Mumbai University > EXTC > Sem 8 > Satellite Communication and Networks

Marks: 10M

The block diagram of transmit receive earth station used for telephone traffic is shown in Fig. Starting at the bottom of the diagram, the first block shows the interconnection equipment required between satellite station and the terrestrial network. For the purpose of explanation, telephone traffic will be assumed. This may consist of a number of telephone channels in a multiplexed format. Multiplexing is a method of grouping telephone channels together, usually in basic groups of 12, without mutual interference

The groupings different from those used in the terrestrial network are required for satellite transmission, and the next block shows the multiplexing equipment in which the reformatting is carried out.

Following along the transmit chain, the multiplexed signal is modulated onto a carrier wave at an intermediate frequency, usually 70 MHz. Parallel IF stages are required, one for each microwave carrier to be transmitted.

After amplification at the 70-MHz IF, the modulated signal is then up converted to the required microwave carrier frequency. A number of carriers may be transmitted simultaneously, and although these are at different frequencies they are generally specified by their nominal frequency, for example, as 6-GHz or 14-GHz carriers.

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