## 3 Noise-Shaped Signaling Definition, Limitation, and Preferences  
The purpose of using noise-shaped signaling is to achieve signal transmission security. Three categories of noise-shaped signaling function are discussed: (1) the eavesdropper is not able to detect the transmitting RF signal (LPD); (2) the eavesdropper is able detect the presence of a transmitting RF signal but not able to interpret the signal characteristics (LPI); and (3) even if the eavesdropper interprets the signal, he or she is not able to exploit it due to an inability to decode the correct message (LPE). The ultimate goal of noise-shaped signaling is to transform the I/Q signal distribution to make it almost equal to a Gaussian distribution. As the signal is transformed into a form almost equal to Gaussian noise, the RF signal may achieve the highest security, achieving LPD, LPI, and LPE functionalities. However, when trying to implement the signaling methods in a real radio, the receiver may face very difficult conditions for decoding the RF signal due to several hardware limitations.  
When adapting Gaussian distributed noise-shaped signaling through SDR, there are several issues to be considered, such as signal power control, signal synchronization, and additional noise mixing processes.  
For signal power control issues, typical radios have a maximum transmission power limitation for several reasons, which is the same as SDR. In addition, SDR receivers control their own gain to protect from overpowered received signals or to increase the performance when the transmission power is too low. Therefore, the noise-shaped signaling process also requires following the maximum power limitation rule. Reshaped signal power should not exceed the maximum power of the original signal and therefore requires the highest bound. Lower power matters to radio performance as well. To correctly decode messages, a certain value of decision distance between I/Q data is required. When lowering the received power, the decision distances decrease as well and can harm the radio performance. Therefore, considering the possibility of the appearance of critical additional noise between the communication channels, noise-shaped signaling should have the lowest bound.  
The basic concept of noise-shaped signaling is to modify the original signal into a noise-like form. The process may be performed by addition, subtraction, multiplication, or division of the original signal and modifying parameters. When the receiver recovers the original signal from the received signal, which is affected by the noise-shaped signaling and channel, the receiver requires almost perfect synchronization in both the frequency and time domains to counter the noise-shaped signaling effect and face only the channel effect. Therefore, the communication system should have a function for perfect synchronization.  
While the additional noise mixing process proceeds, one of the following calculations is used: addition, subtraction, multiplication, or division. Additionally, the mixing process may directly apply to data bits and I and Q data at the same time or independently. In SDR, the transmitter and receiver gain can be set up differently, which can change the I/Q data magnitude. Therefore, addition or subtraction processes may not be feasible when implemented through SDR. In addition, synchronization functions use the inputs as I/Q data forms. Therefore, to achieve the purpose of securing modulation scheme information, application of the noise mixing process to the I/Q data after the modulation function at the transmitter and before demodulation function at the receiver is adapted.  
With the observed definitions, limitations, and preferences, we propose the ring-shaped signaling system algorithm as a suitable noise-signaling algorithm for implementation through SDR. Detailed descriptions of the algorithm, implementation processes, and performance analyses are presented in the following sections.