A Study on Inter-Cell Subcarrier Collisions due to Random Access in OFDM-Based Cognitive Radio Networks

In cognitive radio (CR) systems, one of the main implementation issues is spectrum sensing because of the uncertainties in propagation channel, hidden primary user (PU) problem, sensing duration and security issues. This paper considers an orthogonal frequency-division multiplexing (OFDM)-based CR spectrum sharing system that assumes random access of primary network subcarriers by secondary users (SUs) and absence of the PU´s spectrum utilization information, i.e., no spectrum sensing is employed to acquire information about the PU´s activity or availability of free subcarriers. In the absence of information about the PU´s activity, the SUs randomly access (utilize) the subcarriers of the primary network and collide with the PU´s subcarriers with a certain probability. In addition, inter-cell collisions among the subcarriers of SUs (belonging to different cells) can occur due to the inherent nature of random access scheme. This paper conducts a stochastic analysis of the number of subcarrier collisions between the SUs´ and PU´s subcarriers assuming fixed and random number of subcarriers requirements for each user. The performance of the random scheme in terms of capacity and capacity (rate) loss caused by the subcarrier collisions is investigated by assuming an interference power constraint at PUs to protect their operation.