Research on Anti-interference Synchronization Algorithm in Frequency Hopping Communication

In order to accomplish the proposed frequency hopping synchronization work effectively when less effective frequency peak information is obtained, it is necessary to design a frequency hopping sequence for synchronous hopping at the transmitting end.

If the frequency hopping synchronous search method is used, when the receiving end obtains the effective frequency peak information, the following information can be obtained:

Frequency information, namely F1, F2, F3, F4; the order of the frequencies, namely F1 is connected to F2, not other frequencies; the interval information between the frequencies, namely the interval between F1 and F3 is one hop.

The frequency point sequence of the transmitting end is designed to be F1, F1, F2, F3, F4, F3, F2, F1, F3, that is, according to the above 3 pieces of information analysis, the information of any two frequency points in this sequence can completely determine their positions in this sequence. Since the position information determined by any two frequency points in the transmitting end sequence is unique, the distance between the currently received frequency point information and the synchronous jump random jump position can be inferred, and the time information of the synchronous jump random jump can also be given, which is required, and there are many groups of such sequences composed of the same frequency points, which can ensure that the transmitting end synchronization sequence can have a larger selection space when the frequency point is determined.

2 System Framework

The overall block diagram of the frequency hopping system is shown in Figure 1.

Among them, the frequency tracking synchronization, the determination of the timing information of the synchronous jump random jump and the anti-interference processing are the key points of the algorithm strategy proposed here. Since the algorithm in this paper extracts the required information from the received energy peak, frequency value and the distance between the peaks, rather than the modulation information transmitted by the frequency, the algorithm strategy in this paper has no restrictions on the modulation method and can be well combined with various existing modulation and demodulation methods.

3 Core Algorithms

The algorithm in this paper is mainly divided into the following parts, and the system block diagram is shown in Figure 2.

3.1 Calculation of peak value at the receiving end

The receiving end cycles through the four frequencies F1, F2, F3, and F4 at 4 times the transmitting end speed, and down-converts the received signal to find out its energy, and records the time when the peak value appears. From the design of the synchronous frequency sequence at the transmitting end, it can be seen that the number of consecutive peaks at the same frequency will not exceed two, which is a simple method to remove single-frequency interference information. As shown in Figure 3.

3.2 Receiver Peak Information Template

According to the frequency sequence of the transmitting end, four template sequences are designed for the receiving end, which represent four alignment methods between the local receiving frequency and the transmitting frequency, as shown in Figure 4. Among them, F1~F4 represent the peak values ​​of the four frequency points, and one represents the situation where no peak value appears. These four templates represent the four possible combinations of peak information generated by the receiving end in alignment with the transmitting end frequency point. By comparing the combination of the actual received frequency peak information locally with the four templates, the position of the received frequency peak information in the sequence can be quickly determined, and the time information of the synchronous jump to random jump can be given.

3.3 Receiver frequency peak information combination

From the properties of the designed transmitter sequence, it can be seen that any combination of the frequency peaks at the receiver can correctly give the time information of the synchronous jump to the random jump. Therefore, in order to make full use of the obtained frequency information and improve the anti-interference ability, the received frequency peak information is combined. Each time a new frequency peak information is received, it is combined with the previously received frequency peak information with a certain probability to ensure that the first received frequency peak information has the largest weight.

3.4 Calculating time information using different combinations of frequency information

The combination of various frequency peak information obtained in Section 3.3 is slidingly correlated with the four templates generated locally to obtain the position of the combined sequence in the template, thereby obtaining the time information required for synchronous jump random jump. Since the position indicated by any combination of correct frequency peak information in the sequence is certain, the position indicated by the combination of interference frequency peak information in the sequence is uncertain and divergent. Therefore, after receiving a certain number of frequency peak information, these peak information are fully combined, and then the jump positions indicated by them are searched in the sequence, and the one with the most jump positions indicated in this step is taken as the jump position of synchronous jump random jump.

In this step, since the number of full combinations increases rapidly with the increase of peak information, the amount of calculation also increases significantly. In order to reduce the amount of calculation, under the premise of ensuring anti-interference performance, a method of adaptively adjusting the number of frequency point peak information involved in the calculation is adopted, that is, based on whether the jump position indicated by the full combination of existing frequency point peak information converges, if it cannot converge obviously (here, the indicated convergence position is more than 3 times the number of other positions for obvious convergence), then receive another frequency point peak information to participate in the combination and calculation until the obvious convergence requirement is met.

3.5 The remaining frequency information is used as verification

The required time information of the synchronous jump and random jump is obtained from Section 3.4. The frequency peak information received afterwards can be used as a check of the correctness of the time information obtained in the previous step.

4 Performance Analysis and Simulation Results

The algorithm proposed in this paper uses the frequency peak and the distance information between them as the basis for judgment, so it is not sensitive to whether the information on the specified frequency can be correctly received. Compared with interfering with the information carried on the specified frequency, it is difficult to interfere with the energy information on the specified frequency, so it has strong adaptability. The performance analysis is divided into two parts: one is the anti-interference performance of the system, and the other is the performance index of the system synchronization time.

4.1 Anti-interference performance analysis

There are three main types of interference in the frequency hopping synchronization system, namely broadband low-density interference, narrowband high-density interference and fake synchronization interference. Broadband low-density interference requires a large power to achieve the interference effect, and the interference efficiency is low; narrowband high-density interference uses higher interference energy to suppress some frequency points, and has a strong interference ability to the suppressed frequency points, but the probability that the interfered frequency band can cover the frequency hopping synchronization frequency point is small, and the interference efficiency is not high; fake interference, the interferer uses the synchronization frequency of the interfered party to send fake synchronization information with legal rate, format and algebraic structure. This kind of interference may cause frequent false alarms and disrupt the normal operation of the synchronization system. The anti-interference ability of the frequency hopping synchronization system can be examined from two aspects: capture probability and false alarm probability.

From the above algorithm introduction, we can know that if the system has n correct peak information for calculating the jump position, the combination of the common group can correctly give the jump position. If the system has k correct frequency peak information, the common group can correctly give the jump position combination. The jump position given by the combination of missed or wrongly received frequency peak information due to interference either does not exist or is scattered, which will not have much impact on the judgment of the correct jump position.

FIG5 simulates the influence of different combinations of the number of correct frequency peak information and the number of interference frequency peak information on the correct jump position.

4.2 System time index analysis

System synchronization time refers to the time required to complete synchronization. In the synchronization process based on timing fast scanning, the synchronization time is the time of the synchronization head. In this system, under ideal conditions, the four frequency peak information can accurately complete the calculation of the time information of the synchronization jump random jump, and the synchronization time is very short. Figure 6 shows the relationship between the system synchronization time and the number of frequency peak information involved in the calculation.

4.3 System Complexity Analysis

The frequency hopping synchronization method based on timing information is superior to the waiting search method and the displacement waiting method in shortening the synchronization capture time; it is superior to the precise clock timing synchronization method in terms of reliability; it is superior to the insertion pilot head synchronization method in terms of saving frequency resources. However, in the fast scanning and resident synchronization process at the receiving end, the frequency conversion time of the frequency synthesizer should be short, the frequency hopping rate should be increased to 4 times, and the complexity of signal processing is correspondingly increased in the detection and judgment process of the synchronization head. Therefore, the improvement of the performance of this synchronization method comes at the cost of increasing the technical difficulty and computational complexity of the frequency synthesizer.

5 Conclusion

In order to solve the problem of weak synchronization link in frequency hopping system, this paper proposes a fast frequency hopping synchronization algorithm with strong anti-interference ability. The priority of the frequency peak information received first is fully considered in the implementation of the algorithm, making the algorithm more suitable for the actual application environment. The algorithm has good anti-interference performance and short synchronization time, which can better solve the problem of the synchronization head of the frequency hopping system being easily interfered in a complex electromagnetic environment.