## 7 Conclusions  
As EW attacks have become more varied with the development of modern technologies, a proper defense system for a specific type of attack or a robust defense system is required. In wireless communication systems, the attackers' purposes are to detect the presence of communication, intercept signal characteristics, decode messages that contain military secrets, and disable the communication system. To counter such moves, several covert communication techniques are used to defend against various attacks. In addition, physical-layer defense techniques are studied to achieve LPD, LPI, and LPE communication functionalities.  
In this paper, we proposed the ring-shaped signaling system, which is flexibly adaptable, easily implementable, and improves security in response to LPI and LPE communication functions. The proposed ring-shaped signaling system modifies the generated I/Q signal into a ring shape via shared magnitude- and phase-modifying factors. In addition, the performance modification of the proposed system was formulated and simulated from the perspective of the BER and transmission power. Furthermore, we implemented a noise signaling system through SDR and presented the limitations, requirements, and preferences of configurations of noise signaling when implemented through SDR. Performance evaluation of noise signaling system-installed SDRs was performed with the performance metrics of the BER and PMI.  
From the evaluations, we observed that the recovered BER performance was affected by the values of the magnitude-modifying factors but not affected by the phase-modifying factor. Additionally, the effect of the magnitude-modifying factor was similar for any modulation schemes. With the reduction of the magnitude-modifying factor, the recovered BER performance became worse. However, PMI increased with the reduced magnitude-modifying factor value. At the same time, the eavesdropper could not identify the correct modulation scheme due to the effect of the phase-modifying factor. However, the eavesdropper may have recognized some modulation scheme information that might be used to predict the original modulation scheme. From these observations, we could conclude that combinations of using the magnitude- and phase-modifying factors with proper configuration values could achieve strong LPI and LPE security performance and good recovery performance at the same time.  
For future work, the following studies can advance noise signaling research. We tested and validated the noise signaling algorithm with only the M-PSK system. Therefore, the same comparison and validation processes with the M-QAM system are needed. Using channel states to extract pre-shared modifying factors can solve the limitation of our requirement that the transmitter and receiver must share modifying factors. We only considered  
Figure 17: PMI evaluation with varying \(I_{m}\) and \(I_{p}=4\).  
one-to-one communication networks. However, since the usual communication networks use multiple radios, we need to consider Media Access Control (MAC) as well as upper layer processes. Even though we have presented practical implementation through SDR, this is still a basic presentation. Therefore, further implementation with commercialized products and further experiments in real environments will be needed.  
All the authors have contributed to collecting results, performing analysis, and creating this article. All authors have read and agreed to the published version of the manuscript.  
The authors gratefully acknowledge the support from the Electronic Warfare Research Center at the Gwangju Institute of Science and Technology (GIST), originally funded by the Defense Acquisition Program Administration (DAPA) and the Agency for Defense Development (ADD).  
The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.