Re: [Discuss-gnuradio] Spectrum from 1 to 10 Mhz

[Dave Emery]

On Tue, Aug 31, 2004 at 11:52:50PM -0600, Jung Ko wrote:
> Hi everyone,
> 
> Just out of curiosity, does anyone know what kind of modulation
> scheme(s) is used for the frequency range from 1 to 10 Mhz for over
> the air transmissions? Basically, I would like to know what can I
> expect to receive if I tune the tuner into this frequency range and
> write an application to demodulate the signal.
> 
> Any pointer or help is greatly appreciated! Thanks!!

        Most transmissions in this range fall into three or four
catagories....

        Most listen able (for most folks) are AM broadcasts.   This
includes the MF "AM" broadcast bad from 540-1700 which is all DSB full
carrier AM (except for a tiny number of stereo AM stations and a very
few experimental digital signals).   Channel bandwidth is 10 khz...  
Above 4 mhz or so there are DSB AM shortwave broadcasters from all over
the world, located in a series of shortwave broadcast bands allocated by
international agreement.   These are powerful signals that boom in most
places.   A few shortwave broadcasters transmit SSB, usually with
substantial carrier energy present (not suppressed carrier SSB common
elsewhere), but the great majority are still DSB-AM transmissions.   And
recently a few short wave broadcasters have adopted a digital format
called DRM (nothing to do with the RIAA and its evil schemes) - gnuradio
could no doubt be made to demodulate DRM....

        The second major modulation mode in this region of the spectrum
is SSB - this includes both USB (most common) and LSB (used by hams) -
single sideband suppressed carrier 2-3 khz wide channel voice.   Primary
users here are hams (in a series of ham bands), the US military (almost
all military aircraft carry HF SSB radios), long range commercial
aviation flying over the ocean, and various marine interests including
private sail boats.   Other users include various government agencies,
lots of illegal pirates, and military forces of other nations. HF still
represents a valid world wide communications mode that works without
infrastructure (no satellite and ground control stations needed) so it
is still commonly installed as a last gasp backup system in case all
else fails.

        A variant of simple SSB has grown a lot in popularity in the HF
bands in recent years - namely ALE (automatic link establishment) which
uses a multi tone FSK signaling format on a SSB voice channel to control
a handshake between stations - typically on a whole range of frequencies
chosen to cover all possible propagation conditions - allowing more
automated HF contacts.  Actual traffic on the channel established by the
ALE handshake is normal SSB voice.

        The third modulation mode in this spectrum is narrow band
digital transmission at low baud rates from 45 to 200 baud or so  - the
most common modulation here being frequency shift keying of a carrier
with shifts between 40 hz and 850 hz. This kind of transmission was once
extremely common in the HF bands, and typically carried baudot coded
radio teletype traffic.   Now there is much much less of it, but there
still is a significant amount - almost all of which is encrypted with
modern crypto gear such as the KG-184 and without the keys
unintelligible to any mere mortals.   Aside from ham rtty signals, there
are very few signals that carry unencrypted traffic any more.

        Hams have made another couple of narrow band digital modes
popular in ham bands - narrow band PSK (psk31 etc), and a more complex
modulation and protocol called pactor.   These involve more
sophisticated modulation and coding than simple fsk but are also narrow
band digital at low baud rates.   A few commercial and
government/military users use variants of these modes outside the ham
bands as well.

        And needless to say, hams and a dwindling number of other users
still transmit messages using on off keying of a carrier in morse code. 
This ranges from 5 wpm to maybe 40... usually hand sent and almost
always received by human ears rather than software.   This mode, called
cw,  is the granddaddy of them all but morse has largely been phased out
by everyone else but the ham population over the last 10 years or so as
it takes too much skill and training to learn.   It is still a very
popular mode in the ham bands, however.

        And the final modulation mode common in the HF spectrum is voice
bandwidth digital.   Most of these transmissions are sent through normal
SSB transceivers with 3 khz bandwidth - using DSP modem technology. Both
multi tone OFDM (so called "parallel tone") and single tone 8 and 16PSK
("serial tone") transmissions are common, though more exotic modulation
shows up too.   A few of these transmissions are in the clear (notably
those used for aircraft messaging (ACARS)), most are encrypted with the
same high security crypto gear used for narrow band digital stuff.   And
the US military uses 39 tone OFDM transmissions for secure low rate
digital voice over HF quite a bit...

        Beyond these basic types of signals there are a growing number
of truly exotic wide band modulations that show up - mostly classified
stuff - including frequency hopping (both over a few khz and wider
bands), direct sequence spread spectrum, and various really wide band
signaling (100 khz or more wide) that can be made to support substantial
data rates in NVIS applications.        

-- 
   Dave Emery N1PRE,  address@hidden  DIE Consulting, Weston, Mass 02493