Two of the main factors that affect the performance of wireless communication systems are fading due to rain and interfering signals that reach the receiver. To guarantee an adequate communication link performance, it is usual to limit the percentages of time during which the system performance may be below certain predefined levels. It is important that performance degradation due to both rain and external interferences be such that these performance constraints are met. Degradation due to rain is usually considered in the communication link design, as mathematical models for rain attenuation are widely known. For external interferences, degradation is usually limited by establishing constraints to the statistical behaviour of the interfering signal power. These constraints should be such that, independently of the specific characteristics of the considered interfering system, link performance objectives can be met. Studies aiming the definition of adequate constraints are currently being developed within the International Telecommunication Union. This dissertation defines a constrained optimization problem that allows for the definition of the constraints to be satisfied by the statistical behaviour of the interfering signal power so that link performance are met.