Proactive clipping and filtering for trading in- and out-of-band distortion in OFDM transmitters

The high peak-to-average power ratio (PAPR) of orthogonal frequency-division multiplexing (OFDM) signals combined with nonlinear transmitter characteristics results in poor bit error rate (BER) performance and causes out-of-band spectrum regrowth of the waveforms entering the channel. A popular technique with low complexity to reduce nonlinear distortions in memoryless high power amplifiers (HPA) is amplitude clipping which limits the peak envelope of the HPA input to a predetermined level. This paper characterizes in- and out-of-band distortion as a result of clipping the Nyquist sampled OFDM signal which is then filtered and subsequently passed through the transmitter HPA. For transmitters with limited filtering of the HPA output, this offers a trade-off between out-of-band distortion reduction and in-band distortion increase in the transmitted OFDM signal. To this end, this paper documents spectral regrowth reduction versus irreducible BER using computer simulations. Also, to verify the feasibility of the pre-distortion system, the clipping and filtering has been implemented on a field-programmable gate array (FPGA) platform.