  The successful operation of cognitive radio (CR) between CR transmitter and
CR receiver (CR link) relies on reliable spectrum sensing. To network CRs
requires spectrum sensing at CR transmitter and further information regarding
the spectrum availability at CR receiver. Redefining the spectrum sensing along
with statistical inference suitable for cognitive radio networks (CRN), we
mathematically derive conditions to allow CR transmitter forwarding packets to
CR receiver under guaranteed outage probability, and prove that the correlation
of localized spectrum availability between a cooperative node and CR receiver
determines effectiveness of the cooperative scheme. Applying our novel
mathematical model to potential hidden terminals in CRN, we illustrate that the
allowable transmission region of a CR, defined as neighborhood, is no longer
circular shape even in a pure path loss channel model. This results in
asymmetric CR links to make bidirectional links generally inappropriate in CRN,
though this challenge can be alleviated by cooperative sensing. Therefore,
spectrum sensing capability determines CRN topology. For multiple cooperative
nodes, to fully utilize spectrum availability, the selection methodology of
cooperative nodes is developed due to limited overhead of information exchange.
Defining reliability as information of spectrum availability at CR receiver
provided by a cooperative node and by applying neighborhood area, we can
compare sensing capability of cooperative nodes from both link and network
perspectives. In addition, due to lack of centralized coordination in dynamic
CRN, CRs can only acquire local and partial information within limited sensing
duration, robust spectrum sensing is therefore proposed. Limits of cooperative
schemes and their impacts on network operation are also derived.
