Reducing out-of-band emissions due to nonlinearities in OFDM systems

A major disadvantage of OFDM (orthogonal frequency division multiplexing) is the large peak-to-average power ratio of the transmit signal. Clipping of amplitude peaks by the power amplifier causes interference within and outside the OFDM band. In order to keep the out-of-band emissions within tolerable limits either a sufficiently high input backoff is required or the peak-to-average power ratio of the OFDM signal must be significantly reduced. This paper gives an overview of various approaches discussed in the literature. Furthermore a method of suppressing the amplitude peaks in the OFDM signal by means of signal processing is presented. It is shown that this method allows to reduce the input backoff and hence to use the power amplifier more efficiently while keeping the out-of-band emissions within well-defined limits.