Digital modulator with crest factor minimization in OFDMA is optimize by different technique

The concept of Orthogonal Frequency Division Multiplexing (OFDM) has been known since 1966, but it only reached sufficient maturity for deployment in standard systems during 1990s. OFDM is an attractive modulation technique for transmitting large amounts of digital data over radio waves. One major disadvantage of OFDM is that the time domain OFDM signal which is a sum of several sinusoids leads to high peak to average power ratio (PAPR). Number of techniques have been proposed in this paper for reducing the PAPR in OFDM systems. In this paper the various techniques proposed for reducing the PAPR and the selection criteria for choosing these techniques have been discussed. These techniques can mainly be categorized in to signal scrambling techniques and signal distortion techniques. Signal scrambling techniques are all variations on how to scramble the codes to decrease the PAPR. Coding techniques can be used for signal scrambling. Golay complementary sequences, Shapiro-Rudin sequences, M sequences, Barker codes can be used efficiently to reduce the PAPR. The signal distortion techniques introduce both In band and Out-of-band interference and complexity to the system. The signal distortion techniques reduce high peaks directly by distorting the signal prior to amplification. Clipping the OFDM signal before amplification is a simple method to limit PAPR. More practical solutions are peak windowing, peak cancellation, Peak power suppression, weighted multicarrier transmission, companding etc. Many of the modulation methods currently in use suffer from a high Peak-to-Average power Ratio (PAPR), also known as the Crest Factor (CF). Keywords: Orthogonal Frequency Division Multiplexing (OFDM); peak to average power ratio (PAPR); Crest Factor (CF