WIRELESS networks are currently characterized by a fixed spectrum assignment policy. Due to the proliferation of wireless technologies and services, the demand for the radio spectrum continuously increases. This increasing demand together with the fixed spectrum assignment policy creates a shortage of spectrum. However, this shortage is artificial because studies A Graph-Theoretic Approach to Scheduling in Cognitive Radio Networks show that a very small portion of the assigned spectrum is actually utilized . This situation calls for techniques that utilize the radio spectrum more efficiently. To overcome the inefficiency in the spectrum usage, the dynamic spectrum access (DSA) concept has been introduced by researchers in the wireless networking community. DSA hinges upon the idea of having an intelligent device that opportunistically A Graph-Theoretic Approach to Scheduling in Cognitive Radio Networks utilizes the temporarily unused parts of the spectrum and vacates them as soon as the licensed owner of that spectrum band resumes its operation. These intelligent devices are called cognitive radios. A Graph-Theoretic Approach to Scheduling in Cognitive Radio Networks The licensed owners of the spectrum are called primary users (PUs), and the cognitive radio devices are called the secondary users (SUs). PUs are unaware of the SUs, and SUs are obliged not to disturb the PUs. In a centralized cognitive radio network (CRN), cognitive base station (CBS) is the central entity that has cognitive capabilities; in other words, a CBS is aware of the DSA concept. A Graph-Theoretic Approach to Scheduling in Cognitive Radio Networks The CBS controls and guides the SUs in its service area by ensuring that the PUs in the region are not disturbed by the data communication of the SUs with the CBS. Opportunistic scheduling concept is based on the exploitation of the time-varying channel conditions in wireless networks to increase the overall performance of the system. A Graph-Theoretic Approach to Scheduling in Cognitive Radio Networks All schedulers make frequency, time-slot, and data rate allocations to the SUs. Furthermore, all of them ensure that the PUs in the service area of the CBS are not disturbed, no collisions occur among the SUs, reliable communication of the SUs with the CBS is maintained, each SU is assigned at least one time-slot whenever possible, and the number of frequencies assigned to an SU in a particular time-slot is not more than the number of its transceivers (antennas) for data transmission.