A Multiport Self-Interference Canceller for Wideband SIMO/MIMO-STAR Full-Duplex Arrays

Combining multiple-input and multiple-output (MIMO) with wideband in-band full-duplex (FD) channel access can improve the reliability, robustness, and spectral efficiency of wireless communications systems. FD antenna arrays also bring untold capabilities to multifunctional aperture arrays used in communications, radar, imaging, sensing, and electronic warfare applications. However, the advantages of FD arrays are challenging to achieve. Self-interference (SI) among antennas in an array processor is an especially hard problem. This article begins by presenting a replica antenna (RA)-based approach for SI cancellation (SIC) between two antennas; the concept of which is further extended to the multiantenna case to enable FD-MIMO communications in large arrays. Enhancements of earlier work on RA-based SIC are proposed to increase SIC bandwidth at reduced hardware complexity. The proposed modifications convert the SIC structure to a flexible multistage design that is suitable for implementing FD on Nyquist-spaced antenna arrays with no limits on the number of independent antenna elements. Here, an RA-based first stage acts as an analog computer that emulates the electromagnetic scattering and coupling between elements in a broadband sense, thus enabling coupling to be canceled at the array level and significantly reducing the complexity of later SIC stages. Experimental results from a
$4 \times 4$
MIMO prototype operating from 1.8 to 2.3 GHz demonstrates a four-element wideband FD-MIMO simultaneous transmit and receive (STAR) system with SI down to
${< }{-}65$
dB at a 500 MHz bandwidth in the worst case (all transmit elements active and generating the same data streams).