David,
I totally hear what you are saying, about it just dropping out for a few
seconds at a time.
And that's where having that second link would be great, there are three
approaches that could be taken with this.
1. A second standard radio attached to a second sound card interface.
Digitally selecting the best channel. For RX
2. Make something the the Softrock TXRX with dual tx/rx chains and a Audio
Codec (sound card) built on to it. Both RX and TX.
3. Make an analog mixer to choose the strongest signal.
For now, I am going to start playing around with option 1, but option 2
seems like the best long term goal.
I would like to run some BER tests over both radio with different
polarizations at the same time to see if the errors align or are
totally separate.
Also I would like to see how much a second antenna with the same
polarization helps. (spaced at .8 to 2 Wavelengths)
Basically I want to see how much potential there is.
73s,
Daniel Mundall
On Wed, Mar 6, 2013 at 2:15 PM, David Rowe <[email protected]> wrote:
> Hi Daniel,
>
> Re FEC currently I am currently trying a Golay (23,11) code which is
> about half rate, and can correct 3 errors out of 12 data bits. Lots of
> other possibilities for FEC, for those who are interested in
> experimenting it would be easy to adapt fec_enc.c & fec_dec.c to other
> codes.
>
> One key limitation is the nature of 2.5kHz wide HF channels. Sometimes,
> all the data just gets wiped out for several seconds ...... not much you
> can do about that but fortunately speech is quite redundant. Now if you
> could send the data one one channel, and the FEC parity bits on
> another ... then we might have something.
>
> Narrower carrier spacing won't affect the tx power output. Less
> carriers (like reducing Nc from 14 to 4) might reduce the PAPR and hence
> allow greater tx power, but its hard to squeeze the data we need into 4
> carriers.
>
> Thanks for the diversity links. Good to see you working on this.
>
> Cheers,
>
> David
>
> On Wed, 2013-03-06 at 08:14 -0800, Daniel Mundall wrote:
> > Hi Bruce/David/Barry,
> >
> >
> > I totally like the idea of diversity, and what really needs to happen
> > is actually testing some links to see how much it helps. I am going
> > to see about getting some antennas setup here for experimenting.
> >
> >
> > The receive seems like the easiest to work on, using something like
> > MRC (using the data from the antenna with the best SNR on a frame by
> > frame basis.) Doing that would insure that you had the best possible
> > RX between the two antennas. 2RX = ~3db, 4RX = ~6db
> >
> >
> > Maybe .8 wavelengths would be a good spacing?
> > http://www.jpier.org/PIER/pier64/02.06062401.Khaleghi.pdf
> >
> >
> >
> >
> >
> >
> > It also looks like something like STD (sending every other frame
> > through a different antenna can gain a little bit more) Might be
> > possible with a STDP RF switch and only one radio? Gain: ~2db
> >
> >
> > Bruce, thanks for the links! Very interesting. Now I just want to try
> > it! :)
> >
> >
> > David, maybe it's just me, but FEC (Reed Solomon) seems kind of
> > pathetic for the amount of code word vs correction ability.
> > I think it was something like 35% data added for 7% correction. And
> > going up from there data wise. But then again it might be possible to
> > optimize it.
> >
> >
> > One other thing I was thinking about, and I am sure you guys and
> > already talked about this in times gone by, but would it be possible
> > to narrow the carriers to something less than 75Hz and keep the symbol
> > rate the same? It would allow for more power output, right?
> >
> >
> > Anyways, thanks for your thoughts guys, maybe I can try a link with
> > one of you some time using receive diversity. :)
> >
> >
> > 73s
> > Daniel VA7DRM
> >
> >
> > Sent from Daniel's iPhone
> > image.png
> >
> > image.png
> > On Mar 6, 2013, at 4:33 AM, Barry White <[email protected]>
> > wrote:
> >
> >
> > > An effective fade reduction antenna is to have one vertical and one
> > > horizontal antenna. Due to changes in the ionosphere the signals
> > > can
> > > arrive with constantly changing polarisation.
> > >
> > > Barry VK2AAB
> > >
> > >
> > > David Rowe wrote:
> > > > Hi Daniel,
> > > >
> > > > Yes I have wondered if a separate rx with separate antenna might
> > > > help
> > > > get around fades. I imagine there needs to be a certain
> > > > separation
> > > > (certain number of wavelengths) between the antennas.
> > > >
> > > > Some other ways to get that 6dB are coding gain (if FEC can be
> > > > made to
> > > > work better) and lower Codec bit rates (if that doesn't make the
> > > > robustness worse). At the tx end higher average power (reducing
> > > > PAPR
> > > > issues) would also help the link budget.
> > > >
> > > > Thanks,
> > > >
> > > > David
> > > >
> > > > On Tue, 2013-03-05 at 20:30 -0800, Daniel Mundall wrote:
> > > > > David,
> > > > > Amazing to see all the work that has gone into getting it where
> > > > > it is.
> > > > > Thinking about that 6db you are looking for, wouldn't it be
> > > > > possible
> > > > > to get that at some point with something like SoftRock with dual
> > > > > tx-rx
> > > > > chains (MIMO)? It may not be practical for the lower
> > > > > bands(antenna
> > > > > size), but when possible it may be one of the easiest ways to
> > > > > get that
> > > > > link budget and possibly even lower the tx power.
> > > > > I found some of the ideas here interesting
> > > > > http://www.sequans.com/pdf/SequansUplinkMIMOWP0608_v2.pdf
> > > > > It may not something for right now, but it looks like something
> > > > > with
> > > > > potential for the future.
> > > > > 73s
> > > > > Daniel
> > > > >
> > > > >
> > > > > Sent from Daniel's iPhone
> > > > >
> > > > > On Mar 5, 2013, at 12:09 PM, David Rowe <[email protected]>
> > > > > wrote:
> > > > >
> > > > >
> > > > > > Hello,
> > > > > >
> > > > > > I have just published a new blog post on making FreeDV robust
> > > > > > over
> > > > > > HF
> > > > > > multipath channels:
> > > > > >
> > > > > > http://www.rowetel.com/blog/?p=2905
> > > > > >
> > > > > > Thanks,
> > > > > >
> > > > > > David
> > > > > >
> > > > > >
> > > > > >
> > > > > >
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