Spectrum efficiency of cognitive relay transmissions with cooperative diversity in cognitive radio networks

Cognitive radio is a promising technology that enables unlicensed users to communicate with each other over a licensed band (through spectrum holes). Typically, each cognitive transmission process consists of two required phases, i.e., the spectrum sensing and the data transmission phases. To improve the overall system performance of cognitive transmissions, we propose a cognitive relay scheme where a cognitive relay is used for both the spectrum sensing and the data transmission phases. By jointly considering the two phases, we derive an exact closed-form expression of the spectrum efficiency for the proposed scheme, which is to quantify the percentage of spectrum holes utilized by cognitive users for their successful data transmissions. Numerical results show that a significant performance improvement is obtained in terms of the spectrum efficiency by using the proposed cognitive relay transmission scheme. In addition, it is shown that a maximum spectrum efficiency can be achieved through a tradeoff in determining the time durations for the spectrum sensing and data transmission phases.