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Abstract
Space–air–sea communication networks are of great interest to meet the demand for close and seamless connections between space, land, and ocean environments. Wireless light communication can expand network coverage from land to the sky and even the ocean while offering enhanced anti-interference capabilities. Here, we propose and establish an all-light communication network (ALCN) for space–air–sea integrated interconnection, which merges underwater blue light communication, wireless white light communication, solar-blind deep ultraviolet light communication and laser diode-based space communication. Ethernet switches and the Transmission Control Protocol are used for space–air–sea light interconnection. Experimental results show that the ALCN supports wired and wireless device access simultaneously. Bidirectional data transmission between network nodes is demonstrated, with a maximum packet loss ratio of 5.80% and a transmission delay below 74 ms. The proposed ALCN provides a promising scheme for future space–air–sea interconnections towards multiterminal, multiservice applications.
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1. Introduction
An integrated communication network (ICN) is an extension of free-space communication to the sea, covering the space, air, surface, and underwater domains [1–4]. Such an ICN enables real-time dynamic networking and adapts network methods to meet practical demands. The multidomain interfaces of the ICN facilitate information sharing, allowing image, video, and sensing services to be transferred between any network nodes [5]. In recent years, wireless light communication has received much attention as the subject of studies conducted from various perspectives [6–14]. By using laser diodes (LDs) with wavelengths of 1537 nm and 1563 nm, deep-space full-duplex light communication has been established, with a transmission rate over 20 Mbps at a distance of 580 km [15]. Kojima et al. demonstrated 280 nm light-emitting diode (LED)-based communication at 1.5 m in direct sunlight, achieving a 1.6 Gbps transmission rate modulated via pulse amplitude modulation (PAM) [16]. A data rate of 2 Gbps over a 3 m air–underwater channel was achieved with a 75-µm single-layer quantum dot blue micro-LED [17]. In 2021, Chen et al. reported underwater optical wireless communication with PAM, realizing a data rate of 500 Mbps within 150 m [18]. Previous research efforts in high-speed wireless light communication have primarily focused on offline processing in MATLAB, separate from practical engineering deployments. While some studies have achieved real-time transmission, they are often tailored to specific scenarios and lack interoperability with other communication systems. To address these limitations, we propose and establish an all-light communication network (ALCN) that integrates underwater blue light communication (BLC), wireless white light communication (WLC), solar-blind deep ultraviolet communication (DUVC), and laser diode-based communication (LC). This network enables seamless connectivity across space, land, and ocean environments, facilitating bidirectional real-time data transmission between network nodes in diverse scenarios. Open access is granted to any node within the ALCN, supporting both wired and wireless devices. This paper describes the concept, architecture, and demonstration of the ALCN, providing a feasible scheme for future space–air–sea interconnections towards multiterminal, multiservice deployments.
2. Overview of ALCN
In this paper, we use 4 spectra of light to establish wireless light communication links for distinct application scenarios. Figure 1 shows an overview of the ALCN spanning space, air, and sea. Pure seawater presents a reduced absorption window for blue-green light, allowing it to travel further underwater compared to other wavelengths. Vessels use blue LEDs to control unmanned underwater vehicles (UUVs) to complete exploration operations, and marine intelligent devices establish communications with buoys through blue light to create an underwater interface [19,20]. On the sea, white LEDs are used to transmit information between objects through illumination [21]. The deep ultraviolet (DUV) spectral band (100 nm–280 nm) faces challenges in reaching the surface due to absorption by the ozone layer, which creates a favorable background for communications [22]. In the air, DUV LEDs are used for unmanned aerial vehicle (UAV) cluster solar-blind communication [23,24]. While LED-based communication offers a wider divergence angle, it is hindered by low received optical power. For point-to-point communication in free space, the use of LDs is crucial, as LDs emit directional light with high optical power [25,26]. Neighboring light communication links are connected via a gateway, which provides wired interfaces and wireless fidelity (Wi-Fi) to achieve information sharing within the ALCN.
For the WLC, the light emitted by a white LED was characterized using an Ocean Optics USB4000 spectrometer. The electroluminescence (EL) spectra of the device measured under different injection currents are plotted in Fig. 2(a). The white light generated by this device is a mixture of light produced by yellow phosphor excited by blue LEDs [27,28]. As the injection current of the device is increased from 10 mA to 60 mA, the dominant blue spectral peak shifts by 0.5 nm from 449.0 nm to 449.5 nm, resulting in screening of the polarization-induced electric fields [29], while the excited spectral peak remains at a constant value of 585.0 nm. The bandwidth of the ALCN is associated with each light communication link and depends on the modulation characteristics of the LEDs. The white LED was also pulsed by a Keysight E5080A Network Analyzer, and the light signals were harvested by the analyzer for measurement through a Hamamatsu C12702-11 photodiode module. As shown in Fig. 2(b), as the offset voltage of the device is increased from 10.5 V to 12.5 V, the 3-dB bandwidth of the device shows an upwards trend, with a peak of 1.91 MHz. Considering the heat dissipation of the device, setting the voltage of the white LED near 12 V is a priority for achieving the optimal bandwidth of the ALCN, and this method also works for the LEDs operating in the other spectra. Figure 2(c) illustrates the 3-dB bandwidth at the appropriate offset voltage for the emitting devices employed in the ALCN. Among these devices, the DUV device stands out with the highest 3-dB bandwidth of 42.50 MHz. This device utilizes a thin-film flip-chip geometry, enabling a reduction in forward voltage and an enhancement in light extraction. The sapphire substrate is polished down to 400 μm, while the epitaxial films, approximately 4.5 μm thick, consist of an n-type AlGaN layer, a multiple-quantum-well layer, and a p-type AlGaN layer. However, due to the limited 3-dB bandwidth of 2.30 MHz for blue LEDs and 1.69 MHz for white LEDs, the modulation rates of the BLC and WLC links are lower compared to those employing DUV LEDs and LDs.
Fig. 2. (a) Measured EL spectra of a white LED with the current increasing from 10 mA to 60 mA. (b) Measured 3-dB bandwidth of the white LED with the offset voltage increasing from 10.5 V to 12.5 V. (c) Measured 3-dB bandwidth at the appropriate offset voltage for the emitting devices used in the ALCN.
An ALCN spanning space, air, and sea was formed using light in spectra of 275 nm, 452 nm, 519 nm, and 585 nm, as shown in Fig. 3. The ALCN consists of 4 full-duplex wireless light communication links, which are connected in series via Ethernet switches (ESes). The ESes on each side of the links can be viewed as transit nodes, which provide access to devices such as sensors and personal computers (PCs). If wireless device access is needed, a Wi-Fi module is employed at the transit node to provide data access for PCs and mobile devices. When the interfaces of a transit node cannot accommodate the accessing devices, multiple ESes can be wired together for interface expansion to meet the needs of multiterminal deployment.
Fig. 3. Schematic of the ALCN framework, which is composed of 4 full-duplex light communication links through ESes.
To unify the transmission mode, all of these wireless light communication links use Registered Jack-45 network interfaces. In the case of BLC link, it is divided into two symmetrical ends, and each end has a transmitter and a receiver. As illustrated in Fig. 3, when an underwater Internet Protocol camera (IPC) captures video and feeds it into transit node 1 (T1), the video signals are coded into blue light signals by the BLC transmitter. On the opposite side, the light passing through the lenses is filtered by an optical bandpass filter (OBPF) with a dominant spectrum of 450 nm, a half bandwidth of 10 nm and a transmittance of 45% to isolate the desired light signals from those in the other spectra; this is also the case with the other links. The light signals are converted into electrical signals by an avalanche photodiode (APD) and then decoded and sent to T2 by the BLC receiver. Commercially available 585 nm LEDs are used to form a WLC link, which transmits the IPC signals of T2 in the form of a searchlight. Afterwards, the IPC signals at T3 are sent to the next transit node via the DUVC, for which 275 nm LEDs, quartz lenses with antireflection film, and APDs are adopted for highly sensitive detection of light at a wavelength of 275 nm. Finally, the IPC signals of T4 are coded into laser signals and restored to the original video stream through the LC.
Experimental verification confirms that the BLC link can achieve a transmission distance of 12 m in a swimming pool, where the neutral density (ND) factor is 256 and the nephelometric turbidity unit is 1.7 [30]. Figure 4(a) demonstrates that the BLC link can be conveniently aligned within a 20-degree angle for establishing communication. However, as turbidity and water surges intensify, the received signal weakens, and reflections may cause optical self-interference. To counteract signal deterioration, adjustments to the optical and electrical gains are necessary. On the other hand, the WLC link facilitates communication within a land range of 150 m, as depicted in Fig. 4(b). Consequently, WLC finds application in surface beacons, buoys, and vessels for accurate reporting of oceanographic conditions. In bright sunlight, the DUVC link in Fig. 4(c) can establish solar-blind communication within a 20-degree angle and a maximum range of 7 m, with an ultraviolet index of 8 and a shell temperature of 75 ℃ [31]. Additionally, due to the smaller divergence angle of LDs, the LC link in Fig. 4(d) relies on stabilizers and attitude sensors to sustain aiming during communication, compensating for beam drift caused by mechanical vibrations. As shown in Fig. 4(e), ND filters are used in the BLC and LC links to limit the experimental area, the IPC video signals pass through 4 wireless light communication links, and full-duplex real-time video communication between T1 and T5 is demonstrated. The real-time conditions of sensor 1 and sensor 2 are presented on a PC connected to the ALCN via Wi-Fi. Moreover, addressable transmission of images and audio between the transit nodes is established via the Transmission Control Protocol (TCP), as demonstrated in Visualization 1. Here, any transit node of the ALCN permits open access via TCP, with both wired and wireless devices allowed.
Fig. 4. (a) Underwater channel formed by the BLC link in a swimming pool. (b) Communication during illumination formed by the WLC link. (c) Solar-blind communication in sunlight formed by the DUVC link. (d) Free space communication formed by the LC link. (e) Photograph of the ALCN, in which full-duplex real-time video communication between T1 and T5 is demonstrated (see Visualization 1).
3. Results and discussion
During communication, the average power consumption of the entire ALCN, including network switches and Wi-Fi modules, amounts to 155 W. The four light communication links account for 77.42% of this power. The optical power of each link is measured using a PM100D optical power meter at a distance of 3 m from the transmitter. The power remains stable throughout 3.5 hours of continuous operation, as depicted in Fig. 5(a). When a pseudorandom binary sequence (PRBS) signal is applied with a transmission rate of 4 Mbps to the BLC transmitter, the received eye diagram is as depicted in Fig. 5(b). Similarly, Fig. 5(c) illustrates the eye diagram of the received signals from the DUVC link with a 20 Mbps PRBS signal. In both cases, clear and open eye diagrams are observed.
Fig. 5. (a) Optical power variation of the BLC, WLC, DUVC, and LC links for continuous operation. (b) Eye diagram of the received signals on the WLC link, with the PRBS signals at a transmission rate of 4 Mbps. (c) Eye diagram of the received signals on the LC link, with 20 Mbps PRBS signals. (d) Measured received signals on the sequential links. (e) Actual throughput of each link in the ALCN. (f) PLR results for mutual access between the transit nodes of the ALCN. (g) Delay results for mutual access between the nodes. (h) Jitter results for mutual access between the nodes.
Based on the ALCN framework in Fig. 3, a PC at T5 simultaneously accesses IPC 1, IPC 2, sensor 1, and sensor 2. Meanwhile, the received signals on the BLC, WLC, DUVC, and LC links are sent to a RIGOL DS1104Z digital storage oscilloscope for characterization, with the results plotted in Fig. 5(d). The BLC and WLC links utilize non-return-to-zero on-off keying to drive metal-oxide-semiconductor field-effect transistors, achieving a maximum transmission rate of 2 Mbps, owing to the higher currents and lower 3-dB bandwidths of the blue and white LEDs. The DUVC and LC links, however, achieve a 10 Mbps transmission rate through bias-tee modules, accommodating a greater load of node data while adhering to the timing constraints of the Xilinx Spartan-6 field-programmable gate array. Throughput refers to the effective network bandwidth and represents the network’s maximum capacity to transmit data. When simultaneously feeding 2560 × 1440 pixel real-time videos at 22 frames per second (FPS) into T3 and T4, the actual throughput of the bidirectional DUVC link is shown in Fig. 5(e). With a video segment length of 1460 bytes for measurement, the throughput of the DUVC link measures at 7.78 Mbps, slightly lower than the 10 Mbps transmission rate, due to congestion control and other transmission overheads. At the same time, the BLC and WLC links exhibit a throughput of 1.53 Mbps, allowing bidirectional transmission of 1920 × 1080 pixel videos. Although the ALCN has a maximum throughput of 7.78 Mbps, the rate of services transmitted via BLC or WLC links will be limited to 1.53 Mbps due to a bottleneck. To overcome this bottleneck, it is necessary to prepare LEDs with improved modulation characteristics and adopt more advanced modulation techniques. The triad of red, green, and blue LEDs can emit white light for illumination, offering wavelength-division multiplexing light communication, which has the potential to effectively enhance throughput.
The packet loss ratio (PLR), delay, and jitter are key quantitative indicators for Internet Protocol-based networks and may notably affect the transmission quality of the ALCN [32,33]. Figure 5(f) illustrates the PLR results for mutual access between the transit nodes of the ALCN, where a PC wired to the network acts as the accessing node and an IPC serves as the accessed node. When a maximum transmission unit of 1514 bytes is used for measurement, the mean PLR of the entire ALCN in addition to the self-testing nodes is 2.78%, with a maximum PLR of 5.80%. The PLR shows steady growth towards the edge of the ALCN due to link error accumulation, and the increase in the PLR per link is calculated to be 1.42%. The delay reflects the timeliness of transmission, to which voice services are especially sensitive [34]. As shown in Fig. 5(g), due to the higher throughput of the DUVC and LC links, the mean delays between T3 and T4 and between T4 and T5 are 6.45 ms. However, once a signal passes through the BLC or WLC link, the signal will accumulate a delay of approximately 29.85 ms. Delay jitter degrades real-time transmission, causing lags and interruptions in live streaming. In Fig. 5(h), the maximum jitter is 15 ms, which occurs when signals pass through the longest path in the ALCN. In this case, the delay is approximately 66 ms to 81 ms with a maximum PLR of 5.80%, which is demonstrated in Visualization 1, from which it can be seen that the video communication between T1 and T5 is clear with little lag.
4. Conclusion
In summary, we have established an ALCN for space–air–sea integrated communication, whose coverage can span space, land and ocean environments. Full-duplex real-time video communication between network nodes has been demonstrated, with a maximum PLR of 5.80% and a transmission delay below 74 ms. With the assistance of a modem, the ALCN aims to provide wireless access to the Internet based on the TCP/IP scheme. For multiterminal, multiservice interconnections in Internet of Things applications, it will be crucial to build mobile communication based on this ALCN and utilize advanced modulation to increase the throughput.
Funding
National Key Research and Development Program of China (2022YFE0112000); National Natural Science Foundation of China (61827804, 62005130, 62274096, U21A20495); Natural Science Foundation of Jiangsu Province (BK20200755); 111 Project (D17018); Suzhou Innovation and Entrepreneurship Leading Talent Plan (ZXL2020223).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are available from the authors upon reasonable request.
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