Antonio Jurado-Navas

The growing demand for accurate and timely Earth observation (EO) data has made autonomous mission planning increasingly essential. In particular, data acquisition planning has gained attention in recent years with the advent of agile Earth observation satellites (AEOSs). This process involves two main stages: target identification and observation scheduling. Traditionally, the former is performed manually, while the latter requires solving the agile Earth observation satellite scheduling problem (AEOSSP), a complex combinatorial optimization problem. In this work, we propose a constellation design comprising EO satellites deployed in medium Earth orbit (MEO) and low Earth orbit (LEO). The MEO satellites acquire low-resolution (LR) images for onboard target identification and subsequently schedule high-resolution (HR) observations by a set of LEO AEOSs. We adapt the AEOSSP to this multi-orbit context by explicitly considering communication constraints between MEO and LEO satellites and propose several heuristic solution methods. Among them, the quality-based greedy algorithm yields up to a 35.5% improve in observation profit in simple, low-conflict scenarios, while the structured heuristic algorithm proves the most robust, achieving up to a 21.5% increase in challenging schedules.

The continued performance scaling of AI gigafactories requires the development of energy-efficient devices to meet the rapidly growing global demand for AI services. Emerging materials offer promising opportunities to reduce energy consumption in such systems. In this work, we propose an electro-optic microring modulator that exploits a graphene (Gr) and transition-metal dichalcogenide (TMD) interface for phase modulation of data-bit signals. The interface is configured as a capacitor composed of a top Gr layer and a bottom WSe₂ layer, separated by a dielectric Al₂O₃ film. This multilayer stack is integrated onto a silicon (Si) waveguide such that the microring is partially covered, with coverage ratios varying from 10% to 100%. In the design with the lowest power consumption, the device operates at 26.3 GHz and requires an energy of 5.8 fJ/bit under 10% Gr-TMD coverage while occupying an area of only 20 μm². Moreover, a modulation efficiency of V_πL = 0.203 V·cm and an insertion loss of 6.7 dB are reported for the 10% coverage. The Gr-TMD-based microring modulator can be manufactured with standard fabrication techniques. This work introduces a compact microring modulator designed for dense system integration, supporting high-speed, energy-efficient data modulation and positioning it as a promising solution for sustainable AI gigafactories.

In this work, we explore the use of intelligent reflecting surfaces (IRSs) as transparent relays in free-space optical (FSO) links for low earth orbit (LEO) satellites constellations. We introduce a novel IRS design featuring a slightly convex surface geometry to broaden the reflected beam, thereby mitigating the beam alignment challenges typically encountered in satellite-based FSO systems. We investigate the impact of IRS size, curvature, and transmitter beam width on end-to-end system performance, taking into account attenuation, atmospheric turbulence, and pointing errors. To accommodate the effects of surface curvature, we enhance and refine the channel pointing model and derive novel closed-form expressions for key performance metrics, including bit error rate, outage probability, and ergodic capacity. Numerical results based on realistic system parameters confirm the effectiveness of the proposed approach, albeit at the cost of increased geometric channel losses. Finally, we carry out numerical simulations to validate the new channel model and derived closed-form expressions, providing valuable insights for the design of future non-terrestrial networks architectures based on FSO systems.

The continued performance scaling of AI gigafactories requires the development of energy-efficient devices to meet the rapidly growing global demand for AI services. Emerging materials offer promising opportunities to reduce energy consumption in such systems. In this work, we propose a novel electro-optic microring modulator that exploits a graphene (Gr) and transition-metal dichalcogenide (TMD) interface for phase modulation of data-bit signals. The interface is configured as a capacitor composed of a top graphene layer and a bottom WSe2 layer, separated by a dielectric Al2O3 film. This multilayer stack is integrated onto a silicon (Si) waveguide such that the microring is partially covered, with coverage ratios varying from 10% to 100%. The proposed device design and its key figures of merit, including energy efficiency, are analyzed. Simulation results indicate that the microring modulator achieves low energy consumption and high-speed operation while maintaining a compact footprint. In particular, the device operates at 39.1 GHz with an energy consumption of 8.3 fJ/bit under 25% Gr–TMD coverage, whilst occupying an area of only 20 μm2. Furthermore, a modulation efficiency of VπL= 0.082 V·cm and an insertion loss of 6.8 dB are obtained for the 25% coverage. The proposed Gr–TMD-based microring modulator demonstrates significant potential for high-speed, energy-efficient data modulation, contributing to the development of more sustainable AI gigafactories.

In this paper, we propose, design, and evaluate a new hybrid terrestrial–underwater optical communication link for providing high-speed connectivity between land and underwater systems. A device based on an amplify-and-forward strategy is considered and used for the hybrid optical link. A performance analysis of the proposed hybrid system is then carried out, taking into account both the atmospheric and underwater channels and their respective degradation sources. Different networking scenarios and conditions are evaluated. To this end, the channel model of the terrestrial free-space optical (FSO) link is modeled using the Gamma–Gamma distribution, while the underwater optical link is modeled using the Weibull distribution. The former takes into account atmospheric and turbulence attenuation, geometric spread and pointing errors, while the latter takes into account underwater and turbulence attenuation and geometric spread. Accordingly, a new analytical closed-form expression for the bit error rate (BER), which depends on the cumulative distribution function of the holistic hybrid system, is derived. Analytical results show that pointing errors as well as atmospheric and oceanic turbulence seriously degrade the performance of the hybrid system. In addition, ocean turbulence leads to the occurrence of a BER floor in some scenarios. This is the first time that such a network is proposed and modeled under the assumption of critical channel impairments.

In this paper, we propose a novel hybrid network solution using optical code-division multiple-access and free-space optics (OCDMA-FSO) to support multiple data rates and quality-of-service according to user requirements. The proposed OCDMA-FSO network classifies users into diverse classes based on their requirements and allows for signal transmission in a fully asynchronous manner. The new hybrid network employs 2-D OCDMA codes as well as a variable aperture FSO receiver together with an equal gain combining (EGC) technique. We derive completely new analytical formalisms to evaluate the network performance in terms of average bit error rate (ABER), outage probability, effective data rate during outages, and ergodic capacity. The derived ABER expressions are validated via Monte-Carlo simulations. These new analytical formalisms are derived considering several atmospheric turbulence conditions such as uncorrelated, totally correlated, and partially correlated turbulence. To model this atmospheric turbulence, we derive the gamma-gamma (GG) distribution, which allows us to consider both the scintillation effect and the EGC technique. Finally, we carry out a comprehensive performance evaluation of the proposed hybrid network. Analytical results show that as the number of FSO receiver apertures increases, there is an improvement in overall performance levels. However, the ergodic capacity exhibits minimal improvement even as the number of FSO receiver apertures increases, regardless of the user requirements and turbulence conditions. The proposed network is a potential solution for scenarios that demand flexibility in bandwidth and data rate allocation.

6G networks are expected to use the terahertz (THz) frequency spectrum in some way. However, relatively little is known about the transmission channel in this part of the spectrum. In this paper, we propose a novel and accurate channel loss model that considers both molecular and water vapor continuum absorptions for THz networks based on the frequency range between 100 GHz and 600 GHz. The proposed channel model assumes that the atmospheric absorption is accurately accounted for by the molecular absorption coefficient and the water vapor continuum absorption coefficient. The new model is based on equations developed to accurately match the channel response given by a spectroscopic database. This makes the channel model robust and useful for evaluating the performance of THz networks. The model is validated against a spectroscopic database defined as a benchmark. The performance of the new channel loss model is evaluated against different models, including several error metrics. Numerical results show the new channel model is as accurate as the spectroscopic database by correctly reproducing the provided experimental dataset. Therefore, the proposed channel model is a reliable, robust and simple tool that can be successfully used to carry out performance evaluation of THz networks with accuracy.

High data rate coverage, security, and energy efficiency will play a key role in the continued performance scaling of next-generation mobile systems. Dense, small mobile cells based on a novel network architecture are part of the answer. Motivated by the recent mounting interest in free-space optical (FSO) technologies, this paper addresses a novel mobile fronthaul network architecture based on FSO, spread spectrum codes, and graphene modulators for the creation of dense small cells. The network uses an energy-efficient graphene modulator to send data bits to be coded with spread codes for achieving higher security before their transmission to remote units via high-speed FSO transmitters. Analytical results show the new fronthaul mobile network can accommodate up to 32 remote antennas under error-free transmissions with forward error correction. Furthermore, the modulator is optimized to provide maximum efficiency in terms of energy consumption per bit. The optimization procedure is carried out by optimizing both the amount of graphene used on the ring resonator and the modulator’s design. The optimized graphene modulator is used in the new fronthaul network and requires as low as 4.6 fJ/bit while enabling high-speed performance up to 42.6 GHz and remarkably using one-quarter of graphene only.

Analog beamforming is a key technology to enable millimeter-wave (mm-wave) mobile communications. Nonetheless, the most widespread beamforming solutions are based on electrical implementation, which is inefficient in terms of power consumption, signal bandwidth, and losses. Optical beamforming is a promising alternative for future mm-wave mobile communications due to its inherent benefits, such as large bandwidth, low cross-talk, and low losses. Optical beamforming networks (OBFN) are an outstanding technique to simultaneously generate and radiate multiple beams in an effective manner. True time delays (TTDs) based on optical waveguides are an attractive solution since they offer constant delay in the spectrum to avoid beam squint and allow highly scalable OBFN implementation. In this work, for the first time to the best of the authors' knowledge, an incoherent 4x4 OBFN based on optical waveguides and capable of simultaneously generating four beams is presented, including a thorough explanation of its operating principles and providing a detailed characterization. The presented OBFN is fabricated on Si $_{3}$ N $_{4}$ photonic integrated circuit (PIC), and designed for transmission at 27.5 GHz, within the n257 5 G band. Furthermore, the building blocks forming the OBFN PIC are deeply explained, providing their corresponding theoretical formulation and key design parameters. The fabricated PIC is exhaustively characterized through all its building blocks, showing in detail the realized measurement procedure. The experimental measures match the design parameters with minimal error, showing the feasibility of fabricating future OBFNs with the same technology and topology for larger antenna arrays. The experimental results corroborate the viability of the presented OBFN architecture as an excellent technology to consider in future mm-wave 5 G/6 G networks. The contribution of this work paves the road to turn optical beamforming into a mature, scalable, and efficient technology.

Energy-efficient devices will play a key role in the continued performance scaling of next-generation information and communications technology systems. Graphene has emerged as a key optoelectronic material with unique energy-like properties. But to the best of our knowledge, these advantages have not yet been fully exploited in optical modulators design. In this work, we design and analyze an optical modulator which is composed of two graphene layers and a ring resonator made with different amount of graphene. For performance analysis, the ring resonator’s amount of graphene is varied from 25 to 100% with four discrete steps. The critical coupling condition representing the OFF-state, and the 3-dB transmission level representing the ON-state of the device are obtained. Numerical results show this new optical modulator consumes as little energy as 4.6 fJ/bit whilst achieving a high-speed operation with a bandwidth up to 42.6 GHz when employing surprisingly only 25% of graphene. The 42.6 GHz modulator has a footprint as small as 22.1 µm² with an active area of 1.68 µm² only, the smallest active area to date. Alternatively, the optical modulator achieves up to ∼88.5 GHz at the expense of consuming 17.5 fJ/bit when using 100% of graphene. The proposed graphene-based modulator proved to be a compact, energy-efficient, high-speed device, useful for a myriad of applications including mobile fronthaul, telecom, and datacom.