Enhancing Selectivity in 5G/6G Ultra-Narrowband Applications: Plug-and-Play Frequency Selective Surfaces with Field-Manipulating Vias

This study presents unmatchedly selective ultra-narrowband frequency selective surfaces (FSSs) for a pioneering external RF front-end (RFFE) filtering solution integrated with diverse beamforming antenna modules. The limitations of current base station filters for 28 GHz are demonstrated, and an inventive approach that extends the boundaries of the filtering block by positioning it outside the RFFE for spatial reinforcement is newly devised. An innovative analysis of narrow-bandpass complementary resonator pairs are introduced, unlocking unprecedented selectivity by addressing the challenging issue of recalcitrant mutual coupling arising from a dispersive field distribution. This spatial methodology extends the frontier by seamlessly integrating field analysis with a mixed coupling theory previously established in the circuit domain. Subsequently, an trailblazing technique is proposed for precise control of coupling balance, utilizing field-manipulating vias to centralize the primary current in the complementary resonator. This approach achieves an average rejection of 60 dB across the stopband, ensuring robust stability for incident angles up to 70°, surpassing previous counterparts. Furthermore, it demonstrates a significantly improved roll-off in the transition band and achieves a remarkable 2.7% bandwidth, comparable to the 850 MHz for the n261 band of 5G NR.