## 4 Integrated Signal Design of Communication, Navigation, and TT&C for the Requirements of the NS  
1. Requirements for Modulation Technology Put Forward by the Integrated Channel of NS Communication, Navigation, and TT&C  
Modulation is carried out to match the characteristics of the transmitted signal with those of the channel. Therefore, the selection of the modulation method is determined by the channel characteristics of the system. In the ICNT system of the NS, to effectively utilize the power of the space vehicle, the power amplifier of the transmitted signal usually adopts a high-power nonlinear amplifier (HPA). In this way, due to the nonlinearity of the integrated channel of ICNT in the NS and the amplitude modulation/phase modulation (AM/PM) effect, it is required that the instantaneous amplitude fluctuation of the modulated signal  
Figure 10: ACF, PSD of the noise sequence, and the corresponding distribution of the noise sequence and the fitting curve. (**a**) ACF and PSD. (**b**) Distribution of the noise sequence and the fitting curve. waveform should be as small as possible; therefore, a modulation method with a constant envelope or quasi-constant envelope structure should be adopted. Relevant research results show that the performance gain obtained by using an HPA and (quasi-) constant envelope modulation is higher than that obtained by using a linear power amplifier and a non-constant envelope modulated signal [86]. Therefore, combined with the actual requirements of communication, navigation, and TT&C services, it is necessary to design an integrated constant envelope signal modulation technology. On the other hand, the communication and TT&C tasks in the NS require the real-time transmission of a large amount of data and image information to the ground. This requires the communication system on the vehicle to have an increasingly higher transmission rate, which in turn leads to an increasingly crowded radio frequency spectrum. Information theory research shows [87] that when using a constant envelope signal and there are constraints on its PSD, the capacity of the system will decrease. Therefore, it is difficult for traditional modulation systems such as amplitude shift keying (ASK) and frequency shift keying (FSK) to meet the business requirements. It is necessary to adopt a constant envelope signal to meet the growing demand for high data rates, such as binary phase shift keying (BPSK), continuous phase frequency shift keying (CPFSK), continuous phase modulation (CPM), and other signals. Figure 11 presents the comparison results of the PSDs of ASK, FSK, BPSK, and CPM modulated signals, and based on this, the spectrum utilization efficiency (SUE) of the four signals is statistically compared. Where the symbol rate \(R\)s is 1000 Hz, the number of symbols \(N\) is 1000, the number of samples per symbol is 20, and the modulation index \(h\) of CPM is 0.5 (the main lobe bandwidth is 1.5 \(R\)s), and the pulse length \(L\) is 1; FSK is binary with two different frequencies, and the frequency interval \(\Delta f=Rs\). For the convenience of comparison, we only display the PSD of a single frequency. In addition, we assume that all are binary modulations, and 1 bit is transmitted per symbol, so the data rate is equal to the symbol rate, that is, \(Rb=Rs\). Figure 11: Comparison results of the power spectral density functions and spectrum utilization efficiencies of ASK, FSK, BPSK, and CPM modulated signals. From the simulation results in Figure 11, it can be seen that under the same conditions, the bandwidth of the BPSK is \(B=2 Rs\), so the spectrum utilization efficiency (SUE) is \(\text{SUE}=Rb/B=0.5\). Its side-lobe attenuation is slow, and it is suitable for scenarios with low bandwidth requirements. Similarly, for ASK, its spectral characteristics are similar to those of BPSK because both are linear modulations, with the same bandwidth \(B=2 Rs\), and \(\text{SUE}=Rb/B=0.5\). Its side-lobes also decay slowly, and it is also suitable for scenarios with low bandwidth requirements.