# A Novel Signal Design and Performance Analysis in NavCom Based on LEO Constellation
## 3 Model Design
### System Model  
The NavCom system model of CE-OFDM-PM is shown in Figure 3. On LEO satellites, the navigation payload is responsible for converting the navigation message into a binary navigation data stream after information processing, then sending it to the phase modulation mapper, while the communication payload is responsible for the data streams of different rates, which are converted into binary data streams via data stream processing and sent to the phase modulation mapper. It should be noted that the data stream can be either a pseudo-random stream or a communication data stream containing information entropy. The former can be used as a signal for a low-orbit satellite navigation system in the strict sense, while the latter can also take into account data communication while performing navigation tasks. In the phase of modulation mapping, the first step is to multiply the navigation data stream and the communication data stream by bit to obtain serial binary data, but the rate of the communication data stream is often times the rate of the navigation data stream. In this case, to solve parallel data, the navigation data must be sampled and expanded to the same length of the navigation and the communication data stream and then multiplied by bit. In the second step, the serial binary data are segmented, converted into multiple parallel binary data, and mapped to the time-varying phase function. Each parallel time-varying phase function data point is used as an input sub-band and modulated with an orthogonal subcarrier; then, the modulated parallel sub-band data are converted into serial data. Here, the baseband signal modulation is completed. The modulated IQ baseband signal is sent to the radio modulation module and the module multiplies and mixes branches I and Q to obtain the modulated signal. After the RF front-end amplification and other types of amplification, the radio frequency antenna of the low-orbit satellite achieves matching. Then, the signal is broadcast to receivers.  
However, in the section of the receiver, the NavCom signal is broadcast in the propagation through the RF antenna, the front-end filter, and the amplifier. Then, it is sampled by a downconverter to achieve analog-to-digital conversion and demodulate the RF signal into a digital intermediate frequency signal. After this, signal parameter estimation is carried out to obtain the coarse-grained carrier frequency and data phase information via a locally generated carrier frequency and pseudo-random code phase that can be accurately tracked. After acquisition and tracking are achieved, synchronization is used to obtain the accurate carrier frequency and code phase, which are used to realize the OFDM demodulation of the baseband signal and the inverse mapping of the PM. The parallel data stream is then converted into a serial data stream and the local pseudo-random code is used to separate the navigation message and the communication message.  
Figure 3: NavCom system model with CE–OFDM–PM signal. Navigation data and communication data (or pseudo-random noise) are sent to the phase mapper together via data processing, then the phase of the symbol is generated, OFDM modulation is performed and moved to the target carrier band via radio frequency modulation and transmission, and the receiving end undergoes demodulation and inverse mapping, resulting in the navigation and communication data, respectively, being separated in the final product.