**2.1 Frequency division multiplexing**

Frequency division multiplexing (FDM) is the division of total channel bandwidth into multiple, non-overlapping subbands. Each of these subbands is assigned to a user or a signal by modulating with the appropriate carrier frequency.

The multiplexer from the transmit side is responsible for multiplexing the modulated signals with different carrier frequencies into a total signal for transmission. The demultiplexer at the receiver is responsible for separating the total signal into signals of different users by different frequencies.

**Example 1.** Primary FDM system (**Figure 4**) with total frequency bandwidth from 60 kHz to108 kHz is divided into 12 subbands, each subband has a bandwidth of 4 kHz. At the transmitter, the signal of a user is transmitted through a low pass filter (LPF) which is then a single side band (SSB) modulated with an appropriate carrier frequency. At the receiver, the total signal passes through a band pass filter (BPF) and a single side band demodulator to obtain a signal for the corresponding user.

FDM has some disadvantages:

