In recent studies, visible light communication (VLC) has been predicted to be a prospective technique in the future 6G communication systems. To suit the trend of exponentially growing connectivity, researchers have intensively studied techniques that enable multiple access (MA) in VLC systems, such as the MIMO system based on LED devices to support potential applications in the Internet of Things (IoT) or edge computing in the next-generation access network. However, their transmission rate is limited due to the intrinsic bandwidth of LED. Unfortunately, the majority of visible light laser communication (VLLC) research with beyond 10 Gb/s data rates concentrates on point-to-point links, or using discrete photodetector (PD) devices instead of an integrated array PD. In this paper, we demonstrated an integrated PD array device fabricated with a Si-substrated GaN/InGaN multiple-quantum-well (MQW) structure, which has a 4×4 array of 50μm×50μm micro-PD units with a common cathode and anode. This single-integrated array successfully provides access for two different transmitters simultaneously in the experiment, implementing a 2×2 MIMO-VLLC link at 405 nm. The highest data rate achieved is 13.2 Gb/s, and the corresponding net data rate (NDR) achieved is 12.27 Gb/s after deducing the FEC overhead, using 2.2 GHz bandwidth and superposed PAM signals. Furthermore, we assess the Huffman-coded coding scheme, which brings a fine-grain adjustment in access capacity and enhances the overall data throughput when the user signal power varies drastically due to distance, weather, or other challenges in the channel condition. As far as we know, this is the first demonstration of multiple visible light laser source access based on a single integrated GaN/InGaN receiver module.

Although the 5G wireless network has made significant advances, it is not enough to accommodate the rapidly rising requirement for broader bandwidth in post-5G and 6G eras. As a result, emerging technologies in higher frequencies including visible light communication (VLC), are becoming a hot topic. In particular, LED-based VLC is foreseen as a key enabler for achieving data rates at the Tb/s level in indoor scenarios using multi-color LED arrays with wavelength division multiplexing (WDM) technology. This paper proposes an optimized multi-color LED array chip for high-speed VLC systems. Its long-wavelength GaN-based LED units are remarkably enhanced by V-pit structure in their efficiency, especially in the “yellow gap” region, and it achieves significant improvement in data rate compared with earlier research. This work investigates the V-pit structure and tries to provide insight by introducing a new equivalent circuit model, which provides an explanation of the simulation and experiment results. In the final test using a laboratory communication system, the data rates of eight channels from short to long wavelength are 3.91 Gb/s, 3.77 Gb/s, 3.67 Gb/s, 4.40 Gb/s, 3.78 Gb/s, 3.18 Gb/s, 4.31 Gb/s, and 4.35 Gb/s (31.38 Gb/s in total), with advanced digital signal processing (DSP) techniques including digital equalization technique and bit-power loading discrete multitone (DMT) modulation format.

Visible light communication (VLC) has emerged as a promising communication method in 6G. However, the development of receiving devices is much slower than that of transmitting devices, limited by materials, structures, and fabrication. In this paper, we propose and fabricate an InGaN/GaN multiple-quantum-well-based vertical-structure micro-LED-based photodetector (μPD) on a Si substrate. A comprehensive comparison of the photoelectrical performance and communication performance of three sizes of μPDs, 10, 50, and 100 μm, is presented. The peak responsivity of all three μPDs is achieved at 400 nm, while the passband full-widths at half maxima are 87, 72, and 78 nm for 10, 50, and 100 μm μPDs, respectively. The -20dB cutoff bandwidth is up to 822 MHz for 50 μm μPD. A data rate of 10.14 Gbps is experimentally demonstrated by bit and power loading discrete multitone modulation and the proposed digital pre-equalizer algorithm over 1 m free space utilizing the self-designed 4×4 50 μm μPD array as a receiver and a 450 nm laser diode as a transmitter. This is the first time a more than 10 Gbps VLC system has been achieved utilizing a GaN-based micro-PD, to the best of our knowledge. The investigation fully demonstrates the superiority of Si substrates and vertical structures in InGaN/GaN μPDs and shows its great potential for high-speed VLC links beyond 10 Gbps.

In this paper, we propose a 36-quadrature amplitude modulation (QAM) superposition modulation technique that is featured with uneven symbol probability by nonlinear precoding, named nonlinear coded nonuniform superposition (NCNS) QAM. Its aim is to alleviate the nonlinearity effect caused by high instantaneous power in multi-input single-output (MISO) visible light communication (VLC) system, with an uneven probabilistic-shaped constellation. The transmitter includes two LEDs to send signals independently, and the receiver uses a photo detector to receive the superposed QAM signal. The experiment results show that NCNS has a better robustness against nonlinearity than pulse amplitude modulation 4, approximately gaining a 16% increase in maximum usable peak-to-peak voltage and a 33% enlargement in dynamic range area. It is a simple but effective approach to solve the bandwidth limits related to signal power and hopefully be applied in large power VLC systems such as underwater VLC, or to improve the robustness against power fluctuation.