Improving the Sensing–Throughput Tradeoff for Cognitive Radios in Rayleigh Fading Channels

In-band spectrum sensing (SS) in overlay cognitive radio networks requires that the secondary users (SUs) periodically suspend their communication to determine whether the primary user (PU) has started to utilize the channel. In contrast, in spectrum monitoring (SM), the SU can detect the emergence of the PU from its own receiver statistics such as receiver error count (REC). Previously, it has been shown that in additive white Gaussian noise channels, a hybrid SS/SM system significantly improves the channel utilization of SUs and the detection delay of PUs. In this paper, we investigate the problem of SM in the presence of fading, where the SU employs diversity combining to mitigate the channel fading effects. We show that a decision statistic based on the REC alone does not provide good performance. Next, we introduce new decision statistics based on the REC and the combiner coefficients. It is shown that the new decision statistic achieves significant improvement in the case of maximal ratio combining (MRC). However, for equal gain combining and selection combining, the inclusion of combiner coefficients does not improve the performance over REC alone. In the case of MRC, we evaluate the receiver operating characteristics (ROCs) from analysis and compare the results with those from simulations using a BCH code as well as a convolutional code. The results show a close match between analysis and simulation results. Channel utilization and detection delay are evaluated from simulations which show that with MRC and the proposed decision statistic, the hybrid SS/SM system significantly outperforms the SS alone.