Evolution of Circular Mode Phasing for In-Band Full-Duplex Omnidirectional Antennas

Many in-band full-duplex (IBFD) applications require omnidirectional radiation coverage in addition to multilayered self-interference cancellation (SIC). While system-level SIC approaches often require high-isolation antennas before implementing analog and/or digital methods, maintaining the desired omnidirectional transmit/receive patterns is difficult for compact designs that utilize the same polarization. This article discusses the concept development and design progression of the circular mode phasing technique applied to omnidirectional IBFD antenna arrays. The theoretical fundamentals are presented along with simulation results and then validated through the discussion of two prototype antennas, which were measured to provide up to 60 dB of isolation over 2.4-2.5 GHz, which is the highest reported for this SIC method. Additionally, the evolution of this concept for a collection of measured prototypes is summarized and characterized with a unique figure of merit, which should help advance the application of this technique within future IBFD systems.