Hi Daniel,
Bill Cowley suggested a scheme where we "code across" two diverse
channels. So the data might go on one channel, and the FEC parity bits
on another. For example he has a combined microwave and free space
optical link:
http://www.rowetel.com/blog/?p=657
The idea is that if one channel get wiped out, the other will be OK, so
we can recover the bits. BTW using FEC is more efficient that sending
the same signal twice, the total BW and bit error rate will be better.
In our case we could send the Codec 2 bits on one frequency, and the
parity bits on another frequency, some distance away. This could be
done with two transmitters, or one with a really wide bandwidth.
So we would have two narrowband signals, some distance apart. When a
fade hits one it is unlikely hit the other.
Not sure how "legal" this is on the Ham bands ...... the total BW would
still be less than a single SSB signal, but imagine the confusion in
checking if two frequencies are in use before transmitting!
Of course many other diversity schemes are possible, such as the ones
you suggested below.
Cheers,
David
On Wed, 2013-03-06 at 15:18 -0800, Daniel Mundall wrote:
> 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
> > > > > >
> > > > > >
> > > > > >
> > > > > >
>
> ------------------------------------------------------------------------------
> > > > > > Everyone hates slow websites. So do we.
> > > > > > Make your web apps faster with AppDynamics
> > > > > > Download AppDynamics Lite for free today:
> > > > > > http://p.sf.net/sfu/appdyn_d2d_feb
> > > > > > _______________________________________________
> > > > > > Freetel-codec2 mailing list
> > > > > > [email protected]
> > > > > >
> https://lists.sourceforge.net/lists/listinfo/freetel-codec2
> > > > > >
> > > > >
>
> ------------------------------------------------------------------------------
> > > > > Symantec Endpoint Protection 12 positioned as A LEADER
> in The
> > > > > Forrester
> > > > > Wave(TM): Endpoint Security, Q1 2013 and "remains a
> good choice"
> > > > > in the
> > > > > endpoint security space. For insight on selecting the
> right
> > > > > partner to
> > > > > tackle endpoint security challenges, access the full
> report.
> > > > > http://p.sf.net/sfu/symantec-dev2dev
> > > > > _______________________________________________
> Freetel-codec2
> > > > > mailing list [email protected]
> > > > >
> https://lists.sourceforge.net/lists/listinfo/freetel-codec2
> > > >
> > > >
> > > >
> > > >
>
> ------------------------------------------------------------------------------
> > > > Symantec Endpoint Protection 12 positioned as A LEADER
> in The
> > > > Forrester
> > > > Wave(TM): Endpoint Security, Q1 2013 and "remains a good
> choice"
> > > > in the
> > > > endpoint security space. For insight on selecting the
> right
> > > > partner to
> > > > tackle endpoint security challenges, access the full
> report.
> > > > http://p.sf.net/sfu/symantec-dev2dev
> > > > _______________________________________________
> > > > Freetel-codec2 mailing list
> > > > [email protected]
> > > >
> https://lists.sourceforge.net/lists/listinfo/freetel-codec2
> > > >
> > >
> > >
>
> ------------------------------------------------------------------------------
> > > Symantec Endpoint Protection 12 positioned as A LEADER in
> The
> > > Forrester
> > > Wave(TM): Endpoint Security, Q1 2013 and "remains a good
> choice" in
> > > the
> > > endpoint security space. For insight on selecting the
> right partner
> > > to
> > > tackle endpoint security challenges, access the full
> report.
> > > http://p.sf.net/sfu/symantec-dev2dev
> > > _______________________________________________
> > > Freetel-codec2 mailing list
> > > [email protected]
> > >
> https://lists.sourceforge.net/lists/listinfo/freetel-codec2
> > >
> >
>
> ------------------------------------------------------------------------------
> > Symantec Endpoint Protection 12 positioned as A LEADER in
> The Forrester
> > Wave(TM): Endpoint Security, Q1 2013 and "remains a good
> choice" in the
> > endpoint security space. For insight on selecting the right
> partner to
> > tackle endpoint security challenges, access the full report.
> > http://p.sf.net/sfu/symantec-dev2dev
> > _______________________________________________
> Freetel-codec2 mailing list
> [email protected]
> https://lists.sourceforge.net/lists/listinfo/freetel-codec2
>
>
>
>
> ------------------------------------------------------------------------------
> Symantec Endpoint Protection 12 positioned as A LEADER in The
> Forrester
> Wave(TM): Endpoint Security, Q1 2013 and "remains a good
> choice" in the
> endpoint security space. For insight on selecting the right
> partner to
> tackle endpoint security challenges, access the full report.
> http://p.sf.net/sfu/symantec-dev2dev
> _______________________________________________
> Freetel-codec2 mailing list
> [email protected]
> https://lists.sourceforge.net/lists/listinfo/freetel-codec2
>
>
>
> ------------------------------------------------------------------------------
> Symantec Endpoint Protection 12 positioned as A LEADER in The Forrester
> Wave(TM): Endpoint Security, Q1 2013 and "remains a good choice" in the
> endpoint security space. For insight on selecting the right partner to
> tackle endpoint security challenges, access the full report.
> http://p.sf.net/sfu/symantec-dev2dev
> _______________________________________________ Freetel-codec2 mailing list
> [email protected]
> https://lists.sourceforge.net/lists/listinfo/freetel-codec2
------------------------------------------------------------------------------
Symantec Endpoint Protection 12 positioned as A LEADER in The Forrester
Wave(TM): Endpoint Security, Q1 2013 and "remains a good choice" in the
endpoint security space. For insight on selecting the right partner to
tackle endpoint security challenges, access the full report.
http://p.sf.net/sfu/symantec-dev2dev
_______________________________________________
Freetel-codec2 mailing list
[email protected]
https://lists.sourceforge.net/lists/listinfo/freetel-codec2
