A satellite link is a radio link between a transmitting earth station and receiving earth station through a communications satellite. A satellite link consists of one uplink and one downlink; the satellite electronics (i.e., the transponder) will remap the uplink frequency to the downlink frequency. The transmission channel of a satellite system is a radio channel using a direct-wave approach, operating in at specific RF bands within the overall electromagnetic spectrum as seen below :
The table below shown some key of physical parameters of relevance to satellite communication,
The frequency of operation is in the super high frequency (SHF) range (3-30 GHz). Regulation and practice dictate the frequency of operation, the channel bandwidth, and the bandwidth of the subchannels within the larger channel. Different frequencies are used for the uplink and for the downlink.
Frequencies above about 30 MHz can pass through the ionosphere and, therefore, can be utilized for communicating with satellite (frequencies below 30 MHz are reflected by the ionosphere at certain stages of the sunspot cycle; however, commercial satellite services use much higher frequencies. The range 3-30 GHz represents a useful set of frequencies for geostationary satellite communication; these frequencies are also called "microwave frequencies". Above about 30 GHz, the attenuation in the atmosphere due to clouds, rain, hydrometeors (any water or ice particles that have formed in the atmosphere or at Earth's surface as a result of condensation or sublimation), sand, and dust makes a ground to satellite link unreliable (such frequencies may still be used for satellite-to-satellite link in space, although theses applications have not yet developed commercially).
The frequency bands are further subdivided into smaller channels that can be independently used for a variety of applications. Figure in the below depicts a typical subdivision of the C-band into channels, which are also called colloquially as "transponders". The nominal subchannel bandwidth is (typically) 40 MHz with a usable (typical) bandwidth of 36 MHz.
Similar frequency allocations have been established for the Ku and Ka-bands. Many satellites simultaneously support a C-band and a Ku-band infrastructure (they have dedicated feeds and transponders for each band).
Different frequencies are used for the uplink and downlink to avoid self-interference, following the terrestrial microwave transmission architecture developed by the Bell System in 1940s and 1950s. In system using the C-band the basic parameters are 4 GHz in the downlink, 6 GHz in the uplink, 500 MHz bandwidth over 24 transponders using vertical and horizontal polarization (a form of frequency re-use), resulting in a transponder capacity of 36 MHz or 45-75 Mbps or more of usable throughput - depending on modulation and FEC (Forward Error Correction) scheme.
Most communications systems fall into one of three categories : bandwidth efficient, power efficient, or cost efficient.
Bandwidth efficiency describe the ability of a modulation scheme to accommodate data within a limited bandwidth.
Power efficiency describes the ability of the system to reliably send information at the lowest practical power level.
In satellite communications, both bandwidth efficiency and power efficiency are important.
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