Editing Single channel per carrier

{{short description|Dedicated use of satellite bandwidth}}
{{Unreferenced|date=December 2009}}
{{For|the [[Sickle-cell disease]] symptom sometimes referred to as "SCPC"|Vaso-occlusive crisis}}

'''Single channel per carrier''' ('''SCPC''') refers to using a single [[signal]] at a given [[frequency]] and [[Bandwidth (signal processing)|bandwidth]].  Most often, this is used on [[broadcast satellite]]s to indicate that [[radio station]]s are not [[multiplexing|multiplexed]] as [[subcarrier]]s onto a single [[video]] [[carrier wave|carrier]], but instead independently share a [[transponder]]. It may also be used on other [[communications satellite]]s, or occasionally on non-satellite [[transmission (telecommunications)|transmission]]s.

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 [[Broadcasting|broadcast]]s continuously. Another very common application is voice, where a small amount of fixed bandwidth is required.  However, it does not make sense for [[burst transmission]]s like [[satellite internet]] access or [[telemetry]], since a customer would have to pay for the satellite bandwidth even when they were not using it.

Where multiple access is concerned, SCPC is essentially [[Frequency-division multiple access|FDMA]]. Some applications use SCPC instead of [[Time-division multiple access|TDMA]], because they require guaranteed, unrestricted bandwidth. As satellite TDMA technology improves however, the applications for SCPC are becoming more limited.

==Advantages==
* simple and reliable technology
* low-cost equipment
* any bandwidth (up to a full transponder)
** usually 64 [[kilobit per second|kbit/s]] to 50 [[megabit per second|Mbit/s]]
* easy to add additional receive sites ([[Earth station (communications)|earth station]]s)

==Disadvantages==
* inefficient use of satellite bandwidth for burst transmissions, typically encountered with packet data transmission
* usually requires on-site control
* When used in remote locations, the transmitting dish must be protected.

==MCPC==
SCPC was the successor '''multiple channels per carrier''' ('''MCPC''').

With MCPC, several subcarriers are combined or multiplexed into a single bitstream before being modulated onto a carrier transmitted from a single location to one or more remote sites.  This uses [[time-division multiplexing]] (TDM) as well as [[frequency-division multiplexing]]. It is a [[retronym]] of sorts, as it was the only way [[radio network]]s were transmitted ("[[Piggybacking (data transmission)|piggyback]]ed" on [[television network]]s) until SCPC.

In [[digital radio]] and [[digital television]], an [[DAB ensemble|ensemble]] or other multiplex or multichannel stations can be considered MCPC, though the term is generally only applied to satellites.

The major disadvantage of MCPC is that all of the signals must be sent to a single place first, then combined for retransmission &mdash; a major reason for using SCPC instead.

==Frequency hopping==
SCPC became somewhat obsolete after the invention of [[Frequency-hopping spread spectrum|frequency hopping]].

Frequency hopping ([[FHSS (disambiguation)|FHSS]]) is a system invented by [[Hedy Lamarr]] in which radio signals rapidly change frequency to avoid interception. FHSS required the use of MCPC, as SCPC cannot change frequency mid-broadcast. FHSS was the basis for many technological advancements like [[Wi-Fi]], [[Bluetooth]], and [[GPS]], which could not have been achieved with SCPC.

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[[Category:Broadcast engineering]]