Power Allocation Problem for Fading Channels in Cognitive Radio Networks

Abstract

In this paper, we consider a wireless fading channel in Cognitive Radio (CR) systems. Two types of users try to access to the primary spectrum: the Primary users (PUs) and the Secondary users (SUs). In this context, the spectrum has been licensed to the primary users. The secondary users do not own the spectrum license; however, the secondary communication is allowed to coexist with the primary users (PUs) in the primary communication network provided that the SU does not interfere too much with the PU. In this spectrum sharing strategy, we study the power allocation problem in order to achieve the SU's capacity for different fading channel models. It is shown that fading for the channel between SU transmitter and PU receiver is usually a beneficial factor for ameliorating the SU channel capacities. Both, the average interference power constraint at the PU and the average transmit power constraint of the SU are considered in this power allocation problem. We provide a decoupling method, in order to compute the optimal power allocation policies in our optimization problem. Our method gives an optimal solution and reduces the complexity of the general problem using Lagrangian tools.