# On Self-Interference Cancellation and Non-Idealities Suppression in Full-Duplex Radio Transceivers
## 2 Proposed System Model and Techniques
### Transceiver's Components Linearization
#### 2.1.2 PA and LNA Linearization  
PAs are the critical component at the transmitter end to manage these entire signals and their specific necessities. PA used must be capable of maintaining high power output, high linearity, and high efficiency, and all of them are inter-related quantities. The primary and foremost requirement at the Tx end is high linearity and efficiency.  
Several techniques exist in the literature to satisfy the efficiency and linearity tradeoff. Most of these techniques are characterized based on action to reduce or avoid distortion in a non-linear PA. In this research study, the Doherty technique [53; 54; 55] combined with one of the linearization techniques is used for efficiency and linearity improvement. The Doherty technique is considered a mature technique and is used for efficiency enhancement.  
Doherty technique operation depends on two different transfer function regimes containing two parallel PA, i.e., carrier PA (CPA) and peaking PA (PPA). One is called a low-power and the other as a high-power regime. Low-power is also defined as a linear regime, and high power is a non-linear regime. By combining the two PAs, the linear range increases where the Doherty amplifier operates. Doherty amplifier has a highly complex structure in practice, but it can be endured over its flexible architecture and high-efficiency performance, and this amplifier's topology is the fundamental reason behind high efficiency. CPA and PPA are ideal current generators with zero reactive freeloading. Load impedance of CCA is increased while PPA load impedance is decreased as compared to CCA. No extra or additional circuit is required for any kind of controlling operation. Sufficient bandwidth (BW) is provided, i.e., up to 20 MHz, but is limited to only a single frequency or single band of operation.  
For linearity improvement, two different linearization techniques are used combined with the Doherty technique, i.e., the pre-distortion technique [56; 57] that avoids distortion and the feedforward technique that reduces the distortion being produced by non-linear PA.  
Similarly, for the linearization of LNA, two techniques are implemented, i.e., negative feedback and feedforward [58]. In both LNA and PA, the feedforward linearization technique first compares the attenuated amplifier output with amplifier input, and generates the distortion signal. The distorted signal is then combined destructively at the amplifier output with the help of an auxiliary amplifier, and the resultant signal is a distortion-free signal. However, it can remove or reduce one type of harmonics at a time, i.e., to remove second and third order harmonics. The feedforward technique required an additional degree of freedom, and hence required an additional auxiliary amplifier path. The auxiliary amplifier path power efficiency is decreased, but still provides a reasonable amount of linearization at the BW range of 3-50 MHz. The negative feedback linearization technique improves _A\({}_{1 IP2}\) and A\({}_{1 IP3}\)_ (second and third intercept points) and hence increases the linear operation range of the amplifier. Figure 3 shows the complete block diagram of PA linearization with the efficiency improvement technique. The LNA with negative feedback and feedforward linearization techniques is depicted as well.