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.

To ensure high-quality communication, it’s of great value to use digital pre-distortion (DPD) to linearize the core component power amplifier (PA) of terahertz/mmWave transceiver. In this work, we propose transfer learning with Transformer and LSTM for DPD of terahertz/mmWave transceiver, which uses Transformer based PA behavioral model to train effective and lightweight LSTM model for DPD. To collect and analyze the signal data of terahertz/mmWave transceiver, we set up a physical platform for the D-band system. Through experiments, we show that the proposed method is capable of significantly reducing in-band and out-band distortion while avoiding the excessive complexity of DPD models.

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.

Coherent detection has emerged as a key technology for advancing passive optical networks (PON) beyond 100 Gbps per wavelength, due to its advantages over the intensity and direct-detection (IM/DD) approach, which was previously dominant in PON systems. In coherent PON architectures, the cost of the optical network unit (ONU) is substantial due to the complexity of its transceivers. Consequently, numerous studies have focused on simplifying the transmitters and receivers on the ONU side. Yet, simplifying the ONU transceiver introduces issues like reduced dynamic range and lower data rates. How to make a balance between the performance and the cost, particularly in terms of component complexity, represents a critical challenge for the advancement of coherent PON. Therefore, we propose and demonstrate a hybrid, multi-format, and flexible-rate bidirectional coherent PON system, supporting a compatible OLT and ONUs with different types of transceivers. ONUs are categorized into different tiers based on their performance and cost requirements, with transceivers of varying complexity allocated accordingly. As a demonstration of concept, we have successfully conducted experimental transmissions of 25-GBaud 4/16/64- quadrature-amplitude-modulation signals across 20-km fiber in a bidirectional setup, achieving data rates ranging from 50-Gbps to 300-Gbps. The low-end, middle-end, and high-end ONUs attain power budgets of 39/31/21 dB, 43/36/26 dB, and 40/33/23 dB, respectively. This architecture serves as an effective bridge from the current 50G IM/DD PON to the anticipated 200G coherent PON, meeting the varied requirements of users at different service levels.