15 Even though the PAPR for OFDM that was derived above is an upper bound, it is important to note that the PAPR varies linearly with the number of subcarriers N . To combat impairments in channels that are highly frequency-selective, a large will be preferable in order to flat-fading within a subcarrier. A large N translates to a large PAPR. As a result, OFDM signals have a large PAPR. An obvious method of dealing with the large PAPR is to back off the operating points of non-linear power amplifiers; however, this approach severely reduces the efficiency of the power amplifiers. Therefore, various techniques have been proposed to reduce PAPR of OFDM signals. These techniques exploit the special characteristics of OFDM signals and include clipping and filtering , peak cancellation with a complementary signal, allowing non-linear distortion from the power amplifier (and correcting for it), selected mapping  , special coding techniques  , and partial transmit sequence . 2.6 OFDMA
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