Abstract
In this study, orthogonal frequency division multiple access (OFDMA) based primary and secondary usage in a cognitive radio system is considered and conventional power spectral density (PSD) based technique is used to detect the opportunities in the spectrum. Primary user employs block-based or scattered subcarrier allocation scheme and the overlay system aims to utilize the spectral gaps. Comparison of the allocation schemes and effect of windowing are presented in terms of true detection and false alarm rates. It is observed that when block-based allocation is employed, the unlicensed user will be able to detect more opportunities in the spectrum compared to the case in which the scattered allocation is employed. Simulation results show that threshold selection is crucial since it is the main factor in determining the false alarm and true detection rates. It has been shown that, for either of the schemes all possible spectral gaps cannot be found when PSD is used. Questioning the suitability of PSD method results in that a new definition of opportunity is needed, forming a basis for the second part of this study.