According to the International Radio Consultative Committee (CCIR, current ITU-R), shortwave refers to electromagnetic waves with a wavelength of l00m~l0m and a frequency of 3MHz~30MHz. Radio communication using shortwave is called shortwave communication, also known as high frequency (HF) communication. In fact, in order to make full use of the advantages of short-wave short-range communication, the frequency range actually used for short-wave communication is 1.5 MHz to 30 MHz.
Since an accident in Rome, Italy, in 1921, short-wave communication was discovered to enable long-distance communication, and short-wave communication has developed rapidly. It has become the main means of medium- and long-distance communication in countries all over the world, and is widely used in government, military, and diplomacy. Meteorological, commercial and other departments for transmitting telegrams, telephones, faxes, low-speed data and images, voice broadcasts and other information. Before the advent of satellite communications, shortwaves played a unique and important role in international communications, flood control, shipwreck rescue, and military communications.
Short-wave communication can use ground wave propagation, but mainly uses sky-wave propagation.
The attenuation of ground wave propagation increases with increasing operating frequency. Under the same ground conditions, the higher the frequency, the greater the attenuation. The ground wave is only suitable for short-range communication, and its working frequency is generally selected below 5MHz. The ground wave propagation is less affected by the weather and is relatively stable. The channel parameters do not change with time, so the ground wave propagation channel can be regarded as a constant reference channel.
The sky wave is the part of the radio wave reflected back to the ground by the ionosphere. The obliquely projected electromagnetic wave is reflected by the ionosphere and can be transmitted to the ground thousands of kilometers away. The propagation loss of the sky wave is much smaller than that of the ground wave. After multiple reflections between the ground and the ionosphere (multi-hop propagation), it can reach a very far place. Therefore, the sky wave can be used for global communication. Sky wave propagation is extremely unstable due to the severe effects of ionospheric changes and multipath propagation, and its channel parameters change abruptly with time, so it is called a variable-parameter channel. Skywave can be used not only for long-distance communication, but also for short-range communication.
In areas where the terrain is complex, short-wave ground waves or line-of-sight microwaves are blocked and cannot be reached, communication can be achieved by using sky waves projected at high elevation angles. Compared with satellite communication, terrestrial microwave, coaxial cable, optical cable and other communication means, short-wave communication also has many significant advantages:
1) Short-wave communication can realize long-distance communication without establishing a relay station, so construction and maintenance costs are low, and the construction period is short;
2) The device is simple, and can be fixedly set according to the requirements of use, and fixed-point communication can be performed. It can also be carried or loaded into vehicles, ships, and aircraft for mobile communication;
3) The circuit scheduling is easy, the temporary networking is convenient and rapid, and has great flexibility in use;
4) Strong resistance to natural disasters or war. The communication device is small in size and easy to conceal, and it is easy to change the working frequency to avoid enemy interference and eavesdropping, and it is easy to recover after the destruction.
These are the main reasons why short-wave communication has been retained for a long time and is still widely used today. There are also some obvious shortcomings in shortwave communication:
1) The available frequency band is narrow and the communication capacity is small. According to international regulations, each short-wave radio station occupies a frequency width of 3.7 kHz, while the entire short-wave frequency band has a frequency range of only 28.5 MHz. In order to avoid mutual interference, the world can only accommodate more than 7,700 communication channels, and the communication space is very crowded. And the 3 kHz communication band width largely limits the capacity of communication and the rate of data transmission.
2) The short-wave sky-wave channel is a variable-parameter channel, and the signal transmission stability is poor. Short-wave radio communication mainly relies on the ionosphere for long-distance signal transmission. The weakness of the ionosphere as a signal reflection medium is that the parameters are highly variable. It is characterized by path loss, delayed walks, noise and interference, all of which change with day and night, frequency and location.
On the one hand, changes in the ionosphere cause the signal to fade, and the amplitude and frequency of the fading change continuously;
On the other hand, the sky-wave channel has serious multipath effects, resulting in frequency selective fading and multipath delay. Selective fading distorts the signal, and multipath delay causes the received signal to spread over time, which becomes the main limitation of short-wave link data transmission.
3) Atmospheric and industrial radio noise interference is severe. With the development of industrial electrical appliances, the average intensity of radio noise interference radiated by industrial electrical equipment in the short-wave frequency band is high, coupled with atmospheric radio noise and inter-radio interference. In the past, long-distance can be realized by several watts and dozens of watts of transmit power. Short-wave radio communication, and today, 10 times, dozens of times such power does not necessarily guarantee reliable communication. Atmospheric and industrial radio noise is mainly concentrated at the low end of the radio spectrum, and the intensity gradually decreases as the frequency increases. Although the noise interference in the short-wave band is lower than that in the medium-long band, the intensity is still high, which affects the reliability of short-wave communication, especially the burst-type burst noise, which often causes sudden errors in data transmission and seriously affects communication. quality.
The existence of these problems not only limits the development of short-wave communications, but also does not adapt well to the growing demand for data communications, especially for high-speed data communications services. When satellite communication emerged in the 1960s, short-wave communication was seriously challenged because satellite communication has advantages such as channel stability, high reliability, good communication quality, and large communication capacity compared with short-wave communication. Many important services of the original short-wave communication were replaced by satellite communication; the investment in short-wave communication was drastically reduced, and the status of short-wave communication was greatly reduced. By the late 1970s, some even suspected the value of short-wave communication.
However, practice has proved that satellite communications have high initial construction costs and limited flexibility. Satellite communications, once conceived as possible to replace short-wave communications, do not meet the needs of users in all situations. In fact, not all users need broadband lines.
In addition, during the war, satellite communications are vulnerable to enemy attacks, and the channel is not easy to withstand enemy electromagnetic interference. In contrast, short-wave communication is not only low-cost, easy to implement, but more importantly, it has a natural "relay system" that is not easily "destroyed" - the ionosphere. The satellite relay system may malfunction or be destroyed, and the ionosphere relay system may interrupt it unless a high-altitude atomic bomb explosion, not to mention that the high-altitude atomic bomb explosion is only limited in the ionosphere region for a short time to affect the ionization density.
In February 1980, the Defense Nuclear Agency announced in a report: "A country, after the atomic attack, the most promising solution to resume communication is to use low prices and be able to automatically find channels. High frequency communication system".
In fact, from the late 1970s to the early 1980s, short-wave communication has received renewed attention. Many countries have accelerated the research and development of short-wave communication technologies, and have introduced some new types of equipment and systems with excellent performance. In the comprehensive tactical communication plan revised by the US military in 1979, it also highlighted the status of short-wave communication and listed it as one of the first-line command and control communication means. Since the early 1980s, the US military has implemented a series of short-wave communication improvements throughout the three armed forces. Plan; in the Gulf War, the United States, France and other countries used a large number of short-wave communications, and achieved outstanding results. In recent years, the military of other countries has also listed short-wave communication as one of the important means of communication.
In addition, in some areas of civil communications, the application of short-wave communications has also developed. Especially in the past ten years, due to the application of a variety of new technologies, short-wave communication technology and equipment have made great progress. The shortcomings of short-wave communication have been overcome, and the quality of short-wave communication links has been greatly improved. Whether the quality of telephone transmission or data transmission can be compared with satellite communication, short-wave communication has regained its youth.
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