  As a brain inspired wireless communication scheme, cognitive radio is a novel
approach to promote the efficient use of the scarce radio spectrum by allowing
some users called cognitive users to access the under-utilized spectrum
licensed out to the primary users. Besides highly reliable communication and
efficient utilization of the radio spectrum, the security of information
transmission against eavesdropping is critical in the cognitive radios for many
potential applications. In this paper, this problem is investigated from an
information theoretic viewpoint. Capacity limits are explored for the Cognitive
Radio Channel (CRC) with confidential messages. As an idealized information
theoretic model for the cognitive radio, this channel includes two transmitters
which send independent messages to their corresponding receivers such that one
transmitter, i.e., the cognitive transmitter, has access non-causally to the
message of the other transmitter, i.e., the primary transmitter. The message
designated to each receiver is required to be kept confidential with respect to
the other receiver. The secrecy level for each message is evaluated using the
equivocation rate. Novel inner and outer bounds for the capacity-equivocation
region are established. It is shown that these bounds coincide for some special
cases. Specifically, the capacity-equivocation region is derived for a class of
less-noisy CRCs and also a class of semi-deterministic CRCs. For the case where
only the message of the cognitive transmitter is required to be kept
confidential, the capacity-equivocation region is also established for the
Gaussian CRC with weak interference.
