All literature is made up of a system of signs, and fiction is a kind of literature.
The discourse of SF differs from that of other literary forms in that it takes the form of a thought experiment. The story is told using assumptions of science about the nature of the world. SF stories are always related to science, to humanity's ways of coming to know itself and its environment through observation, hypothesis and experiment. See if you can find web pages for Blish, Anderson, and Lafferty on the Internet. Explain what you find.