The transmission of radio or optical signals in free space, i.e., space devoid of physical obstructions that might hinder signal propagation. In this context, the term physical obstruction suggests trees, buildings, hills, mountains, and other significant material objects.The term does not suggest atomic, molecular, or particulate matter that commonly is present in the atmosphere. Neither does it suggest water vapor, rain, snow, sleet, or hail. Free space transmission does not include radio or optical transmission through waveguides. See also free space, FSO, transmission medium, and waveguide.