2 × 2 MIMO In-Band Full Duplex Radio Front-End With Two-Stage Passive Suppression and Four Three-Tap RF Cancellers

This article presents a
$2\times 2$
multiple-input multiple-output (MIMO) in-band full duplex radio (FDR) front-end with 55-dB self-interference cancellation (SIC) over 200-MHz bandwidth and 60-dB SIC targeting 100 MHz within the entire 200-MHz bandwidth. A passive 30-dB suppression is achieved in two stages at the antenna board. Crosstalk suppression is achieved through antenna separation with a decoupling network (DN), and self-talk suppression is achieved through a circulator with a reflection coefficient controller (RCC). Additional active cancellation is achieved by using an RF self-interference cancellation (RFSIC) board composed of four RF cancellers for each channel. Each RF canceller should be composed of at least two taps taking into account the two-stage passive suppression and it needs one additional tap for the reflected path from the environment. Each tap of the RF canceller is composed of a variable attenuator, a phase shifter (PS), a group delay controller (GDC), and a replaceable delay filter. To determine the required
$S$
-parameters of the RF canceller, a power delay profile (PDP) analysis and gradient optimization are used and to implement it on the RF canceller, the bias voltages of each tap are determined by using the proposed control method. The RF cancellers can adapt to the entire 200-MHz band as well as any channel band within the range of 3.6–3.8 GHz on the antenna board, leading to improved SIC. By adjusting the RF cancellers to the lower and upper 100-MHz bandwidth, it is possible to achieve active 30-dB cancellations and a total SIC of 60 dB.