Low-Cost Broadband RF Front-End Design for Enhanced Isolation in In-Band Full Duplex Radio

Future radios face the challenge of delivering faster data rates while efficiently utilizing the limited and regulated electromagnetic spectrum. In this context, broadband spectrum access in-band full-duplex (IBFD) operation using a simultaneous transmit and receive (STAR) radio architecture is attractive. These radios&#x0027; features are not only enhancing throughput but also improving multi-user access. However, in designing STAR radios, it is necessary to suppress self-interference (SI) from the transmitted and received signals. The challenge of suppressing this interference becomes even more severe for wideband operation due to the dynamic and time-varying nature of wireless channels. In fact, the SI leakage at the receiver can be up to 10 billion (<inline-formula><tex-math notation="LaTeX">$10^{9}$</tex-math></inline-formula>) times greater in magnitude than the intended signal power. This necessitates a multi-stage cancellation approach. To address this, we propose a custom-designed and cost-effective multi-stage SI cancellation strategy. The first two stages incorporate passive components to achieve high isolation across a wide bandwidth. Our innovative RF-SIC (self-interference cancellation) filter, combined with a multi-antenna setup, provides <inline-formula><tex-math notation="LaTeX">$&gt;60\;\text{dB}$</tex-math></inline-formula> of isolation in the analog domain across a contiguous 1 GHz bandwidth from 2&#x2013;3 GHz. Past STAR radios delivered up to 500 MHz bandwidths. Therefore, this is the first paper to present a RF-SIC filter that works across 1 GHz. The paper is focused on the RF-SIC filter using a microstrip fabrication process. Also, in contrast to past FIR filters, the proposed one doesn&#x0027;t employ discrete components. Instead, it uses the impedance and delay of the shunt section as control parameters.