A preamble is used in Digital Communication Systems to train the VCO of the receiver to the incoming signals's clock so as to produce a clocking in the reciver that is synchronized to the received signal, and so a perfect sampling and/or demodulation can be done.
The Pilot symbols
The pilot symbols are used in wireless communication systems for the sake of channel estimation and correction purposes. Where the wireless channel have great effect on the phase and the frequency of the wireless signal and alters them by some value, which have many bad side effects and may affect the process of demodulation. Some of these effects as :Phase rotation, doppler frequency shift, degradation of the amplitude and phase distortion. All of these cause poor SNR,SIR and one important thing is it may alter the place of the frequncy of some subcarriers in an OFDM which may cause the loss of othogonality between them and the OFDM symbol is destructed. So, at the transmitter a well known symbol (its freqency,amplitude and phase) is inserted among the subcarriers to carry the effects of the channel, and at the receiver it demodulated and then all the effects are calculated and the received signal is corrected by those calculated amounts
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