What is SCPC in satellite communication and VSAT

The term "Single Channel Per Carrier" or SCPC is widely used term in satellite communication and VSAT technology. We know that satellite transponder will have some bandwidth and it is divided into number of carriers considering bandwidth across each carrier. For example C band satellite having total bandwidth of 500MHz is divided among many transponders having 36MHz bandwidth. Each transponder's band is used to carry information in the form of voice or data. This voice or data lines are called as channel.

SCPC means single channel (i.e. either voice channel or data channel) per RF carrier of a satellite transponder. As shown in the figure below either voice or data channel is modulated over RF carrier and transmitted using VSAT to the satellite and the same modulated signal down converted/amplified and sent down to the receiving VSAT on the other side by transponder. This is for mesh configuration. For Star configuration it will have two hops. Information flows from Transmitting VSAT to satellite, satellite to hub station, hub to satellite and further from satellite to receiving VSAT.

In this SCPC system, MUX can be configured for various bit rates as per channel and also data/voice can be configured. Various voice formats such as 2 wire E&M; 4wire E&M and so on can also be configured. The voice line can be interfaced with EPABX/PBX.

As it is imperative from SCPC concept that only single channel is occupying the carrier all the time, hence it is not an efficient system in the use of satellite resource (i.e. carrier for FDMA access assignment). Hence VOX feature is enabled to save power, which will only enable RF when voice activity is detected.

SCPC is mainly useful for defense applications requiring dedicated connection from one end to the other with encryption always enabled. SCPC is well-suited for applications where a constant, dedicated, and reliable connection is essential, such as video conferencing, dedicated data links, and mission-critical communications.

Advantages of SCPC

Following are the advantages of SCPC in satellite and VSAT technology.
1. SCPC provides a dedicated carrier frequency for each VSAT terminal, ensuring a constant and predictable bandwidth for point-to-point communication.
2. With a fixed allocation of bandwidth, SCPC offers predictable and consistent performance.
3. SCPC systems typically exhibit lower latency compared to shared systems, as there is no contention for bandwidth among multiple users.
4. The dedicated nature of SCPC can enhance security, as communication is confined to specific endpoints with allocated frequencies.
5. It is preferred for constant, dedicated and reliable connection requirements.

Disadvantages of SCPC

Following are the disadvantages of SCPC in satellite communication.
1. SCPC may lead to underutilization of bandwidth, especially in scenarios where the traffic patterns vary, as the allocated bandwidth remains constant.
2. The fixed allocation of bandwidth can be inflexible in situations where dynamic adjustments are required to accommodate varying data rate demands.
3. SCPC is less efficient in point-to-multipoint scenarios.
4. SCPC solutions can be more expensive, particularly when dedicated and constant bandwidth is needed for each VSAT terminal as per their unique data requirements.
5. Managing and coordinating dedicated carrier frequencies for each point-to-point link can be more complex, especially in large-scale deployments.

Conclusion : In summary, while SCPC offers dedicated and reliable communication for specific applications, its inflexibility and potential for underutilization of bandwidth can be limiting factors in certain scenarios. It is suitable for point to point VSAT links.

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