edVirtus Reference

What Is a Subcarrier?

What Is a Subcarrier Signal?

Preview: Learn more about subcarriers and how they allow multiple information signals to share a single communication channel.

A subcarrier is a secondary carrier signal used to carry additional information within a communication channel. Rather than modulating the main radio-frequency carrier directly, the information first modulates a lower-frequency subcarrier, which then modulates the main carrier for transmission. This technique allows several independent information streams to be transmitted simultaneously over the same radio-frequency channel and is widely used in broadcasting, satellite communications, navigation systems, and digital communication systems.

The basic principle is straightforward. A transmitter first generates a low-frequency sinusoidal signal known as the subcarrier. The information signal modulates this subcarrier using an appropriate modulation technique, such as amplitude, frequency, or phase modulation. The resulting subcarrier signal then becomes part of the modulation applied to the main RF carrier. At the receiver, the reverse process first recovers the subcarrier before extracting the original information.

A useful analogy is placing several smaller packages inside a larger shipping container. The container represents the main carrier, while each package represents a separate subcarrier carrying its own information. Although all the packages travel together, each can be identified and unpacked independently at the destination.

One of the earliest and most familiar applications of subcarriers was FM stereo broadcasting. In addition to the main monophonic audio signal, FM broadcasts include a 38 kHz suppressed subcarrier carrying the left-minus-right (L − R) stereo information. A 19 kHz pilot tone allows the receiver to regenerate the suppressed subcarrier and reconstruct the separate left and right audio channels. Additional services, such as the Radio Data System (RDS), are transmitted using another subcarrier at 57 kHz, allowing station identification, programme information, and traffic messages to be broadcast simultaneously.

Subcarriers are also widely used in satellite communications. Digital television, satellite radio, telemetry, and command signals may all be transmitted on separate subcarriers sharing the same RF carrier. Historically, analogue satellite systems frequently employed frequency-modulated subcarriers to carry multiple voice channels or telemetry signals alongside television transmissions.

Modern digital communication systems have extended the concept significantly. In Orthogonal Frequency Division Multiplexing (OFDM), hundreds or even thousands of closely spaced orthogonal subcarriers are transmitted simultaneously, each carrying a portion of the total data stream. Because the subcarriers are mathematically orthogonal, they do not interfere with one another despite their close frequency spacing. OFDM has become the basis of technologies such as Wi-Fi, LTE, 5G, digital television, and many broadband communication systems.

Subcarriers are also fundamental to Global Navigation Satellite Systems (GNSS) such as GPS, Galileo, and BeiDou. Navigation signals often employ subcarrier modulation to improve ranging accuracy, reduce interference, and separate different navigation services operating on the same carrier frequency.

It is important to distinguish a subcarrier from the main carrier. The main carrier is the high-frequency radio signal transmitted through the communication channel, whereas the subcarrier is an intermediate signal that carries information before it is applied to the main carrier. In many systems, several subcarriers may coexist within the same transmitted signal, each carrying different information.

Today, subcarriers are found throughout modern communications. From FM stereo broadcasting and satellite communications to OFDM-based wireless networks and satellite navigation systems, they provide an efficient means of multiplexing multiple information streams within a single transmission channel. Their flexibility and efficiency have made them one of the fundamental techniques of modern communications engineering.