A lots of literature can be found on OFDM. Here a sample code is attached which justifies the line "Multipath fading channel will be converted to flat fading channel using OFDM". At the same time one can also finds how using Cyclic Prefix of length greater or equal to (L-1) converts linear convolution into circular convolution. One can find "Circular convolution and DFT as fourier transform pair" which says that for discrete signals the circular convolution in time domain is multiplication of their respective DFT values. This examples can be calculated by hand and can be verified with the given code.