# Difference between OFDM and FDM

In Frequency Division Multiplexing we provide one channel to one user.

Example like BandWidth of user1's voice is 200Khz. I provide one channel of 200KHz in FDM for uplink as shown in the figure. Now I will provide another 200KHz Bandwidth(channel 2) for user2's voice. Similarly for user3. I will be having a guard band of 100 Hz for this.

Now What I understand about OFDM is that channel1 will be orthogonal to all other channels. Also, we divide channel 1 into orthogonal sub-carriers with bandwidth less than 200KHz.
Am I correct? If yes then will there be no ISI as my required Bandwidth is 200KHz. Both are orthogonal in your example, in different ways. The FDM example provides orthogonality by properly spacing the signals in the frequency domain (their dot product is 0). OFDM is far more spectrally efficient where you have overlapping carriers that are orthogonal.

From an implementation standpoint OFDM can be achieved with a DFT. Each of the carriers is a much lower bit rate that turns a frequency selective fading channel into flat fading. This is not true for your FDM example. There are many other differences but these are a few.

You can have the channel of 200kHz split into orthogonal sub-carriers. But for this you need to meet certain requirements to eliminate ICI (Inter-Carrier Interference, Not ISI Inter Symbol Interference). If you lose orthogonality, the data in one sub-carrier will be affected by data in another sub-carrier (hence inter-carrier interference).

In order to achieve orthogonality here, your total BW is 200kHz, so you can split them into 200 sub-carriers of 1kHz each. In that case you OFDM symbol duration should be 1/1kHz = 1ms (200/200kHz). Or you can split into 20 sub-carriers of 10kHz each. In this case, one OFDM symbol duration is 1/10kHz = 0.1ms (20/200kHz). You can choose any combination like this to achieve orthogonality. In general, $$f_{sc} \times N = 200kHz$$

NOTE: For OFDM, to maintain orthogonality, you also need to add cyclic prefix. Normally this is 25% of each OFDM symbol set. But you can vary it depending on your channel and transmission requirement. So the disadvantage of OFDM compared to FDM is loss of resources because of addition of prefix.

Orthogonality in the time domain means you can separate the signals even if their spectrum crosses into each other. It allows increased spectral efficiency because of this.

On the other hand you can still view the resulting spectrum as being split up allowing for simpler equalization which is another big reason for OFDM.