Single channel per carrier (SCPC) refers to using a single signal at a given frequency and bandwidth. Most often, this is used on broadcast satellites to indicate that radio stations are not multiplexed as subcarriers onto a single video carrier, but instead independently share a transponder. It may also be used on other communications satellites, or occasionally on non-satellite transmissions. In an SCPC system, satellite bandwidth is dedicated to a single source. This makes sense if it is being used for something like satellite radio, which broadcasts continuously. Another very common application is voice, where a small amount of fixed bandwidth is required. However, it does not make sense for burst transmissions like satellite internet access or telemetry, since a customer would have to pay for the satellite bandwidth even when they were not using it.

SCPC is currently provided in the C band. A single 36-MHz channel is sub- divided into 800 45-kHz analog channels, each dedicated to a simplex VF link, using FM. There is also digital SCPC, using QPSK, which provides 64-kbps service in the same 45-kHz bandwidth, enough for digitized voice.

SCPC is attractive for remote areas where there are few user stations near each site. Whereas FDMA is used as a trunk facility in the long-haul telecommunication system, SCPC provides direct end-user service. Although SCPC is more efficient of bandwidth than FDMA, it does suffer from the inefficiency of fixed assignment. This is especially unsuitable in very remote areas, where it is typical that each earth station serves one or a very few user stations. To achieve greater efficiency, DAMA is used. With DAMA, the set of subchannels in a channel is treated as a pool of available links. To establish a full-duplex link between two earth stations, a pair of subchannels is dynamically assigned on demand.

The first commercially available DAMA SCPC system was SPADE (single channel per carrier, pulse code modulation, multiple-access, demand-assignment equipment), introduced in the early 1970s and currently available on INTELSAT satellites. Each subchannel carries a 64-kbps QPSK signal, which occupies 38 kHz, plus a 7-kHz guardband.

Fig: SPADE Satellite Communication System for Switched SCPC Service: