SATCOM Systems Research Articles

Performance Analysis of RIS-Assisted SatComs Based on a ZFBF and Co-Phasing Scheme

In recent high-throughput satellite communication (SatCom) systems, the use of reconfigurable intelligent surfaces (RISs) has emerged as a promising solution to improve spectral efficiency and extend coverage in areas with limited terrestrial network access. However, the RIS may amplify the inter-beam interference (IBI) caused by multibeam transmission at the satellite, and multiple RISs can also cause inter-RIS interference (IRI) to terrestrial users. In this paper, the performance of the RIS-assisted SatCom system is asymptotically analyzed for both full and partial channel state information (CSI) scenarios. In particular, zero-forcing beamforming is considered as the active beamforming for data transmission, while the co-phasing scheme is considered as the passive beamforming for RIS reflection. Based on the asymptotic analyses, deterministic active and passive beamforming techniques using partial CSI are proposed that can gradually eliminate both IBI and IRI, ultimately achieving ideal performance. Simulation results validate the accuracy of asymptotic analyses and demonstrate the superiority of deterministic active and passive beamforming techniques using partial CSI. The simulation results also confirm that the proposed beamforming can achieve approximately 92.8% of the ideal performance, even though it only requires partial CSI.

Low-loss microwave photonic switching for satellite communication.

For traditional switching architecture, packet switching performs fine granularity data packet forwarding, but its digital signal processing (DSP) has high power consumption (PC). All-optical switching provides rapid exchange of wavelength resources, which has coarse granularity. In scenarios where the PC is limited, such as broadband satcom, a switching architecture with lower PC and finer granularity than optical switching would be useful. In this paper, we propose a novel, to the best of our knowledge, low-loss microwave photonic switching architecture that can exchange subband signals across beams and frequency bands. The switching process is realized by exchanging optical carriers instead of payload signals, which does not degrade the signal power, guaranteeing the signal-to-noise ratio (SNR). We conducted a proof-of-concept experiment of 2 × 2 switching with two 1.2-GBaud quadrature phase-shift keying (QPSK) signals; an error vector magnitude (EVM) of or less than 13.87% is realized after forwarding. The proposed system has the advantages of low PC, high SNR, and fine granularity, and is very promising for flexible forwarding in future satcom systems.

Feasibility study on a plasma based reflective surface for SatCom systems

Gaseous Plasma Antennas (GPAs) exploit an ionized gas to transmit and receive electromagnetic waves. GPAs offer several advantages over metal antennas since, while in use, they are electronically reconfigurable in terms of radiation pattern and operation frequency. When not used, the plasma can be turned “off”, and the GPA reverts to a dielectric tube with a very low radar cross-section. This makes GPAs suitable to be stacked into arrays, providing they can reduce co-site interference. Thus, GPAs are very appealing for Satellite Communications (SatCom). The antenna pointing and tracking obtained by steering the beam electronically, rather than using mechanically moving parts, can enable several space missions. A plasma-based reflective surface has been recently proposed in this framework to maximize reconfigurability. Such a device consists of many plasma discharges placed on top of a metallic ground plane, and the reflection of radio signals can be controlled by electronically varying the plasma properties (e.g., density). This work presents the first step toward practically implementing a plasma-based reflective surface in the X-band capable of electrically, rather than mechanically, tuning its beam steering and focusing capabilities. The study here presented combines numerical and experimental approaches. First, a target plasma discharge has been characterized experimentally to provide the plasma parameters needed to estimate the electromagnetic (EM) response using numerical simulations. Then, the numerical simulations were performed to preliminary design a plasma-based reflective surface. At first, a simplified single-element plasma cylinder was analysed. Then, the glass envelope needed to contain the plasma was added to the column, and its impact on the performance was evaluated. Finally, a finite surface of ten plasma cylinders (including glass vessels) has been considered. The preliminary results prove the capability to obtain beam-steering by tuning the plasma density within the discharges.

Guest editorial IJSCN special issue on 3GPP NTN standards for future satellite communications

Guest editorial IJSCN special issue on 3GPP NTN standards for future satellite communications

Onboard Processing in Satellite Communications Using AI Accelerators

Satellite communication (SatCom) systems operations centers currently require high human intervention, which leads to increased operational expenditure (OPEX) and implicit latency in human action that causes degradation in the quality of service (QoS). Consequently, new SatCom systems leverage artificial intelligence and machine learning (AI/ML) to provide higher levels of autonomy and control. Onboard processing for advanced AI/ML algorithms, especially deep learning algorithms, requires an improvement of several magnitudes in computing power compared to what is available with legacy, radiation-tolerant, space-grade processors in space vehicles today. The next generation of onboard AI/ML space processors will likely include a diverse landscape of heterogeneous systems. This manuscript identifies the key requirements for onboard AI/ML processing, defines a reference architecture, evaluates different use case scenarios, and assesses the hardware landscape for current and next-generation space AI processors.

Hybrid SAG-FSO/SH-FSO/RF Transmission for Next-Generation Satellite Communication Systems

Recent advances demonstrate satellite communication (SatCom) as a potent enabler for future Terabit/s global connectivity. Existing SatCom systems, however, rely mostly on radio frequency (RF) transmission, whose limited available bandwidth is the main bottleneck for further data rate increases. Free-space optical (FSO) communication links, with huge license-free bandwidth, have emerged as an attractive alternative. Despite their ability to deliver high-throughput transmissions, FSO communications are weather-dependent and susceptible to atmospheric turbulence. Hybrid FSO/RF and space-air-ground (SAG) FSO transmissions are possible solutions to increase the reliability of FSO-based transmission systems. A strategically deployed unmanned-aerial vehicle, acting as a relay, can successfully mitigate the adverse effect of atmospheric turbulence, while the hybrid FSO/RF transmission can address weather-dependent effects. These solutions can also be integrated to create a system with significantly improved performance and reliability. To evaluate the performance of the resulting integrated transmission system, we analyze the outage probability and average symbol error probability in this work. Asymptotic expressions are also derived to get further insight into the system behavior and calculate the overall diversity gain. Furthermore, we consider the optimal design of switching thresholds. The numerical results show that the integrated transmission system achieves about 10 dB performance gain over existing solutions for both downlink and uplink scenarios.

Ergodic Capacity Analysis of Satellite Communication Systems With SAG-FSO/SH-FSO/RF Transmission

Future non-terrestrial networks aim to achieve a throughput of Terabits/s. Therefore, free-space optical (FSO) communications have been adopted as a candidate solution due to their ability to achieve an extremely high data rate. Nonetheless, FSO communications are sensitive to the adverse effects of beam scintillation, beam-wander-induced pointing errors, free-space loss, and weather conditions. Space-air-ground (SAG) FSO transmission and hybrid single-hop (SH) FSO/radio frequency (RF) transmission are promising solutions to improve the performance of FSO links and can be integrated into a satellite communication (Satcom) system. In this work, we carry out a thorough capacity analysis of the resulting integrated SAG-FSO/SH-FSO/RF Satcom system, where Gamma-Gamma and Rician distributions are used to characterize FSO and RF links, respectively. The exact analytical expressions are derived and validated by Monte-Carlo simulations. We also obtain asymptotic expressions for the ergodic capacity in the high signal-to-noise ratio region. The numerical results highlight the significant potential of the integrated SAG-FSO/SH-FSO/RF Satcom system over existing solutions. We also show that the integrated Satcom system with intensity modulation and direct detection can achieve a capacity gain over that with a heterodyne detection technique over all satellite zenith angles.

Satellite-based communications security: A survey of threats, solutions, and research challenges

Satellite-based Communication systems are gaining renewed momentum in Industry and Academia, thanks to innovative services introduced by leading tech companies and the promising impact they can deliver towards the global connectivity objective tackled by early 6G initiatives. On the one hand, the emergence of new manufacturing processes and radio technologies promises to reduce service costs while guaranteeing outstanding communication latency, available bandwidth, flexibility, and coverage range. On the other hand, cybersecurity techniques and solutions applied in SATCOM links should be updated to reflect the substantial advancements in attacker capabilities characterizing the last two decades. However, business urgency and opportunities are leading operators towards challenging system trade-offs, resulting in an increased attack surface and a general relaxation of the available security services. In this paper, we tackle the cited problems and present a comprehensive survey on the link-layer security threats, solutions, and challenges faced when deploying and operating SATCOM systems.Specifically, we classify the literature on security for SATCOM systems into two main branches, i.e., physical-layer security and cryptography schemes.Then, we further identify specific research domains for each of the identified branches, focusing on dedicated security issues, including, e.g., physical-layer confidentiality, anti-jamming schemes, anti-spoofing strategies, and quantum-based key distribution schemes. For each of the above domains, we highlight the most essential techniques, peculiarities, advantages, disadvantages, lessons learned, and future directions.Finally, we also identify emerging research topics whose additional investigation by Academia and Industry could further attract researchers and investors, ultimately unleashing the full potential behind ubiquitous satellite communications.

A smart reactive jamming approach to counter reinforcement learning‐based antijamming strategies in GEO SATCOM scenario

SummaryReinforcement learning (RL) is being considered for future SATCOM systems due to its inherent capability to self‐learn the optimum decision‐making policy under different scenarios. This capability enables SATCOM systems to manage their resources judiciously and mitigate jamming attacks autonomously without prior jammer type classification. We propose a novel smart reactive SATCOM jamming approach that would not only counter these RL based anti‐jamming strategies but would also be effective against conventional anti‐jamming schemes, that is, FHSS and DSSS. The proposed jamming approach exploits the limitations in learning patterns of Q‐learning‐based RL agent and achieves effective jamming while conserving considerable amount of jamming power. To achieve this, we propose an intelligent jamming engine (IJE) along with few potent jamming algorithms and evaluate their performance in terms of throughput degradation of victim SATCOM link, jamming power conservation, and design complexity of the jammer. Software simulations successfully demonstrate the effectiveness of our proposed smart reactive jamming approach which outperforms the standard reactive jammer against RL‐based antijamming schemes.

Highly integrated transmitting and receiving phased array with multi‐channels and high efficiency in K/ Ka‐band SatCom application

A highly integrated K/Ka-band phased array with duplexing channels for transmitting and dual circularly polarized channels for receiving is presented. To increase both the efficiency and channel isolation, a novel architecture is proposed to integrate the horn antenna, polarizer, ortho-mode transducer (OMT), and filters within a very compact radiating aperture as the element. Based on this element, one large-scale phased array with 416 elements was developed, fabricated, and measured. Experimental results show that the proposed phased array has high aperture efficiency of larger than 56% in both transmitting and receiving channels, high channel isolations of 50 dB between different frequency channels and 20 dB between different polarization channels. Furthermore, beam scanning performance within ±20° and ±18° at 20 and 30 GHz are achieved, respectively. The developed multifunctional phased array is an attractive and feasible frontend for K-/Ka- band SatCom system.

Evaluation of Time Diversity Technique for Satellite Mitigation in Tropical Climate

Time diversity is one of the mitigation techniques for satellite communications (SatCom) systems that aims to combat deep fades cause by propagation impairments. This study presents the performance of time diversity technique for rain attenuation at Ku- and Ka-band SatCom systems in Johor Bahru, Malaysia. The Synthetic-Storm Technique (SST) has been utilized to obtain rain attenuation time series based on rain rate data measured by local rain gauge. The time diversity has been quantified through diversity gain as a function of retransmission time delay, in which shows the effectiveness of time diversity technique to improve the Quality of Service (QoS) and link availability. The results demonstrated it is feasible to use time diversity as mitigation technique at higher frequencies in a mission to support future SatCom operation in tropical region.

Secrecy Analysis for Orthogonal Time Frequency Space Scheme Based Uplink LEO Satellite Communication

Recently, satellite communication (Sat-Com) has developed rapidly, and its security is critical. In this letter, we investigate the security performance for an uplink low earth orbit (LEO) Sat-Com system, where the cooperative UAV is employed to send jamming signals against the reconnaissance satellite. In particular, the uplink transmission is based on the orthogonal time frequency space (OTFS) technique, in order to combat the severe Doppler effect caused by the high mobility of the LEO satellite. As a pioneer work, we derive the closed-form expression for the security outage probability (SOP) at the legitimate receiver, where a novel moment matching approach is used for finding a summed probability distribution function (PDF) of the shadowed-Rician (SR) distributed terms. Furthermore, we analyze the SOP advantage achieved by the OTFS scheme over the traditional orthogonal frequency division multiplexing (OFDM) scheme. Simulation results validate the theoretical results for SOP performance of the uplink LEO Sat-Com, and show the effectiveness of employing UAV jammer in terms of enhancing secrecy transmission.

위성 통신에서 이론적 재밍 검출 기반 주파수 영역 노치 필터링 기법

위성 통신에서 이론적 재밍 검출 기반 주파수 영역 노치 필터링 기법

Validation of a Digital Noise Power Integration Technique for Radiometric Clear Sky Attenuation Estimation at Q-Band

This article presents the validation of a digital signal processing technique that can be used to estimate radiometric sky noise, and hence atmospheric absorption, within existing digital receivers at little/no additional cost. To demonstrate this, a receiver was constructed that simultaneously records the beacon signal power from the ALPHASAT Aldo Paraboni technology demonstration payload, as well as the integrated noise power in the adjacent band. Calibration from the digital radiometer is performed using tip-curve calibration procedures. Atmospheric fading is then obtained by observing the beacon as well as the radiometric signals. This enables the comparison of fading obtained by the two techniques and provides a means to calibrate the received beacon power level to obtain total atmospheric attenuation. It is shown that for low levels of fading, up to a few dB, the two techniques provide good agreement. This approach can, therefore, provide a low-cost option for geostationary mm-wave satellite channel measurements in the low fading regime, which can be useful in the design and operation of the feeder links in emerging satcom systems.

Microwave Liquid Crystal Enabling Technology for Electronically Steerable Antennas in SATCOM and 5G Millimeter-Wave Systems

Future satellite platforms and 5G millimeter wave systems require Electronically Steerable Antennas (ESAs), which can be enabled by Microwave Liquid Crystal (MLC) technology. This paper reviews some fundamentals and the progress of microwave LCs concerning its performance metric, and it also reviews the MLC technology to deploy phase shifters in different topologies, starting from well-known toward innovative concepts with the newest results. Two of these phase shifter topologies are dedicated for implementation in array antennas: (1) wideband, high-performance metallic waveguide phase shifters to plug into a waveguide horn array for a relay satellite in geostationary orbit to track low Earth orbit satellites with maximum phase change rates of 5.1°/s to 45.4°/s, depending on the applied voltages, and (2) low-profile planar delay-line phase shifter stacks with very thin integrated MLC varactors for fast tuning, which are assembled into a multi-stack, flat-panel, beam-steering phased array, being able to scan the beam from −60° to +60° in about 10 ms. The loaded-line phase shifters have an insertion loss of about 3 dB at 30 GHz for a 400° differential phase shift and a figure-of-merit (FoM) > 120°/dB over a bandwidth of about 2.5 GHz. The critical switch-off response time to change the orientation of the microwave LCs from parallel to perpendicular with respect to the RF field (worst case), which corresponds to the time for 90 to 10% decay in the differential phase shift, is in the range of 30 ms for a LC layer height of about 4 µm. These MLC phase shifter stacks are fabricated in a standard Liquid Crystal Display (LCD) process for manufacturing low-cost large-scale ESAs, featuring single- and multiple-beam steering with very low power consumption, high linearity, and high power-handling capability. With a modular concept and hybrid analog/digital architecture, these smart antennas are flexible in size to meet the specific requirements for operating in satellite ground and user terminals, but also in 5G mm-wave systems.

Implementation of Mixing Sequence Optimized Modulated Wideband Converter for Ultra-Wideband Frequency Hopping Signals Detection

Frequency hopping (FH) communications are widely used in Satcom systems. The latest FH systems have exceeded 3 GHz with hop rates up to 100 000 hops/s. Traditional spectrum sensing systems hardly monitor such signals. Modulated wideband converter (MWC) is an emerging compressive sampling structure applied to sample multiband signals. This article analyses the effects of the nonlinearity of analog devices and proposes an optimization method based on a greedy algorithm for mixing sequences to make reconstruction performance uniform across sub-bands. To correct the errors caused by the nonideality of analog front end circuits between the theoretical sensing matrix and the practical one, we also develop a calibrating method that obtains all the sensing matrix coefficients through a single measurement. In conventional MWC, out-band noise will be blended during reconstruction. Instead of matrix factorization, we reserve signal characters and designed sliding filters to improve narrowband signals reconstruction sensitivities. We produced a four-branch MWC principle prototype for FH signal detection. The sensing bandwidth is 3 GHz, and the sampling rate is 400 MHz, whereas the reconstruction sensitivity is as low as 13-dB in-band signal-to-noise.

Physical Layer Security for Multiuser Satellite Communication Systems With Threshold-Based Scheduling Scheme

Satellite communication (SatCom) has attracted much attention due to its inherent characteristics. Security issues have gained severe concerns in SatCom since it is susceptible to be illegally eavesdropped by malicious ground stations within large-scale wireless coverage. In this paper, we investigate the physical layer security of a multiuser SatCom system in the presence of multiple eavesdroppers. Particularly, we propose a threshold-based scheduling scheme, where the geographically clustered eavesdroppers with both the colluded and collaborated eavesdropping scenarios are assumed. Specifically, closed-form expression for the secrecy outage probability (SOP) is derived for the passive eavesdropping scenario when the channel state information (CSI) of the eavesdroppers is unavailable. Moreover, we obtain a closed-form expression for the average secrecy capacity (ASC) of the considered system under the proposed user scheduling scheme. In order to get further insights of the proposed scheduling scheme at high signal-to-noise ratios (SNRs), the asymptotic analysis for the SOP and ASC is also demonstrated. Moreover, the reduced percentage with respect to number of user examination is also given, which validates the simplicity and efficiency of our proposed scheme compared to the traditional approaches. Numerical results deduce that with the proposed scheme, a comparable system performance with regard to the maximal selection (MS) scheme can be achieved.

Near Optimal Timing and Frequency Offset Estimation for 5G Integrated LEO Satellite Communication System

The integration of satellite and terrestrial 5G networks aims to provide ubiquitous coverage, improve service reliability, and enable the network scalability. However, the inherent characteristics of satellite channels bring challenges on the air interface design of integrated terrestrial-satellite networks. For example, for low earth orbits (LEO) mobile satellite communication (SatCom) system, it is unclear so far whether the 5G new radio (5G-NR) synchronization signals could meet the requirement of timing and frequency offset estimation in the presence of large Doppler shifts. In this paper, we investigate time and frequency synchronization for the downlink transmission of 5G-NR signals over LEO satellite channels. Starting from the maximum log-likelihood criterion for timing offset estimation given the observation of the received primary synchronization signals (PSS), we derive an upper bound of the objective function for simplicity. With a priori information that the maximum Doppler shift of LEO satellite-ground link is within a specific range, we construct the local synchronization sequence via using the modulated discrete prolate spheroidal sequences (DPSS) vectors. Then the timing offset estimation can be recast into a one-dimensional peak search problem. Moreover, the cyclic prefix (CP) structure of orthogonal frequency division multiplexing (OFDM) can be utilized to improve the estimation performance further. Once the timing and frequency offset are captured in the above initial synchronization phase, the tracking synchronization can be much simplified as the variation of both the timing and frequency offset is very small between two adjacent synchronization blocks. Simulation results show that by using the proposed algorithms, the 5G-NR signals can achieve near optimal downlink time and frequency synchronization in typical LEO SatCom systems.

Thermal Effects on a 4.2 m Center-Fed Cassegrain Antenna Strut in the Alphasat Ka- and Q-/V-Band Aldo Paraboni Propagation Measurements

Large gateways of future satellite communications (Satcom) systems will likely operate at Q-/V-band frequencies (40–50 GHz) to take advantage of the larger available bandwidth. On the other hand, Q-/V-band Satcom systems suffer from highly variable propagation losses, which must be accurately assessed and modeled. The Alphasat Aldo Paraboni propagation experiment, conceived and realized by the Italian Space Agency (ASI), Rome, Italy, has been running since 2014 to carefully investigate the propagation effects in the Ka- and Q-band and, thus, to support the design of high-throughput systems and propagation impairment mitigation techniques. In the framework of this experiment, ASI deployed two large ground terminals (Cassegrain antenna with 4.2 m diameter) in Tito Scalo (South of Italy) and Spino d’Adda (North of Italy). This contribution investigates the effect of antenna strut elongation on propagation measurements, which causes an additional reduction of 1–2 dB in the received signal during sunny days.

Throughput Enhancement for SATCOM Systems Using Dynamic Spectrum Controlled Channel Allocation under Variable Propagation Conditions

Throughput Enhancement for SATCOM Systems Using Dynamic Spectrum Controlled Channel Allocation under Variable Propagation Conditions