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

Marks: 10 M

Year: May 2013, Dec 2013

FDM/FM/FDMA earth station

FDM/FM/FDMA acronyms indicate, the signals are frequency-division multiplexed, frequency modulated, with frequency-division multiple access to the satellite. The voice-frequency signals are first SSBSC amplitude modulated onto voice carriers in order to generate the single side bands needed for the 60 channel supergroup. Each 60 channel supergroup is then frequency modulated onto a carrier which is then up-converted to a frequency in the satellite uplink band . Consider an example of three earth station; one in San Francisco, one in Washington, one in Chicago. All three earth station access a single satellite transponder channel simultaneously, and each communicates with both of the others. Each earth station transmits one uplink carrier modulated with a 60 channel sub supergroup and receives two similar downlink carriers.

The earth station at Washington is shown in more detail. One transmit chain is used and this carries telephone traffic for both Chicago and San Francisco. On the receive side, two receive chains must be provided, one for the Chicago-originated carrier and one for the San Francisco-originated carrier. Each of these carriers will have a mixture of traffic, and in the demultiplexing unit, only those telephone channels intended for Washington are passed through. These are remultiplexed into an FDM/FM format which is transmitted out along the terrestrial line to the Washington switching office.

Uplink carrier frequencies of 6253, 6273 and 6278 MHz For the satellite transponder arrangement those carriers would be translated down to frequencies of 4028, 4048 and 4053 MHz Typically a 60 channel FDM/FM carrier occupies 5MHz of transponder bandwidth, including guardbands. A total frequency allowance of 15MHz is therefore required. For the three stations, an each station receives all the traffic. The remainder of the transponder bandwidth may be unused or it may be occupied by other carriers.

As an example of preassignment, suppose that each earth station can transmit upto 60 voice circuits, and that 40 of these are preassigned to the Washington and San Francisco route. If these 40 circuits are fully loaded, additional calls on the Washington San Francisco route will be located even though there may be ideal circuits on the other preassigned routes.