Dynamic spectrum access in cognitive radios

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With the rapid growth of wireless communications and their applications, the demand for broadband multimedia services will keep increasing in future wireless networks, which will trigger a huge demand for spectral bandwidth. However, most of the related radio spectrum has been assigned to various licensed applications in most regions, thus leading to a potential spectrum scarcity problem. On the other hand, recent real measurements of the spectrum have indicated that at any given location and any given time, much of the licensed spectrum is not utilized. Therefore, dynamic spectrum access has recently emerged as a promising solution to alleviate the spectrum scarcity problem by letting secondary (unlicensed) users borrow the spectrum when the spectrum is temporarily not in use by the primary (licensed) users. As a key enabler for dynamic spectrum access, cognitive radios have received much attention, where secondary users can intelligently adapt to the changing environment. For secondary users, the lower priority to access the spectrum poses a big challenge to the design and analysis of their channel access protocols because primary users should not be affected whenever possible. The situation becomes more complicated when multimedia traffic with heterogeneous quality-of-service (QoS) requirements is supported.