Optical Fronthauling and mm-Wave/Sub-THz Signal Generation Techniques for the 6G and Beyond 6G Wireless Systems

The combined approach of spectrally efficient analog radio-over-fiber (A-RoF) fronthauling and the compatible optical heterodyning technique provides an efficient solution for the simultaneous generation and distribution of high-frequency mm-wave/sub-THz signals for 5th/6th generation (5G/6G) and beyond 6G (B6G) wireless signals. The choice of optical source, i.e. free-running lasers or a frequency comb, determines the impairments and complexity of the optical heterodyne A-RoF link, and the specific wireless applications that can be supported. The frequency fluctuations and phase noise impairment of the optical carriers used for heterodyning limits the performance of such links — especially for the transmission of low subcarrier spacing orthogonal frequency division multiplexed (OFDM) signals synonymous with 5G. This is due to transfer of frequency and phase noise from the optical domain to the RF domain during the heterodyning process. This presentation will outline the major elements of the optical heterodyne A-RoF link, i.e., the choice of optical sources, detector technology for high-frequency mixing and impairment compensation techniques. The presentation will focus on the use of optical heterodyning for transmitting low subcarrier spacing OFDM signals. The performance of 60 GHz mm-wave and 180–260 GHz sub-THz signals generated using heterodyning of optical carriers obtained from free-running fiber lasers and a gain-switched comb source will be discussed and analysed.