When I think of the benefits of using something like Direct Sequence
Spread Spectrum modulation over visible light frequencies, with
bandwidths in, say, the terahertz range, I don't see any reason why an
advanced civilization would be emitting much radio at all. Worse yet,
such a signal would look just like white light, or starlight, or any type
of source desired. It would not be decipherable or even detectably
artificial unless the transmitting folk wanted it to be. For that
matter, any number of "stars" we see could just as well be transmissions.
Sorry, SETI fans.
- PW