FDM principle; SCPC and DAMA FDMA

Principle of FDM

Frequency division multiplexing shares the bandwidth of one channel among many signals by giving each signal its own frequency band at the same time. Each message is band-limited and modulates a different carrier $f_{c1}, f_{c2}, \dots$. The modulated signals are added and sent together. Guard bands between bands prevent overlap. At the receiver, band-pass filters tuned to each band separate the signals, and demodulators recover the messages.

 Spectrum of composite FDM signal
   | ch1 |g| ch2 |g| ch3 |g| ch4 |
   fc1      fc2     fc3     fc4      f
   (g = guard band)

When the same idea is used to let many earth stations share one satellite transponder, it is called FDMA.

SCPC (Single Channel Per Carrier)

• Each voice or data channel gets its own carrier inside the transponder band, instead of first being grouped into a multi-channel FDM baseband.
• Example: a 36 MHz transponder divided into narrow channel slots (e.g. 45 kHz each).
• Often used with voice activation: the carrier is switched off during silence, which saves transponder power.
• Suits thin-route traffic (small stations with only a few circuits).
• In pre-assigned SCPC, channels are fixed to station pairs, so they sit idle when not in use.

DAMA (Demand Assigned Multiple Access)

• Channels are not fixed. All stations share a common pool of carrier frequencies, and a channel is given to a station only when a call is requested. It is returned to the pool when the call ends.
• A common signalling channel (central or distributed control) handles the requests.
• Example: the INTELSAT SPADE system (SCPC, PCM, multiple access, demand assignment) uses about 800 SCPC channels at 45 kHz spacing in a 36 MHz transponder.
• Advantage: far better use of the transponder when traffic is random and many stations have light traffic.