## 1 Introduction
The main limitation of HTs is that \(3\) dB loss is incurred between the TX and the antenna as well as between the antenna and the RX, degrading power efficiency and sensitivity. Furthermore, in typical HT configurations, the TX signal appears as a large common-mode signal at the LNA input due to parasitic coupling capacitors. A large common-mode leakage could saturate the LNA. The common-mode leakage could also cross-modulate with a jammer, thereby degrading the receiver linearity. The noise of the broadband common-mode signal could also appear at the RX signal thereby degrading the receiver sensitivity. The author [7] proposed a differential HT that would cancel out this common mode leakage signal, but this would nearly double the power consumption and area and additional loss from the balun that is needed to convert the transceiver differential signal to single ended at the antenna port. In [8], the issue of common-mode coupling due to parasitic capacitor was not addressed. The author relied on the linearity of the LNA design, which tolerated blocker of up to \(-15\) dBm. Due to the frequency variability of the antenna impedance, transmitter-to-receiver isolation is in the order of \(50\) dB for a signal bandwidth of \(6\) MHz as illustrated on the TX/RX isolation plot. Other single antenna self-interference cancellation works have also been proposed in [9; 10; 11; 12; 13; 14; 15; 16]. The use of a dual antenna architecture can significantly relax the design challenges since the limited coupling provides isolation between TX and TX without incurring any signal loss. However, the usual isolation between a pair of antennas in a mobile device is typically in the order of \(20\)-\(30\) dB [17] in contrast to a HT-based transceiver. Hence, a highly linear and tunable self-interference canceler (for FDD and FD) is required to further attenuate the TX leakage. Other works have advocated the replacement of the off-chip filters with SI cancellation techniques [11; 18] or the use of N-path filters [13; 19] to either achieve FDD or FD operations. The SI canceler must have variable gain and it must be able to track and adjust its phase over the entire \(360^{\circ}\) phase shift. In the two-point cancellation architecture proposed in [18] the LNA is based on the noise-cancelling approach and is able to tolerate up to \(2\) dBm TX leakage power while achieving high effective OOB IIP3. However, the TX leakage is effectively cancelled only after down-conversion and recombination between common-gate and common-source paths. As a result, it does not address SNR degradation due to IIP2 and reciprocal mixing. An N-path filter based canceler, which was designed for both FDD and FD applications has been proposed in [20] using dual antenna architecture. The use of passive mixer switches in the N-path filter results in large power dissipation and the N-path LO is prone to reciprocal mixing. With \(30\) dB isolation between the TX and RX, up to \(-4\) dBm TX leakage power was tolerated by the filter. However, the effective IIP3 is only improved by \(10\) dB for more than 25 dB of leakage suppression, which highlights an intrinsic linearity limitation from the canceler. A low-power mixer-first receiver with passive mixer based self-interference cancellation is proposed in [21] for dual antenna FD. It handles a SI of up to \(-10\) dBm and 25 dB of suppression is achieved over a limited bandwidth. Moreover, the effective IIP3 improvement is limited. The SI cancellation is done after the mixer, hence, reciprocal mixing is an issue for this architecture. A passive canceler based on an analog FIR filter in the RF side and another FIR filter in the baseband was proposed by [22] for dual antenna FD systems. The FIR filter taps consist basically of a true-time-delay circuit, a buffer and a variable gain amplifier. A 40 MHz cancellation bandwidth was achieved over a 50 dB leakage cancellation, however at the expense of high power consumption and noise degradation which scales with the number of FIR taps.