Transnlission Characteristics

Transnlission Characteristics 

Transnlission Characteristics The optimum frequency range for satellite
transmission is in the range 1 to 10 GHz. Below 1 GHz, there is significant noise
from natural sources, including galactic, solar, and atmospheric noise, and humanmade
interference from various electronic devices. Above 10 GHz, the signal is
severely attenuated by atmospheric absorption and precipitation.
Most satellites providing point-to-point service today use a frequency bandwidth
in the range 5.925 to 6.425 GHz for transmission from earth to satellite
(uplink) and a bandwidth in the range 3.7 to 4.2 GHz for transmission from satellite
to earth (downlink). This combination is referred to as the 4/6-GHz band. Note that
the uplink and downlink frequencies differ. For continuous operation without interference,
a satellite cannot transmit and receive on the same frequency. Thus signals
received from a ground station on one frequency must be transmitted back on
another.
The 4/6-GHz band is within the optimum zone of 1 to 10 GHz but has become
saturated. Other frequencies in that range are unavailable because of sources of
interference operating at those frequencies, usually terrestrial microwave. Therefore,
the 12/14-GHz band has been developed (uplink: 14 to 14.5 GHz; downlink:
11.7 to 12.2 GHz). At this frequency band, attenuation problems must be overcome.
However, smaller and cheaper earth-station receivers can be used. It is anticipated
that this band will also saturate, and use is projected for the 20/30-GHz band
(uplink: 27.5 to 30.0 GHz; downlink: 17.7 to 20.2 GHz). This band experiences even
greater attenuation problems but will allow greater bandwidth (2500 MHz versus
500 MHz) and even smaller and cheaper receivers.
Several properties of satellite communication should be noted. First, because
of the long distances involved, there is a propagation delay of about a quarter second
from transmission from one earth station to reception by another earth station.
This delay is noticeable in ordinary telephone conversations. It also introduces
problems in the areas of error control and flow control, which we discuss in later
chapters. Second, satellite microwave is inherently a broadcast facility. Many stations
can transmit to the satellite, and a transmission from a satellite can be received
by many stations.