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[hfsig] What can you do with an OFDM signal in a 3.5 KHzband pass?

Kerry Berland kberland at ix.netcom.com
Sat Aug 20 22:53:40 UTC 2005


Here are some first thoughts on estimating OFDM throughput over a voice
bandwidth channel.

 

Here are some assumptions:

12 audio channels (carriers), spaced at multiples of 100 Hz over the audio
band.

Each carrier is DPSK modulated, differential phase shift keying, 4 different
phase angles, therefore encoding two bits per symbol period (2 bits per
baud).

Baud rate is 100 per second.

So the net raw throughput is 12 carriers x 2 bits/symbol/channel x 100
symbols/second or 2400 bits per second.

 

However this is the raw data rate. This gets reduced by several factors-

Protocol overhead within each packet, that is, need for headers, send
sequence numbers, checksums, etc.

Coding overhead, that is, need to send extra bits, implementing some form of
FEC, forward error correction.

Ack/nak time, that is, need to stop sending periodically to allow the
receiving unit to acknowledge packets.

Poor transmission conditions, which often cause one more more of the 12
carrier channels not to be useful in transmitting data.

 

50% net throughput might be a goal in good HF conditions-that is, equivalent
of 1200 bps.

 

Many of the above details could be changed or optimized but my guess is that
the above is not off by a large amount.

 

Comments from those with experience in this area would of course be welcome.

 

Kerry Berland @ home

kberland at ix.netcom.com

773-267-1563

 

-----Original Message-----
From: hfsig-bounces at lists.tapr.org [mailto:hfsig-bounces at lists.tapr.org] On
Behalf Of tattje
Sent: Saturday, August 20, 2005 3:14 PM
To: TAPR HF Modes SIG Mailing List
Subject: RE: [hfsig] What can you do with an OFDM signal in a 3.5 KHzband
pass?

 

The POTS will give you 56 kbit/sec in the same bandwidth.

Using OFDM some channels will not be available, fec is not enough, so one
has to spread the info over all the channels.

Reread the fs-1052, why do you think are so many different throughput speeds
defined?

10 kbits in 3 kHz? Forget it, may be sometimes, but not always.

HF is a naughty channel,

Regards

Henk

-----Oorspronkelijk bericht-----
Van: hfsig-bounces at lists.tapr.org
[mailto:hfsig-bounces at lists.tapr.org]Namens DuBose Walt Civ AETC CONS/LGCA
Verzonden: vrijdag 19 augustus 2005 22:25
Aan: 'TAPR HF Modes SIG Mailing List'
Onderwerp: RE: [hfsig] What can you do with an OFDM signal in a 3.5 KHz band
pass?

When talking to EOC, disaster relief and military contingency personnel
about HF data modes, they no longer ask how many words per minute is it, now
they are asking how many pages per minute is your capability.

We can no longer think in WPM capability; but rather, pages per minute
(ppm). 

73, 

Walt/K5YFW 

-----Original Message----- 
From: hfsig-bounces at lists.tapr.org [mailto:hfsig-bounces at lists.tapr.org]On
Behalf Of DuBose Walt Civ AETC CONS/LGCA 
Sent: Friday, August 19, 2005 2:41 PM 
To: 'hfsig at lists.tapr.org' 
Subject: [hfsig] What can you do with an OFDM signal in a 3.5 KHz bandpass? 

 

What can you do (what RAW throughput) can you get in a 3.5 Khz bandwidth
using an OFDM mode? 
Here's what I found using some figures based on the FS-1052, Appendix A
modem. 
Your User Throughput will be determined on how much RAW througput you use
for FEC. 
Walt/K5YFW 
  
  

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