TABLE OF CONTENTS In order to utilize the band more efficiently and provide more data or users, two different schemes are employed. The first scheme uses beam pointing at two different points on earth using the same satellite. Each beam is focused on one area of the earth so that the same antenna can be used for two systems in the same band and frequency. The narrower the beamwidth, the less coverage area is illuminated on the earth surface. However, this provides for more EIRP so that the earth stations can be less expensive.
Another way to increase data capacity for a given band is to use polarization. If orthogonal polarizations are used for two different channels, the channels can use the same antenna and bandwidth with minimal interference to each other. In theory, horizontal and vertical polarizations are orthogonal, and LHCP and RHCP are also orthogonal. Cross-polarization degrades the separation of the two channels that are orthogonally polarized. Frequently, orthogonal polarization is used to provide increased isolation between the transmitter and the receiver for transceiver operation.
In order for a system to use polarization for frequency re-use in a satellite communication system, the isolation between the channels should be at least 25 to 30 dB. The main causes that degrade this isolation are the Faraday effect (earth s magnetic field) and atmospheric effects (rain or ice crystals). Also multipath can alter the polarization during reflection of the signals on a surface and propagation through the...
