Frequency division multiplexing explained

Frequency division multiplexing (FDM) is an analog multiplexing technique in which the link bandwidth is divided into non-overlapping frequency bands, and each signal is modulated onto its own carrier frequency and sent at the same time over the shared link.

Operation

1. Each source signal (e.g. a voice signal of 4 kHz) modulates a different carrier frequency f1, f2, f3 (AM, FM or SSB). This shifts each signal to its own band.
2. The modulated signals are added (combined) in the multiplexer and transmitted together on the link.
3. At the receiver, the demultiplexer uses band-pass filters to separate the bands. Each is then demodulated with its carrier to recover the original signal.
4. Narrow guard bands are left between channels to prevent adjacent channel interference.

 m1(t) -> [Mod f1] \                     / [BPF f1]->[Demod]-> m1
 m2(t) -> [Mod f2] -->[ + ]==== link ===< [BPF f2]->[Demod]-> m2
 m3(t) -> [Mod f3] /                     \ [BPF f3]->[Demod]-> m3

 Frequency spectrum on the link:
 | ch1 |g| ch2 |g| ch3 |
 f1        f2        f3        (g = guard band)

Key points

• Total link bandwidth ≥ sum of channel bandwidths + guard bands. For n channels of bandwidth B: Blink ≥ nB + (n−1)Bguard.
• All channels are transmitted simultaneously, each continuously.
• Example: three voice channels of 4 kHz with 1 kHz guards need 3×4 + 2×1 = 14 kHz.
• Used in AM/FM radio, TV broadcasting, cable TV, first-generation cellular (AMPS) and telephone trunks.
• Drawbacks: crosstalk, noise accumulates in the analog signal, bandwidth wasted by guard bands, and intermodulation distortion.