## 1 Introduction  
The evolution of mobile communication technologies ranging from GSM to the future 5G has continuously required increased data rates and quality of service and, in general, the miniaturization of devices and costs reduction. Off-chip Surface Acoustic Wave (SAW) filters are typically used to reject out-of-band interferers and Self-Interference (SI) signals (or transmitter leakage) but cause a significant form factor burden, even more so when the device operates over a large number of bands and uses multiple antennas. Also, the reconfigurability of receivers with SAW filters is difficult to achieve. Much attention is now focused toward SAW-less receivers, and several architectures have been proposed that are capable of tolerating large SI signals. There are various challenges that arise with the SAW-less receivers. For instance, when operating in Frequency Division Multiplexing (FDD) mode, where the transmitter (TX) and receiver (RX) operate simultaneously in different frequency bands, the TX leakage could inter-modulate with the out-of-band (OOB) blockers, thereby resulting in in-band third-order and second-order intermodulation, as well as generate reciprocal mixing. Furthermore, wideband TX noise leakage in the receive band can severely degrade receiver sensitivity [1]. Transceivers operating in Full Duplex (FD) mode, where the TX and RX overlap in time and frequency, are inherently SAW-less and pose an even greater SI challenge since a very large (in the order of 110 dB [2]) TX-RX isolation must be ensured, which also requires very high in-band linearity. In a single-antenna transceiver, a circulator can be used to provide isolation between transmitter and receiver. Alternatively, isolation can be achieved using separate antennas for transmitter and receiver. This basic isolation can be improved using RF cancelation architectures, either based on an RF canceller [3, 4] or on an auxiliary transmitter [5]. The use on an auxiliary transmitter allows to achieve very high cancellation levels and bandwidths even in the presence of strong power amplifier nonlinearity. However, it does not address the issue of transmitter noise leaking into the receiver. With a typical TX noise level in the order \(-160\) dBc/Hz [6], if the transmitter leakage is sufficiently low (e.g., \(-20\) dBm or less), this is not a major concern. If instead the transmitter leakage approaches \(0\) dBm, sensitivity degradation becomes large, and an RF canceller is potentially more attractive. A major issue of RF cancellation is given by the limited cancellation bandwidth, which results from the substantial delay (in the order of ns) between the TX signal and the SI at the RX input. Several board-level demonstrators have been implemented using off-the-shelf components, reporting strong cancellation levels over broad bandwidths. In [3], using a circulator showing multiple coupling paths with delays from \(0.5\) ns to \(2\) ns, \(33\) dB cancellation over \(20\) MHz and \(18\) dB cancellation over \(100\) MHz was reported using an RF canceller based on a two-tap delay line followed by programmable vector modulators under self-adaptive control. In [4], a three-tap RF canceller achieved \(41\) dB cancellation over \(80\) MHz bandwidth. Moving from the above mentioned solutions--based on off-the-shelf components mounted on a printed circuit board or connected using cables--to fully-integrated solutions, several implementation challenges related to component integration (e.g., RF delay lines) and dynamic range limitations are faced, typically leading to much smaller form factors but inferior cancellation levels and bandwidth. Prior works have proposed several integrated architectures to substitute the off-chip filters with tunable integrated solutions. Early works have advocated the use of passive hybrid transformers (HT) [7; 8]. The HT allows us to connect TX and RX to the antenna port while ensuring isolation between TX and RX.