The increasing demand for wireless communication introduces efficient spectrum utilization challenge. To address this challenge, cognitive radio has emerged as the key technology, which enables opportunistic access to the spectrum. The main potential advantages introduced by cognitive radio are improving spectrum utilization and increasing communication quality. In this paper, the authors consider the high Peak-to-Average Power Ratio (PAPR) problem of Orthogonal Frequency Division Multiplexing (OFDM) signals in cognitive radio systems. A high PAPR can lead to saturation in the Power Amplifier (PA) of Secondary Users (SUs) and consequently increase spectral spreading, and cause interference to adjacent Primary Users (PUs).