# Physical Layer Security Based on Non-Orthogonal Communication Technique with Coded FTN Signaling
## 1 Introduction
Unlike general FTN signaling, the proposed technique uses a different symbol interval for each transmission symbol. Therefore, each symbol has a different symbol position and interference value. Furthermore, the information about the symbol interval is shared between the transmitter and legitimate receivers. The legitimate receivers can remove the interference and detect the transmitted symbols. However, the eavesdropper cannot execute the interference cancellation operation. Furthermore, the eavesdropper cannot even find the position of each symbol. In this paper, we introduce three transmission schemes. The first scheme is symbol-wise coded FTN signaling. In this scheme, the symbol interval values are randomly generated. The number of symbol interval values is equal to the frame length of the transmitted signal, and the generated symbol interval is allocated to each symbol. The second method is block-wise coded FTN signaling. Block-wise coded FTN signaling divides a transmission frame into symbol blocks of identical length, and different symbol intervals are assigned to the respective symbol blocks. Symbols in the same block have an identical symbol interval. The third method is variable-length block-wise coded FTN signaling. The third scheme adjusts the block length according to the symbol interval of the block. In blocks with short symbol intervals, interference values are high due to the large overlapping area between adjacent symbols. Therefore, the third method assigns short block lengths to blocks with short symbol intervals and long block lengths to blocks with long symbol intervals. For performance evaluation, the digital video broadcasting-satellite-second generation extensions (DVB-S2X) system [31] is considered. To improve the DVB-S2X system, there have been efforts to apply an FTN signaling scheme to the DVB-S2X system [32; 33; 34]. Therefore, the proposed technique is applied to the DVB-S2X system, and a performance evaluation of the designed system is carried out. The contributions of this study are summarized as follows:  
Introduction of a novel physical layer security technique: a novel security method is proposed that exploits the non-orthogonality of Faster-than-Nyquist (FTN) signals with variable symbol intervals to enable secure communication without relying on artificial noise. Development of coded FTN signaling schemes: three innovative schemes (symbol-wise coded FTN signals, block-wise coded FTN signals, and variable-length block-wise coded FTN signals) are introduced to balance complexity, bit error rate (BER) performance, and interference management. Application to real communication systems: the proposed technique is integrated into a DVB-S2X system to demonstrate its practical applicability and effectiveness in a real digital broadcasting environment. The rest of this paper is organized as follows. Section 2 presents the FTN system model. Section 3 describes the proposed physical layer security techniques based on coded FTN signaling. Next, the DVB-S2X system operation with a coded FTN signaling and iterative decoding and detection (IDD) scheme is described in Section 4. The simulation results are reported in Section 5, followed by some concluding remarks in Section 6.