Confusion about the impact of flat fading and frequency selective fading on OFDM
Because of wide-band nature of OFDM signals they are so prone and sensitive to multi-path delayed channel. The problem is that at the receiver side you will receive several copies of original transmitted signal, each with a different magnitude and delay. This "delays" are so destructive at the receiver side. The mathematics behind this: imagine the channel with multi-path problem. so u can model this channel as a linear-system with an "impulse response". this impulse response consists of several impulses with different magnitude and delays. so, your sent OFDM signal passes through this channel. the result can be guessed. at the output this system, the OFDM signal is convolved with the impulse response of the channel (system) in time domain. In frequency, it is like as the freq-domain OFDM signal is multiplied by the frequency-response of the channel. That's why we face a distorted received signal. At the receiver, when you apply FFT on the received signal you have to compensate that errors. This is called channel estimation (means estimating the freq-response of the channel). Then you should correct the signal. This is so crucial because in OFDM we usually have QAM modulation &amp; demodulation. and in QAM the magnitude of signal is important which is already distorted by the channel.