In response to the growing capacities and various losses in satellite communication, coherent detection has been investigated because of its high sensitivity and spectral efficiency. However, owing to the optical hybrid structure, conventional homodyne coherent detection schemes inherently suffer a 3 dB loss when the modulation order is increased from binary to quadrature or higher. In this paper, a novel coherent detection scheme based on a single balanced detector is proposed. By using the half-symbol rate as the frequency offset of the local oscillator, the OQAM signal was transmitted with an improved signal-to-noise ratio and a simpler structure than those of conventional coherent homodyne QAM detection. Mathematical analyses, simulations, and proof-of-concept experiments were performed on the proposed schemes.

Numerous researchers have studied innovations in future sixth-generation (6G) wireless communications. Indeed, a critical issue that has emerged is to contend with society's insatiable demand for high data rates and massive 6G connectivity. Some scholars consider one innovation to be a break-through-the application of free-space optical (FSO) communication. Owing to its exceedingly high carrier frequency/bandwidth and the potential of the unlicensed spectrum domain, FSO communication provides an excellent opportunity to develop ultrafast data links that can be applied in a variety of 6G applications, including heterogeneous networks with enormous connectivity and wireless backhauls for cellular systems. In this study, we perform video signal transmissions via an FPGA-based FSO communication prototype to investigate the feasibility of an FSO link with a distance of up to 20 km. We use a channel emulator to reliably model turbulence, scintillation, and power attenuation of the long-range FSO channel. We use the FPGA-based real-time SDR prototype to process the transmitted and received video signals. Our study also presents the channel-generation process of a given long-distance FSO link. To enhance the link quality, we apply spatial selective filtering to suppress the background noise generated by sun-light. To measure the misalignment of the transceiver, we use sampling-based pointing, acquisition, and tracking to compensate for it by improving the signal-to-noise ratio. For the main video signal transmission testbed, we consider various environments by changing the amount of turbulence and wind speed. We demonstrate that the testbed even permits the successful transmission of ultra-high-definition (UHD: 3840 × 2160 resolution) 60 fps videos under severe turbulence and high wind speeds.

In response to the growing capacities and various losses in satellite communication, coherent detection has been investigated because of its high sensitivity and spectral efficiency. However, owing to the optical hybrid structure, conventional homodyne coherent detection schemes inherently suffer a 3 dB loss when the modulation order is increased from binary to quadrature or higher. In this paper, a novel coherent detection scheme based on a single balanced detector is proposed. By using the half-symbol rate as the frequency offset of the local oscillator, the OQAM signal was transmitted with an improved signal-to-noise ratio and a simpler structure than those of conventional coherent homodyne QAM detection. Mathematical analyses, simulations, and proof-of-concept experiments were performed on the proposed schemes.

Numerous researchers have studied innovations in future sixth-generation (6G) wireless communications. Indeed, a critical issue that has emerged is to contend with society's insatiable demand for high data rates and massive 6G connectivity. Some scholars consider one innovation to be a break-through-the application of free-space optical (FSO) communication. Owing to its exceedingly high carrier frequency/bandwidth and the potential of the unlicensed spectrum domain, FSO communication provides an excellent opportunity to develop ultrafast data links that can be applied in a variety of 6G applications, including heterogeneous networks with enormous connectivity and wireless backhauls for cellular systems. In this study, we perform video signal transmissions via an FPGA-based FSO communication prototype to investigate the feasibility of an FSO link with a distance of up to 20 km. We use a channel emulator to reliably model turbulence, scintillation, and power attenuation of the long-range FSO channel. We use the FPGA-based real-time SDR prototype to process the transmitted and received video signals. Our study also presents the channel-generation process of a given long-distance FSO link. To enhance the link quality, we apply spatial selective filtering to suppress the background noise generated by sun-light. To measure the misalignment of the transceiver, we use sampling-based pointing, acquisition, and tracking to compensate for it by improving the signal-to-noise ratio. For the main video signal transmission testbed, we consider various environments by changing the amount of turbulence and wind speed. We demonstrate that the testbed even permits the successful transmission of ultra-high-definition (UHD: 3840 × 2160 resolution) 60 fps videos under severe turbulence and high wind speeds.

Abstract A flexible full-color micro-LED display with high mechanical robustness was fabricated by printing quantum dots (QDs) on a blue micro-LED array using standard photolithography. The red and green colors yielded from QDs exhibit a better color gamut than conventional color filters. The light conversion efficiency was enhanced by adding TiO 2 nanoparticles to the QD-photoresist composite. This full-color micro-LED display was successfully mounted on various unusual substrates such as curved glass, fabrics, and human skin, enabling diverse optoelectronic applications. In addition, wireless multi-channel visible light communication (VLC) based on the wavelength-division-multiplexing orthogonal-frequency-division-multiplexing (WDM-OFDM) technique was demonstrated using a QD-based color micro-LED panel. A high data transmission rate of 1.9 Gbps was successfully obtained owing to the high electrical–optical modulation bandwidth of the QD-based micro-LED panel.

We propose an optical IQ modulator-based nonlinear analog self-interference cancellation which can mitigate performance degradation caused by nonlinearity of the analog circuit. We achieved 29 dB cancellation with enhance signal-of-interest recovery performance in proof-of-concept experiment.

We propose a wavelength-reuse coherent orthogonal frequency division multiplexing (OFDM) uplink optical transmission scheme that employs downlink intensity modulation (IM) and uplink phase modulation (PM). In the proposed approach, the uplink OFDM signal is generated by a single optical phase modulator, which uses the intensity-modulated downlink signal as its optical source. Consequently, the uplink signal contains both IM and PM components. By leveraging the orthogonality between IM and PM in coherent detection, the PM component—which represents the desired uplink signal—can be effectively recovered. Since optical phase modulation provides a larger Euclidean distance than optical intensity modulation, higher signal power can be reliably achieved through coherent detection. Furthermore, the proposed uplink transmitter eliminates the need for an optical source as well as temperature or bias controllers, thereby simplifying the overall transmission scheme. We experimentally demonstrated bidirectional 10-km optical transmission using two 1-GHz 16QAM-OFDM signals centered at 1.9 and 3.4 GHz, successfully validating the proposed technique.

We propose a novel optical transmission technique based on orthogonal frequency division multiplexing non-orthogonal multiples access (OFDM-NOMA) for multi-distance passive optical network with direct detection. This method uses diversity of dispersion-induced power fading in single-mode fiber channels to suppress the interference caused by non-orthogonally multiplexed OFDM. Since chromatic dispersion in optical fiber involves the conversion between optical intensity and phase modulations, an arbitrary frequency can be suppressed at a specific transmission distance by adjusting the intensity-to-phase modulation ratio at the transmitter. In the proposed technique, two optical signals are polarization-multiplexed to suppress the NOMA frequency band across different transmission distances. As chromatic dispersion suppresses interference between NOMA signals, direct detection using a single photodetector is possible without polarization demultiplexing and successive interference cancellation. 23 and 43 km transmission of 8 Gb/s polarization-multiplexed OFDM-NOMA was performed and successfully verified the proposed technique.