Advanced Over-the-horizon Communications with Microwave Photonic Technologies （Invited）

This research is driven by the critical need to overcome the inherent limitations of conventional Radio Frequency （RF）-based Over-The-Horizon （OTH） communication systems. Traditional systems， which rely on the propagation of microwave signals through the atmosphere or via ionospheric reflection， face significant challenges that constrain their performance in modern， high-demand scenarios. These challenges include severely limited bandwidth capacity， which restricts data throughput； vulnerability to both intentional jamming and unintentional electromagnetic interference， compromising reliability； and a lack of operational flexibility， making real-time reconfiguration and multi-channel parallel processing difficult. Furthermore， achieving wideband， cross-frequency operation often necessitates complex， bulky， and power-intensive electronic components. The primary objective of this work is to conceptualize， design， and validate a next-generation OTH communication system architecture that fundamentally transcends these electronic bottlenecks. By strategically integrating the field of microwave photonics—which utilizes light to generate， process， and distribute microwave signals—this study aims to create a system prototype capable of delivering unprecedented levels of bandwidth， robust anti-interference performance， and dynamic reconfigurability. The ultimate goal is to establish a concrete technological foundation for future OTH platforms that can meet the escalating demands for secure， high-capacity， and resilient long-distance tactical and strategic communication links.To achieve the stated objectives， a comprehensive system design and simulation-based research methodology was employed， centered on a novel architecture that synergizes three core microwave photonic subsystems. The system's front-end is a microwave photonic phased-array antenna. This antenna comprises 64 independent elements designed to simultaneously capture weak wireless signals from free space. The received RF signals from each element are first amplified by a dedicated Low-Noise Amplifier （LNA） to mitigate front-end noise. Subsequently， each amplified RF signal is used to directly modulate the intensity of a continuous-wave optical carrier generated by a Directly Modulated Laser （DML）， thereby translating the electrical signals into the optical domain. This process creates 64 parallel optical RF channels. A critical innovation lies in the beamforming network. The 64 optical signals are fed into a tunable optical delay and attenuation module. A centralized digital control unit precisely and independently adjusts the time delay and attenuation for each of the 64 optical paths. This optical True-Time-Delay （TTD） approach is frequency-independent， enabling wideband， squint-free beam steering and shaping. The individually processed optical signals are then coherently combined via a Wavelength Division Multiplexing （WDM） stage， effectively synthesizing the desired radiation pattern in the optical domain before detection. The combined optical beam is converted back into a consolidated electrical RF signal using a high-speed Photodetector （PD）. This aggregated RF signal is then routed to the second core subsystem： the microwave photonic frequency conversion unit. The heart of this unit is a coherent dual Optical Frequency Comb （OFC） setup. One OFC serves as a multi-wavelength Local Oscillator （LO）， while the other is used for signal modulation. Electro-Optic Modulators （EOMs） are used for precise signal imprinting and frequency shifting. A key technique implemented is Carrier-Suppressed Single-Sideband （CS-SSB） modulation， achieved by carefully biasing the EOMs， which eliminates LO leakage and provides excellent channel isolation. This setup allows for the simultaneous up-conversion or down-conversion of multiple RF channels across a ultra-wide bandwidth. The final subsystem is an integrated communication terminal， which performs demodulation， decoding， and protocol processing on the recovered baseband signals. The feasibility and performance of this integrated architecture were rigorously validated through detailed theoretical modeling and extensive link-level simulations using the industry-standard Optisystem software， which models the complex interactions between optical and RF components.The simulation and analysis of the proposed microwave photonic OTH system yielded highly promising results across multiple performance dimensions， confirming the effectiveness of the chosen architectural approach. Core Communication Performance： The system successfully demonstrated stable and high-fidelity signal transmission across the entire 4~16 GHz operational bandwidth. Within this spectrum， a sustained communication data rate of 200 Mbps was achieved， representing a significant improvement over typical narrowband OTH links and validating the system's high-bandwidth capability. Reconfigurable Channelized Processing： A major accomplishment was the implementation of reconfigurable microwave photonic channelized frequency conversion. The system possesses the ability to dynamically process signals across 13 independent wavelength channels. This channelization allows for simultaneous， parallel handling of multiple communication streams or signal sub-bands within the wide RF aperture， greatly enhancing spectral efficiency and multi-user capacity. Phased Array Link Performance： For the 64-element microwave photonic phased array link， detailed simulation confirmed that multi-channel parallel processing was feasible with minimal signal quality degradation. Critically， the complex beamforming and signal combining processes were achieved with a signal-to-noise ratio （SNR） degradation of less than 3 dB， indicating highly efficient optical processing and minimal noise introduction from the photonic beamforming network. Optical Frequency Comb Conversion Performance： The dual-OFC frequency conversion subsystem exhibited exceptional precision and spectral purity. Using an array of 23 finely spaced optical comb lines， the system realized the reconfigurable switching functionality for the 13 channels. Specifically， it accomplished the up-conversion of a 336 MHz Intermediate Frequency （IF） signal to the target 4~16 GHz RF band. The converted spectrum showed excellent characteristics： a well-defined channel spacing of 1 GHz， a spurious suppression ratio better than 20 dB （indicating minimal unwanted harmonic generation）， and a high in-band amplitude flatness with fluctuations not exceeding 4 dB across the entire 12 GHz range. This suite of metrics confirms the system's ability to perform precise， MHz-level frequency control and management with high spectral integrity.In conclusion， this research has successfully established the technical viability and superior potential of a microwave photonic approach to revolutionizing over-the-horizon communication. The proposed and analyzed system architecture， integrating a photonic phased array， a dual-optical-frequency-comb-based channelizer， and an integrated terminal， directly addresses and overcomes the fundamental bandwidth， interference， and flexibility constraints inherent in traditional electronic OTH systems. The simulation results provide compelling evidence that such a system can deliver wideband （4~16 GHz）， high-data-rate （200 Mbps）， multi-channel （13 channels）， and reconfigurable communication links with high spectral purity and efficient beamforming. Beyond the immediate performance metrics， this work lays a critical and substantial foundation for the future engineering and deployment of advanced OTH communication systems. It demonstrates a clear pathway toward building more secure， resilient， and high-capacity strategic communication infrastructures. Future work will naturally focus on the practical implementation of this architecture， including the development of integrated photonic chips to reduce size， weight， and power consumption （SWaP）， experimental validation in real-world propagation environments， and the exploration of advanced signal processing algorithms to further enhance performance. This study marks a significant step forward in the convergence of photonics and wireless communications for national security and civilian long-range connectivity applications.