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Re: Digital Radios



>> This is especially true for slow speed textual communications, as
>> contrary to popular amateur folklore CW is actually *extremely*
>> power-inefficient for the data rate it supports.

>Thats not how I have ever seen it. CW is very good for very weak signal
>working requiring minimum equipment for maximum readability under weak
>signal conditions.

"Low power" and "power efficiency" are not the same thing. CW only
*seems* efficient (i.e., low power) because the user data rate is so
extremely low.  What counts is not the transmitter power in watts, but
the transmitted *energy per bit* (in watt-seconds or joules).

If you compare apples with apples, you'll find that CW can easily
require 20 dB or more transmitter power than a good coded digital
scheme operating at the same user data rate on the same channel.

Listen to the sound file I put on my web site the other day demoing
what my proposed coded AO-40 telemetry format can sound like while
still being fully decodeable without errors. The URL is

http://people.qualcomm.com/karn/ao40/testmessage_ebno7.wav

Here my user data rate is 160 bps (a FEC code rate of 0.4 times the
channel data rate of 400 bps). If we consider a Morse "word" as
equivalent to 25 bits (5 bits/character times 5 characters/word), then
160 bps corresponds to a Morse speed of (160/25) * 60 = 384 words per
minute. I doubt you could communicate effectively with Morse at even
10 WPM on that particular channel given all the noise and severe
fading.

Now imagine what I could do with my digital scheme if I slowed it down
to typical Morse speeds. See the web pages I did way back in 1995 on
exactly this topic:

http://cache.qualcomm.com/karn/fecdemo/index.html (CW vs slow digital)
http://people.qualcomm.com/karn/voicedemo/index.html (SSB vs digital voice)

>Amateur Radio is not the Internet, the signalling speed is often far less
>important than making a minimal contact. That is the essence of DX work
>on most bands.

Yes, but this isn't an argument for CW. It's a good argument for a
very low speed, strongly coded digital mode that can be much more
effective than CW.

> EME is very power inefficient, so what ?

EME may be "power inefficient" in that the path loss is far greater
than alternative propagation modes, but we're talking about modulation
& coding schemes, not propagation modes.

One of the best reasons for hams to pursue more power-efficient and
robust digital modulation and coding schemes is to enable us to use
those more challenging propagation modes, and EME is probably the most
challenging of them all. Check out the paper W3IWI and I did on "DSP
Moonbounce" at Central States some years ago.

Phil


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