1.

INTRODUCTION

Frequency hopping synchronization includes two steps: synchronization acquisition and synchronization tracking.

First, synchronous capture. It can be divided into two steps: initial acquisition and synchronous identification: 1) initial acquisition refers to finding the general time position of the transmitter by acquiring the TOD information of the transmitter, and initially making the clock error of the transceiver less than the allowable value. 2) Synchronization identification refers to further checking whether the clock error of both the transmitter and the receiver is less than the allowable value based on the completion of the initial acquisition.

Second, synchronous tracking. It refers to fine-tuning the clock based on the completion of synchronization acquisition, further reducing the time error of the frequency hopping sequence of the transmitter and receiver until it is accurately aligned, so as to keep the synchronization. Synchronization tracking is usually realized by using phase locked loop. Figure 1 shows the time-frequency matrix of both the sending and receiving sides before synchronization. Figure 2 shows the time-frequency matrix of both the sending and receiving sides after synchronization.

Figure1.

The time-frequency matrix diagram of the transmitter and the receiver before synchronization

Figure 2

The time-frequency matrix diagram of the transmitter and the receiver after synchronization

Frequency hopping communication system Due to the characteristics of its carrier frequency changing with time, frequency hopping synchronization usually refers to frequency synchronization, that is, the transmitter and receiver use the same frequency at the same time. The advantages and disadvantages of frequency hopping synchronization performance determine the performance of frequency hopping communication system. The frequency of each hop of the transmitter and receiver is not the same before synchronization, and the frequency of each hop of the transmitter and receiver becomes the same after synchronization. Synchronous capture is the key to frequency hopping synchronization, and its performance largely determines the performance of frequency hopping synchronization, and most researchers mainly improve synchronization performance by improving the performance of synchronous capture.

2.

PERFORMANCE ANALYSIS

2.1

Average capture time

Suppose that the number of frequencies used for synchronization in the DSFH communication system is the number of times, the hopping period is, and the initial phase difference between the receiver and the transmitter is evenly distributed. From the principle of the above matching filter method, it can be seen that each time the received synchronization signal is fed into a branch at the same time, which is equivalent to searching for frequencies at the same time, which greatly increases the search rate and reduces the capture time, as long as the time of receiving a cycle can be completed, so the expected value of the synchronous capture time is:

2.2

Capture probability, imaginary capture probability, missed capture probability

Suppose the received hop signal is

The signal is fed into each branch, and after mixing, filtering, and A/D conversion, the signal is obtained only in the corresponding branch:

Fm is a preset intermediate frequency, and the detection performance of the energy detection in each branch is the same as that of the sliding correlation method, so the detection probability Pd, false alarm probability P_f, and missed detection probability Pm of each branch are:

2.3

Solve for the capture probability

The capture probability PA, the false capture probability PFA, and the leakage capture probability PMA can be obtained by the principle of the matching filter method, respectively:

As the probability of capture increases as N increases, the probability of virtual capture decreases; If N is left unchanged, the probability of false capture decreases as the signal-to-noise ratio increases. As is shown in Figure 3.

Figure3.

Effect of N on PA of the matched filter method

3.

FREQUENCY HOPPING SYNCHRONIZATION PERFORMANCE

The advantages and disadvantages of frequency hopping synchronization performance determine the performance of frequency hopping communication system, and this paper measures the advantages and disadvantages of frequency hopping synchronization performance mainly in these three aspects: reliability, anti-reconnaissance and anti-interference. Reliability is mainly based on the average capture time of synchronization as a metric, according to the process of synchronous capture of quantitative calculation of the average capture time, the shorter the average capture time, the higher the reliability; Anti-reconnaissance is mainly based on the number of frequency points used for synchronization, frequency point jump law for qualitative analysis, the more frequency points, the more pseudo-random anti-reconnaissance performance is better; Anti-interference is measured by two indicators of synchronous capture probability and synchronous virtual capture probability, synchronous capture probability and synchronous virtual capture probability can be quantitatively calculated, the larger the synchronous capture probability, the smaller the synchronous virtual capture probability, the better the anti-interference performance. In addition, the complexity of the system needs to be analyzed, and the lower the complexity of the system, the easier it is to engineer.

Usually according to the way the receiver obtains frequency hopping synchronization information, the frequency hopping synchronization method is generally divided into four types: independent channel method, reference clock method, synchronous prefix method and self-synchronization method.

The independent channel method refers to the use of a specially selected channel to transmit synchronization information, and the sending and receiving parties synchronize by adjusting the local clock according to the synchronization information. The advantage of this method is that the amount of information that can be transmitted synchronously is large, the synchronization time is short, and the synchronization can be maintained for a long time by continuously transmitting synchronization information. The disadvantage of the independent channel method is that it requires a special channel to transmit synchronous information, its concealment is very poor, it is very easy to be found and interfered with by the enemy, and it consumes a small number of frequency resources and signal power on the battlefield, which is not suitable for use in the communication environment.

The synchronous prefix method refers to the transmitter sending a synchronous prefix to the receiver containing all the information of the generated frequency hopping sequence, and the receiver realizes synchronization accordingly. The advantage of this method is that the synchronization time is short and synchronization can be achieved quickly. The disadvantage of the synchronous word header method is that although it uses a communication channel to transmit synchronous information, it still occupies the frequency resources of a small number of communication channels and consumes signal power, the disadvantages are similar to the independent channel method, and there are also shortcomings that are not hidden enough to be easily discovered and interfered with by the enemy, and are not suitable for communication.

4.

SIMULATION EXPERIMENTS

Using Matlab simulation, and sequence using a general model based on the m sequence on GF(P), and using a linear feedback shift register structure to generate a frequency hopping sequence with a period of 2³⁶ bits, the frequency point number is 512, the jump speed is 200h/s, the maximum time difference between the sending and receiving sides is set to 5 min, and the channel is AWGN channel.

Set the number of samples for different energy detections, and the synchronization capture probability changes with the signal-to-noise ratio as shown in Figure 4. As can be seen from the figure, the horizontal comparison of synchronous capture probability, with the increase of the signal-to-noise ratio, the capture probability increases and tends to 1, longitudinal comparison, the same signal-to-noise ratio, with the increase of the number of samples, the detection probability increases. Increasing the number of samples can improve the probability of synchronous capture of the signal.

Figure 4

The PA of different N

5.

CONCLUSION

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