Short note: Frequency division multiplexing

Frequency Division Multiplexing (FDM) sends several signals at the same time over one channel by giving each signal a separate frequency band. Each message modulates a different carrier; the modulated signals are added and transmitted together.

Tx: m1->[Mod fc1]-+          Rx: +->[BPF1]->[Demod]->m1
    m2->[Mod fc2]-+->(sum)->ch->-+->[BPF2]->[Demod]->m2
    m3->[Mod fc3]-+              +->[BPF3]->[Demod]->m3

Working

1. Each message is band-limited by a LPF.
2. It modulates its own subcarrier (often SSB-SC to save bandwidth).
3. Band-pass filters limit each modulated signal to its slot; guard bands separate adjacent slots.
4. All are summed and sent (possibly modulating a main carrier).
5. At the receiver, band-pass filters separate the channels and demodulators recover the messages.

Total bandwidth $\approx N \times (\text{channel bandwidth}) + \text{guard bands}$; e.g. 12 voice channels in 4 kHz slots need 48 kHz (telephone group, 60–108 kHz).

Applications: analog telephone trunks (group, supergroup), AM/FM broadcasting, cable TV, FM stereo, satellite FDMA, and OFDM in Wi-Fi/4G as a digital form.

Advantages: no synchronization between channels, simple for analog signals, continuous transmission. Disadvantages: crosstalk from nonlinearity (intermodulation) and imperfect filters, needs many sharp filters and stable oscillators, wasted guard bands, and fading affects a whole channel.