Capacity bounds for the three-user Cognitive Z-Interference Channel

In this paper, we consider a three-user cognitive radio network with two primary users and one cognitive radio. We concentrate on a three-user Interference Channel (IFC) where one of the transmitters has cognitive capabilities and non-causally knows the messages of the other two transmitters. Moreover, we assume that the cognitive transmitter does not cause any interference at the receivers of the primary users and we introduce three-user Cognitive Z-Interference Channel (C-ZIFC). We first obtain an inner bound on the capacity region of three-user C-ZIFC, where our coding scheme makes use of collaborative strategy by rate splitting and cooperative strategy by superposition coding. Moreover, the cognitive user uses Gel'fand-Pinsker binning in order to reduce the interference at its own receiver. We also provide an outer bound on the capacity region of this channel and show that characterizing the capacity region of a three-user C-ZIFC is non-trivial based on the two-user case; the reason lies in the fact that the three-user setup captures the specifications of the IFC like independent channel inputs. We also consider the Gaussian case and find an achievable rate region for the Gaussian C-ZIFC.