Satcom refers to the use of artificial earth satellites as relay stations to forward radio signals for communication between two or more earth stations.
The frequency band used for satcom communication is also the microwave frequency band, so some people say that it is a special case of ground-based microwave communication. It sets up microwave relay stations in the air, expands the coverage area, and reduces the number of relay times. Therefore, satcom communication includes two parts: the relay station-communication satellite part and the terminal station-earth station part.
The communication satellite is an artificial earth satellite used for information communication. Therefore, it is a huge system, including satellite technology and communication technology.
Therefore, people divide satcom network into two parts: space platform and effective payload. Space platform is the satellite technology category, mainly including satellite control and monitoring. Effective payload is the communication technology category, which is the part we should focus on and introduce here. Since it is a relay station set up in the air, the effective payload part is nothing more than a combination of feeder lines and transceiver equipment. In the communication satellite, the transceiver equipment is called a communication relay.
It is divided into transparent transceiver (only low noise amplification, frequency conversion, and high power amplification relay process) and processing transceiver (in addition to the relay process, there are digital signal regeneration, signal exchange between beam and other things) two categories.
Since the transceiver is located above the satellite, the minimum requirements for it are: the smallest additional noise and distortion, sufficient working bandwidth and output power to effectively and reliably relay radio signals for various earth stations.
Communication satellites can be divided into low earth orbit (LE0) communication satellites, medium earth orbit (ME0) communication satellites, high elliptical orbit (HEO) communication satellites, and geostationary orbit (GE0) communication satellites according to the orbit they are located in. Geostationary orbit communication satellites are commonly used in satcom communication. They are 3600 kilometers away from the earth's surface and are located on a circular orbit above the equator. Satellites on this orbit are synchronized with the earth's rotation speed and are stationary relative to the earth's surface.
GE0 is mainly suitable for low latitude areas; HEO is mainly suitable for high latitude areas; LEO/ME0 requires many satellites to achieve continuous global coverage.
The earth station of satcom communication is equivalent to the terminal station of ground-based microwave communication. It also includes baseband equipment, radio frequency equipment, feeder line equipment, monitoring equipment, etc., but in order to ensure that the earth station antenna and the satellite antenna are aligned, servo tracking equipment is added. The outstanding feature of satcom communication earth stations is low noise reception and high power output. The frequency band of satcom communication will develop from a single C-band to multiple frequency bands such as UHF, L, Ku, and Ka.
The multi-access connection technology of satcom also includes commonly used frequency division multiple access (FDMA), time division multiple access (TDMA), code division multiple access (CDMA), space division multiple access (SDMA), and random multiple access (ALOHA). In satcom communication, the channel allocation technology closely related to multi-access connection technology is channel allocation methods, which include pre-allocation method (PA), on-demand allocation method (DAMA), and random allocation method (RA), etc.
