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，这将对频谱带宽产生巨大的需求。然而，大多数相关的无线电频谱已被分配给大多数地区的各种许可应用，从而导致潜在的频谱稀缺问题。另一方面，最近的频谱的真实的测量已经表明，在任何给定的位置和任何给定的时间，大部分许可频谱没有被利用。因此，动态频谱接入最近已经成为一种有前途的解决方案，以通过在频谱暂时不被主要（许可）用户使用时让次要（未许可）用户借用频谱来缓解频谱稀缺问题。认知无线电作为实现动态频谱接入的关键技术，能够使次用户智能地适应不断变化的环境，受到了广泛的关注。对于次用户，接入频谱的较低优先级对其信道接入协议的设计和分析提出了很大的挑战，因为主用户应该尽可能不受影响。当支持具有异构服务质量（QoS）要求的多媒体业务时，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.