The performance is decline because Delay Spread: Frequency Selective Fading
causing the Inter-Symbol Interference (ISI) and the signal power to be weakened and because Doppler Shift causing the frequency signal wave to be changed or distorted.
However based on the time delay spread small scale fading can be categorized as flat fading and frequency selective fading
. As per Doppler spread small scale fading can be categorized as fast fading and slow fading.
Orthogonal frequency division multiplexing (OFDM) as a multi-carrier communication technique, has been adopted in many applications because of its performance against the broadband mobile wireless communications' frequency selective fading
This makes the power line channel cause multipath effect, and the transmission characteristics show a certain frequency selective fading
OFDM is appealing techniques for wireless high-rate information transmission with high bandwidth efficiency because of minimizing impact over frequency selective fading
channels .OFDM have a point of interest in that modulation and demodulation can be done by utilizing the Inverse Fast Fourier Transform (IFFT) and Fast Fourier Transform (FFT) operations, which are computationally productive.
Several papers have analysed performance of DS-CDMA systems based on cooperative diversity in which direct transmission is considered in addition to two-hop relaying over different fading channels such as Rayleigh, Nakagamim and frequency selective fading
WOFDM performs channel modeling through accurate characterization of time variant and frequency selective fading
However, suffering from the frequency selective fading
of the dispersive wireless channel, some subchannels may face deep fading and degrade the overall system performance.
The first is interference caused by echoes exceeding the guard time and the second is frequency selective fading
when the symbol duration of each subcarrier is not long enough.
Figure 2 shows an example of Rayleigh fading as well as Rician fading channel responses for frequency flat as well as frequency selective fading
SC-FDE is frequency domain equalization technique used for reduction of frequency selective fading
effects in LTE uplink.