Subcarrier and Power Allocation for OFDMA-Based Cognitive Radio Systems With Joint Overlay and Underlay Spectrum Access Mechanism

In this paper, we study a subcarrier-and-power-allocation problem for orthogonal-frequency-division multiple-access (OFDMA)-based cognitive radio (CR) systems. In a departure from existing works in the literature that considered resource allocation for either an overlay spectrum access mechanism (OSAM) or an underlay spectrum access mechanism (USAM), we propose subcarrier-and-power-allocation schemes for a joint overlay and underlay spectrum access mechanism (JOUSAM). In particular, for such a CR system, the total transmission rate of CR users is maximized for a given power budget while keeping the interference introduced to the primary-user (PU) receivers' below given thresholds with a certain probability. As the optimal scheme can be highly complex, we also propose a low-complexity suboptimal subcarrier-and-power-allocation scheme. The selected numerical results show that a significant gain in terms of total achievable transmission rate can be obtained over an USAM or an OSAM. These selected numerical results also show that the proposed suboptimal scheme, which has a very low complexity, provides a significant improvement in performance over either underlay or overlay spectrum access mechanism. The proposed optimal scheme can lead to an unfairness among CR users in sharing the total transmission rate; therefore, we also propose a suboptimal subcarrier-allocation scheme that can guarantee a certain level of fairness among CR users.