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[hfsig] RE: What can you do with an OFDM signal in a 3.5 KHz band pass?

W6AFK w6afk at amsat.org
Sun Aug 21 17:13:27 UTC 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 ;-)

Bruno, W6AFK/VE2EQ
Oceanside California
Grid: DM13ie






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