Electromagnetic waves in different frequency bands have different propagation modes and characteristics, and their uses are different. The special problem that needs to be stored in the radio frequency allocation is the interference problem, because the electromagnetic waves are transmitted according to the characteristics of the frequency band, and there is no other law to constrain, so the two stations have the same frequency or very similar frequencies working in the same area. In the same time period, it will cause interference. The range of available modern radio frequencies is limited and cannot be used out of order, but needs to be carefully utilized.
The use of spectrum includes two aspects:1. Distribution of spectrum: It is allocated according to different services to avoid confusion in frequency usage.
2. Spectrum savings.
Due to the limited spectrum range, the spectrum resources occupied by each station should be reduced to accommodate more stations and reduce interference. This requires compressing the bandwidth of each station as much as possible and reducing the spacing between channels and spurious emissions.
Electromagnetic waves are transmitted globally. They cannot be decided by a certain country, but need to be resolved according to international agreements. The world organization that now performs the work of frequency allocation is the ITU. The main basis for frequency allocation is the propagation characteristics of electromagnetic waves in various bands and the characteristics and sharing requirements of various services, as well as historical conditions and technological development.
The ITU Geneva Conference divides the world into three frequency zones, namely “Europe and Africaâ€, “South-North America†and “Asia-Australiaâ€. The frequency division of different frequency bands is also different. Interested can be consulted.
The types of radio frequency services are classified into the following 16 types according to the ITU protocol:
Fixed-point communication service
2. Aviation fixed-point communication service
3. Broadcasting service
4. Mobile communication services
5. Aviation mobile communication service
6. Marine mobile communication services
7. On the road mobile communication service
8. Radio navigation service
9. Radiolocation service
10. Space communication service
11. Radio astronomy business
12. Meteorological services
13. Amateur radio service
14. Standard frequency service
15. Timing signal service
16. Industrial Technology Medical Frequency
Items 11 to 15 are recognized as being undisturbed, and other services allocated to these services are not authorized or used only without interference.
Here I mainly introduce the frequency range in which the most discussed 850MHz and 900MHz are located and the frequency band in which the current mobile communication service is located:
1.30 to 1000 MHz band:This band is part of the high frequency (meter wave) and ultra high frequency (decimeter wave). This band is an "intermediate" band. It is basically not reflected by the ionosphere, but can also be reflected at the lower end of the meter wave band (in the year of high solar activity and E1 layer), generally below 60 MHz. This is often also a cause of interference. The distance traveled by ground waves is shorter, but military tactical radio stations also use ground waves for short-range communication. Mainly use the low end of this band.
The mode of propagation that plays a major role in this band is the spatial wave propagation within the line of sight, as well as tropospheric and ionospheric scattering.
The advantage of this band compared to the high frequency band is that small size antennas can be used for low volume systems. Obviously, this feature is particularly suitable for mobile communications. In radio relay systems, higher frequencies are used, and although the propagation loss increases, the high antenna gain can compensate for this loss. Therefore, the high frequency end of this frequency band is suitable, and the capacity can also be increased, and more ways can be passed.
Tropospheric scoping replaces the radio-relay system in some cases because it can be used for hundreds of kilometers without a relay station, and can also have large capacity (multiplexed), which is not possible at low frequencies.
The allocation of frequencies in this band. Mainly distributed in broadcasting, land mobile communications, aeronautical mobile communications, maritime mobile communications, fixed-point communications, space communications, radar, etc.
1. Broadcasting services: FM broadcasting is allocated at 88 to 108 MHz, while television broadcasting is allocated at 41 to 100 MHz, 170 to 216 MHz, and 470 to 960 MHz (varies vary from country to country).
Land mobile communications are mainly used by vehicle radio stations or piggybacked radio stations, with a main frequency band below 500 MHz. At the lower frequency end, due to the large atmospheric noise interference, it is not suitable for use in cities (because the urban man-made noise level is also high). The city should be used at around 450 MHz.
2. Aeronautical mobile communications: Air-to-ground communications use 118 to 139 MHz. They are close-range mobile communications, in line-of-sight. When the flying height is 1500 meters, the viewing distance is about 130 kilometers. When the height is 12,000 meters, it is about 320 kilometers. Most of this communication uses a pre-tuned channel to connect to the radiotelephone.
Maritime mobile communications are mainly used for communication (short-range) between waterways on the harbour, within the harbour or between ships on the high seas. Among them, 156.8 MHz is the internationally mandated frequency band for crying.
3. Fixed-point communication: It is almost in the entire range of 30 to 1000 MHz. It works in a variety of ways: line of sight, troposcatter and ionospheric scattering. However, from the current point of view, stations operating in this frequency range are not increasing fast. The reason is that if the frequency is greater than 1000 MHz, the antenna gain is greatly improved, and the large capacity is easy to be multiplexed, and the control of the interference is also easier. So the frequency uses more radio stations above 1 megahertz.
Ionospheric scattering works in the range of 30 to 60 MHz with a minimum communication distance of 1200 km. It requires High Power and large antennas, which is a disadvantage, but it provides more reliable communication than high frequency sky wave propagation.
Tropospheric scatter is the long-distance communication of over-the-horizon with meters and decimeter waves. It is superior to the short-wave channel. It can jump up to 800 km (several channels) or transmit at a close distance. The road.
4. Space communication: 136 to 137 MHz - telemetry and tracking for space research, telemetry and tracking for 137 to 138 MHz operating systems,
400 to 401 MHz - for meteorological satellites, 401 to 402 MHz - for space telemetry.
5. Applications such as aeronautical navigation: 108 to 118 MHz are allocated to the blind landing system. 75 MHz is used for aviation airport beacons. 420 to 460 MHz is used for radio altimeters.
6. Radio astronomy: Only a few narrow bands are specified for use in radio astronomy, ie 38, 80, 405, 610 MHz, etc. Others include radar (designated at 216-225, 400-450, 890-942 MHz), amateur radio, and standard frequency and timing signals. The engineering, medical, and medical frequencies are designated as 40.68 MHz.
Two: 1000 to 10000 MHz bandThis band belongs to the band of decimeter waves to centimeter waves (30 cm to 3 cm). At the radio conference held in Geneva in 1959, this band was allocated to fixed-point and mobile communications, navigation, radar, meteorology, radio astronomy, space communications, amateur radio and engineering, medical and medical use.
The propagation characteristics of this band are line-of-sight propagation, and the atmospheric noise is low, but in some frequency regions (3 cm wavelength), the atmospheric (water vapor) absorption is relatively large. In addition, this band is also transmitted by scattering. Since the frequency band is not too crowded, the current allocation problem is not significant.
The allocation of frequencies in this band. The fixed-point communication and mobile communication services are in this frequency range, mainly radio-radio relay systems, and are mostly carried out with a large capacity. In addition, due to the development of tropospheric scatter communication, many fixed communication stations use tropospheric scatter.
Mobile communication occurs in this frequency band in the form of vehicle radio stations, most of which are temporary fixed-point communications (ie, they are portable devices), and in sports communications, this band is rarely used.
In this frequency band, radio navigation and radar are particularly suitable because there is no interference from atmospheric noise and the beam of a short-wavelength antenna is easily narrowed. In the range of l000 to 10000 MHz, 18% is allocated for navigation, and 30% is allocated to radar and positioning. In fact, navigation in this band is basically using radar technology.
1. Space communication: This band is the most important band of space communication, because this band has a wide bandwidth and can accommodate a large number of channels. At the same time, the development of radio technology and the situation of electromagnetic wave propagation are suitable for this band. In the range of 3400 to 8500 MHz, a total of 200 MHz is allocated for use by communication satellites, which is shared with low-power mobile communication.
2. Radio astronomy business: allocated 1400 ~ l427 MHz and 16644 ~ 16684 MHz. These are the frequency bands observed for the radiation of atomic hydrogen and hydroxide (OH) ions, respectively. In addition, 2690-2700 and 4990-5000 MHz are allocated for observation of the radiation of the continuous spectrum.
Engineering, department, medical frequency allocation 2450 ± 50 MHz.
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