The major advantage of OFDM is its robustness against multi path propagation. Thus, it is suitable to be implemented in wireless environments. The introduction of cyclic prefix made OFDM system resistance to time dispersion. OFDM symbol rate is low since a data stream is divided into several parallel streams before transmission. This make the fading is slow enough for the channel to be considered as constant during one OFDM symbol interval.
Cyclic prefix is a crucial feature of OFDM used to combat the inter-symbol interference (ISI) and inter-channel-interference (ICI) introduced by the multi-path channel through which the signal is propagated. The basic idea is to replicate part of the OFDM time-domain waveform from the back to the front to create a guard period. The duration of the guard period Tg should be longer than the worst-case delay spread of the target multi-path environment. The use
of a cyclic prefix instead of a plain guard interval, simplifies the channel equalization in the demodulator.
One of the major disadvantages of OFDM is its requirement for high peak-to average power ratio. This put high demand on linearity in amplifiers.
Second, the synchronization error can destroy the orthogonality and cause interference. Phase noise error and Doppler shift can cause degradation to OFDM system. A lot of effort is required to design accurate frequency synchronizers for OFDM.
Summery for Adv. & DisAdv. of OFDM :
• N narrowband transmissions are done, this way is easily to accomplish that Ts (of each channel) is larger than T (of the channel), this means the transmission is not affected by the channel.
• High transmission bitrates.
• Chance to cancel any cannel if is affected by fading.
• High synchronism accuracy.
• Multipath propagation must be avoided in other orthogonallity not beaffected.
• Large peak-to-mean power ratio due to the superposition of all subcarrier signals, this can become a distortion problem.