This contribution discusses several recently proposed remote antenna units (RAUs) based on air-filled substrate-integrated-waveguide technology, specifically focusing on the co-design between antenna elements and optoelectronic transducers. These include two passive sub-6 GHz transmit RAUs and one active mmWave transmit RAU. It is concluded that the RAU’s performance benefits from a thorough co-design, aiming for a conjugate-match between both components, when compared to a diakoptic design procedure, optimizing both components separately. Specifically, this approach resulted in a significant miniaturization of the sub-6 GHz RAU, while in case of the mmWave RAU, the efficiency was improved.