[hfsig] RE: What can you do with an OFDM signal in a 3.5 KHz band pass?
w6afk at amsat.org
Sun Aug 21 12:13:27 CDT 2005
On Sun, 21 Aug 2005 12:41:31 +0100 (BST), Trevor AKA wrote:
Raw data rates of up to 12800 kbps have available commercially for a number
of years now using standard HF SSB rigs. There are several products
available using MIL-STD-188-110B waveforms an example is
http://www.rapidm.com/rm6.shtml sure they drop down to lower speed if the
path isn't great, but on many paths the higher rates are viable.
However, I am not aware of any Amateur implementation of these waveforms,
has anybody tried ? For some of our HF bands I believe there are valid
arguments for restricting b/w to 1.8 kHz or less but for 28 MHz there is no
reason why 3.5 kHz or 7.5 kHz shouldn't be used.
73 Trevor M5AKA
IMHO, it's not really a matter of bit rates but of throughput.
Through experience, I can state that 9600 bps (SSB) and 12800 bps (2x ISB)
rates are rarely maintained over HF links that require the ionosphere to
propagate, be it NVIS or multi-hop. So even if a system occasionally
attains the highest rate for a few ARQ data blocs, it will drop to lower
ones with the associated overhead and retry requests. Again, through
experience, one must NOT only rely on these rates for throughput
calculations as they are practically unrealistic unless you are using
surface wave and/or the propagation is excellent (both path quality and
also no interference).
110B modems offer an advantage over 110A models in that they can maintain
3200+ bps over the nominal 1200 bps (I know, 110A raw bit rates are higher,
but not sustainable). I am aware of the excellent RapidM products, as well
as Titan-L3's series, Rockwell's Q-series and NSG Datacom's products...
having used all of them.
The improvement ---> One way to increase the general throughput is via data
compression prior to transmission. You can get amazing throuputs by using
the right compression algorithm for the file you are sending. Naturally,
you want to compress the file prior to encryption as good encryption will
leave no room for compression ;-)
If the goal is to measure pages (or parts of) per minute, then you would be
surprised at throughput calculations when the HF E-mail application
combines the right compression algorithms with the higher bit rates. Text
files, word files, spreadsheets, databases can all be dramatically
compressed. Pre-compressed images and pre-encrypted messages do not. So a
5kB text file will be sent much faster than a 5kB ciphered file. Perception
is everything ;-)
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