SIGNAL PROCESSING ADVANCED TECHNIQUES FOR COGNITIVE RADIO

General description

The recent proliferation of wireless communication services (cellular systems, broadcast systems, personal, local and metropolitan area networks, etc.) clearly shows the importance that the radioelectric spectrum resource has for our society. The fact that many of these highly successful services have blossomed in free access frequency bands has brought into reconsideration the traditional fixed spectrum assignment policy, in order to obtain a more efficient use of this resource.

Many field measurements and studies reflect that a large portion of the assigned spectrum is used sporadically: at a given location, the usage of a given licensed band can be quite low. Usage is usually concentrated on certain portions of the licensed spectrum, whereas a significant amount of this spectrum remains underutilized. However, under the fixed spectrum assignment policy, these sporadically used bands are banned to unauthorized potential users, even if their transmissions did not disrupt normal operation of the licensed service.

This fact, together with the limited available radio spectrum, is spurring interest in a new spectrum access paradigm. The key idea is to allow operation in the licensed bands to unlicensed agents (termed secondary users) in an opportunistic manner, as long as their operation does not cause interference to incumbent (or primary) users, which may or may not operate under a license. In this way, secondary users must determine which parts of the spectrum are available before transmitting; and, while transmission is in progress, they must detect any new primary users accesing the channel in order to act accordingly, for example vacating the channel if need be. Thus, secondary users must operate on a "first, do no harm" basis.

The key enabling technology of this paradigm is Cognitive Radio: a wireless device that can change its transmission parameters based on interaction with the radio environment. Adoption of this technology will deeply affect communication protocol design at many levels. At the physical layer, the key functionalities that a cognitive radio must include are:

• Cognitive ability, in order to be able to determine the optimal set of operation parameters. It requires the addition of spectrum sensing and monitoring capabilities to discern which bands are available and under what conditions.
• Reconfigurability, in order to automatically adjust the operation parameters (coding rate, modulation format, transmission power, carrier frequency, etc.) without having to modify the constituent hardware. This holds at the beginning of the communication process as well as while transmission is in progress.