A 67–110 GHz Multi-Carrier Communication Front End Based on Broadband Diplexer and Filtering Waveguide-to-Microstrip Transitions

This paper presents a 67–110 GHz millimeter-wave RF front end for 6G high-speed wireless communications. A broadband diplexer based on a staggered branch-waveguide structure is first proposed to achieve low insertion loss and high isolation simultaneously. The fabricated diplexer exhibits better than 12 dB return loss, 48.6% fractional bandwidth, and less than 1.5 dB insertion loss. A filtering waveguide-to-microstrip transition is then developed to suppress harmonics and spurious signals while maintaining efficient mode conversion. Back-to-back measurements demonstrate over 50 dB out-of-band suppression at 99.5 GHz with a center frequency of 89 GHz. Finally, the proposed components are integrated with broadband amplifiers, filters, and a local oscillator to realize a 67–110 GHz RF front end with 4–26 GHz IF output. Experimental results show harmonic suppression better than 60 dBc, demonstrating excellent spectral purity and validating the proposed architecture as a compact, broadband, and highly integrated solution for future millimeter-wave and terahertz communication systems.