The rapid advancement of global information infrastructure, particularly in the context of global satellite internet and integrated space-air-ground networks, presents unprecedented challenges for modern communication systems regarding capacity and performance. While radio frequency (RF) communication has traditionally been the cornerstone of communication systems, it increasingly exhibits fundamental limitations at the physical layer: restricted spectrum availability, limited data transmission rates, and vulnerability to interference. These constraints have become particularly evident in B5G and subsequent-generation communication systems. Free space optical communication (FSOC) technology, characterized by its ultra-wide bandwidth, high-speed transmission, robust anti-interference capabilities, and enhanced security, has emerged as a promising solution for next-generation high-performance communication systems. It is widely acknowledged as a crucial enabling technology for establishing integrated space-air-ground information networks.

FSOC technology demonstrates a clear trajectory toward enhanced capacity, expanded coverage, and functional integration. A notable research direction involves the convergence of laser communication and ranging technologies. This integrated approach combines high-speed laser communication with high-precision laser ranging, enabling both large-capacity information transmission and precise navigation capabilities, spatial positioning, and multi-mission functionality. The increasing complexity of space missions necessitates enhanced satellite payload capabilities in terms of integration, environmental adaptability, and functional versatility. Integrated systems exhibit superior mission adaptability and system scalability, establishing themselves as the predominant direction for future satellite communication payloads.

Although there have been many advancements in establishing satellite-ground links, interstellar laser communication, network architecture design, modulation mechanisms, and system integration, and there has also been a comprehensive review of key FSOC technologies, a unified analytical framework that covers various FSOC system architectures and their critical technical components is still needed. Future FSOC systems must address complex scenarios involving multiple modulation formats and communication-perception coordination, placing increased demands on communication terminals regarding modulation adaptability and environmental awareness. This paper provides a systematic review of recent FSOC research achievements by major international space agencies and Chinese research institutions, analyzes the development status of various FSOC systems, and incorporates recent advances in integrated communication-ranging studies. Based on this analysis, the paper outlines the evolution toward multi-modal architectures supporting multiple modulation formats and integrated communication-ranging functionalities in future communication terminals. Additionally, this work presents design specifications and field experimental results from a self-developed multi-modal communication terminal board, offering theoretical guidance and research references for understanding FSOC system evolution, identifying technical challenges, and developing future-oriented integrated space-air-ground optical communication networks.

Table 1 summarizes representative FSOC achievements and specific parameters from various countries. The EDRS-A satellite, launched in 2016, successfully achieved its designated orbit, featuring a laser communication terminal with a 45000 km communication range and 1.8 Gb/s transmission rate. SA plans to launch DRS-D in 2025, targeting data communication capabilities of 80000 km range and 3.6 Gb/s transmission rate. In 2021, the Japanese JDRS satellite established successful two-way FSOC links with ground stations, achieving downlink data rates of 2.5 Gb/s from geosynchronous orbits. China initiated related projects in the 1990s. In 2024, the Changguang Satellite completed interstellar laser communication experiments at 10 Gb/s and 100 Gb/s, demonstrating technical capability for direct, high-speed transmission of high-resolution remote sensing images. Recent research has emphasized integrated laser communication and ranging technology, as detailed in Table 2. The U.S. lunar laser communication program in 2013 implemented an innovative combination of 1550 nm wavelength and pulse position modulation (PPM) technology, achieving sub-centimeter ranging accuracy (<1 cm) while enabling uplink speeds of 20 Mb/s and downlink speeds of 622 Mb/s. In 2024, the Beijing Institute of Telemetry Technology team achieved 14.1 mm ranging error using code element phase difference measurement with 625 Mb/s BPSK modulation.

While these studies have advanced multi-modulation compatibility mechanisms, hardware schemes, and sensitivity indices, most research remains limited to offline simulations or laboratory verifications. A comprehensive multimode modulation and demodulation link has not been established, and system functions remain predominantly separated. Furthermore, integrated communication and ranging design requirements have not been fully addressed.

This paper proposes and develops a multimodal communication ranging integrated system for space laser communication, building upon previous research findings. To evaluate the performance of the multimodal communication terminal under authentic long-distance spatial channel conditions, an FSO link spanning approximately 510 meters was established. The experiments were conducted using BPSK format at 625 Mb/s rate, with a symbol period of 1.6 ns. The achieved ranging root mean square error of 47 ps, approximately 2.9% of the codeword period, as presented in Table 4, demonstrates that the proposed system achieves high-accuracy ranging capability at the sub-codeword level while maintaining stable operation under real-time communication conditions. Fig. 8 illustrates the sensitivity of the multimodal receiver at various communication rates (0.625 Gb/s, 1.25 Gb/s, 2.5 Gb/s, 5 Gb/s, 10 Gb/s). The BER threshold of the FEC was established at 3.8×10^-3. The sensitivity measurement for the 10 Gb/s DP-QPSK signal reached -46 dBm.

This paper presents a systematic review of research advancements and engineering implementations in the field of air-heaven-heaven integrated FSOC network, both domestically and internationally. The FSOC system is evolving from single-link operations to a sophisticated system characterized by multi-layer architecture, broad coverage, and enhanced coordination capabilities. It is emerging as a crucial supporting technology for establishing high-bandwidth, low-latency, ubiquitous interconnected communication networks. This paper thoroughly examines the key technological challenges confronting FSOC systems, particularly regarding diversification of modulation formats, heterogeneity of link types, and integration of communication and ranging capabilities.

Based on the above discussion, this paper presents and develops a multimodal communication and ranging integrated terminal system designed for air-to-air fusion network applications. The system features multimodal modulation format compatibility, adaptive rate control, and sub-code level ranging fusion capabilities. The system employs a unified hardware and software architecture, supporting mainstream modulation modes including OOK, BPSK, QPSK, and DP-QPSK. It implements signal modulation/demodulation and integrated ranging functionality through an FPGA integration platform.

The system design methodology and experimental verification results outlined in this paper provide viable engineering approaches and technical support for developing next-generation FSOC communication terminals characterized by high adaptability, flexibility, and scalability. The research findings are anticipated to contribute significantly to the development of future air-to-air integrated network systems.