Cognitive Radio Technology

Professor Danijela Cabric

A major shift in wireless communications is now emerging with the development of cognitive radios, which attempt to share spectrum in a fundamentally new way. These radios are addressing the fact that spectrum is actually poorly utilized in many bands, in spite of the increasing demand for wireless connectivity (Figure 1). On a conceptual level, cognitive radio networks sense the spectral environment and adapt transmission parameters to dynamically reuse available spectrum. This is not just a hypothetical concept, there is actual evidence that the FCC supports this technology and is currently working on the rules for cognitive radio operation in licensed TV bands. The FCC's interest also extends to higher frequencies, where the spectrum utilization is even lower. The realization of this vision could open up to 100 GHz of spectrum and a new frontier of opportunities for radio designers and wireless application developers. However, the novelty of this approach requires new mechanisms for using radio frequencies through sharing rather than fi xed allocations.

Professor Cabric is exploring fundamental questions in cognitive radio system design by bridging the theoretical and practical aspects of the physical and network layers. She proposed a cognitive radio system architecture that presents the basis for integration of core cognitive radio functions, spectrum sensing and adaptive transmission over wide bandwidths, with protocols and control channels for cognitive network management. One of her main contributions is the understanding of implementation issues and limitations of spectrum sensing, which is considered the key enabling functionality for cognitive radios. Spectrum sensing presents a major challenge as it requires detection of very weak signals of different types in a minimum time with high reliability (Figure 2). The most comprehensive approach is to address it as a cross-layer design problem that involves radio RF front-end, digital signal processing and networking solutions. Professor Cabric created a spectrum sensing design framework for analysis of techniques across these three layers and selection of the optimal system level sensing design based on performance and implementation cost. Her work involves a closed loop research approach connecting theoretical analysis and development of new algorithms with their implementation and experimental verifi cation. This system level design perspective is needed to show the feasibility of cognitive radio technology.

Looking forward, Professor Cabric believes that use of spatial dimension and multiple antennas presents many new opportunities in cognitive radio spectrum sharing. Resolving signals in angular domain allows selective processing of interference sources and localization of wireless nodes. From the higher layer perspective, she is interested in exploring how spectrum sensing in both temporal and spatial dimensions can provide better physical layer security and network management.