ii) ground stations within same beam with distances about 50Km and finally iii) ground covering completely independent beams. The third node of the EDRS system, namely EDRS-D is expected to cover Pacific areas. Therefore, hypothetical SD scenarios over Australia and Japan will be included. Comparisons in terms of site diversity gain and system throughput will be given.
References
[1] T. Araki, “A Study of the Future Optical Data Relay System; Requirements, Problems and Solution”, IEEE International Conference on Space Optical Systems and Applications (ICSOS) 2017.
[2] A. D. Panagopoulos, P.M. Arapoglou, P.G. Cottis “Site vs. Orbital Diversity: Performance Comparison based on Propagation Characteristics at Ku band and above”, IEEE Antennas and Wireless Propagation Letters, vol. 23. Issue 3, pp. 26-29, 2004.
[3] H. Hauschildt, S. Mezzasoma, H. L. Moeller, M. Witting and J. Herrmann, "European data relay system goes global," 2017 IEEE International Conference on Space Optical Systems and Applications (ICSOS), Naha, 2017, pp. 15-18. doi: 10.1109/ICSOS.2017.8357204
[4] A. D. Panagopoulos, P. M. Arapoglou and P. G. Cottis, "Satellite communications at KU, KA, and V bands: Propagation impairments and mitigation techniques," in IEEE Communications Surveys & Tutorials, vol. 6, no. 3, pp. 2-14, Third Quarter 2004.
doi: 10.1109/COMST.2004.5342290
[5] https://www.ffg.at/sites/default/files/mtg_ground_segment_presentation_to_austrian_industry.pdf

In recent years, the integration of SATCOM into 5G framework became a hot topic. The integration is planned to be realized in the higher layers of the ISO-OSI stack using Software-defined networking (SDN), and network functions virtualisation (NFV). The integration of the terrestrial and the satellite system will be limited unless the channel capacity of the satellite systems can be increased proportionally to the terrestrial counterparts. One of the most effective technique to increase the capacity of a Multi-beam satellites system is represented by precoding [1–4]. In satellite communication, the DVB-S2X standard [5] has been introduced to enable the application of precoding.
While standard technologies, as for DVB-S2 [6], are designed to operate using an interference avoidance approach through a proper re-use of the available spectrum amongst beams, precoding goes in the opposite direction through the management and the exploitation of the interference amongst beams. The objective is clearly to maximize the use of the user link available spectrum (in terms of spectral efficiency) which represents a limited resource of the system.
To increase the Technology Readiness Level of precoding technique in satellite communication we develop an hardware demonstrator of precoding technique using Software Defined Radio (SDR). To this aim, our demonstrator will implement all the required functionalities of a precoding-based system to correctly exploit the achievable gain coming from the usage of this technique. In particular, the functionalities to be included on the User Terminal (UT) side are synchronization and proper collection of the estimated Channel State Information (CSI). On the Gateway (GW) side precoding-specific smart user-scheduling techniques, Modulation and Coding (Modcod) allocation, computationally-efficient calculation of the precoder and application of the resulting precoder to the vectors of symbol streams, one per each beam have to been developed. The benchmark for this study is represented by an equivalent system architecture operating in a 4-colour reuse scheme, which entails an interference avoidance approach, without the use of precoding. Based on recent studies and depending on the system assumptions, expected gains compared with conventional satellite schemes are in the range of 30% to 100%.
The main functional blocks of the Precoding demonstrator are a precoding enabled gateway, a satellite MIMO channel emulator and a set of user terminal receivers. The channel emulator encompasses the effect of the whole satellite forward link, from the IF input at the RF equipment in the gateway to the IF output of the LNA/LNB in the terminal user equipment. In this way, a transition from the laboratory demonstrator to a real live satellite scenario will be performed without any substantial modification. Furthermore, the demonstrator should utilizes the IF central frequencies in L band that are typically used by the conventional commercial LNB equipment.
It is worth noting that the use of a satellite channel emulator that implements not only the payload and receiver impairments, but also the linear combinations of the different carriers through a channel matrix is fundamental for a proper execution of the in-lab testing and for the performance characterisation. The emulator, being software definable, can load payload and channel characteristics (TWTA characteristics, IMUX/OMUX taps, PN mask of OLs, H matrix which can be defined based on a given antenna pattern and link budget).

The development of Entry Descent & Landing (EDL) system technologies ensuring safe landing on Mars is one of the critical areas of work in the frame of the Mars Robotic Exploration Program (MREP). The rate of failure of Mars missions is still very high. Whether it is landing a probe on the Martian surface, orbiting the planet or merely conducting a flyby, only 40% of past attempts have proven successful. After the Beagle 2 failure, it has been recommended that future ESA missions shall implement means of sending essential telemetry data related to events occurring during the critical EDL phase. This new requirement calls for the implementation of a communication system capable of transmitting information during the EDL phase. Moreover, since communications with an orbiter cannot always be guaranteed, it is necessary to investigate a solution compatible with the transmission of information from the lander directly to Earth (DTE link) during EDL. This recommendation has been cascaded to all MREP related studies and projects, such as ExoMars, INSPIRE and Phootprint. These missions are different in terms of scenario and objectives, nonetheless they share the need to safely bring landing assets to a celestial body (e.g. Mars, Phobos).
This paper provides an overview of the activities carried out and the results achieved in the frame of the EDL Communications Technology (ECOMTEC) Assessment study. The two main objectives of the study are to propose an end-to-end system architecture demonstrating the feasibility of DTE reception of telemetry during EDL, and to investigate Ground Segment architectures that would significantly improve spacecraft tracking accuracy during the EDL phase, using large aperture Radio Telescopes operating as Very Large Baseline Interferometry (VLBI) receivers.
The overview starts from a classification of EDL scenarios based on the entry environment (atmospheric vs. non-atmospheric), the distance to Earth, and the communications link architecture (Direct-to-Earth vs. Orbiter Data Relay). The paper then focuses on the more complex and challenging atmospheric entry scenarios, with communications via a DTE link. The DTE acquisition, tracking and demodulation of the Essential TM signal transmitted during EDL has to cope with very low S/N0 ratios, and extreme amplitude and Doppler shift/rate dynamics. During the study several modulation schemes were studied and down-selected. The paper describes the baseline solution, which is based on two robust non-coherent modulation schemes and a highly-parallel open-loop processing architecture at the receiving end. The predicted performance is presented for some representative configurations, showing that essential telemetry (TM) transmitted at 8bps can be correctly demodulated at signal power to noise spectral density ratios, S/N0, as low as 16dB-Hz.
After a detailed trade-off that took into account on-board RF amplification technology, on-board antenna, power, size and mass, two on-board architecture were defined: one operating in UHF, the other in X-Band. On the other side of the link, a number of candidate Ground Stations and Radio Astronomy facilities have been identified with capability for Deep Space operations. The predicted end-to-end performance was validated with an extensive link budget analysis campaign.
The proposed modulation schemes are also suitable to improve the tracking of the EDLM during EDL, using Same Beam Interferometry (SBI) techniques. Since the EDLM and its carrier spacecraft (or any other orbiting asset) are in the same beam of the on ground antenna, double-differential phase delay measurements can be made, reaching accuracies several orders of magnitude better than equivalent DDOR measurements. An SBI experiment architecture, combining the measurements performed at two ground stations, was defined during the study. The proposed phase estimation processor relies on FFT-based methods to jointly estimate frequency, frequency rate and frequency acceleration. The estimation of the frequency rate and acceleration is needed to increase the integration time, thus to detect the signal phase in very low S/N0.
Finally, during the last part of the study, the Edl communications End-to-End Performance Simulator (E3PSim) was developed. This is a parametric simulator capable of reproducing the end-to-end performance of the selected DTE architectures, including signal detection and SBI phase measurement accuracy, considering the signal dynamics model, the on-board performances and on ground performances. The paper will conclude showing the performance of the selected end-to-end architectures, evaluated with the E3PSim on the ExoMars EDL mission scenario.