  We propose a novel paradigm for spectrum sensing in cognitive radio networks
that provides diversity and capacity benefits using a single antenna at the
Secondary User (SU) receiver. The proposed scheme is based on a reconfigurable
antenna: an antenna that is capable of altering its radiation characteristics
by changing its geometric configuration. Each configuration is designated as an
antenna mode or state and corresponds to a distinct channel realization. Based
on an abstract model for the reconfigurable antenna, we tackle two different
settings for the cognitive radio problem and present fundamental limits on the
achievable diversity and throughput gains. First, we explore the (to cooperate
or not to cooperate) tradeoff between the diversity and coding gains in
conventional cooperative and noncooperative spectrum sensing schemes, showing
that cooperation is not always beneficial. Based on this analysis, we propose
two sensing schemes based on reconfigurable antennas that we term as state
switching and state selection. It is shown that each of these schemes
outperform both cooperative and non-cooperative spectrum sensing under a global
energy constraint. Next, we study the (sensing-throughput) trade-off, and
demonstrate that using reconfigurable antennas, the optimal sensing time is
reduced allowing for a longer transmission time, and thus better throughput.
Moreover, state selection can be applied to boost the capacity of SU
transmission.
