Parallel Transmission in OFDM:
In wideband wireless systems, the bandwidth of the channel is much larger than its coherence bandwidth and thus the channel is frequency selective. With a single carrier transmission system, the symbols are transmitted sequentially, and the signals occupy the entire available bandwidth. Therefore, adaptive equalization techniques are essential at the receiver for successful decoding of information. The equalizer estimates the channel impulse response to compensate for the distortion due to the multipath fading. As the symbol period becomes shorter with increasing data rates, the adaptive equalization becomes more difficult. The shorter the symbol duration, the more the number of adjacent symbols affected by a single fade will be.
On the other hand, in a multi-carrier system, the entire available bandwidth is divided into many narrow band subcarriers and each subcarrier experiences flat fading. Equalization is performed on a subcarrier basis and thus is much simpler than the single carrier transmission systems .Fading across different subcarriers provides frequency diversity. A single fade only affects a limited number of the subcarriers. By coding across the symbols in different subcarriers, the information conveyed by those affected subcarriers can be recovered at the receiver.
The approach of parallel transmission also reduces the sensitivity of the system to the multipath delay spread. Since each subcarrier carries only a small fraction of the overall data rate, the symbol period of the signal is increased. The increased symbol period improves the robustness of OFDM to delay spread. This in effect reduces the effect of ISI. By introducing a guard interval called cyclic prefix, , the ISI can further be reduced or even eliminated completely