Well, I was really thinking of GPS locked PSK rather than any incoherent
mode.  WIthout the need for clock and frame timing recovery we could scrape
every possible advantage for coherent signalling.  With such superb accuarcy
of timing even without carrier recovery, it seems a pity to throw all the
advantages away by using on-off keyed power detection.

I did some basic tests last year with GPS locked 1 B/s PSK and with a very
simple decoder managed to receive down to frighteningly low signal levels.

Andy  G4JNT