Two of the main factors that affect the performance of satellite communication systems are the occurrence of rain fading in the transmission path and external interferences due to other systems sharing the same frequency band. To guarantee an adequate link performance, it is usual to establish limits to 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 performance requirements are met. Degradation due to rain is usually considered in a communication link design, as mathematical models of rain attenuation are widely known. Degradation due to external interference is limited by the establishing constraints to be imposed on the statistical behaviour of the interfering signal power. These constraints, object of study within the International Telecommunication Union, should be such that, independently of the specific characteristics of the considered interfering system, guarantees that link performance objectives are met. This dissertation defines a constrained optimization problem such that its solution permits to establish the conditions to be imposed to the statistical behaviour of the single-entry interfering signal power so that link performance objetives are met.