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The nonlinear behavior of power amplifier (PA) in orthogonal frequency-division multiplexing (OFDM)-based cognitive radio (CR) system introduces in-band and adjacent channel interferences. The power in the adjacent channels is an important issue in the CR because it can act as interference to primary users. In this paper, we derive a closed-form expression for the leakage power at adjacent channels when CR devices use OFDM modulation. Moreover, the PA nonlinearity creates in-band distortion that produces intercarrier interference and reduces the secondary system capacity. To control this effect, we derive the optimum power scaling factor for the input signal that maximizes the secondary user rate. The optimization process is executed by considering nonlinear effects on signal-to-interference-and-noise ratio under the constraints of the powers in the adjacent channels. The power in the adjacent channels …