  In this contribution, we investigate the idea of using cognitive radio to
reuse locally unused spectrum to increase the total system capacity. We
consider a multiband/wideband system in which the primary and cognitive users
wish to communicate to different receivers, subject to mutual interference and
assume that each user knows only his channel and the unused spectrum through
adequate sensing. The basic idea under the proposed scheme is based on the
notion of spectrum pooling. The idea is quite simple: a cognitive radio will
listen to the channel and, if sensed idle, will transmit during the voids. It
turns out that, although its simplicity, the proposed scheme showed very
interesting features with respect to the spectral efficiency and the maximum
number of possible pairwise cognitive communications. We impose the constraint
that users successively transmit over available bands through selfish water
filling. For the first time, our study has quantified the asymptotic (with
respect to the band) achievable gain of using spectrum pooling in terms of
spectral efficiency compared to classical radio systems. We then derive the
total spectral efficiency as well as the maximum number of possible pairwise
communications of such a spectrum pooling system.
