Frequency Division Multiplexing combines multiple signals into one transmission channel by dividing it into frequency bands. Each signal gets its own band. They travel together without interfering. At the receiver, filters separate them. FDM is used in radio, television, and telephone networks.

FDM was used in early telephone trunk lines. Many voice channels shared one cable. Each voice channel occupied a different frequency band. The same principle applies to cable TV. Many channels share one coaxial cable. Each channel has its own frequency. FM radio uses FDM to broadcast multiple stations.

FDM concepts

• Subcarriers: individual frequency bands
• Guard bands: unused spectrum between channels
• Modulation: each subcarrier carries a signal
• Multiplexer: combines signals
• Demultiplexer: separates signals

FDM is related to FDMA. FDM combines signals from one source. FDMA gives different users different frequencies. OFDM, used in Wi-Fi and 5G, uses many orthogonal subcarriers. The subcarriers overlap without interfering. FDM was the beginning. OFDM is the modern evolution.