Dynamic spectrum access in cognitive radios

随着无线通信及其应用的快速发展，未来无线网络对宽带多媒体业务的需求将不断增加，这将引发对频谱带宽的巨大需求。然而，在大多数地区，大部分相关的无线电频谱已分配给各种许可应用，从而导致潜在的频谱短缺问题。另一方面，最近对频谱的实际测量表明，在任何给定地点和任何给定时间，大部分许可频谱未被利用。因此，动态频谱接入在频谱暂时不被主用户（授权用户）使用的情况下，允许二次用户（未授权用户）借用频谱，成为缓解频谱短缺问题的一种很有前景的解决方案。认知无线电作为动态频谱接入的关键实现手段，由于辅助用户能够智能地适应不断变化的环境，因此受到了广泛的关注。对于次要用户来说，访问频谱的较低优先级对其信道访问协议的设计和分析提出了很大的挑战，因为只要可能，主用户不应受到影响。当支持具有异构服务质量（QoS）需求的多媒体流量时，情况变得更加复杂。

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.