# Physical Layer Security Based on Non-Orthogonal Communication Technique with Coded FTN Signaling
## 3 Physical Layer Security Technology Based on Coded FTN Signaling
### Variable-Length Block-Wise Coded FTN Signaling  
This subsection considers the performance improvement method for the coded FTN signaling. As mentioned above, in block-wise coded FTN signaling, the symbol block with a low \(\tau\) value degrades the overall system performance because of the continuously high ISI. To solve this problem, this scheme adjusts the symbol block length according to the \(\tau\) value. To prevent high ISI being generated from a low \(\tau\) value, this method assigns a  
Figure 2: Comparison between general FTN signaling and coded FTN signaling.  
short symbol block length to a symbol block with a low \(\tau\) value. In the opposite case, a long symbol block length is assigned.  
In this paper, the symbol block length is selected as follows:  
\[B_{l}=\lfloor 10\times\tau_{l}\rfloor \tag{7}\]  
where the \(B_{l}\) and \(\tau_{l}\) are the block length and \(\tau\) value for the \(l\)-th block, and \(\lfloor\Delta\rfloor\) is the largest integer not exceeding \(\Delta\). Therefore, in the case of \(\tau_{l}=0.55\), \(B_{l}=5\). Figure 3 shows the conceptual diagrams of the block-wise coded FTN signaling and the variable-length block-wise coded FTN signaling. In the case of block-wise coded FTN signaling, an identical block length is assigned to all symbol blocks regardless of \(\tau\) value; while in Figure 3b, a different block length is assigned to each symbol block according to its \(\tau\) value.  
Figure 4 shows a block/flow diagram of the proposed technique. The block/flow diagram shows the end-to-end system design for the proposed methodology, highlighting the coded FTN signaling scheme for physical layer security. The flow is divided into the following key steps: The system starts with a channel encoder to prepare the input data by adding error correction codes to ensure reliable transmission. The data are then processed by an interleaver to rearrange the data sequence in order to increase the burst error tolerance. Next, an M-ary mapper converts the encoded data into modulation symbols such as QPSK or 16-QAM for efficient transmission. Then, an oversampler using the coded FTN coefficient \(\tau\) adjusts the symbol spacing to enable non-orthogonal signals. Here, the oversampling interval for each symbol is adjusted according to the coded FTN \(\tau\) value, so that different \(\tau\) values can be assigned to each symbol for the coded FTN. The signal is then passed through a pulse-shaping filter which is root-raised cosine (RRC). Finally, the signal is transmitted through the channel and is subject to noise and interference during transmission. At the receiver side, the signal is first processed by a matched filter (RRC) to extract the transmitted pulses. The filtered signal is downsampled using the coded \(\tau\) and aligned for further processing. Then, the interference canceller removes the interference using the coded \(\tau\) values shared by the legitimate transmitter and receiver. The cleaned signal is demodulated by the M-ary demapper to convert the symbols back into data bits. The bits are resequenced by the deinterleaver to restore the original order. Finally, the channel decoder recovers the original data and corrects the errors that occurred during transmission.  
Figure 3: Conceptual diagram for block-wise coded FTN and variable-length block-wise coded FTN signaling.