The Case for Spread-spectrum Signaling in Interstellar Messaging

A particular challenge for SETI searches is choosing the type of signal to seek. Past applications of communication theory to this issue have failed to infer any credible concrete characteristics of a signal on the basis of standard measures of detection sensitivity such as detection and false alarm probabilities. However, these efforts have also ignored radiofrequency interference, which is a growing challenge in the earth’s environs and may also be recognized by a transmitting civilization. We demonstrate that a transmitted interstellar beacon signal can be chosen in such a way that it is intrinsically immune to interference. Adopting as our criterion robustness of interference immunity regardless of the interference characteristics, which we argue is an appropriate perspective for the transmitter designer, elementary probability theory is employed to prove that the transmitted signal must possess the statistical characteristics of a burst of white Gaussian noise. Detecting such a signal requires that the receiver successfully guess and correlate against the signal. Such a signal can be generated by a pseudorandom algorithm based on elementary mathematics, such as the geometry of a square or a circle. As a concrete example, the binary expansion of π can form the basis of a signal with the appropriate characteristics.