Short note: FDMA in satellite communications

FDMA (Frequency Division Multiple Access) in satellite communication lets many earth stations share one satellite transponder by giving each station its own frequency band within the transponder bandwidth. All stations transmit continuously and at the same time.

   Transponder (e.g. 36 MHz)
 |  ES-A  |g|  ES-B  |g|  ES-C  |g| ...
 <------------ frequency ------------>

Types

• MCPC (Multiple Channels Per Carrier): each earth station FDM-multiplexes many voice channels onto one carrier (FDM/FM/FDMA). Pre-assigned, suited to heavy fixed routes.
• SCPC (Single Channel Per Carrier): each voice/data channel has its own carrier. Can be demand assigned (DAMA), e.g. INTELSAT SPADE system: about 800 channels at 45 kHz spacing in a 36 MHz transponder, assigned on request.

Features and problems

• Simple: no network-wide timing or synchronization needed.
• Intermodulation: the transponder's TWT amplifier is nonlinear; many carriers produce intermodulation products, so the amplifier is operated with input/output back-off (a few dB), which reduces capacity and power.
• Guard bands between carriers waste spectrum.
• Power control needed so that a strong carrier does not suppress weak ones (capture effect).
• Capacity falls sharply as the number of carriers rises.

Comparison: FDMA is the oldest and simplest satellite access method; TDMA uses the transponder more efficiently (single carrier at saturation) but needs precise timing; CDMA allows overlapping use with spread spectrum.