# 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. 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, $$f_{sc} \times N = 200kHz$$