I wonder if there's much reason to be interested in ultra-low-speed
digital radio, by which I mean two or three bits per second, or even 1
bps, using spread spectrum modulation over a voice channel or across a
whole band. The idea is to have a reasonably reliable HF
communications channel for portable, low-powered equipment in remote
areas. You'd use data compression so that a 100 character text
message would compress down to maybe 40 bytes. At 2 bits/sec, sending
the 40 bytes would take about 2.5 minutes, maybe a little longer if
you add some FEC. 100 characters in 2.5 minutes is about 5 words per
minute, a speed which has proven usable to novice ham CW operators for
many decades.

The energy per symbol at 2 bps would be over 1000x higher than using
voice, since packet operators routinely get 2400 bps through voice
channels. So in terms of DX capability, using a 1 watt transmitter at
2 bps is like using a 1 kw transmitter for voice. Similarly,
receiving data at 2 bps is like having 30 db of antenna gain over
receiving voice.

I'm imagining an HT-sized tranceiver that you'd use with a collapsible
vertical or wire antenna. You could be on some remote island or
mountainside, crossing the Atlantic in a balloon or whatever, and
still be able to send and receive text messages using a keypad and LCD
display. Coverage would be equal to having a much larger transmitter
and antenna at higher bit rates. You wouldn't necessarily have to be
able to reach your destination, since any station that could hear you
could forward your messages, either by digital radio or through the
internet. You'd have a low-tech substitute for a satellite phone,
that wouldn't let you have voice conversations, but that didn't depend
on being able to launch stuff into space.

Of course, propagation permitting, your tranceiver could increase the
bit rate up to near the chip rate (say 3 kbps for a voice-sized
channel); or in really bad conditions, it could back off to 1 bps or
even lower. I'm using the 2 bps example just to illustrate the kinds
of applications I have in mind. I think most of the time, higher
speeds should be possible. At low speeds, you'd have the usual spread
spectrum advantages: low speed signals in the same bandspace shouldn't
interfere with each other, since the spreading sequences would be
uncorrelated.

I don't follow digital radio very closely but most new developments
that I hear about aim at getting higher and higher bit rates to move
more data, rather than lowering the bit rate to get better DX
capabilities.

Thoughts?