Optimal power allocation for secondary users in cognitive relay networks

We consider a cognitive relay network (CRN) where the secondary users (SUs) are involved as cooperative relays in a primary user´s (PU) communication. To avoid generating interference to PU transmissions, it is assumed that SUs can transmit only when the PU´s channel is idle. On one hand, SUs use some relay powers to speed up the PU´s transmissions. Consequently, PU´s buffer will be depleted faster, resulting in more channel idle times (i.e., more transmission opportunities for SUs). On the other hand, due to the energy limit, less power can be used for SU´s own transmissions if it uses too much power on relaying. Thus, an optimal power allocation strategy is necessary to address this tradeoff. In this paper, the power allocation problem for both single-SU case and multiple-SU case is investigated. The former is formulated as a utility maximization problem, while the latter is modeled as a non-cooperative game. The existence and uniqueness of the Nash equilibrium (NE) are proved, and the impact of different system parameters on NE is comprehensively analyzed through numerical results.