In frequency division multiple
access system, each earth station is assigned a segment
of the satellite usable bandwidth. To ensure interference
free communication, sufficient guard band is provided
between these segments.
The station 'A'
Transmits FDM
FM signals in a frequency
segment of (5930 - 5950) MHz to the satellite repeater
which receives these signals and converts them to a
frequency range 3705 - 3725 MHz by mixing it with a
carrier frequency of 2225 MHz generated by the local
oscillator of the satellite repeater. Similarly, signals
transmitted to the satellite by stations B and C are
converted to lower frequencies in the satellite repeater
and retransmitted to their destinations.
Station
B UP-Signal
= 5990 - 6010 MHz
Re-transmitted
signal frequency
= 3765 - 3785 MHz
Station C
UP-Signal
= 6220 - 6240 MHz
Re-transmitted
signal frequency
= 3995 - 4015 MHz
Thus it can be seen that
signals transmitted in the UP direction (to satellite)
lie in the band 5925 to 6425 MHz while the down signals
(transmitted by the satellite) lie in the frequency
band 3700 - 4200 MHz. Salient features of the FDMA system
are:
Each earth station can transmit one carrier up towards
the satellite. The signals frequency is changed by
the satellite and the signal is re-radiated omni-
directionally.
The re-radiated signal can be received by any of
the earth-stations.
Satellite repeater is capable of receiving simultaneously
signals from different earth stations and reradiating
them at new frequencies.
It should be remembered
that 20 MHz frequency segments transmitted by different
earth stations contain a base band of 120 telephone
channels frequency division multiplexed, whose frequency
modulated by UHF carrier.
