Advances in wireless communications technologies (e.g., 3G, 4G and beyond) entail demands for higher data rates. A well-known popular solution to fulfill this requirement was to allocate additional bandwidth, which unfortunately is not anymore viable due to radio-frequency (RF) spectrum scarcity. Nonetheless, spectrum measurements around the globe have revealed the fact that the available spectrum is under-utilized. One of the most remarkable solutions to cope with the under-utilization of spectrum is the concept of cognitive radio (CR). In CR systems, the main implementation issues are spectrum sensing because of the uncertainties in propagation channel, hidden primary user (PU) problem, sensing duration and security issues. Hence, the accuracy and reliability of the spectrum sensing information can be suspicious and questionable inherently. There has been no study to date that investigates the impacts of absence of spectrum sensing information in CR networks. In this work, due to the imprecise and unreliable spectrum sensing information, we investigate the performance of an orthogonal frequency-division multiplexing (OFDM)-based (4G and beyond) CR spectrum sharing communication system that assumes random allocation and absence of the PU's channel occupation information, i.e., no spectrum sensing is employed to acquire information about the availability of unused subcarriers or the PU's activity. The results show that due to the lack of information of the PUs' activities, the SU randomly allocates the subcarriers of the primary network and collides with the PUs' subcarriers with a certain probability. The number of subcarrier collisions is found to be following hypergeometric distribution. The SU's capacity with subcarrier collisions is employed as a performance measure to investigate the proposed random allocation scheme for both general and Rayleigh channel fading models. To avoid the subcarrier collisions at the SUs due to the random allocation and to obtain the maximum sum rate for SUs based on the available subcarriers, an efficient centralized sequential algorithm is proposed and analyzed. The performance of such a communication set-up can provide various insights into the studies in the CR literature, and it these be utilized as a valid candidates for performance comparison benchmarks in CR spectrum sharing systems with the availability of spectrum sensing information.


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