# A Capacity Enhancement Method for Frequency-Hopping Anti-Jamming Communication Systems
## 3 Results  
### Design of Demodulation Algorithm for FH Capacity Boost Signal  
Demodulation Algorithm Flow  
As shown in Figure 2, for Information A, coherent demodulation is used to ensure BER performance. Before demodulating Information A, the carrier frequency of the FH capacity boost signal must be determined. After the demodulation of Information B, the carrier reconstruction is performed according to the correspondence between Information B and the carrier frequency, and then Information A is demodulated using the reconstructed carrier of Information B. For Information B, the demodulation performance of both the envelope detection and coherent demodulation converge as the modulation order increases; for simplicity, the demodulation method of envelope detection is used for Information B. The specific flow of the demodulation algorithm is as follows:  
The received fixed-frequency signal is input into the Gardner loop for timing synchronization after passing through the matched filter, after which the reconstructed carrier of Information B, demodulated using the power detector method, is used to perform carrier recovery out of Information A. First, it is determined whether the timing synchronization is completed or not, and the frequency hopping begins once the timing synchronization is completed, as shown in Figure 5.  
Based on the proposed signal generation and demodulation methods, a simulation verification system is established to verify the functional performance parameters. In the simulation verification model, [PERSON]'s algorithm is used for the timing error detection of Information A. The carrier recovery of Information A is performed using a Costas loop. Before the information transmission link is established, a certain synchronization time is reserved, and the frequency is fixed during the synchronization time without the transmission of Information B. * Power Detection Method for Demodulation of Information B  
The demodulation of Information B is equivalent to the demodulation of FSK signals. There are two different methods that can be used to demodulate FSK signals: the coherent  
Figure 5: Demodulation flowchart. demodulation method and the power detector method. When the number of frequency points gradually increases, the demodulation performance of these two methods is similar. However, the power detector method is relatively simple, so this paper adopts the envelope detector method. The realization block diagram is shown in Figure 6.  
In order to improve the received signal-to-noise ratio of Information B, the BER can be reduced at the expense of the information rate by using a joint detector judgement of multiple symbols, in which the J frequency-hopping symbols are jointly estimated as one Information B symbol. When J = 16, the Information B symbol keying selects the carrier frequency group of 16 frequency points, and Information B is recovered using the envelope detection method according to the correspondence between different frequency groups and Information B, as shown in Figure 7.  
Figure 6: Block diagram of the power detection method. Figure 7: Schematic diagram of joint envelope detection for J = 16 symbols. Figure 7A shows a received signal containing multiple carrier frequencies, and Figure 7B shows the received signal passing through bandpass filters at different frequencies to separate the different frequency carriers, respectively. As shown in Figure 7C, the signals are integrated to take the absolute value, and Figure 7D,E show the combined signals and the correspondence with Information B in accordance with Information B, respectively. Finally, the baseband Information B is recovered by the low-pass filter. Through simulation, the spectra after the multi-symbol joint detector for J of one, two, four, eight, and sixteen are shown in Figure 8. The signal power spectral densities are all enhanced with respect to the J = 1 signal, and the power spectral density values at the highest points are enhanced by 4 dB, 9 dB, 12 dB, and 15 dB, respectively.