In this paper, we study the transmission performances of a cognitive radio (CR) system which coexists with a primary system. We focus at first, on the power control of the CR network. We define then, the maximum allowable power for a cognitive transmitter whose modelisation is related to a flat reflecting surface. This model contributes also to establish a closed-form expression of the tolerated minimum distance between a CR transmitter and its nearest primary user. Finally, we study the impact of this modelisation on the uplink ergodic capacity of a cognitive transmitter communicating with a CR base station (CR BS). The implementation results prove that our work offers, at the same time, an accurate power control and a precise localization to the cognitive transmitter.