Advantages and Disadvantages of Shortwave Communication (1)
In areas where the terrain is complex and 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 communication means such as satellite communication, terrestrial microwave, coaxial cable, optical cable, etc., short-wave communication also has many significant advantages:
1) Short-wave communication can realize long-distance communication without establishing a relay station, so the construction and maintenance costs are low, and the construction period is short.
2) The equipment is simple and can be fixed according to the requirements of use, and fixed-point fixed communication can be carried out. 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 fast, and it has great flexibility in use.
4) Strong resistance to natural disasters or wars. The communication equipment is small in size, easy to conceal, easy to change the working frequency to avoid enemy interference and eavesdropping, and easy to recover after damage.
These are the main reasons why shortwave communication has been retained for a long time and is still widely used today. Shortwave communication also has some obvious disadvantages:
1) The available frequency band is narrow, and the communication capacity is small. According to international regulations, each shortwave station occupies a frequency width of 3.7kHz, while the available frequency range of the entire shortwave band is only 28.5MHz. In order to avoid mutual interference, the world can only accommodate more than 7,700 communicable channels, and the communication space is very crowded. And the 3kHz communication bandwidth limits the capacity of communication and the rate of data transmission to a large extent.
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 have great variability. It is characterized by path loss, delay spread, noise, and interference, which are constantly changing with day and night, frequency, and location.
On the one hand, the change of the ionosphere causes signal fading, and the amplitude and frequency of fading change constantly.
On the other hand, the sky wave channel has a serious multipath effect, which causes frequency selective fading and multipath delay. Selective fading distorts the signal, and multipath delay spreads the received signal in time, which is the main limitation of data transmission on shortwave links.
