Shortwave communication, also known as high frequency (HF) communication, refers to a communication method that uses ionospheric reflection for long-distance transmission or ground waves for short-distance transmission in the 3M-30MHz frequency range. Compared with other communication methods, shortwave communication has its own advantages: the communication distance is long, and the shortwave can communicate beyond the line of sight without a transponder within a range of thousands of kilometers; the shortwave is the only one that is not affected by the network hub The long-distance communication method agreed by the active relay system, once a war or disaster occurs, various communication networks may be damaged, and satellites may also be attacked, but the ionosphere is indestructible; the receiving equipment is simple, and for broadcasting services, the receiving end Only need to configure the shortwave receiver. It is precisely because of these advantages that shortwave communication has always been the main means of medium and long-distance communication in countries around the world, and is widely used in government, military, diplomatic, meteorological, commercial and other departments.
Shortwave communication also has inherent shortcomings: the phenomenon of multipath fading is serious, and the signal amplitude changes dozens of times or even hundreds of times during the propagation of shortwave reflections in the ionosphere; communication in blind areas is difficult. Generally speaking, shortwaves pass through ground waves. The longest propagation distance is about 30 kilometers, and the shortest distance for sky waves to first reflect from the ionosphere to land is about 100 kilometers. Therefore, the area from 30 kilometers to 100 kilometers forms a blind zone for shortwave communication; the ionospheric storm seriously interferes with shortwave Communication, ionospheric parameters are affected by the sun and other external influences. The electron concentration, effective height and ionospheric structure of the F2 layer will produce irregular changes, which will reduce the highest available frequency of the ionosphere, or even completely destroy it and interrupt shortwave communication.
In recent years, with the widespread application of shortwave communication in aviation navigation, maritime safety, emergency rescue and disaster relief, military communications, etc., its shortcomings of poor stability and reliability have become increasingly prominent, which has brought great challenges to shortwave communication research. The development of new shortwave communication technologies is also facing unprecedented opportunities.
The shortwave channel (ionosphere) is a typical time-varying dispersion channel. Its path loss, delay spread, noise and interference are constantly changing with the changes of frequency, location, season, day and night. Therefore, the working frequency in shortwave communication is not Arbitrarily chosen. Statistics show that even in the worst case of the communication environment at night, there are about 4% noise-free channels in the shortwave frequency band, and about 27% of the channels have little or no interference at noon. Therefore, avoiding interference in real time and finding a noise-free channel with good propagation conditions is the most effective way to improve the quality of shortwave communication. The key to achieving this goal is to adopt shortwave adaptive frequency technology. At present, adaptive frequency has gone through two mature stages of shortwave frequency management and 2G-ALE, and is developing towards 3G-ALE.
Frequency Management System
The shortwave frequency management system forms a frequency management grid in a certain area, measures and analyzes various channel parameters and interference distribution within the shortwave range, and obtains a frequency ranking list of the quality of communication according to the comprehensive analysis and calculation results, and assigns them uniformly All shortwave communication users in the area enable users to establish communication links on the best working frequency. The essence of shortwave frequency management is to provide real-time frequency forecasts to users in the area. The technology used is called Real Time Channel Evaluation (RTCE) technology. The frequency management system is characterized by the separation of communication and detection, and the detection equipment is expensive. This development process is also known as the 1G-ALE stage of shortwave adaptive technology.
2G-ALE communication system
In the mid-1980s, there appeared shortwave adaptive radio stations that directly used RTCE technology in the communication system to detect, evaluate and communicate with shortwave channels. This kind of radio station can select the best shortwave communication channel in real time, so that the shortwave operating frequency changes with channel conditions, ensuring that communication is always carried out on the best quality channel. The 2G-ALE communication system has the following functions:
Link Quality Analysis (LQA)
In the 2G-ALE communication system, the RTCE function is called link quality analysis LQA. Generally, LQA is performed before or during the communication, and is only performed on a limited shortwave channel, and the obtained data is stored in the LQA matrix. In actual communication, the system selects the preferred operating frequency according to the order of the channels in the LQA matrix.
Automatic scanning reception
In order to receive selective calls and carry out LQA tests, all radio stations in the network have automatic scanning reception functions, which can cyclically scan on several pre-defined channels, waiting for call signals or LQA detection signals.
Automatic link establishment ALE (Automatic Link Establishment)
According to the LQA matrix, the system automatically establishes a communication link. This function is called the automatic link establishment ALE function. It is based on the comprehensive application of automatic scanning, selective calling and LQA. It is also the biggest difference between 2G-ALE and 1G-ALE systems.
Automatic channel switching
In the communication process, when the radio wave propagation conditions deteriorate or severe interference, the short-wave adaptive communication system can switch channels, so that the communication frequency is automatically adjusted to the second best frequency in the LQA matrix.
3G-ALE communication system
On the basis of the 2G-ALE communication system, the 3G-ALE communication system has made many improvements:
1. Resident group division
Introduced the concept of Dwell Group, which divides all stations in the network into multiple Dwell Groups. At the same time, the stations in the same group work on the same channel, while different groups work on different channels, reducing system congestion.
2. Address structure
Each station in the network is assigned a separate 11-bit address, the lower 5 bits are the resident group number, and the upper 6 bits are the member numbers in the group. There are a maximum of 32 resident groups in the network, and each group has a maximum of 60 stations. The network can accommodate up to 32×60 = 1920 stations.
3. Channel separation technology
Separate the call channel and the data stream channel, and keep the call channel and the data stream channel adjacent to make them consistent in transmission characteristics, which is conducive to monitoring the transmission channel, ensuring high efficiency of information transmission and fast link establishment Sex.
4. Slot structure
The dwell time of the radio on each channel is 5.4s, divided into 6 time slots. The first time slot is used for tuning and monitoring, the second to fifth time slots are used for calling and answering, and the sixth time slot is reserved for handshake and notification. Time slot division technology reduces channel congestion.
The ALE communication system solves the frequency selection problem of shortwave communication well, but it also inevitably brings about problems such as detection calls occupying a large amount of network communication time, selection and adaptation of frequency limitations, and slow establishment of communication links. Recently, the latest self-optimizing radio station under NG developed by Curton of Australia is based on the CALM intelligent link quality database established based on the channel (frequency), transceiver link (contact information of both parties) and landing time used in the past. Solve these problems well.