Single channel per carrier
Encyclopedia
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 station
s are not multiplexed
as subcarrier
s onto a single video
carrier
, but instead independently share a transponder
. It may also be used on other communications satellite
s, 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 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 FDMA
. Some applications use SCPC instead of TDMA
, because they require guaranteed, unrestricted bandwidth. As satellite TDMA technology improves however, the applications for SCPC are becoming more limited.
(TDM). It is a retronym
of sorts, as it was the only way radio network
s were transmitted ("piggybacked" on television network
s) until SCPC.
In digital radio
and digital television
, an 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 — a major reason for using SCPC instead.
Frequency
Frequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency...
and bandwidth. Most often, this is used on broadcast satellites to indicate that radio station
Radio station
Radio broadcasting is a one-way wireless transmission over radio waves intended to reach a wide audience. Stations can be linked in radio networks to broadcast a common radio format, either in broadcast syndication or simulcast or both...
s are not multiplexed
Multiplexing
The multiplexed signal is transmitted over a communication channel, which may be a physical transmission medium. The multiplexing divides the capacity of the low-level communication channel into several higher-level logical channels, one for each message signal or data stream to be transferred...
as subcarrier
Subcarrier
A subcarrier is a separate analog or digital signal carried on a main radio transmission, which carries extra information such as voice or data. More technically, it is an already-modulated signal, which is then modulated into another signal of higher frequency and bandwidth...
s onto a single video
Video
Video is the technology of electronically capturing, recording, processing, storing, transmitting, and reconstructing a sequence of still images representing scenes in motion.- History :...
carrier
Carrier wave
In telecommunications, a carrier wave or carrier is a waveform that is modulated with an input signal for the purpose of conveying information. This carrier wave is usually a much higher frequency than the input signal...
, but instead independently share a transponder
Transponder
In telecommunication, the term transponder has the following meanings:...
. It may also be used on other communications satellite
Communications satellite
A communications satellite is an artificial satellite stationed in space for the purpose of telecommunications...
s, 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
Satellite radio
Satellite radio is an analogue or digital radio signal that is relayed through one or more satellites and thus can be received in a much wider geographical area than terrestrial FM radio stations...
, which broadcast
Broadcasting
Broadcasting is the distribution of audio and video content to a dispersed audience via any audio visual medium. Receiving parties may include the general public or a relatively large subset of thereof...
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
Burst transmission
In telecommunication, the term burst transmission or data burst has the following meanings:# Any relatively high-bandwidth transmission over a short period of time...
s like satellite internet access or telemetry
Telemetry
Telemetry is a technology that allows measurements to be made at a distance, usually via radio wave transmission and reception of the information. The word is derived from Greek roots: tele = remote, and metron = measure...
, 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 FDMA
Frequency-division multiple access
Frequency Division Multiple Access or FDMA is a channel access method used in multiple-access protocols as a channelization protocol. FDMA gives users an individual allocation of one or several frequency bands, or channels. It is particularly commonplace in satellite communication. FDMA, like...
. Some applications use SCPC instead of TDMA
Time division multiple access
Time division multiple access is a channel access method for shared medium networks. It allows several users to share the same frequency channel by dividing the signal into different time slots. The users transmit in rapid succession, one after the other, each using its own time slot. This...
, 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 kbit/s to 50 Mbit/s
- easy to add additional receive sites (earth stations)
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.
- A dish which is moved out of alignment can result in fines as high as $1,100 per minute (as of 2003) from the satellite operator.
MCPC
With multiple channels per carrier (MCPC), several subcarriers are combined 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 multiplexingTime-division multiplexing
Time-division multiplexing is a type of digital multiplexing in which two or more bit streams or signals are transferred apparently simultaneously as sub-channels in one communication channel, but are physically taking turns on the channel. The time domain is divided into several recurrent...
(TDM). It is a retronym
Retronym
A retronym is a type of neologism that provides a new name for an object or concept to differentiate the original form or version of it from a more recent form or version. The original name is most often augmented with an adjective to account for later developments of the object or concept itself...
of sorts, as it was the only way radio network
Radio network
There are two types of radio networks currently in use around the world: the one-to-many broadcast type commonly used for public information and mass media entertainment; and the two-way type used more commonly for public safety and public services such as police, fire, taxicabs, and delivery...
s were transmitted ("piggybacked" on television network
Television network
A television network is a telecommunications network for distribution of television program content, whereby a central operation provides programming to many television stations or pay TV providers. Until the mid-1980s, television programming in most countries of the world was dominated by a small...
s) until SCPC.
In digital radio
Digital radio
Digital radio has several meanings:1. Today the most common meaning is digital radio broadcasting technologies, such as the digital audio broadcasting system, also known as Eureka 147. In these systems, the analog audio signal is digitized into zeros and ones, compressed using formats such as...
and digital television
Digital television
Digital television is the transmission of audio and video by digital signals, in contrast to the analog signals used by analog TV...
, an ensemble
DAB ensemble
DAB ensembles are groups of Digital audio broadcasting broadcasters transmitting multiple digital radio channels on a single radio transmission....
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 — a major reason for using SCPC instead.