Reducing the Peak to Average Power problem for OFDM
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Abstract
A major disadvantage of Orthogonal Frequency Division Multiplexing (OFDM) is that it results in a large Peak to Average Power Ratio (PAPR). This significantly decreases the efficiency of the transmitter power amplifier. First of all in this report the basics of OFDM are given and the size of the problem of having a large PAPR is examined. The cdf’s for the PAPR’s are given for different amounts of carriers and it is seen that the PAPR increases with the amount of subcarriers used. Also this report treats multiple methods -Clipping, Selective Backoff, Windowing and Symbol Substraction- to decrease the PAPR problem. This is done by applying some kind of predistortion after making the OFDM symbol (before amplifying). A scrambling scheme is also treated. It is then seen that it is possible to decrease the PAPR when using symbol subtraction to about 6 dB for a different amount of subcarriers without distorting the frequency spectrum. Some increase in error rate has to be accepted.