Subcarriers

A subcarrier is a secondary frequency component used within a broader transmission to carry information. In many communication systems, a channel can be divided into multiple subcarriers, each modulated with its own data stream. This approach, called subcarrier multiplexing, enables parallel transmission and more efficient use of the spectrum. A widely used form is orthogonal frequency-division multiplexing (OFDM), where the available bandwidth is split into many subcarriers that are mutually orthogonal.

In OFDM, each subcarrier is a narrowband carrier carrying a modulated symbol, typically using BPSK, QPSK, or higher-order constellations such as 16QAM. The subcarriers are spaced so they overlap in frequency without interfering, thanks to orthogonality. Some subcarriers may be reserved as pilots to aid channel estimation and equalization.

Examples of subcarriers in real systems include FM broadcasting, where a 19 kHz pilot tone helps recover the stereo signal and a 38 kHz subcarrier carries the stereo difference information; additional data such as RDS can travel on a 57 kHz subcarrier. In digital communications, subcarriers are central to discrete multi-tone modulation used in ADSL and other broadband links, and they are a fundamental element of digital broadcasting standards such as DVB and DVB-T/T2, which rely on OFDM to deliver multiple data streams within a single channel. Subcarriers thus enable higher spectral efficiency and robust performance in the presence of multipath and frequency-selective channels.