Downlink Capacity and Optimal Power Allocation in Hybrid Underlay–Interweave Secondary Networks

In this paper, the downlink capacity region of a secondary network in a multiuser spectrum sharing system is obtained for a hybrid underlay-interweave paradigm, where the current channel side information (CSI) of the channel between a secondary transmitter-receiver pair is assumed at the secondary transmitter, whereas the outdated CSI of the interference channel is available to the secondary transmitter. In the hybrid paradigm, the secondary network cycles through the interweave or the underlay paradigms depending on whether spectrum holes are detected or the primary network is active, respectively. The outdated CSI is described via a correlation model, and the interference outage probability constraint (IOPC) and the average interference power constraint are considered. In general, determining the boundary of this region is a nonconvex problem. To navigate around solving this nonconvex problem, we replace the IOPC with its equivalent constraint on the peak transmit power, and we propose a power allocation algorithm for solving the latter problem. Simulations show that utilizing the outdated CSI of the interference channel reduces the secondary network's capacity as compared with when the current CSI is utilized. This reduction is a function of the correlation between the current and outdated CSI values, the interference outage probability in the primary network, and imperfect spectrum sensing.