Q5Satellite Communication

Multiple access techniques allow many earth stations to share the limited bandwidth and power resources of a single satellite transponder simultaneously, and different techniques offer different trade-offs in efficiency, complexity, and suitability for different traffic patterns.

FAMA (Fixed/Frequency Assignment Multiple Access)

In FAMA, each earth station is permanently (or semi-permanently) assigned a specific frequency sub-band (channel) within the transponder's available bandwidth, similar in concept to conventional FDMA, and continues to use that assigned channel regardless of whether it currently has traffic to send, based on a fixed allocation plan established in advance. This is simple to implement and coordinate but is inefficient for bursty or variable traffic patterns, since a station's exclusively assigned channel remains unused (wasting valuable transponder capacity) whenever that particular station has no traffic to send at a given moment.

DAMA (Demand Assignment Multiple Access)

DAMA improves on fixed assignment by dynamically allocating transponder channels to earth stations only when they actually have traffic to send, based on real-time demand — a station requests a channel assignment from a central network control system (or, in distributed DAMA schemes, through a shared signaling/request channel) when it needs to transmit, is granted a channel for the duration of its call/session, and releases the channel back to the shared pool once finished, allowing the same limited pool of channels to serve a much larger number of earth stations than could be supported under fixed assignment, since not all stations need to transmit simultaneously — significantly improving overall transponder capacity utilization for variable, bursty traffic (such as voice telephony) compared to FAMA.

TDMA (Time Division Multiple Access)

In TDMA, all earth stations share a single common carrier frequency, but each is assigned a specific, precisely synchronized time slot within a repeating frame during which it may transmit its burst of data; since only one station transmits at any given instant on the shared carrier, TDMA avoids the intermodulation distortion problems that arise when a satellite's nonlinear transponder amplifier (TWTA) must simultaneously amplify multiple different-frequency FDMA carriers (since a nonlinear amplifier operating on multiple simultaneous signals generates unwanted intermodulation products); this allows the transponder's power amplifier to be operated much closer to saturation (its most power-efficient operating point) for TDMA than would be acceptable for multi-carrier FDMA operation, giving TDMA a significant power-efficiency advantage.

Comparative summary: FAMA is simplest but least efficient for variable traffic; DAMA substantially improves capacity utilization over FAMA for bursty demand while retaining a frequency-division structure; and TDMA, by using a single shared carrier with time-division access, offers the greatest overall efficiency and flexibility for modern digital traffic (allowing dynamic, statistically-multiplexed bandwidth allocation among many users while maximizing transponder power-amplifier efficiency), which is why TDMA (and its refinements, including packet-based and multi-frequency TDMA variants) has become the dominant access technique in modern digital satellite communication systems, though DAMA-based FDMA schemes remain in use for certain legacy and specialized (e.g., maritime, thin-route telephony) applications.

Synchronization and framing requirements: TDMA's efficiency advantage comes with a significant implementation cost not present in FAMA/DAMA — every earth station sharing the TDMA carrier must transmit its assigned burst with precise timing accuracy (typically to within a fraction of a symbol period) so that its burst arrives at the satellite transponder exactly within its allocated time slot without overlapping adjacent stations' bursts, requiring a dedicated network reference/timing station and a guard-time allowance between bursts to absorb residual timing uncertainty and propagation-delay variations (which change slightly as a GEO satellite drifts within its station-keeping box). FAMA and DAMA, by contrast, require no such tight burst-level timing synchronization, since each station's continuously-transmitted carrier is separated from others purely by frequency, making frequency-division schemes considerably simpler to deploy and operate, particularly for small, low-cost VSAT-class terminals lacking sophisticated burst-timing hardware.

Capacity allocation efficiency compared quantitatively: for a transponder shared by N earth stations each with intermittent (bursty) traffic requirring only a fraction p of the time, a fixed-assignment FAMA scheme must statically reserve 1/N of the transponder bandwidth for each station regardless of actual usage, so overall utilization efficiency is at best p; DAMA instead pools the transponder capacity and assigns channels only to currently active stations, so the required number of simultaneous channels approaches the statistically expected number of concurrently active stations (Np) rather than the full N, allowing many more total stations to be supported by the same transponder bandwidth, following the same trunking-efficiency principle used in terrestrial telephone exchange design (Erlang traffic theory); TDMA extends this efficiency further by allowing burst durations (not just channel assignments) to be dynamically sized in proportion to each station's actual instantaneous traffic demand within each frame, achieving still finer-grained statistical multiplexing gain than channel-level DAMA alone.