# On Self-Interference Cancellation and Non-Idealities Suppression in Full-Duplex Radio Transceivers
## 2 Proposed System Model and Techniques
### SI Cancellation Techniques  
#### 2.2.1 Analog/RF SI Cancellation Techniques  
Several analog/RF cancellation techniques exist in the literature, as described in the previous section, to suppress the SI. After applying RF/analog cancellation techniques, the signal at the input of LNA can be described as:  
\[y_{RF}(t)=y_{ch}(t)-y_{est}(t) \tag{7}\]  
Figure 3: Linearization and efficiency improvement techniques for PA in transmitter RF front end, and linearization techniques for LNA in receiver RF front end. where \"\(y_{est}(t)\)\" is the estimated signal attained from RF/analog cancellation techniques and can be expressed as:  
\[y_{est}(t)=h_{est}(t)*x_{PA}(t) \tag{8}\]  
As stated above, the significant bottleneck in the efficiency and performance of FD transceivers is the self-interfering signal, i.e., coupling of transmit signal to its Rx. The critical challenge is to cancel the SI signal with one part per billion accuracies, and to cancel the effect of the SI signal over the desired signal up to 100% approximately. To achieve this accuracy to be the replica of the transmitted signal, SI signal is needed. It is difficult to replicate the SI signal because of multi-path channel effects or the signal from the nearby environment. Two automated operations are required to eliminate the SI in the analog domain efficiently: (1) sensing the self-interfering signal at the Rx, and (2) using adaptive integrated procedure or filter to cancel SI signal to a higher degree of accuracy. Reliable FD communication systems need to develop an algorithm that can control SI, which is altered by the environment, i.e., reflections from nearby buildings and multi-path channel effects. Tx does not have any knowledge about the signal being transmitted over the channel. The signal that a Tx knows is a baseband signal (the signal that is sent after digital to analog conversion and amplification) and the signal present at the output of the PA (the signal before transmission over the channel). Due to the multi-path coupling channel between Tx and Rx, the Rx receives multiple copies of the transmitted signal, i.e., delayed, attenuated, and phase-shifted copies of the transmitted signal. Here, multi-path coupling channel characteristic and statistical information is unknown to the Rx [59]. This statistical information of the multi-path coupling channel is crucial for the Rx to boost the received signal or to remove the SI signal. The knowledge about channel statistical information, i.e., fading distribution type, average channel gain, line of sight (LOS) component, spatial correlation, and impulse response, is termed as channel state information (CSI) [60]. CSI depicts the flow of the signal and how it propagates from the Tx end to the Rx end. CSI illustrates the properties of the communication link between Tx and Rx. CSI also includes the information about the statistics of received interferences and thermal noise. Using CSI in wireless communication systems aims to improve the channel's capacity, maximize the SNR, and minimize the BER at the detection stage. Primarily, CSI is categorized as: (1) CSI is known to both Tx and Rx (i.e., channel state information at the Tx (CSIT) and channel state information at Rx (CSIR)); (2) CSI is known to Rx only and Tx is unaware of it (i.e., (CSIR)); and (3) both Tx and Rx are unaware of CSI. CSIR is obtained on the Rx side and is used at the Tx side through either the reciprocity principle or feedback method. CSI helps the Tx or Rx adapt to a current channel state for reliable communication with a high data rate. As the channel is primarily a time-varying entity, its statistical information varies with time, so CSI must be predicted on a short-term basis, i.e., must be predicted after a regular interval or period. Usually, a training-based approach is used for estimating the current state of the channel. This approach is considered a popular approach for channel estimation, but the significant overhead of this approach is reduced transmission efficiency [61].