Shortwave frequency management system and its application in communication
1. Development background of shortwave frequency management
Shortwave communication has many unique advantages. It is mobile, flexible, low cost, and can communicate over long distances. Therefore, high-frequency communication has always been an important means in military communication, and has always enjoyed an important position in wireless communication. However, in the early 1970s, satellite communications emerged suddenly. Due to the stable and reliable performance of satellite communications, it replaced high-frequency communications to a considerable extent. However, with people's understanding of the limitations of satellite communications such as vulnerability and huge cost, high-frequency communications have once again received attention. From the perspective of military communications, it can be concluded that there is no single long-distance communication method that can meet all requirements. Therefore, high-frequency systems are deployed in the C4I systems of major developed countries in the world, and are increasingly playing Important role. For example, in the US Army ’s 21st century digital battlefield, a battlefield information transmission system? BITS? Was planned. The wideband HF? WBHF? Project is to study the application of high-frequency systems in the digital battlefield, and hopes to expand the tactical Internet to HF band. These all put forward new requirements for short-wave frequency selection.
Shortwave communication frequency selection has mainly gone through three main stages:
(1) Long-term frequency prediction. According to the number of sunspots and the time of the quarter month, the highest available frequency MUF of the circuit is predicted. This method is based on the concept of monthly median, so the operating frequency is difficult to track changes in the ionosphere, thus affecting the high-frequency communication effect.
(2) Real-time frequency prediction. Since the 1960s, countries such as the United States and Canada have developed real-time frequency selection technologies, and special real-time frequency selection systems such as CURTS, CHEC, and Chirp have appeared. Real-time frequency selection technology plays a great role in ensuring high-quality short-wave trunk communication. In recent years, the Chirp frequency selection system has been greatly developed. In 1994, ITU Recommendation 720 has recommended FM / CW “Chirp†detection technology as the standard for dynamic frequency management. The United States, Britain and other countries conducted field scientific experiments on the Chirp system, and achieved good results in the Gulf War.
(3) Adaptive communication technology. This is a new technology developed in the 1980s. It combines frequency selection and communication, and can automatically communicate circuits on the best channel. Because frequency selection and communication are integrated, and communication is the mainstay, the frequency selection quality is lower than the frequency quality provided by the dedicated real-time frequency selection system. The future development direction should be to combine the dedicated frequency selection system and the adaptive communication system to further improve the quality of shortwave communication. At present, the third-generation adaptive high-frequency system proposed by the United States has embodied the combination of the Chirp frequency selection system and the short-wave communication system.
China's scientific research on short-wave communications and short-wave real-time frequency selection can basically track the latest international level. During the "Eighth Five-Year Plan" period, the short-wave frequency management and forecast system completed by the Seventh Institute of Electronics, the 713th Plant, and the 22nd Institute of China marked a leap to a new level in this field. The system has played a huge advantage in practical applications. China's vast area and sea area will surely place more and higher demands on short-wave communications. During the "Ninth Five-Year Plan", the Seventh Institute of the Ministry of Electronics undertook the task of "communication frequency management". After the completion of this task, the short-wave real-time frequency selection will be raised to a new level, and the frequency range from 300kHz to 2000MHz will be managed in a wide area.
2. Basic principles of short-wave frequency management
"Chirp short-wave frequency management forecast system" consists of a detection transmitter, a detection receiver, a spectrum monitor and a frequency management terminal. Each detection receiver can receive signals from three transmitters.
The system can perform real-time detection of the ionosphere, accumulate detection data, and perform real-time dynamic management of short-wave frequency resources; it has a three-transmission and one-receiving networking function; and has real-time, short-term, and long-term frequency prediction functions. Real-time forecasting is based on the signal-to-noise ratio and delay spread value of real-time detection to sort and select the frequencies to provide the best communication frequency for short-wave trunk communication. In addition, high-frequency communication frequencies can also be provided for short-wave communications outside the detection circuit.
The detection principle of the "Chirp short-wave frequency management forecasting system" belongs to the chirp detection system? Chirp Sonding System ?, and the detection signal adopts FM continuous wave? FM / CW? Signal.
When the system is working, both the detection transmitter and the detection receiver are accurately timed. When the transmitter transmits the FM / CW detection signal, the receiver's internal clock controls the receiver to simultaneously generate an internal reference signal in the same form as the transmitted signal, precisely synchronized with the transmitter signal. Any radio energy that can propagate through the HF circuit? Surface waves, one-hop two-hop F-layer propagation, etc. can be received by the detection receiver. The propagation time of the probe signal from the transmitter to the receiver makes the time at which the signal arrives at the receiver lag the receiver's tuning frequency point. Therefore, when the receiver receives the probe signal and mixes it with the reference signal in the machine, it will be obtained Difference frequency signal. The receiver amplifies and converts the difference frequency signal into a baseband audio signal. An audio signal of 0 Hz means that there is no time delay. The increase in the frequency of the audio signal? The maximum is 500 Hz? Represents the increase in the delay of the ionospheric reflected signal. In fact, the receiver baseband audio output is a multi-tone signal that represents the signal and various delays caused by reflections from different regions of the ionosphere. The relationship between the signal delay Dt and the receiver's audio output Df is: Df =? Df / dt? × Dt, where df / dt is the slope of the swept frequency signal, and it can be seen that the frequency of the receiver's baseband audio signal is proportional to the delay. By analyzing these multi-tone signals with an audio spectrum analyzer, the number of propagation modes and the differential delay of each propagation mode can be determined, and finally the required ionization map can be obtained.
The so-called ionization diagram refers to the distribution of the delay of different propagation modes with frequency. The signals of different propagation modes have a time interval of milliseconds. When the frequency changes, the delay time of these multipath signals also changes accordingly.
The delay parameters can be obtained from the ionization diagram. When there are multiple propagation modes, the time difference between the earliest and latest signals arriving at the receiving end is the multipath delay. The signal-to-noise ratio can be obtained by measuring the Chirp signal energy and local interference. Therefore, the communication quality of each frequency point can be evaluated.
The Chirp detection equipment can scan with a continuous phase signal in the useful frequency band at a rate of 100 kHz per second. Since the detection receiver works in a relatively narrow receiving bandwidth, on the order of a few hundred hertz, only a relatively low transmit power is needed to complete long-distance detection. After many tests, it has been shown that the short-wave frequency management system developed by our country can complete the detection of the frequency of 2000km communication circuit in the whole frequency band with a detection power of 20W.
3. How to apply the short-wave frequency management system to civil communications
In civil communications, a national short-wave trunk frequency selection network can be established through a short-wave frequency management system. The system of short-wave frequency management system adopts FM / CW system. A three-layer structure can be installed nationwide.
Taking Beijing as the center, the first-class frequency selection network with the capitals of the major administrative regions as the end point, the network has a total of six detection circuits. The headquarters is equipped with two detection receivers, one or more interference noise monitors, and a frequency management terminal. The frequency management terminal shall be connected to the communication system controller and the central network management equipment. Corresponding detection transmitters are set up in the capitals of major administrative regions. The first-level frequency selection network is responsible for short-wave frequency management in Beijing and major administrative regions.
Taking the capital of the large administrative region as the center and the provincial capital as the endpoint, a secondary frequency selection network is formed. The network is generally composed of 2-4 detection circuits. Set up a detection receiver, an interference spectrum monitor, and a frequency management terminal in the capital of the large administrative region. The frequency management terminal is connected to the communication system controller and network management equipment of the large administrative region. Set up the corresponding detection transmitter in the provincial capital. The secondary frequency selection network is mainly responsible for frequency management in the large administrative area.
Below the provincial capitals, you can set up a three-level frequency selection network.
For communication circuits that deviate from the detection circuit, frequency correction can be performed by extrapolation or pseudo-sunspot number method? PPSN ?.
4. How to apply the short-wave frequency management system to military communications
In the next stage of scientific research, the frequency management system and the communication system should be organically integrated for 2-2000MHz frequency band management. The short-wave frequency management system is only part of the communication frequency management system. The frequency management system can not only detect the communication channel in real time, but also plan the frequency. At the same time, it is necessary to understand the user's demand for frequency, the background of the on-site electromagnetic spectrum, and the change in network topology, so as to adjust the frequency used to achieve dynamic frequency management. Through the communication system centralized controller or network management equipment for frequency distribution, and realize the frequency change operation of the radio station.
In a mobile battlefield environment, a mobile frequency selection system can be used for frequency selection. The short-wave frequency management system can be loaded into vehicles and ships. With the flow of the battlefield, vehicle-mounted and shipborne frequency selection systems can be used for frontier battlefield frequency management. This can solve the short-wave communication problem in various complex situations.
When the radio environment requires silence, probe transmission is not allowed, then passive real-time frequency selection technology can be used for frequency management.
5. Future development trend of short-wave frequency management
The main development trend is used in the third generation high frequency communication system.
The first-generation high-frequency communication system does not have an adaptive frequency selection function; the second-generation high-frequency communication system is based on the US military standard MIL-STD-188-141A and has an adaptive frequency selection function, but because it is an asynchronous system, Therefore, the circuit linking process is relatively long; the third-generation high-frequency communication system is based on the US military standard MIL-STD-188-141B and has an adaptive frequency selection function. Because it is a synchronous system, it can quickly establish circuits.
The basic concept of the third-generation high-frequency adaptive communication system is: when working in synchronous mode, all search receivers change the frequency at the same time. However, all stations are not monitoring the same call channel at the same time. A group of stations monitoring the same channel at the same time is called a resident group. In this way, calls initiated to network members will be separated in time and frequency, so the probability of collision in the call is greatly reduced. When a calling station wants to link with the called station, the calling station will calculate the search frequency of the called station in the next dwell period and use this frequency for calling. Compared with the second-generation adaptive communication system, Reduced call time. The calling period of the sending station is the same as the searching dwell time of the receiving station on a certain frequency.
In the third-generation adaptive communication system, the communication system itself usually does not perform channel quality analysis? LQA ?, but uses the chirp frequency management system to provide the communication frequency, and makes the search frequency in the communication system express the best. Therefore, the chirp frequency management system is the basis of the third-generation adaptive communication system.
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