SCPC in Satellite Communication and VSAT: Definition, Advantages, and Disadvantages

The term “Single Channel Per Carrier” or SCPC is widely used in satellite communication and VSAT technology. Satellite transponders have a certain bandwidth, which is divided into multiple carriers, each with its own bandwidth. For example, a C-band satellite with a total bandwidth of 500MHz might be divided into multiple transponders having 36MHz bandwidth. Each transponder’s band is used to carry information in the form of voice or data. These voice or data lines are referred to as channels. SCPC means using a single channel (either voice or data) per RF carrier of a satellite transponder.

As shown in the figure below, either voice or data is modulated onto an RF carrier and transmitted using a VSAT to the satellite. The same modulated signal is then down-converted/amplified and sent down to the receiving VSAT on the other side by the transponder. This describes a mesh configuration. For a Star configuration, there are two hops. Information flows from the Transmitting VSAT to the satellite, satellite to the hub station, hub to satellite and further from the satellite to the receiving VSAT.

In an SCPC system, a MUX can be configured for various bit rates as per the needs of the channel, and data/voice settings can also be adjusted. Various voice formats such as 2-wire E&M and 4-wire E&M can also be configured. The voice line can be interfaced with an EPABX/PBX.

Since SCPC dedicates a single channel to each carrier all the time, it’s not necessarily the most efficient use of satellite resources (i.e., carrier for FDMA access assignment). Hence, the VOX (Voice Operated eXchange) feature is often enabled to save power, which only activates the RF when voice activity is detected.

SCPC is mainly useful for defense applications that require a 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

The following are the advantages of SCPC in satellite and VSAT technology:

1. Dedicated Bandwidth: SCPC provides a dedicated carrier frequency for each VSAT terminal, ensuring a constant and predictable bandwidth for point-to-point communication.

2. Predictable Performance: With a fixed allocation of bandwidth, SCPC offers predictable and consistent performance.

3. Lower Latency: SCPC systems typically exhibit lower latency compared to shared systems, as there is no contention for bandwidth among multiple users.

4. Enhanced Security: The dedicated nature of SCPC can enhance security, as communication is confined to specific endpoints with allocated frequencies.

5. Reliable Connection: It is preferred for constant, dedicated and reliable connection requirements.

Disadvantages of SCPC

The following are the disadvantages of SCPC in satellite communication:

1. Bandwidth Underutilization: SCPC may lead to underutilization of bandwidth, especially in scenarios where traffic patterns vary, as the allocated bandwidth remains constant.

2. Inflexibility: The fixed allocation of bandwidth can be inflexible in situations where dynamic adjustments are required to accommodate varying data rate demands.

3. Inefficient for Point-to-Multipoint: SCPC is less efficient in point-to-multipoint scenarios.

4. Higher Cost: 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. Complex Management: 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.