10 years.
The term of a radio frequency use license shall not exceed 10 years. For temporary use of radio frequencies, the period of radio frequency use license shall not exceed 12 months. If the corresponding use license obtained by a radio frequency user does not determine the frequency use period, if the frequency has been used for more than 10 years and needs to continue to be used, the extension should be applied for within 6 months from the date of implementation of the "Radio Frequency Use License Management Measures" formalities.
Radio
Radio waves refer to electromagnetic waves in the radio frequency band that propagate in free space (including air and vacuum). The shorter the wavelength and higher the frequency of radio waves, the more information can be transmitted in the same time. Radio waves propagate in space in the following ways: direct radiation, reflection, refraction, penetration, diffraction (diffraction) and scattering.
Characteristics of radio waves
Fading characteristics of radio waves
The fading of radio waves in the propagation process is a very important characteristic of it, which can be viewed from a large and Described in three sizes: medium and small.
Large scale is used to describe the median signal (regional mean). It has power law propagation characteristics, that is, the median signal power is inversely proportional to a certain power raised by the distance length.
Mesoscale is used to describe slow fading. It is the average power change superimposed on the median level of the large-scale propagation characteristics. When expressed in decibels, this variation tends to be normally distributed.
Small scale is used to describe fast fading. It usually obeys the Rayleigh probability density function, also known as Rayleigh fading.
Radio wave applications
The earliest form of broadcasting was marine radiotelegraphy. It uses a switch to control whether the continuous wave is transmitted or not, and the signal generated at the receiver is Morse code. Cellular phones or mobile phones are currently the most common form of wireless communication. Cellular phone coverage areas are typically divided into multiple cells. Each cell is covered by a base station transmitter. Common analog TV signals use amplitude modulation, frequency modulation and synthesis of images to be transmitted in the same signal.