h distortion. The discrete symbols are filtered and converted to analog for RF up-conversion. The reverse process is performed at the receiver. A one-tap equalizer is usually used for each sub-carrier to correct channel distortion. The tap coefficients are calculated based on channel information. As shown in Figures 9-4 and 9-5, one can understand that the OFDM signal spectrum is approximately rectangular. Since the OFDM modulator is an IFFT processor, its physical meaning is to convert data from the frequency-domain to the time-domain and, transmit them in the time-domain over the channel. If a spectrum analyzer, which converts the input from the time-omain to the frequency-domain, is used to monitor an OFDM signal, what displayed is the original data. When there is multipath distortion, a conventional single carrier wideband transmission system suffers from frequency selective fading. A complex adaptive equalizer must be used to equalize the in-band fading. The number of taps required for the equalizer is proportional to the symbol rate and the multipath delay. For an OFDM system, if the guard interval is larger than the multipath delay, the ISI can be eliminated and orthogonality can be maintained among sub-carriers. Since each OFDM sub-carrier occupies a very narrow spectrum, in the order of a few kHz, even under severe multipath distortions, the sub-carriers are only subject to flat-fading. In other words, the OFDM converts a wideband frequency-selective fading channel to a series of narrowband frequency non-selective fading sub- channels by using the parallel multi-carrier transmission scheme. Since OFDM data sub-carriers are statistically independent and identically distributed, based on the central limit theorem, when the number of sub-carriers NFFT is large, the OFDM signal distribution tends to be Gaussian. The peak-to-average power ratio (PAPR) for a single carrier modulation signal depends on constellation and the pulse shaping filter roll-off factor. For a Gaussian distributed OFDM signal, the cumula