Photonic THz Communications Based on Radio-Over-Fiber Technology for 6G Mobile Network: Design and Opportunity

Abundant available spectral resources in terahertz (THz) bands enables opportunities to develop next-generation mobile communications. However, devices with large operating bandwidths are required to exploit the benefits offered by the THz band, which significantly increases the cost and complexity of implementation. In addition, THz frequencies experience a relatively large propagation loss, which inevitably reduces network coverage; thus, numerous antenna sites are required to provide reliable mobile services. Therefore, implementing antenna-sites for a THz-band 6G mobile network may be limited by various design restrictions, such as the high power, complexity, and cost requirements. A photonic-based approach for a THz communication system can offer an efficient solution for simplified antenna-site architecture. THz-wave generation using optical heterodyne can reduce the implementation burden on the antenna-site through the use of complex electrical signal up-conversion. When combined with radio-over-fiber (RoF) technology, the benefits of analog-waveform transmission can be achieved, such as bandwidth-efficient transmission in a mobile transport link. This study presents a comprehensive investigation of design guidelines for implementing an RoF-based photonic THz communication system. We investigate a major performance-limiting factor by analyzing noise contribution in a fiber-wireless link through theoretical and experimental verifications considering the quality of an RoF-based analog photonic link. Furthermore, we demonstrate the THz-band fiber-wireless transmission of an aggregated mobile signal using multiple intermediate frequency (IF) carriers to confirm the technical feasibility of the RoF-based photonic THz system for a 6G mobile network. The experimental results show that a mobile signal of 32 IF carriers with a total occupied bandwidth of 12.8 GHz was successfully transmitted over a 5-m free-space wireless link at a frequency of 0.3 THz, which satisfies the 16-quadrature amplitude modulation requirements. We also discuss the design issues related to the optical-link budget considering potential architectures of the RoF-based photonic THz communication system.