| |
VE2HAR > MT63 07.03.05 15:19l 132 Lines 4573 Bytes #-7502 (0) @ WW
BID : 62753SENTTO
Read: GUEST
Subj: Re: [MT63] 20m Olivia experiment results
Path: DB0FHN<DB0RGB<OK0PPL<DB0RES<ON0AR<VA2HAR<VE2HAR
Sent: 050307/1318z @:VE2HAR.#MTL.QC.CAN.NOAM Laval #:40059 $:62753sentto
Dave,
Your results are interesting.
At 12:46 AM 3/7/2005, you wrote:
>Steve and I started by establishing communications in Olivia's default
>mode of 32
>tones within 1,000 Hz bandwidth. We both experienced 100 percent print at
>both 50
>W and 5 W, notably after we moved up 500Hz to avoid a packet station.
>
>Settings of 16 tones and 500 Hz produced the same results at both power
>settings,
>just with slightly slower print. At 50 W, Steve reported that I was S7 on
>his meter,
>while I sent him an S6. The noise level at my QTH was sitting right around
>S3, and
>when we went QRP, Steve's signals hovered between S3 and S4, but I never
>lost a
>character.
>
>Then we went to a setting of 32 tones in 250 Hz of bandwidth. What a
>change! The
>text, while running between 75 to 100 percent copy, slowed to a crawl at
>7.81 baud
>and 0.6 characters per second. The spacing of the tones was quite narrow.
This is interesting. I wonder if either station was able to reduce
receiver bandwidth to 1000Hz, 500Hz, and 250 Hz? There is a penalty for
increasing the number of tones, but that can be offset by reducing the
bandwidth thus increasing the S/N ratio. The other affect with Olivia is
increasing the interleaving depth, which seems to have had little affect
during your test. I would guess that the QSB was not too bad (boy talk
about relative terms). Another way of saying this is that when your fade
margin is higher, increasing the number of tones has little constructive
affect.
>Next we went to 8/250 and the print was much better, near perfect at both
>at 50 and
>5 W. As a tuning aid, I was sampling the audio out of the back of the 706
>and feeding
>that to a old laptop running DigiPan, so I could use the panoramic
>display. The
>difference between 32 tones and 8 tones in the same bandwidth was visually
>appreciable.
I don't think interleaving was helping you that much, and I suspect the
receiver bandwidth did not change, so you were paying less penalty for the
number of tones.
>Much wider tones made for a more robust contact. While a tad slower
>throughput than with the default 32/1000 (2.4 cps at 32/1000 vs. 1.5 cps
>at 8/250),
>we were having a solid QSO and taking up one-quarter of the bandwidth,
>which all
>digital operators can appreciate.
I think you are saying that the tone spacing was wider, as the tone
duration is a function of baud rate.
>Then Steve suggested we go for broke and set up at 4 tones and 125 Hz
>bandwidth.
>Surprisingly, this set-up was as good or better than 8/250. The baud rate
>was the
>same (31.25), although we lost one-third of the character rate, dropping
>to 1.0 cps.
This shows again that the interleaving depth was not a factor.
>We decided to go to low power again, and at the end of Steve's
>transmission, he
>revealed he'd been at 200 mW, as low as the Mk V Field would go. Whoa! At
>times his
>signal faded out altogether, but on the receiving end, I only lost one
>character.
This is an example where interleaving depth and Viterbi decoding did help
>The
>software was actually copying below the noise!
I doubt that it was copying below the noise. Even on an AGWN channel when
the average noise and average signal are equal, 50 % of the time the signal
is above the noise, and 50% of the time the signal is below the noise. If
we spread out our bit errors by interleaving, and we have FEC, the protocol
can "fill in the gaps" during the time that the signal falls below the
noise. I hear this "below the noise" claim from PSKers all of the
time. The average signal may be below the average noise, but there are
times when that signal pops its head above the noise. Otherwise you would
not copy.
Very nice report Dave. It certainly gives us some things to think about.
73,
Mark N5RFX
------------------------ Yahoo! Groups Sponsor --------------------~-->
Tired of hearing the same songs over and over?
Listen to Internet Radio! Skip songs. Click to listen to LAUNCHcast!
http://us.click.yahoo.com/.mKGzA/HARHAA/kkyPAA/CPMolB/TM
--------------------------------------------------------------------~->
<< Try MT63 on 80m - great fun!>>
- The MT63 Reflector -
MT63@egroups.com
(To unsubscribe. send email to
MT63-unsubscribe@onelist.com)
Yahoo! Groups Links
<*> To visit your group on the web, go to:
http://groups.yahoo.com/group/MT63/
<*> To unsubscribe from this group, send an email to:
MT63-unsubscribe@yahoogroups.com
<*> Your use of Yahoo! Groups is subject to:
http://docs.yahoo.com/info/terms/
Read previous mail | Read next mail
| |
