Abstract : SoftWare Radio (SWR) is an enabling technology for Cognitive Radio (CR) systems which promises to (de) modulate any signal, at any frequency. SWR signal, therefore, is composed of different standard's signals and each standard's signal is either multicarrier or multiplex of single carriers. This combination leads to high temporal fluctuations and thus SWR signal inherits high Peak to Average Power Ratio (PAPR) or simply high Power Ratio (PR). Non-linear analogue components (amplifiers, converters etc.) cause distortions (in and out of band distortion) for high PR signals which result in system performance degradation. Usually PR problem is addressed in time domain while this paper presents frequency domain interpretation of PR which is more appropriate in SWR context. Gaussian equivalence between SWR signal and Orthogonal Frequency Division Multiplexing (OFDM) signal is proved first to accentuate high PR issue in SWR as OFDM suffers the same problem. Then frequency domain interpretation of PR metric is discussed which results in a PR upper bound. This PR upper bound depends only upon spectral values of the signal thus associates spectrum with PR. As a result this bound assists in spectrum access for CR systems by providing PR metric information related to any available bandwidth. Thus bandwidth allocation in a spectrum access scenario under PR constraint is simplified.