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.