The case for multiband sensing

Cognitive radios achieve opportunistic use of underutilized spectrum by sensing and using frequency bands only if no primary user is detected. Previous approaches to spectrum sensing have examined one band at a time. We argue that this ignores the sparsity inherent to the problem: not only are a large fraction of bands unused, but the number of primary antenna sites is far smaller than the number of occupied bands due to the expense of towers. Exploiting this sparsity leads to better performance and more robustness to fading models. Fading consists of two major components: multipath and shadowing. Multipath can be counted on to be qualitatively independent across both frequencies and space. Shadowing has an uncertain distribution that is not guaranteed to be independent across space, but it can be counted on to not be very frequency selective. Multiband sensing leverages this to reduce the impact of the shadowing uncertainty on the non-interference guarantee given to the primary users. Finally, cooperation can behave qualitatively differently from cooperation. While cooperation among singleband sensors is needed to meet a target probability of harmful interference to others, cooperation for multiband sensors can be used instead to reduce the probability of missed opportunity for ourselves.