  The fading cognitive multiple-access channel with confidential messages
(CMAC-CM) is investigated, in which two users attempt to transmit common
information to a destination and user 1 also has confidential information
intended for the destination. User 1 views user 2 as an eavesdropper and wishes
to keep its confidential information as secret as possible from user 2. The
multiple-access channel (both the user-to-user channel and the
user-to-destination channel) is corrupted by multiplicative fading gain
coefficients in addition to additive white Gaussian noise. The channel state
information (CSI) is assumed to be known at both the users and the destination.
A parallel CMAC-CM with independent subchannels is first studied. The secrecy
capacity region of the parallel CMAC-CM is established, which yields the
secrecy capacity region of the parallel CMAC-CM with degraded subchannels.
Next, the secrecy capacity region is established for the parallel Gaussian
CMAC-CM, which is used to study the fading CMAC-CM. When both users know the
CSI, they can dynamically change their transmission powers with the channel
realization to achieve the optimal performance. The closed-form power
allocation function that achieves every boundary point of the secrecy capacity
region is derived.
