Working principle of FDM

Multiplexing is the technique of sending several independent signals simultaneously over one common transmission medium, so that the medium's capacity is shared and the cost per channel is reduced.

Principle of FDM

In Frequency Division Multiplexing, the total bandwidth of the medium is divided into non-overlapping frequency bands. Each message signal is shifted by modulation to its own band; all the shifted signals are added and sent at the same time. At the receiver, filters separate the bands and demodulators bring each one back to baseband.

Block diagram

 TRANSMITTER
 ch1 -[LPF]-[Mod f1]-[BPF]-+
 ch2 -[LPF]-[Mod f2]-[BPF]-+--(SUM)--> line
 chN -[LPF]-[Mod fN]-[BPF]-+

 RECEIVER
       +-[BPF f1]-[Demod f1]-[LPF]-> ch1
 line -+-[BPF f2]-[Demod f2]-[LPF]-> ch2
       +-[BPF fN]-[Demod fN]-[LPF]-> chN

Working

1. Band limiting: each input (e.g. voice) passes through an LPF to limit it to 0.3–3.4 kHz.
2. Modulation: each signal modulates a different sub-carrier $f_1, f_2, \ldots, f_N$. Telephone FDM uses SSB-SC so each voice channel needs only 4 kHz (3.1 kHz speech + guard band).
3. Band-pass filtering: removes the unwanted sideband and carrier.
4. Combining: a summing network adds all channels into a composite baseband signal, which may then modulate a final radio carrier.
5. Receiver: BPFs centred on each band pick out each channel; product demodulators with locally generated carriers (same frequency and phase) recover the baseband, and LPFs remove the remaining high-frequency components.

 amplitude
   |  [ch1]  [ch2]  [ch3]  [ch4]
   |  |   |g |   |g |   |g |   |
   +--+---+--+---+--+---+--+---+--> f
      f1     f2     f3     f4
 g = guard band

Guard bands between channels allow for practical filter roll-off and reduce crosstalk.

Example – FDM telephone hierarchy (CCITT)

Other uses: AM/FM radio and TV broadcasting, cable TV, first-generation (AMPS) mobile.