  Achievable rate regions and outer bounds are derived for three-user
interference channels where the transmitters cooperate in a unidirectional
manner via a noncausal message-sharing mechanism. The three-user channel
facilitates different ways of message-sharing between the primary and secondary
(or cognitive) transmitters. Three natural extensions of unidirectional
message-sharing from two users to three users are introduced: (i) Cumulative
message sharing; (ii) primary-only message sharing; and (iii) cognitive-only
message sharing. To emphasize the notion of interference management, channels
are classified based on different rate-splitting strategies at the
transmitters. Standard techniques, superposition coding and Gel'fand-Pinsker's
binning principle, are employed to derive an achievable rate region for each of
the cognitive interference channels. Simulation results for the Gaussian
channel case are presented; they enable visual comparison of the achievable
rate regions for different message-sharing schemes along with the outer bounds.
These results also provide useful insights into the effect of rate-splitting at
the transmitters, which aids in better interference management at the
receivers.
