High-altitude and low Earth orbit (LEO) platforms assisted free-space optical (FSO) communication systems have emerged as a promising solution to facilitate the development of future integrated space-air-ground networks, owing to their expansive coverage, high bandwidth, unlicensed spectrum, and large-capacity. However, these systems are highly susceptible to the adverse effects of atmospheric turbulence, degrading overall system performance and posing a risk of signal outages. In this paper, we propose a novel semantic communication paradigm, termed prior probability-based vector quantized autoencoder (PVQAE), to enable uninterrupted and efficient image transmission in free-space optical communication systems under turbulent conditions. Specifically, it utilizes a neural network-based encoder-decoder architecture for semantic information extraction and image reconstruction, and achieves uninterrupted transmission of semantic information in turbulent channels via vector beams shift keying without the need of any adaptive optics for beam compensation. A shared discrete codebook, designed for both the transmitter and receiver, facilitates latent feature representation and enables the quantization of semantic information for light-field shift keying transmission. Moreover, a power differential detection (PDD) algorithm is derived to reduce the complexity associated with receiver-side detection. Simulation results demonstrate the superior performance of PVQAE over benchmark schemes, such as conventional methods using JPEG for image source coding and Reed-Solomon (RS) for channel coding, in terms of structural similarity index (SSIM) and compression ratio (CR) product, and classification accuracy. These results highlight the effectiveness of the proposed framework in achieving robust and efficient transmission without interruption in turbulent channels.

The next generation of mobile communication puts forward higher requirements for communication rate and capacity. However, the current congested spectrum resources also make the expansion of higher frequency band communication more urgent. Visible light communication (VLC) has nearly 400 THz of spectrum resources and the potential for high-speed communication. To expand communication rate, it is necessary to explore multiplexing technologies of higher dimensions. This paper constructed a 4-wavelength wavelength division multiplexing (WDM) system based on commercial and homemade red and green lasers. To obtain a high beam quality for mode control, we utilized a lithium niobate (LiNbO3) external modulator to perform intensity modulation on the output beam of a laser driven by a stable bias current, and based on laser mode converters (LMCs), obtained 4 transverse electromagnetic modes (TEMs) for mode division multiplexing (MDM). Finally, based on these 16 channels, a total transmission rate of 269.37 Gbps was achieved in 1m of free space.

The advancement of current data-intensive services imposes increasingly demands on next-generation mobile communication systems. Visible light communication (VLC) offers significant potential for the development of next-generation communication networks. It has an abundant spectrum resource of up to 400 THz, exhibits immunity to electromagnetic interference, and its blue-green band coincides with the underwater transmission window, enabling high-speed communication with high signal-to-noise ratio (SNR). In this paper, we demonstrated a 532 nm visible light coherent communication (VLCC) system based on a lithium niobate phase modulator, and root raised-cosine (RRC) pulse shaped binary phase-shift keying (BPSK) and coherent detection scheme. Compared to intensity modulation with direct detection schemes, our system mitigates signal distortion induced by frequency chirp and relaxes the demand for high sensitivity at the receiver photodetector. Finally, we successfully achieved 12 Gbaud BPSK signal transmission. To the best of our knowledge, this represents the highest reported data rate for blue-green band visible light coherent communication to date.

Over the past two decades, advances in materials science and electronics have greatly supported the progress of the communication industry, and the emergence of new materials has made it possible to design higher performance communication modules. As a potential key technology for future 6G networks, the standardisation and industrialisation of visible light communication (VLC) cannot be separated from the improvement of the communication devices. In this paper, we first analyse and summarise the performance enhancement of new materials and designs of VLC devices. Then the support of improved capabilities of novel VLC devices for 6G new services is prospected. As device performance improves, VLC is expected to support more scenarios in 6G.

Visible light communications (VLC) is considered a potential key technology for 6G. However, the performance of transceiver devices of VLC determines the upper limit of system. InGaN/GaN based light-emitting diodes (LEDs) have widely been used as the transmitter. In this paper, a VLC transceiver system is designed on the basis of InGaN/GaN blue light laser diode (LD). The transmitted signals are designed based on the 5G new radio (NR) physical layer protocol. The experiment verifies the feasibility of 256 quadrature amplitude modulation (QAM) and the blue LD-based VLC system can achieve up to 14Gbit/s system sum rate when adopting 5G NR protocol, which contributes to the application of VLC in 6G systems.