Characterizing feasible interference region for underlay cognitive radio networks

In an underlay cognitive radio network (CRN), in order to guarantee protection of primary users (PUs) (i.e., all PUs achieve their target signal-to-interference-plus-noise ratios [SINRs]), the interference caused by all secondary users (SUs) to the primary receiving-points should be controlled. To do so, the feasible cognitive interference region (FCIR), i.e., the region for allowable values of interference at all of the primary receiving-points, which guarantee protecting the PUs, needs to be formally characterized. In the state-of-the-art interference management schemes for underlay CRNs, it is considered that all PUs are protected if the cognitive interference for each primary receiving-point is lower than a maximum threshold, the so called interference temperature limit (ITL) for the corresponding receiving-point. This is assumed to be fixed and independent of ITL values for other primary receiving-points. This corresponds to a box-like FCIR, which is not correct. In this paper, we analytically obtain the FCIR and show that, the FCIR is a polyhedron (i.e., the maximum feasible cognitive interference threshold for each primary receiving-point is not constant, and it depends on that for other primary receiving-points). Therefore, in practical interference management algorithms, it is not proper to consider a constant and independent ITL value for each of the primary receiving-points. This finding would significantly affect the design of practical interference management schemes for CRNs.