Optimal Output Back-Off in OFDM Systems with Nonlinear Power Amplifiers

A well-known problem of OFDM signals is their high peak-to-average power ratio, which can cause the clipping of peaks in the transmitted signal due to the limited linear range of high power amplifiers. Operating the amplifier solely in the linear region, i.e., with a large output back-off, results in an undistorted transmitted signal, but at the expense of a low SNR at the receiver. Driving the amplifier into saturation, on the other hand, maximizes the received SNR, but causes severe nonlinear signal distortions. In this paper we study the resulting tradeoff. We show both theoretically and by simulations that, despite the distortions, it is optimal to operate the amplifier close to saturation. Furthermore, we analytically derive a modification of the Bussgang noise cancellation algorithm which was recently shown in the literature to improve the EXIT characteristic of the algorithm.