This question appears in Mumbai University > Satellite Communication and Network subject

Marks: 8 M

Year: May 2015

The basic elements for a redundant earth station are shown in figure 1. Redundancy means that certain units are duplicated. A duplicate, or redundant, unit is automatically switched into a circuit to replace a correspond ding unit that has failed. Redundant units are shown by dashed lines.

Block diagram of a redundant earth station.

Working –

• Referring again to the block diagram, after passing through the up-converters, the carriers are combined, and the resulting wideband signal is amplified. The wideband power signal is fed to the antenna through a diplexer, which allows the antenna to handle transmit and receive signals simultaneously
• The station’s antenna functions in both - the transmit and receive modes - but at different frequencies. In the C band, the nominal uplink (or transmit) frequency is 6 GHz and the downlink (or receive) frequency is nominally 4 GHz. In the Ku band, the uplink frequency is nominally 14 GHz, and the downlink, 12 GHz. High-gain antennas are employed in both bands, which also means narrow antenna beams. A narrow beam is necessary to prevent interference between neighbouring satellite links. In the case of C band, interference to and from terrestrial microwave links also must be avoided. Terrestrial microwave links do not operate at Ku-band frequencies
• In the receive branch (the right-hand side of the figure), the incoming wideband signal is amplified in an LNA and passed to a divider network, which separates out the individual microwave carriers. These are each down-converted to an IF band and passed on to the multiplex block, where the multiplexed signals are reformatted as required by the terrestrial network
• It should be noted that, in general, the signal traffic flow on the receive side will differ from that on the transmit side. The incoming microwave carriers will be different in number and in the amount of traffic carried, and the multiplexed output will carry telephone circuits not necessarily carried on the transmit side
• A number of different classes of earth stations are available, depending on the service requirements. Traffic can be broadly classified as heavy route, medium route, and thin route.