A Wideband, High Isolation, Shared-Aperture MIMO IBFD Front End With CMS Balun and Fewer Multitap Cancellers

This article presents a wideband, high transmitters and receivers (TRX) isolation, shared-aperture, multi-inputmulti-output (MIMO) in-band full-duplex (IBFD) front-end system. In the antenna domain, a high common-mode-suppression balun (CMSB) is developed to enhance port isolation within a wideband range. A passive self-interference cancellation (SIC) level of 54.5 dB is achieved. The RF multitap canceller scheme is implemented in the analog domain to improve the decoupling effects. By exploiting the isolation properties among adjacent elements within the antenna array, the number of multitap cancellers is reduced from <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$2^{2}$</tex-math> </inline-formula> to 2 in the two transmitting and two receiving (2T2R) front-end systems, significantly decreasing the hardware cost and iteration time. The measurement results demonstrate that the proposed 2T2R antenna system exhibits high isolation levels, exceeding 64.5 dB, between any TX and any RX within the frequency range from 6.4 to 6.8 GHz.