# Design and Performance Analysis of Navigation Signal Based on OFDM
## 4 Analysis of ACF and PSD
### Modulation Parameter Analysis  
We set the receiver RF front-end bandwidth to 36 M, and selected several groups of typical navigation signals for analysis. The binary phase shift modulation BPSK-R is a widely used navigation modulation method. The modulation and demodulation processes are simple. BPSK-R(1) and BPSK-R(10) are used in GPS civil and military navigation signals, respectively. Binary square wave subcarrier BOC modulation in the construction process is a new navigation signal modulation method proposed by Galileo and BeiDou. The BOC(1,1) and BOC(10,5) are adopted by GPS and BeiDou navigation signals. In this paper, two kinds of OFDM-modulated navigation signals are selected, OFDM(8) and OFDM(128). The number of OFDM(8) subcarriers is 8, subcarrier spacing is 2.4 MHz, cyclic prefix mode is adopted, and cyclic prefix length is \(1/16T_{u}\); for OFDM(128), the number of carriers is 128, subcarrier spacing is 0.15 MHz, and remaining parameters are similar to OFDM(8).  
Figure 8: Effect of the guard interval length on the ACF and PSD. (**a**) The ACF of the OFDM navigation signals with different cyclic prefix lengths; (**b**) The PSD of the OFDM navigation signals with different cyclic prefix lengths.  
Figure 9 shows the ACF and PSD of several different types of navigation signals and analyzes the characteristics of these types of navigation signals. Figure 9a shows that the main peak of the ACF of the OFDM navigation signal is the same as that of the BOC(10,5). The main peak is relatively narrow and has a higher tracking accuracy of the code loop. In contrast to the BOC(10,5), the OFDM-modulated navigation signal's secondary peaks near the main peak of the ACF are effectively suppressed, and the amplitude of the secondary peaks is significantly improved compared with the BOC(10,5). The code loop tracking loop locking is more accurate and false locks are not prone to occur. There is no need to adopt the algorithm of unambiguous tracking, which helps to eliminate the problem of the mislocking of the main peak of the BOC signal receiver and reduces the complexity of the receiver design. In Figure 9b, when comparing the OFDM-modulated navigation signal with the BOC signal, it can be found that the main lobe area of the OFDM navigation signal is basically flat, showing the form of a band-pass white noise PSD function, and the power is evenly distributed to the main board. The PSD curve amplitude is low. These characteristics make the OFDM navigation signal not only have a strong anti-interference ability, but also make it easy to integrate with other signals. Compared with the OFDM modulation, the power spectral density of the BOC signal and BPSK signal is composed of multiple PSD peaks. The position of the main lobe is relatively scattered in the spectrum range and the energy is not concentrated, which can easily cause interference with other signals.  
In summary, the OFDM navigation signal has a better tracking accuracy of the code loop, and the tracking of the code loop is stable, therefore there is no need to consider the unambiguous tracking. The power of the signal can also be evenly distributed in the range of the frequency spectrum, so the OFDM navigation signals have a good navigation performance.