Xicong Li

Free space optics (FSO) provides innovative and cost-effective connectivity for future sixth generation (6G) terrestrial backhaul networks (TBNs). However, atmospheric conditions such as fog and smog can severely disrupt FSO performance. In this letter we proposed for the first time the use of an acoustic wave strategy to mitigate dense fog in FSO to optimise the performance of TBNs. Acoustic agglomeration has previously been studied for fog removal or air purification systems, but seldomly for FSO applications. In this study, an indoor optical chamber was designed to evaluate the proposed system over a propagation link range of 0.6-1.0 m. The results showed that FSO link visibility was improved by 126 m in dense fog, with an improved transmittance up to 65%.

In this paper, two techniques including uplink positioning and multiple-input multiple output (MIMO) singular value decomposition (SVD) beamforming are proposed and utilized to provide a novel physical layer security (PLS) scheme for visible light communication (VLC) systems, which much enhance the integrated location-sensing and data security strength. The paper first develops a closed-form mathematical model for ubiquitous uplink positioning using a single light-emitting diode with the trilateration algorithm, considering the field-of-view constraints due to the user’s optical transmitter and access point receivers. SVD in MIMO VLC is subsequently introduced to exploit the acquired user’s location to form a pair of public and private keys for secure data transfer over the physical medium. The proposed system performance is evaluated in terms of the root mean square positioning error ε_X and the achieved bit error rate (BER) for the uplink positioning and the downlink data transmission, respectively. Results show that the effective area of the photodiode significantly impacts ε_X, with smaller areas resulting in higher ε_X. It is also observed that the ε_X directly relates to the strength of ambient light inducing noise in the system. However, the proposed uplink positioning is less prone to natural ambient light as compared to the traditional downlink visible light positioning. The BER analysis of the downlink data transmission shows that the proposed SVD-based PLS enhances the system’s confidentiality by significantly narrowing the communication zone area by over 90% in comparison to the zero-forcing beamforming scheme and achieves reasonable secrecy capacity.

This study investigates the effect of helium-implanted edge termination (ET) on GaN-on-GaN Schottky barrier diode (SBD) temperature sensors. Devices with and without ET of three different diameters are fabricated and characterized from 298 to 473 K. Based on the conventional calculation method, the results show that the sensitivity (Sm) of the device with ET is lower than that of the device without ET at the same given current with the same anode area. Technology Computer Aided Design (TCAD) simulation reveals that ET can restrict the current to a smaller area, resulting in a larger actual current density (JA) for devices with ET. After correcting (JA) in the sensitivity calculation, the two types of devices exhibit the same corrected sensitivity (Sc) values, which also follow the fundamental linearity theory of sensitivity versus the natural logarithm of JA with high sensitivity corresponding to the lower current density. This pseudo-degradation phenomenon of sensitivity after the ET process can serve as a reference for in situ temperature monitoring in the gallium nitride (GaN)-based integrated-circuit design.

In this study, a Ni/GaN Schottky barrier diode (SBD) with both groove beveled and oxygen plasma terminations was fabricated and evaluated. The mixed termination structure was formed by inductive coupled plasma (ICP) in oxygen atmosphere. Confirmed by the measurement results of transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS), a 3.5-nm-thick GaO _x passivation layer and beveled termination with an angle of 65° were identified around the edge of the electrode. Compared with the conventional samples, the treatment samples demonstrated the reverse leakage current reduced by one order of magnitude, the ON-resistance reduced by approximately 20%, and the breakdown voltage increased by 80%. Further technology computer-aided design (TCAD) simulation shows that the mixed termination structure can effectively inhibit the electric field concentration effect. Finally, temperature dependence characteristics show that the zero-temperature coefficient (ZTC) bias points of treatment samples locate in the low-voltage region, indicating that the devices are more suitable for the integrated circuit (IC).

With an emphasis on the real-time implementation of a practical carrier-less amplitude and phase (CAP) receiver for visible light communication (VLC), this paper proposes a full-digital architecture for the $M$ -CAP receiver with synchronisation and adaptive blind equalisation. The architecture is mainly based on the observation that a CAP signal can be demodulated using a quadrature and amplitude (QAM) receiver with an added counterclockwise rotation operation. The proposed CAP receiver employs two digital phase-locked loops (DPLL) to realise the symbol timing and carrier synchronisation, which are the most critical issues in practical systems. With the constant-modulus algorithm (CMA) offering the benefit of decoupling carrier and symbol timing synchronisation, the receiver can blindly equalise the distorted signal at the output of symbol timing synchronisation. Finally, the performance of 4/16/64-CAP receivers, which are designed using this architecture and operates at 80/160/240 Mbit/s through a low-pass light-emitting diode (LED) with a 3-dB bandwidth of 7 MHz cascaded with three measured optical wireless channels, is evaluated by Monte Carlo simulations. Results show that the receiver architecture can successfully solve the symbol timing and carrier synchronisation problems and mitigate inter-symbol interference caused by LED and/or wireless channels. The design framework for a full-digital CAP receiver has been laid out in this paper.

An equivalent circuit model for a typical commercial light-emitting diode (LED) used in visible light communication (VLC) applications is proposed. The parameters of the elements in the model are extracted from the measured impedance using a vector network analyser. The model shows high accuracy in fitting the measured results and shows the effect of bonding/packaging parasitics and the electrostatic discharge (ESD) component on the LED's impedance and bandwidth. This study aims to raise the designer's awareness when designing and fabricating high-speed LED/ μLED devices to achieve the optimal modulation bandwidth in the emerging VLC applications.

This paper experimentally demonstrates a VLC sys-tem using a large off-the-shelf LED panel with dimensions of 60 × 60 cm ² and achieves a data rate of 40 Mb/s based on multi-band carrier-less amplitude and phase (CAP) modulation. The proposed system demonstrates the potential of utilising the existing LED-based lighting infrastructure for simultaneous indoor illumination and data communication.

In this Letter, we propose a new configuration for visible light communication systems, which results in doubling of the data rate due to the use of polarization division multiplexing. As light-emitting diodes are unpolarized incoherent light sources, we isolate both the perpendicular s and parallel p modes for independent modulation. For the first time, to the best of our knowledge, we show that it is possible to transmit and successfully recover two separate orthogonal frequency division multiplexing (OFDM) signals on each polarization (pol-OFDM). Furthermore, we compare the performance of the pol-OFDM system with the transmission of a single conventional OFDM signal without a polarizer over the same physical link. We show that similar bit error rates can be achieved while obtaining ∼ 45 % improvement in both the data rate and spectral efficiency due to polarization multiplexing.