Satellite radio communications fundamentals and link budgets

Abstract

Satellite communications makes use of radiofrequency links. Particular frequencies are allocated for satellite communications through international regulatory registration and coordination processes which prevents interference between systems. In typical operation, a satellite's transponder receives an uplinked signal from Earth, changes its frequency slightly to avoid self-interference, and retransmits it on a downlink to Earth. Antennas provide gain by focusing the transmitted energy. Path loss describes a natural spreading out of the transmitted wave front as it travels through space. A link budget is an accounting of gains and losses throughout a system that is used as a design tool to provide sufficient power (or gain) to allow a satellite connection to be established. The link margin is the excess amount of received signal power above what is required. Shannon's Law implies that there are trade-offs possible in a communications system design between power, bandwidth, and complexity.