## 4 Integrated Signal Design of Communication, Navigation, and TT&C for the Requirements of the NS
For binary FSK, the frequency interval between the two frequencies of binary FSK affects the bandwidth. When the frequency interval \(\Delta f=Rs\), the bandwidth \(B=2\Delta f+2 Rs\), and \(\text{SUE}=Rb/B=0.25\). It can be seen that it occupies a relatively large amount of spectrum resources. In contrast, the bandwidth of CPM is more compact than that of BPSK, with a bandwidth of \(B=1.5 Rs\) and \(\text{SUE}=Rb/B=0.6667\). Its phase continuity makes the side-lobe attenuation faster, and it is suitable for applications with limited bandwidth and high power-efficiency requirements. General Considerations of the Modulation Technology for the Integration of NS Communication, Navigation, and TT&C  
In the ICNT system of the NS, when selecting a modulation method, in addition to adapting to the ICNT channel of NS, various factors need to be comprehensively considered, for example, good anti-interference performance, high spectral efficiency, low inter-spectrum interference, etc. At the same time, the difficulty of implementing the modulation and demodulation equipment should also be taken into account. Therefore, for the ICNT system, it is of great importance to select an appropriate modulation method that can be applied to both the communication and navigation systems as well as the TT&C system, so as to give due consideration to the utilization rate of the NS signal bandwidth. Integrating navigation signals such as BPSK, binary offset carrier (BOC), multiplexed binary offset carrier (MBOC), binary coded symbols (BCS), multiplexed binary coded symbols (MBCS), and communication signals such as CPFSK, CPM, orthogonal frequency division multiplexing (OFDM) to obtain a hybrid modulated signal is indeed a reference solution. Where the navigation signals serve both navigation and TT&C tasks, the communication signals are responsible for communication and high-speed data transmission. This solution has also been preliminarily verified in references [88; 89; 90; 91; 92]. Figure 12 shows the simulation results of the PSD and ACF curves for a typical single signal and an integrated signal. From the simulation results in Figure 12, it can be seen that for the PSD curves, the integrated signal combines the functions of single signals. For example, if the CPM signal is used for communication and TT&C, and the BOC (1,1) signal is used for navigation, then the CMP-BOC (1,1) signal can simultaneously carry out communication, TT&C, and navigation services. The same is true for other signals such as CMP-OFDM, OFDM-BOC (1,1), and BPSK-BOC (1,1). From the perspective of the ACF curves, the integrated signal can also improve the autocorrelation characteristics of the signal to a certain extent. For instance, it can reduce the number of side-lobes, which is beneficial for improving the ranging accuracy and tracking accuracy of the signal. According to the research conclusions of (1) and (2), it can be seen that the design method of using integrated signals is indeed a solution for the integrated signal design of NS communication, navigation, and TT&C. It is expected to enhance the spectral utilization rate of the signal, reduce the power consumption, volume, and cost of the system, and meet the current requirements of communication, navigation, and TT&C tasks for hypersonic aircraft entering the atmosphere.