Harmonic Suppression for Electro-Optic Communication Systems

For future networks to function properly, it is necessary to have synchronized clock and radio frequency carriers for precise positioning and highly dependable, low-latency communications. Multiple RF bands must be accessible by remote units (RUs) and wireless devices. Future 6G infrastructures face the challenge of offering precise clock and timing services to customers in addition to high rate data connections and to have such signals distributed widely across various parts of the network hierarchy. Radio-over-fiber (RoF) systems may be utilised to address such challenge, where RF signals are sent through low loss optical fibers, transmitted on optical carriers or frequency combs and received using broadband photodetectors. This paper deals with such systems, where a filtered optoelectronic frequency comb, with fixed frequency/wavelength spacing is advantageously used. The comb generates synchronized RF tones across a wide bandwidth. The work reported here allows multi-frequency RF signal transmission to several radio units. The study utilises cutting-edge RoF technology to combine optical and electrical signals and transmit them over fibre to radio units. In such RoF systems, however, extracting the desired electrical signal from the multi-frequency signal set suffers from harmonic interference, across a wide frequency band, which depends on the comb generation and the system frequency map particulars. For this work such band ranges from 5 GHz to 40 GHz. The work reported here considers the design and implementation of low-pass and band-pass filter combination for signal extraction and harmonic suppression, thus eliminating undesired harmonics and delivering the desired signals with frequencies at 6.25 GHz and 12.5 GHz. The paper reports the design method and experimental results, showing the efficacy of the techniques proposed.