## 2 Full-Duplex Transceiver with Passive/Active Eavesdrop Transmission System
### Conventional Schemes without Feedback  
The conventional blind scheme without feedback and the semi-blind scheme without feedback have been studied in [27; 28], respectively, and are presented in Figure 2. At the receiver side, the received signal \(\mathbf{y}_{A}\) will pass through a Digital Self-Interference Cancellation (DSIC) process based on an adaptive filter with the Recursive Least Square (RLS) algorithm [36] to firstly estimate the SI channel and then reconstruct and cancel the SI component. Then, the residual signal will go to an equalizer to firstly estimate the intended channel and then obtain the equalized signal. In the semi-blind scheme without feedback, it is noted that the pilot symbols are attached to the information sequence at the transmitter side, and they are also used for the DSIC and equalizer processes. Then, they will be removed from the equalized signal, and this signal continuously goes to the demodulator to obtain the Log Likelihood Ratio (LLR) belief sequence. Finally, the Sum Product Algorithm (SPA) decoding algorithm [37; 38] with an efficient message-passing schedule will be implemented in the 5G LDPC decoding process at user A and user E to reconstruct the binary input signal \(\mathbf{\hat{x}}_{Sol}\) of user B. The principle of SPA involves the message repetitively passing from the check nodes to the symbol nodes for guessing the transmitted bits from each other at each iteration \(j\) until it reaches the maximum number of iterations, \(j_{max}\). For the rest of this paper, this DSIC process and decoding scheme are called blind scheme without feedback or semi-blind scheme without feedback, respectively.  
In this paper, we assume the following hypotheses:  
* In case of a passive eavesdropper, only blind channel estimation is used, where there is no knowledge about the channel state information at all communication users;
* In case of an active eavesdropper, both blind and semi-blind channel estimations, where all transceivers share a few pilot symbols, are mainly implemented;
* User E knows the parity check matrix \(\mathbf{H}\) of user B and performs the SPA decoding mechanism; user E also uses an RLS algorithm in the DSIC process of user A in case of an active eavesdropper;
* Both user A and user E have equal computation capabilities, and the location of user E is close enough to user A to broadcast its jamming signal as well as to be attacked by the self-jamming signal from user A;
* The channel gains at the receiver and the eavesdropper are constant within a code word and change from one to another in fading channels;
* The impact of hardware impairments on the SI cancellation is not considered (which is outside the scope of this study but essential in practice). Moreover, the problem of the synchronization process between the transceivers is also not taken into account. Last but not least, the bit resolution of DAC/ADC is chosen to be high enough to bypass the effect of the quantization noise, i.e., larger than 6 bits for both DAC/ADC process. Alternatively, the oversampling should be applied in the ADC process if the green communication system and IoT applications are considered with low-bit ADC [39].  
Figure 2: Conventional schemes without feedback.