Cognitive radio networks enabling techniques

随着每项新业务在非常理想的频带（例如1至3ghz）上的引入，带宽短缺似乎变得更加严重。几十年来，对频谱有效利用的大部分需求已经通过调制、编码和多址技术的改进得到了解决。通过这些传统方法进一步提高效率似乎是有限的。更重要的是，人们已经确定，通常并不存在真正的频谱短缺，无法满足日益增长的需求。稀缺是由于频段只静态和永久地分配给许可证持有者。考虑到频谱的时空分布，在大多数情况下，许多位置的理想频段都没有得到充分利用。一旦对传统模式进行修改，允许未经许可的用户共享频谱，这种稀缺性就会消失。因此，关键问题是找到动态利用可用带宽的方法。CR （Cognitive Radio）允许未授权的从用户共享授权给主用户的频谱。如果次要用户在主用户不活跃时进行传输，或者以对许可证持有人无害的级别进行传输，则可以进行这种共享。

It appears that with the introduction of each new service at highly desirable frequency bands (e.g. 1 to 3 GHz), bandwidth scarcity becomes more acute. For decades, most of the demand for efficient utilization of spectrum has been addressed by improvements in modulation, coding and multiple access techniques. Further efficiency improvements by these conventional approaches seem to be limited. More importantly, it has been determined that often there is no real scarcity of spectrum to meet the increased demand. Scarcity is due to static and permanent assignment of the bands only to license holders. Considering space-time distribution of spectrum, the desirable frequency bands are severely underutilized at many locations for most of the time. Once the conventional model is modified to allow unlicensed users to share the spectrum, the scarcity can disappear. Hence the critical issue is to find ways to dynamically utilize the available bandwidth. Cognitive Radio (CR) allows unlicensed secondary users to share the spectrum that is licensed to primary users. This sharing may take place if secondary users transmit when primary user is not active, or by transmitting at a level not harmful to the license holder.