Doesn't OFDM inherently deals with ISI? why cyclic prefix is needed?

1. Why do we need cyclic prefix in OFDM?
2. How does OFDM avoid ISI?
3. How does cyclic prefix remove ISI?
4. What will happen if we append zeros instead of a cyclic prefix?

Why do we need cyclic prefix in OFDM?

Cyclic prefix is inserted to avoid intersymbol interference due to the multipath channel. In practice, CP-OFDM modulator is implemented via the computationally efficient fast Fourier transform (FFT).

How does OFDM avoid ISI?

The basic principal of OFDM is to split the channel in to N number of sub channels and transmission of signal using orthogonal carriers, through these sub channels. OFDM converts frequency selective channel in to flat frequency channel so it can completely eliminate the Inter symbol interference (ISI) [5] .

How does cyclic prefix remove ISI?

The cyclic prefix acts as a buffer region or guard interval to protect the OFDM signals from inter symbol interference. We have learned that we minimize the ISI a lot when we make the size of the symbol larger (the size of the delay spread becomes relatively minor compared to the larger size).

What will happen if we append zeros instead of a cyclic prefix?

The main idea of using Cyclic prefix in OFDM systems is because of 2 reasons, 1) To eliminate ISI from previous symbol. If an OFDM symbol is Zero padded instead of cyclic prefix, ISI due to adjacent symbol still gets eliminated, but it cannot be modeled using circular convolution.